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What is the ideal directivity pattern for stereo speakers?
What is the ideal directivity pattern for stereo speakers?
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Old 23rd August 2011, 07:54 PM   #41
DBMandrake is offline DBMandrake  Scotland
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Originally Posted by ra7 View Post
It always amazes me when people say that reflections are bad. Reflections are necessary for proper localization of the phantom image. In an ideal anechoic chamber, with no reflected sound, where do you think the phantom image from two sources will form? It will form inside your head, just like in the case of headphones.
What makes you think room reflections are necessary for the formation of a centre phantom channel in a stereo playback system ? Is this just a hypothesis of yours, or have you actually tested it ?

Headphones localize the image inside your head because the HRTF of the outer ear is bypassed. (and in the case of in-ear-canal headphones the pinnae is also bypassed)

If you haven't tried it before, set up your speakers outdoors at normal indoor listening distances and angles but well away from any walls or objects - you'll hear a perfectly good centre phantom channel, in fact it will usually be better than indoors.
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It is the room reflections that tell your brain exactly where the image is.
The human brain is perfectly able to localize images without room reflections - can you figure out which direction a noise is coming from outdoors ? Of course you can.

In fact the opposite of what you say is true - the brain has to do a lot of extra processing to try to ignore or "window out" reflections so that the direction of the original sound (the first arrival) can be determined despite time delayed reflections coming from different directions also being present.

This is all part of the precedence effect, and probably the reason the precedence effect exists in the first place, as an adaption to help determine the true direction of a sound source in a confusing acoustic environment.
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Last edited by DBMandrake; 23rd August 2011 at 08:00 PM.
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Old 23rd August 2011, 08:10 PM   #42
6.283 is offline 6.283  Germany
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Hi Keyser,
brave but good attempt
I think people here will not come to a final agreement. History in forums shows that there does not seem to be a consensus. In the end it all boils down to preference and duty and probably there is no right or wrong.

My current simplified picture of the two extreme positions:
It seems that people, who are after enjoying the music as such, prefer speakers with wide dispersion and more reflections. People, who are doing a recording, mixing or whatever job will likely prefer pinpoint type of speakers with narrower dispersion because they need the feedback if the mics have been placed in the right spot for what they are trying to capture. The "music lover" does typically not care so much about that aspect and in turn prefers a representation that is closer to a live venue with a nice mixture of direct and indirect sound; also on the recording itself. Pinpointing does not play a major role as “there is no pinpointing in Carnegie Hall either”.
It is no secret that I am more in the camp of the music lovers. On the other hand it is my experience that dipoles and an omni like Pluto are able to combine the best of the two worlds, although I believe that there is no "one fits all" approach. That means, if implemented well, such speakers could cover the needs of a good part of a population if they have an appropriate room, which is no real challenge.
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Old 23rd August 2011, 08:24 PM   #43
Bob Richards is offline Bob Richards  United States
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Originally Posted by DBMandrake View Post
The human brain is perfectly able to localize images without room reflections - can you figure out which direction a noise is coming from outdoors ? Of course you can.
The ear-brain mechanism is fully intriguing. Since the only axis 2 horizontally displaced ears can analyze for image location DIECTLY is left to right (X axis), it appears that we use reflection and frequency response cues to sense Y and Z axis. Say what?! I've been researching this for decades.

The pinea of the ear has a unique shape that is well understood by the ear-brain mechanism (from experience if not genetics). The brain knows that when sounds go up and down physically, there is a well known variation in relative frequency response in the approx. 8kHZ region that is created by the reflections in the pinea (outer ear). This works best when there is change, and less when the response shape is static.

To sense where on the Z axis a sound is, I don't see how else we could detect that with two horizontally displaced ears except by analyzing all related reflections, and placing it by relation to all other sounds. A comparative analysis of the reverb/reflections.

As far as the X axis (left to right), it appears that we sense image location more by amplitude comparisons in the upper frequencies, and more by timing comparisons in the lower frequencies (below about 1kHZ).
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Old 23rd August 2011, 08:50 PM   #44
dantheman is offline dantheman  United States
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A nice demo on distance and reflections: Initial Time Delay Gap ITDG

FWIW, the most spacious stereo I've ever heard cam from narrow pattern loudspeakers crossfired midway between the listening position and the loudspeaker.

