After reading the books by Toole (Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms) and Geddes (Premium Home Theater : Design & Construction) I have a few residual questions. It is very likely that I did not comprehended 100% of what I read and that these leftover thoughts and questions have been answered but I was looking for further direction and to start a conversation about peoples thoughts on such issues.
I am looking at most things through the lens of stereo music listening and soundstage and specifically focusing on setting up the L&R speakers (as I firmly believe that the best way to set up the L&R channels is for stereo music listening, and then if one wants to integrate that into a 5.1 or home theater setup they will be already optimized and in the right position). So I am wondering specifically about setting up the L&R channels and specifically imaging / soundstage and creating a wide sweet spot.
Here is what I would like to discuss:
1) Speaker positioning from the front wall and soundstage depth:
There was not too much discussion in terms of ideal positioning from the front wall outside of knowing about speaker boundary interference and adjacent boundary effects and how it will affect the bass response. It seems that bass response is the main consideration with positioning the speaker in/flush/in front/away from the front wall. Toole says “Adding absorption to the front wall, behind the loudspeakers, reportedly improved image localization and reduced coloration... Memo for Listening room recommendations: add sound absorbing material to front wall”
It seems the general audiophile advice is to pull speakers away from the front wall to help create a deeper soundstage etc yet I did not see this explicitly mentioned by Toole or Geddes. They seem to be ok with positioning the L&R speakers close to the front wall as long as one pays attention to bass response and possibly absorbs.
What I am wondering is if there truly is a benefit in soundstage depth with pulling the speakers away from the front wall?
Or do people think it is a psychoacoustic effect by having the front wall further away from the source (not saying that it is not a very real psychological effect, but merely one that cannot be seen on the frequency response, nor reproducible with blind tests)?
I wonder if this explains it: “Generations of listeners have noted the obvious differences in directional and spatial impressions created by sounds panned to the real left and right loudspeakers and those panned to intermediate positions, including center. The difference is that the extreme left and right locations are created by monophonic signals, delivered to single loudspeakers, whereas the intermediate image locations result from “stereo” signals, delivered to both loudspeakers simultaneously, with amplitude biases and/or delays appropriate to define the direction. The common impression is that the left and right panned sounds appear to originate in the loudspeakers themselves, whereas the intermediate images appear to originate further back, in a more spacious setting, and sometimes elevated. Instead of a soundstage extending across a line between the loudspeakers, the center images tend to drift back-ward.” - Toole
As then if the sound is hard panned to the L or R then it comes further forward (from the speaker), and then if intermediate positions then it appears higher up and further back and hence creates the impression of depth? Do people think that explains what we observe / hear?
What are peoples thoughts on the optimal distance from the front wall and imaging dept?
2) Speaker positioning from the side wall and side reflections:
Despite popular belief, it seems that the side wall reflections are important and desired. Specifically those that can come from wide angles (60°) as Toole writes “Start to think in terms of “preference,” “spaciousness,” “low interaural cross-correlation (IACC),” and “lateral reflections” as positively correlated with each other…. For maximum “preference” from the Ando (1977) data, it seems that reflections from about 30° to 90° are most effective. When IACC is measured, a broad minimum is seen around 60°, corresponding to a maximum in the preference ratings. Preference, therefore, is associated with low interaural cross-correlation….IACC exhibits a broad minimum around 60°.”
So should this goal of 60° determine the side wall distance goals? I think a lot of people would have a hard time getting 60° reflections from the near side wall but would be getting these angles from the far side wall.
Should we be setting up our speakers to ensure ~60° angle of the near wall reflections? If we do not have enough lateral room for this should this call into question the orientation of our rooms (having the width of the room be larger than the length if it allows for 60 angle reflection)?
3) Precedence Effect and side reflections
I am sure I just missed something somewhere when it comes to the precedence effect. But my understanding is that the primary sound will be heard first and that all reflected sounds will be combined into the first arriving sound. My understanding is that this is commonly misunderstood to mean that the delayed signals are ignored which is certainly not the case, as they are combined with the original signal, but that the original signal has all these other delayed signals added into the interpretation of it.
Toole mentions that delays greater than 30-40ms can then be interpreted as separate sources and thus everything under falls into the precedence effect does it not? So if the signals are being combined to that first signal, then does the distance of reflections matter? Does it matter if the side walls are mere inches vs feet vs meters away? Should it not all be interpreted under the primary signal? If that is the case, then would not all reflections in small rooms (under 30ms), be of no consequence when it comes to imaging? Once again, I am sure I am missing something here, but I like to think I understood that the delayed signal is combined with the original (and not ignored), but that the original signal trumps the delayed signal in terms of localization etc. But now it has me questioning the effects of all reflections and if they truly are damaging to soundstage.
4) Tooles low interaural cross-correlation (IACC) for imaging vs Geddes time intensity trading for image stability?
It seems like there are two opposing forces at play when it comes to toeing in of the speakers.
If you want truly low IACC and to isolate the L&R signals to the L&R ears (as used to the extreme with Ambiophonics or placing a mattress in between the speakers and up to your face) then one would not want to toe in the speakers to an extreme (at maximum aiming directly at the listening position, but probably further outward as ones off axis response allows). This should help with the soundstage and imaging. So I wonder if absorbing the far wall reflections (even though they are at a wider angle and thus less IACC, would help with soundstage would it not (although maybe at the cost of envelopment / spaciousness)
If you want a more stable “sweet spot” then using time intensity trading and aggressively toeing in the speakers as recommended by Geddes sure makes a lot of sense, and is easily experienced when I have personally done it. But is this improved sweet spot and image stability coming at the cost of a larger soundstage? I have not been able to definitively test this as I am not able to do blind tests myself, but I wonder what people impression is.