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Old 23rd August 2011, 09:02 PM   #45
ra7 is offline ra7  United States
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What is the ideal directivity pattern for stereo speakers?
Quote:
Originally Posted by DBMandrake View Post
What makes you think room reflections are necessary for the formation of a centre phantom channel in a stereo playback system ? Is this just a hypothesis of yours, or have you actually tested it ?

Headphones localize the image inside your head because the HRTF of the outer ear is bypassed. (and in the case of in-ear-canal headphones the pinnae is also bypassed)

If you haven't tried it before, set up your speakers outdoors at normal indoor listening distances and angles but well away from any walls or objects - you'll hear a perfectly good centre phantom channel, in fact it will usually be better than indoors.
The human brain is perfectly able to localize images without room reflections - can you figure out which direction a noise is coming from outdoors ? Of course you can.

In fact the opposite of what you say is true - the brain has to do a lot of extra processing to try to ignore or "window out" reflections so that the direction of the original sound (the first arrival) can be determined despite time delayed reflections coming from different directions also being present.

This is all part of the precedence effect, and probably the reason the precedence effect exists in the first place, as an adaption to help determine the true direction of a sound source in a confusing acoustic environment.
There is still the reflection from the ground outside. If there are no reflections and only direct sound from source to ear, and of course you have to be equidistant and at the centerline of the two sources, the image will form in your head.

The reason, perhaps, that it images better outside is because the ground reflection is early enough to enhance the sense of localization. Also, in my limited experience, it is most certainly more tiring to listen to a speaker with narrow dispersion than one with a wide dispersion. The brain is working harder to localize the sound, but it is not getting enough cues from the reflected sound.

Only when reflections start coming in late that the sound becomes muddy and hard to follow.

And this is not only my hypothesis:
Linkwitz-Publications

Listen to "Talk with Q&A"... its pretty long, but worth it.

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Old 23rd August 2011, 09:10 PM   #46
dantheman is offline dantheman  United States
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This book can help with understanding localization: http://human-factors.arc.nasa.gov/pu...Multimedia.pdf

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Old 23rd August 2011, 09:22 PM   #47
5th element is offline 5th element  United Kingdom
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There is perhaps a different way of looking at the sound inside the head thing.

With headphones the left ear receives only sound from the left headphone and visa versa. But with a pair of speakers, even if there are no reflections present the left ear still receives a lot of information from the right loudspeaker and the same is true for the right ear and the left loudspeaker.

Lets not forget that we also determine the position of sound sources by the difference in sound between the left and right of the head principally because the shape of the head and then the ear alters the frequency response of the incoming sound so that what the left and right ear hear are quite different. The brain then works its magic and we say - it's over there.

If there happened to be a single loudspeaker replaying sound in a reflection-less environment you'd still be able to determine where it is because of these differences. You don't need reflections to know that it is over there. If you then brought in a second loudspeaker, say in a typical triangle configuration, as would be used in stereo listening, you'd know that the left speaker is over there and the right one were over there. The sound would not be 'in your head' for each individual loudspeaker, so why would it suddenly jump inside your head if you turned both on at once?
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Old 23rd August 2011, 09:26 PM   #48
DBMandrake is offline DBMandrake  Scotland
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Quote:
Originally Posted by speaker dave View Post
I'm not sure what psychoacoustic factors will come into play. Even the seperation of sources is likely to be inaudible since the angular split is fairly low and it is in a vertical orrientation where we aren't very discerning.

Best frequency range? I'd have to do some simulations for that. The Allison picture shows a typical arrangement with the woofer center maybe 9 or 10 inches off the floor and the midrange just below a tweeter at the usual 40 inches up. Simulate or measure that and it will show you where the crossover needs to be. Note that for both drivers you will get a comb filtered response. For the woofer all the comb filtering can be in the stop band and thus ignored. For the mid the comb filtering is in the pass band so your hope is to ignore the first or second null and use it in a region where the response has settled down.
You're looking at it only from the perspective of comb filtering here, and while that's a part of the issue it's not the whole thing, and possibly not even the most important aspect.