5) Finding the sweet spot:
I have never found an objective way to find the sweet spot for speakers until I read this from Toole:
“On the matter of the sound quality of the center phantom image in stereo, I recommend a simple experiment: Arrange for monophonic pink noise to be delivered to both loudspeakers. When seated in the symmetrical sweet spot, this should create a well-defi ned center image midway between the loudspeakers. If it does not, something is seriously wrong. If it does, consider what you hear as you lean very slightly to the left and to the right of the symmetrical axis. The timbre of the noise changes and more obviously the closer you sit to the loudspeakers. In fact, it is possible to fi nd the exact sweet spot by simply listening to when the sound is dullest. Moving even slightly left or right of the sweet spot causes the sound to get audibly brighter; there is more treble. It is much more exact to find the sweet spot by listening to the timbre change than by trying to judge when the center image is precisely localized in the center position. There is nothing faulty with the equipment or setup; this is simply stereo as it is—flawed…. Figure 9.7d shows the difference between the curves, revealing the result of acoustical interference. This can be confirmed by a simple calculation. The time differential between the ears for a sound source at 30° away from the frontal axis is about 0.27 ms for an average head. A destructive acoustical interference will occur at the frequency at which this is one-half of a period: 1.85 kHz. It won’t be a perfect cancellation because of a tiny propagation loss and a signifi cant diffraction effect. The wavelength is just over 7 in. (178 mm), which, because it is similar in dimension to the head, will experience a substantial head-shadowing effect at the ear opposite to the sound source. There will be an interaural amplitude difference of the order of 6 dB in this frequency range”
Does this seem like the best way to find and judge the sweet spot in ones room?
In the past I have also used repeating short clips from songs recorded in QSound (I have taken a few seconds from the openings to a song in Amused to Death and made a loop on perpetual repat and then inverted the R&L channels to test each speaker and its positioning). Has anyone else tried something similar or what do people think is the best way to test imaging (outside of grossly assessing soundstage from well-known songs)?
If anyone wants to chat and discuss any of the above topics that would be much appreciated!
Thanks!
I am looking at most things through the lens of stereo music listening and soundstage and specifically focusing on setting up the L&R speakers (as I firmly believe that the best way to set up the L&R channels is for stereo music listening, and then if one wants to integrate that into a 5.1 or home theater setup they will be already optimized and in the right position). So I am wondering specifically about setting up the L&R channels and specifically imaging / soundstage and creating a wide sweet spot.
Here is what I would like to discuss:
1) Speaker positioning from the front wall and soundstage depth:
There was not too much discussion in terms of ideal positioning from the front wall outside of knowing about speaker boundary interference and adjacent boundary effects and how it will affect the bass response. It seems that bass response is the main consideration with positioning the speaker in/flush/in front/away from the front wall. Toole says “Adding absorption to the front wall, behind the loudspeakers, reportedly improved image localization and reduced coloration... Memo for Listening room recommendations: add sound absorbing material to front wall”
It seems the general audiophile advice is to pull speakers away from the front wall to help create a deeper soundstage etc yet I did not see this explicitly mentioned by Toole or Geddes. They seem to be ok with positioning the L&R speakers close to the front wall as long as one pays attention to bass response and possibly absorbs.
What I am wondering is if there truly is a benefit in soundstage depth with pulling the speakers away from the front wall?
Or do people think it is a psychoacoustic effect by having the front wall further away from the source (not saying that it is not a very real psychological effect, but merely one that cannot be seen on the frequency response, nor reproducible with blind tests)?
I wonder if this explains it: “Generations of listeners have noted the obvious differences in directional and spatial impressions created by sounds panned to the real left and right loudspeakers and those panned to intermediate positions, including center. The difference is that the extreme left and right locations are created by monophonic signals, delivered to single loudspeakers, whereas the intermediate image locations result from “stereo” signals, delivered to both loudspeakers simultaneously, with amplitude biases and/or delays appropriate to define the direction. The common impression is that the left and right panned sounds appear to originate in the loudspeakers themselves, whereas the intermediate images appear to originate further back, in a more spacious setting, and sometimes elevated. Instead of a soundstage extending across a line between the loudspeakers, the center images tend to drift back-ward.” - Toole
As then if the sound is hard panned to the L or R then it comes further forward (from the speaker), and then if intermediate positions then it appears higher up and further back and hence creates the impression of depth? Do people think that explains what we observe / hear?
What are peoples thoughts on the optimal distance from the front wall and imaging dept?
2) Speaker positioning from the side wall and side reflections:
Despite popular belief, it seems that the side wall reflections are important and desired. Specifically those that can come from wide angles (60°) as Toole writes “Start to think in terms of “preference,” “spaciousness,” “low interaural cross-correlation (IACC),” and “lateral reflections” as positively correlated with each other…. For maximum “preference” from the Ando (1977) data, it seems that reflections from about 30° to 90° are most effective. When IACC is measured, a broad minimum is seen around 60°, corresponding to a maximum in the preference ratings. Preference, therefore, is associated with low interaural cross-correlation….IACC exhibits a broad minimum around 60°.”
So should this goal of 60° determine the side wall distance goals? I think a lot of people would have a hard time getting 60° reflections from the near side wall but would be getting these angles from the far side wall.
Should we be setting up our speakers to ensure ~60° angle of the near wall reflections? If we do not have enough lateral room for this should this call into question the orientation of our rooms (having the width of the room be larger than the length if it allows for 60 angle reflection)?
3) Precedence Effect and side reflections
I am sure I just missed something somewhere when it comes to the precedence effect. But my understanding is that the primary sound will be heard first and that all reflected sounds will be combined into the first arriving sound. My understanding is that this is commonly misunderstood to mean that the delayed signals are ignored which is certainly not the case, as they are combined with the original signal, but that the original signal has all these other delayed signals added into the interpretation of it.
Toole mentions that delays greater than 30-40ms can then be interpreted as separate sources and thus everything under falls into the precedence effect does it not? So if the signals are being combined to that first signal, then does the distance of reflections matter? Does it matter if the side walls are mere inches vs feet vs meters away? Should it not all be interpreted under the primary signal? If that is the case, then would not all reflections in small rooms (under 30ms), be of no consequence when it comes to imaging? Once again, I am sure I am missing something here, but I like to think I understood that the delayed signal is combined with the original (and not ignored), but that the original signal trumps the delayed signal in terms of localization etc. But now it has me questioning the effects of all reflections and if they truly are damaging to soundstage.
4) Tooles low interaural cross-correlation (IACC) for imaging vs Geddes time intensity trading for image stability?
It seems like there are two opposing forces at play when it comes to toeing in of the speakers.
If you want truly low IACC and to isolate the L&R signals to the L&R ears (as used to the extreme with Ambiophonics or placing a mattress in between the speakers and up to your face) then one would not want to toe in the speakers to an extreme (at maximum aiming directly at the listening position, but probably further outward as ones off axis response allows). This should help with the soundstage and imaging. So I wonder if absorbing the far wall reflections (even though they are at a wider angle and thus less IACC, would help with soundstage would it not (although maybe at the cost of envelopment / spaciousness)
If you want a more stable “sweet spot” then using time intensity trading and aggressively toeing in the speakers as recommended by Geddes sure makes a lot of sense, and is easily experienced when I have personally done it. But is this improved sweet spot and image stability coming at the cost of a larger soundstage? I have not been able to definitively test this as I am not able to do blind tests myself, but I wonder what people impression is.
5) Finding the sweet spot:
I have never found an objective way to find the sweet spot for speakers until I read this from Toole:
“On the matter of the sound quality of the center phantom image in stereo, I recommend a simple experiment: Arrange for monophonic pink noise to be delivered to both loudspeakers. When seated in the symmetrical sweet spot, this should create a well-defi ned center image midway between the loudspeakers. If it does not, something is seriously wrong. If it does, consider what you hear as you lean very slightly to the left and to the right of the symmetrical axis. The timbre of the noise changes and more obviously the closer you sit to the loudspeakers. In fact, it is possible to fi nd the exact sweet spot by simply listening to when the sound is dullest. Moving even slightly left or right of the sweet spot causes the sound to get audibly brighter; there is more treble. It is much more exact to find the sweet spot by listening to the timbre change than by trying to judge when the center image is precisely localized in the center position. There is nothing faulty with the equipment or setup; this is simply stereo as it is—flawed…. Figure 9.7d shows the difference between the curves, revealing the result of acoustical interference. This can be confirmed by a simple calculation. The time differential between the ears for a sound source at 30° away from the frontal axis is about 0.27 ms for an average head. A destructive acoustical interference will occur at the frequency at which this is one-half of a period: 1.85 kHz. It won’t be a perfect cancellation because of a tiny propagation loss and a signifi cant diffraction effect. The wavelength is just over 7 in. (178 mm), which, because it is similar in dimension to the head, will experience a substantial head-shadowing effect at the ear opposite to the sound source. There will be an interaural amplitude difference of the order of 6 dB in this frequency range”
Does this seem like the best way to find and judge the sweet spot in ones room?
In the past I have also used repeating short clips from songs recorded in QSound (I have taken a few seconds from the openings to a song in Amused to Death and made a loop on perpetual repat and then inverted the R&L channels to test each speaker and its positioning). Has anyone else tried something similar or what do people think is the best way to test imaging (outside of grossly assessing soundstage from well-known songs)?
If anyone wants to chat and discuss any of the above topics that would be much appreciated!
Thanks!
Hi,
Lots of questions in there! Some will be controversial so expect differents opinions expressed and to find what works for yourself.
I will answer about some but not in the order you gave.
Yes.
Yes, a lot.
Yes it is as it'll influence the way you'll perceive the recorded image: if the Early Reflection are to short they create combfiltering effect. It'll change the frequency response but not only: it'll mess up transient too (time smearing).
But time is not the only things to take into account: their level is of primary importance too.
That is why Tooles recommend to use absorbsion in frontwall ( it give you control over the level of ER). But this is one way to adress it, other exist too: reflection and diffusion (scattering).
Each one as its pro and cons.
They are but it'll depend of what you want to achieve ( your own preference regarding the rendering of the recorded image). There is two extremes about it : 'you are there'/'they are there'. Off course there is an infinity of intermediate position between them.
'You are there': you favour the image in the recording and thus you'll try to limit at most room's influence on it.
'They are there': you favour the illusion to have the musician in your room so you'll use the room characteristics to influence the rendering.
Both ways have their pro and cons.
Now as you talk about 'all reflections' just take a stepback and look at your room: as distance is of primary importance do you think the side walls are the most offending ones?
Isn't here a dimension ( of the room) which is shorter than the others? If your room is 'typical' you'll soon realise that ceiling is the most offending place regarding early reflections (ground is too but for practical reason except a carpet you can't do many things about it).
Lots of questions in there! Some will be controversial so expect differents opinions expressed and to find what works for yourself.
I will answer about some but not in the order you gave.
Yes.
Yes, a lot.
Yes it is as it'll influence the way you'll perceive the recorded image: if the Early Reflection are to short they create combfiltering effect. It'll change the frequency response but not only: it'll mess up transient too (time smearing).
But time is not the only things to take into account: their level is of primary importance too.
That is why Tooles recommend to use absorbsion in frontwall ( it give you control over the level of ER). But this is one way to adress it, other exist too: reflection and diffusion (scattering).
Each one as its pro and cons.
They are but it'll depend of what you want to achieve ( your own preference regarding the rendering of the recorded image). There is two extremes about it : 'you are there'/'they are there'. Off course there is an infinity of intermediate position between them.
'You are there': you favour the image in the recording and thus you'll try to limit at most room's influence on it.
'They are there': you favour the illusion to have the musician in your room so you'll use the room characteristics to influence the rendering.
Both ways have their pro and cons.
Now as you talk about 'all reflections' just take a stepback and look at your room: as distance is of primary importance do you think the side walls are the most offending ones?
Isn't here a dimension ( of the room) which is shorter than the others? If your room is 'typical' you'll soon realise that ceiling is the most offending place regarding early reflections (ground is too but for practical reason except a carpet you can't do many things about it).
Some food for thoughts about ER and how it is managed in proworld ( i don't suggest you do do same things but it'll give perspective on Tooles explanations):
https://www.diyaudio.com/forums/mul...gineer-mastering-monitors-30.html#post6172211
Controlled Image Design: The management of stereophonic image quality - BBC R&D
Tom Hidley - Non Environment Rooms – Acoustic Fields
https://www.diyaudio.com/forums/mul...gineer-mastering-monitors-30.html#post6172211
Controlled Image Design: The management of stereophonic image quality - BBC R&D
Tom Hidley - Non Environment Rooms – Acoustic Fields
Thanks for the input and information. Yeah I must have gotten bogged down and not comprehended all the info in the book.
Any thoughts on objective ways to optimize the speaker position / toe in for the best soundstage?
It is difficult to answer without knowing what you prefer about 'rendering', the loudspeakers you use and your room.
The 'ideal' position of loudspeaker/sweetspot is a isoceles triangle (60*angle) but you can do differently... there is no hard rules about enjoyment.
About toe in it depend mostly from the type of loudspeakers: constant directivity loudspeakers will benefit from toe in ( if you want a large sweetspot) others type may be indifferent to toe in ( omni design).
About constant directivity loudspeakers it is worth to take a look at Wayne Payrham ( Pi speakers) whitepapers too. He found an interesting answer to the question with his cornerloaded constant directivity PI7. He present an 'ideal room' for them in one of the papers and after studying it i came to the conclusion this is smart approach : it'll give from lateral reflections ER in the range of 15ms and -15db without any treatments of side walls ( attenuation comes from the distance the ER has to travel - you loose 6db each time you double the travel distance from 1m to 2m, to 4m, etc,etc...).
This is a compromise i like a lot as it give a rendering close to what someone experience in first row center of a concert hall: wide and 'in your face' about the presentation.
Don't worry if you don't grab everything at first, this is complicated as many things interacts in acoustic. Wait for other point of view than mine too as i'm biased...
I will give links to some example of ER tayloring in the next few days-audio example- (it is late at night atm and i'm lazy) as i should have some month ago but got sidetracked...
The 'ideal' position of loudspeaker/sweetspot is a isoceles triangle (60*angle) but you can do differently... there is no hard rules about enjoyment.
About toe in it depend mostly from the type of loudspeakers: constant directivity loudspeakers will benefit from toe in ( if you want a large sweetspot) others type may be indifferent to toe in ( omni design).
About constant directivity loudspeakers it is worth to take a look at Wayne Payrham ( Pi speakers) whitepapers too. He found an interesting answer to the question with his cornerloaded constant directivity PI7. He present an 'ideal room' for them in one of the papers and after studying it i came to the conclusion this is smart approach : it'll give from lateral reflections ER in the range of 15ms and -15db without any treatments of side walls ( attenuation comes from the distance the ER has to travel - you loose 6db each time you double the travel distance from 1m to 2m, to 4m, etc,etc...).
This is a compromise i like a lot as it give a rendering close to what someone experience in first row center of a concert hall: wide and 'in your face' about the presentation.
Don't worry if you don't grab everything at first, this is complicated as many things interacts in acoustic. Wait for other point of view than mine too as i'm biased...
I will give links to some example of ER tayloring in the next few days-audio example- (it is late at night atm and i'm lazy) as i should have some month ago but got sidetracked...
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Yes. The main reasons for inwall is about destructive interference in the low end and the fact that once the loudspeakers are integrated into the wall you change the radiating conditions in low end ( the situation about Baffle Step Compensation change as instead of a loss you have an increase in gain ( the hp radiates their power into an hemisphere rather than a sphere). It helps to reduce diffraction ( from the box) too.
Another reason is it 'fixes' the sweetspot at a defined place and for treatment of ER too: in pro control room the walls in which the loudspeakers are puts are angled ( taking into account the directivity behavior of loudspeakers) and there is usually another angling of adjacent walls to help redirect reflections toward the back of the room to help create an Reflection Free Zone around the sweetspot.
It is a bit of an overgeneralization about absorbption. In fact this is the most convenient way to attenuate the level of ER - materials are relatively cheap and doesn't need to modify the architectural shape of the room: with something 20cm deep hanging on the wall you can have pretty efficient treatments.
However as can be seen in the CID documents linked before you can achieve same thing by angling some panels and redirect the sound outside the listening spot. In practice you've got same overall results but with a different 'flavor' as energy put into the room isn't 'robbed' ( by absorption) and it gives a different behavior about how the room react to other source into it (eg:when you talk into the room).
You could use diffusion too and it could be very effective but you'll need a very large room because for the treatments to be efficients at a given frequency you need a minimum distance from it. Usually it is considered to be 3x the wavelength of the lower freq of interest ( same thing happen for reflection).
In the case of diffusion the point is to have an uncorrelated ( non specular) 'backwash' to fool your mind about the room size. You don't really change the time the ER have but you make them more random and spread over a wider area.
What do you call a deeper soundstage?
The ability to 'read' deeper into the recorded image or the ability to have the illusion of the event to 'happen' in the room you are listening to your loudspeaker?
If you want the first then inwall ( and the associated treatments) is your best bet.
If you want the second then it may help to have the loudspeaker more into the room and to angle ( toe in) in order to create some ER which will help to achieve the effect.
Of course it is not black and white and you could achieve first goal with second approach ( or vice versa) but you'll need some treatments to helps ( and to follow 'a plan' about it).
It is definetely at play and very real effect: we are 'visuals' by nature, this sense is the one our brain mainly focus on. Subconsciously we make relation between the cue our auditory system send us and the ones our view send too.
Since our birth we make a library of acoustic cues ( size of room, materials in it,...) and link them to what our eyes register. When we experience something which isn't linked between both senses it can make you feel weird and is a condition for audible 'illusion' to happen.
That is why i recommend to close your eyes when listening. As you won't have visual cues you focus on auditory and it is easier to 'detach' image rendering and have a finer analysis of what happen.
No. The impression of depth is embeded in the reproduced message: reverbs cues ( and ER from the space the message was recorded into), level and eq ( the closer the signal the more hf) give you this informations.
What your room/loudspeakers couple will induce for depth are an effect you put on top.
What you talk about are artefacts from the attempt to reproduce the reality we experience. Is it a big issue? Well i don't believe it is this problematic and one of the many artefacts you'll encounter into the recording/reproducing chain from the instrument to your ears ( eg microphone couples introduce artefacts as they doesn't behave as our ears, nevertheless we reach a point were the illusion is credible to our mind).
There is no conscensus about an optimal distance and will never be as it can be adapted through acoustic treatments and mainly because this is a preference things.
This point is controversial because the type of loudspeakers come into play too: talk about close front wall location for dipole users and wait for reaction ( talk about ER management too and see what happens.
). In a way this reactions are right because if you choosed dipole you are looking for an effect you like.Thus said some diffusion will always be benificial in this case but it could be better if someone with a different bias than mine put it into light.
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Ok Ambiphonic ( iirc) is like binaural: once there is interaural crosstalk it loose it's intrinsic interest ( well this is not as simple but let's assume that for clarity of explanation).
About the back wall, well absorption can help but mainly for lowend, at least in the room treated like that in which i've been.
What is coming from behind you ( back wall) benefit mainly from diffusion. It'll help as it'll give you the feeling the room is bigger than it really is. The same principle than i already given are at play: by making the reflections uncorelated and spread over a wider area we 'trick' our brain that analyse it thinking the room is very deep or even that backwall doesn't exist.
The fact that reflections from diffusors are 'somewhat' predictible ( there is a pattern of diffusion associated with each kind of diffusor) could even help to dissociate the direct signal from the reflected sounds as we can detect recurence/artificiality in it. Even more if it arrive past 30ms or so ( Haas/precedence effect). For this to happen you need at least 3/3,5m behind the sweetspot .
It'll fix the lower freq the diffusion will be effective too.
I would say if you want a wider sweetspot rather than a more stable one. Dr E.Geddes answer is a set of compromise as all choices are.
It won't nescessarely come at the cost of a wider soundstage but it'll depend of the time of arrival and intensity of ER so it ultimately depend of the size of your room.
The guide given by Tooles does absolute make sense. There is other signals than purely pink noise which can be as good ( or even better*) to localize it.
That said here again this is a bit of an oversimplification given: all this does assume you'll listen to direct sound mainly.
Truth is this is ( here again!) dependent of room size: there is a parameter most enthousiast overlook which is called 'critical distance'.
This is the distance in room at which you listen equally to direct sound and soundfield ( reverb).
This is room size AND loudspeaker directivity dependant. And it can de deceptly short! In typical European's sized rooms it can be as low as 50cm...
If you are located above this distance what you listen to is the sound of your room, not your loudspeaker's anymore. Hence the need for a balanced acoustic treatment if this is what you like about presentation!
In that case i don't think the guide given by Tooles is valid anymore...
Hmmm. Qsound is a psychoacoustic heavily treated process, i would not rely on this to determine sweetspot as it is already 'corrupted' and play with trickery of our brain.
Better find a simpler signal in my view ( or something dedicated to it*).
Trust your ears.
Use a well known track it'll most of the time suffice to assess 'close enough' or pink noise.
* i will find a link to a dedicated signal to help.
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Ok here is a test signal: you should have a 'rainbow' like rendering if not well you'll have to modify set up and/or treat the room:
Online LEDR™ Sound Test | Listening Environment Diagnostic Recording Test
There is other tests of interest too.
Well i haven't heard about the study Tooles is talking about.
There is one thing which is sure ( and you made it too) which is the implementation of it. You'll need a huge room.
So would i use this goal to determine sidewall distance?: no because there is more issues about destructive interference with this than any concern about ER for my own preference.
That said nothing stop you to experiment using movable panels to try to have this behavior ( if you can accept it in your living room of course). For 1m width panels you can effectively redirect from 1khz and up. You can even use absorpsion on it too.
Yes you could shift orientation but you'll run into issues about backwall. You'll absolutely need absorbtion on your backwall. But like the rest it'll be a compromise.
If you want to play with this there is a technic used by acoustician which is called 'ray tracing' which help to understand what happen with reflections ( this is an approximation but it works pretty well in practice).
You have an example of it in the CID article as R.Walker used it to determine the profile of rooms they experimented with.
Here is a link for an online calculator:
amcoustics.com: let's help each other to build the best acoustics
Amray - The Raytracing Sketchpad
Ok for now i stop. Sorry for the very long posts but it isn't easy to sum this things up. All this is a simplification* and there is more to it but i don't want to overflow with details ( which may not be details but...).
* i'm sure i will be corrected on points and i'm totally open to it. As i've said i'm biased and as such this is a pov. Other pov can be valid too. One important point however is that you have to stick to a 'concept' ( copyright Wesayso!) about it and not try things randomly if you want good results.
Online LEDR™ Sound Test | Listening Environment Diagnostic Recording Test
There is other tests of interest too.
Well i haven't heard about the study Tooles is talking about.
There is one thing which is sure ( and you made it too) which is the implementation of it. You'll need a huge room.
So would i use this goal to determine sidewall distance?: no because there is more issues about destructive interference with this than any concern about ER for my own preference.
That said nothing stop you to experiment using movable panels to try to have this behavior ( if you can accept it in your living room of course). For 1m width panels you can effectively redirect from 1khz and up. You can even use absorpsion on it too.
Yes you could shift orientation but you'll run into issues about backwall. You'll absolutely need absorbtion on your backwall. But like the rest it'll be a compromise.
If you want to play with this there is a technic used by acoustician which is called 'ray tracing' which help to understand what happen with reflections ( this is an approximation but it works pretty well in practice).
You have an example of it in the CID article as R.Walker used it to determine the profile of rooms they experimented with.
Here is a link for an online calculator:
amcoustics.com: let's help each other to build the best acoustics
Amray - The Raytracing Sketchpad
Ok for now i stop. Sorry for the very long posts but it isn't easy to sum this things up. All this is a simplification* and there is more to it but i don't want to overflow with details ( which may not be details but...).
* i'm sure i will be corrected on points and i'm totally open to it. As i've said i'm biased and as such this is a pov. Other pov can be valid too. One important point however is that you have to stick to a 'concept' ( copyright Wesayso!) about it and not try things randomly if you want good results.
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Thanks! I will take a look! As I do have more constant directivity waveguide speakers so that looks like a good resource.
Perfect thanks!
I haven't read all the responses so I may repeat or disagree, but that's life.
The first thing that I would say is that the question is kind of open ended. My concern is from what direction are we approaching these issues: 1) do we have given speakers and want to find how to design or setup a room that best suites them, or 2) do we have a given room (by far the most common situation) and we want to determine the best speakers and placement for say imaging and/or spaciousness.
I will assume 2), but much of the OPs questions seem to imply that we have room options when we usually don't.
so to the questions:
1) both Floyd and I agree that front wall (which I sometimes call the "back" wall because it is "in back" of the speakers) should be well damped. I have found that no amount of damping on the front wall is too much, the more the better. This absolutely enhances imaging. Few will have the option of placing the speakers out from this wall as there just isn't enough room, so damping it is the key. My front wall has about 1 foot of foam covering it and so very little reflection. SO what is the "optimal" depth? As much as you can, but this is usually not practical, so use heavy damping.
2) side wall reflections are a continued source of disagreement. They will enhance spaciousness but degrade imaging. With a narrow DI speaker we can control how much sound reflects off of which wall. A low DI speaker cannot do this. In experimentation this effect is obvious. Since my speaker are laterally symmetric and very high DI, I can place them such that the direct response stays the same but the side walls get illuminated differently. I put my speakers on rotational bearing and I could change their pointing. The effect is quite pronounced, but as one listener said preference was not clear. What was clear is that illuminating the near wall for high early reflections enhanced spaciousness while the opposite enhanced imaging. My preference was for imaging, but someone who listens to concert hall reproductions would likely prefer the opposite.
3) I think that the precedence effect is widely misused, so I will avoid this term and its implied effects. Our hearing processes signals in vastly different ways as the frequency changes. This is due to neural response as well as cochlear effects. Our ears neural process cannot keep up with a signal above about 700 Hz synchronously and the nerves begin to fire randomly at HFs. Thus a transition from temporal detection to place theory where tone is based on the location on the cochlea. So the processing is completely different above and below this transition. This is why there is such a steep change in loudness below about 500 Hz.
The best way to understand my point is to look at the Gammatone filter responses for a human ear. You will see that they are very short at HFs and very long at LFs. At HFs a delayed signal does not "fuse" with the direct unless it is < a ms or so, while at LFs this fusion occurs for 10s of ms. There simply isn't a clear transition for a broadband signal. This is why one cannot state a delay gap that is accurate across the bandwidth. The greater the delay gap the lower in frequency fusion will not occur. Simply put all reflections are relevant but not always of the same importance. Of course at 30 ms no reflections fuse, at 1 ms virtually all of them do. In between is a mix that will depend highly on the characteristics of the signal.
The direct sound does "trump" the reflections as far as the perception of direction goes, but strong reflections will tend to blur that image making it less precise in location. The "precedence effect" does not consider "blurring" only primary direction. In other words it does not speak to imaging quality at all.
3) "But is this improved sweet spot and image stability coming at the cost of a larger soundstage?" That is almost certainly the case. What Is not clear is if the tradeoffs are comparable. In other words imaging falls fast with more lateral reflections, but spaciousness does not fall as fast with lower early lateral reflections. In fact one can design a room where the back of the room is very reverberant adding to spaciousness while not detracting from imaging, but illuminating the near walls to enhance spaciousness will always degrade imaging.
It's a tradeoff that you have to make based on your speakers room design/size and your personal preferences. To me high DI speakers, toed-in, in a room with low HF damping yields the best of the set of compromises that I had when building my listening room. I have not found another setup that I prefer with some 60 years of experience.
5) I am sure that Floyd has this right, so I won't comment.
The first thing that I would say is that the question is kind of open ended. My concern is from what direction are we approaching these issues: 1) do we have given speakers and want to find how to design or setup a room that best suites them, or 2) do we have a given room (by far the most common situation) and we want to determine the best speakers and placement for say imaging and/or spaciousness.
I will assume 2), but much of the OPs questions seem to imply that we have room options when we usually don't.
so to the questions:
1) both Floyd and I agree that front wall (which I sometimes call the "back" wall because it is "in back" of the speakers) should be well damped. I have found that no amount of damping on the front wall is too much, the more the better. This absolutely enhances imaging. Few will have the option of placing the speakers out from this wall as there just isn't enough room, so damping it is the key. My front wall has about 1 foot of foam covering it and so very little reflection. SO what is the "optimal" depth? As much as you can, but this is usually not practical, so use heavy damping.
2) side wall reflections are a continued source of disagreement. They will enhance spaciousness but degrade imaging. With a narrow DI speaker we can control how much sound reflects off of which wall. A low DI speaker cannot do this. In experimentation this effect is obvious. Since my speaker are laterally symmetric and very high DI, I can place them such that the direct response stays the same but the side walls get illuminated differently. I put my speakers on rotational bearing and I could change their pointing. The effect is quite pronounced, but as one listener said preference was not clear. What was clear is that illuminating the near wall for high early reflections enhanced spaciousness while the opposite enhanced imaging. My preference was for imaging, but someone who listens to concert hall reproductions would likely prefer the opposite.
3) I think that the precedence effect is widely misused, so I will avoid this term and its implied effects. Our hearing processes signals in vastly different ways as the frequency changes. This is due to neural response as well as cochlear effects. Our ears neural process cannot keep up with a signal above about 700 Hz synchronously and the nerves begin to fire randomly at HFs. Thus a transition from temporal detection to place theory where tone is based on the location on the cochlea. So the processing is completely different above and below this transition. This is why there is such a steep change in loudness below about 500 Hz.
The best way to understand my point is to look at the Gammatone filter responses for a human ear. You will see that they are very short at HFs and very long at LFs. At HFs a delayed signal does not "fuse" with the direct unless it is < a ms or so, while at LFs this fusion occurs for 10s of ms. There simply isn't a clear transition for a broadband signal. This is why one cannot state a delay gap that is accurate across the bandwidth. The greater the delay gap the lower in frequency fusion will not occur. Simply put all reflections are relevant but not always of the same importance. Of course at 30 ms no reflections fuse, at 1 ms virtually all of them do. In between is a mix that will depend highly on the characteristics of the signal.
The direct sound does "trump" the reflections as far as the perception of direction goes, but strong reflections will tend to blur that image making it less precise in location. The "precedence effect" does not consider "blurring" only primary direction. In other words it does not speak to imaging quality at all.
3) "But is this improved sweet spot and image stability coming at the cost of a larger soundstage?" That is almost certainly the case. What Is not clear is if the tradeoffs are comparable. In other words imaging falls fast with more lateral reflections, but spaciousness does not fall as fast with lower early lateral reflections. In fact one can design a room where the back of the room is very reverberant adding to spaciousness while not detracting from imaging, but illuminating the near walls to enhance spaciousness will always degrade imaging.
It's a tradeoff that you have to make based on your speakers room design/size and your personal preferences. To me high DI speakers, toed-in, in a room with low HF damping yields the best of the set of compromises that I had when building my listening room. I have not found another setup that I prefer with some 60 years of experience.
5) I am sure that Floyd has this right, so I won't comment.
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Thanks for all the info and taking the time to reply, I certainly appreciate it!
That totally makes sense. If I change the orientation of the room, then that might actually be a possibility.
Yeah I am looking more for the second I believe.
It is impressive how large of an effect that does have. Certainly shows the need for better blind tests when one compares or review or studies speakers etc.
Perfect! Thanks so much! I will look into it.
Yeah that is what I was thinking, and then had me call into question the advice that one should have the longer distance be the length of the room.
But then the backwall would be a major issue in most cases.
Thanks again for all the input and taking the time to post thoughtful replies!
That totally makes sense. If I change the orientation of the room, then that might actually be a possibility.
Yeah I am looking more for the second I believe.
It is impressive how large of an effect that does have. Certainly shows the need for better blind tests when one compares or review or studies speakers etc.
Perfect! Thanks so much! I will look into it.
Yeah that is what I was thinking, and then had me call into question the advice that one should have the longer distance be the length of the room.
But then the backwall would be a major issue in most cases.
Thanks again for all the input and taking the time to post thoughtful replies!
Yes, I initially was trying to ask in terms of general principals, but I sadly fall into the latter category 2) where I am very constrained by the room I have.
So maybe I will answer in terms of specifics to my room.
The room I have is ~33ft x 10ft and prior to knowing anything technical about audio (a mere 12 months and a pandemic ago) I set it up using the shorter distance as the length of the room and thus it is very shallow and very wide. I have a ~110" projector screen and the L&R speakers ~8ft apart on either side.
After a bit of reading I built two HTM-12s (which have a 12" woofer and SEOS-15 waveguides for my main L&R speakers and then with further reading I realized that I was lucky to have picked speakers with good sensitivity, a waveguide, and good directivity (although unlucky to have not built 3 of them for the full LCR - as my prior experience with a center speaker below a large projector screen were less than ideal compared to a phantom center so I only got two of them for the L&R)
After reading more and more about audio I realized that an overhaul pf my room was needed and hence greatly enjoyed your book Dr. Geddes for further direction.
All that to say, I will now be changing the orientation of the room to use the longer 36ft as the length of the room and the 10ft as the width.
What I am now contemplating is how to place these two front speakers.
I have lots of real estate in terms of the length of the room, so I could pull the speakers out as much as desired.
But I do have limited width (10ft), and I really don't want to sacrefice on a large and wide soundstage.
All that being said, I don't know how far out I should pull the speakers (have lots of flexibility), and how wide to have them (10ft width).
Thankfully I do have lots of room in this regard, so would you say a good 2-5ft? Would you add as much damping in addition to this?
Also, a random thought: If you hung the screen level with the speakers (3-5ft into the room) and the speakers off to either side of the screen then would having a acoustically absorbent / dense screen be beneficial? Would it 'trap' the sound from the front wall behind the speakers... but it would only work on high frequencies... so probably not worth doing?
Totally makes sense.
In my room that is 10ft wide with good directivity speakers, how wide would you place them? As my goal to to have as wide of a soundstage as possible without sacrificing imaging.
Perfect! That certainly makes sense and also makes sense with low frequencies and subwoofer placement and much that you have written on the low frequencies not being as critical in the time/location domain.
I will think of it more as a gradation that moves as the frequency increases. I guess that is another reason why higher directivity speakers are easier to manage in small rooms.
Ok, so I am justified in all my toe-in tinkering. Ill make sure to tell my wife that.
Thanks again everyone for their input and expertise it is greatly appreciated!
I am always amazed at the level and quality of information that can be had if someone knows where to look and the topic is specific enough.
While I have you Dr. Geddes.... Do you have any opinions on near-field subwoofer placement (within 1-2x the cone diameter)? And any advice on where one can find cheap waveguide plug foam?
Thanks again!
Nice to have Dr Earl Geddes to give you answer.
I'm glad not to be to much off about what i wrote (Tbh from what i've read i've got preference which aren't really different from his) and learned some things along the way.
One more thing : when i say front wall: where the loudspeakers are located, back wall: behind listening place - on your back.
I'm glad not to be to much off about what i wrote (Tbh from what i've read i've got preference which aren't really different from his) and learned some things along the way.
One more thing : when i say front wall: where the loudspeakers are located, back wall: behind listening place - on your back.
I strongly recommend that you have three front speakers as almost anyone will. But they should all be identical, which makes for the only solution is the speakers behind the screen. If I were you I'd just screen off a whole end of the room and put all the speakers and lots of damping behind it. It doesn't have to cover the whole area, but the vast majority of it. See, that's exactly what I do.
See, that's not possible. Going one way enhances one and the other not and visa-versa. The relative rates may change, but not the overall situation - if that makes any sense.
As to pointing, I'd always prefer a toe-in - I'd put you speakers somewhat near the side walls point in almost 45 degrees - your room is so narrow that getting the speakers away from the walls will be difficult. The expected listening range for this would be back from the speaker plane about the same distance as the speakers are apart. That's the sweat spot in narrow rooms like ours - mine is 14 x 22 ft. though, a bit better ratio than yours.
I've heard that near field works very well, especially if sound leakage is a problem. But I don't think that it would compare with a well sealed room with some 5 subs around it. But, I expect that it's a great solution when needed.
E-mail me and I can get you foam.
I have to chuckle at that one. My "front wall absorption" is a giant wall of waveguide foam scraps all piled high from floor to ceiling, about 3 feet thick. Scrap from years of cutting foam for speakers. Scrap was > product - ouch! It is one heck-of-a sound absorber! Think anechoic chamber wall.
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