There's a couple of psychoacoustic effects I'm thinking of applicable to the high midrange low woofer approach (there may be others too) - one is the widely different time delay of the floor reflection of each driver and how it relates to the precedence effect, the other is apparent source localisation of sounds whose different frequency components are produced by drivers at different heights, and how both of these relate to possible crossover frequencies.

I think everyone agrees that for good quality midrange you can't have the midrange driver too close to a side-wall or floor boundary, but why ? I would say that the reason is to make sure that there is a certain minimum time delay for the first reflection, such that the precedence effect can work its magic.

With sufficient time delay we perceive the direct arrival above ~300Hz, and we don't really hear the bad effects of comb filtering. Too short a delay and it all fuses together, allowing us to hear the response irregularities of the comb filtering.

Another possible aspect of this (just a hypothesis though) is that any resonant behaviours of the reflective surface (such as a panel resonances on a wall) will be audible if the time delay is short enough to allow aural fusing, but largely ignored as a component of the delayed signal if the time delay is in the precedence window. (The same probably applies to the floor and wall behind the speaker too)

So although a steady state measurement might show show the 2nd, 3rd, 4th harmonic notches in the comb filtering from the midrange driver being present, we won't really hear them above the Schroeder frequency due to sufficient time delay of the reflection. (Also, with multi-path reflections any deep notches would tend to be eliminated at the listening position even in the steady state signal)

The implication from this is that the crossover frequency should be at or close to the Schroeder frequency - for higher frequencies where the brain can separate first arrivals from reflections, we want those produced by a driver with a sufficiently delayed 1st reflection, provided to us by sufficient height. On the other hand for frequencies below the Schroeder frequency where we tend to hear the steady state response we want those produced by a driver close to the floor, whose close proximity puts its comb filtering artefacts above it's passband.

The second factor is localisation of the sound source when you split bass and midrange into two drivers with considerable vertical separation. Low frequencies can't be localised, midrange frequencies obviously can, but what is the cut-off point ? If the source of bass can be any direction in the room, most people say 80Hz, but if it's much closer to the midrange driver we can go a lot higher. In fact if the woofer is vertically below the midrange driver, I think we can go up to about 300Hz without the localisation shifting from the midrange driver. (empirically derived)

So if the crossover frequency is too low (say 80Hz) audible holes in the steady state response below the Schroeder frequency will open up due to boundary cancellation, while if the crossover frequency is too high (say 500Hz) midrange clarity will be compromised due to a significant part of the midrange being produced by a driver close to the floor, as well as the localisation of the sound source starting to stretch vertically - the woofer will start to draw attention to itself, when ideally the sound should sound like it's coming from half way between midrange and tweeter with the woofer not being noticed as a separate sound source.

Another consideration I'd throw into the mix is that the proximity of the woofer to the floor has a lot more effect on the 2nd harmonic floor/ceiling mode (typically between 100-150Hz) than it does at the floor bounce differential frequency (typically ~300Hz for a high mid-woofer) and that a lot of the benefit of putting the woofer close to the floor applies in this 100-150Hz region rather than the calculated floor bounce frequency. I've typically found the calculated floor bounce notch disappears once you get more than a couple of metres away in a room back due to multi-path reflections from multiple boundaries, whereas the effects of the floor/ceiling mode cancellation tend to persist at most listening locations, so aren't as temporal.

One final thought (more questions than thoughts actually) is the effects on the directivity index / power response of a low mounted woofer, versus a high mounted one. If you design a speaker with a high mounted woofer (or a 2 way using a mid-bass) and apply a theoretical 6dB baffle step correction you'll end up with a certain frequency response in room and a certain shift in directivity index from bass to midrange.

If you then move the woofer so that it's close to the floor, the measured bass response at the listening position, particularly in the upper bass region will increase by many dB. To get a balanced overall result it's necessary to equalise this in the network somehow, for example by lifting the drive level of the midrange and tweeter.

What does this do to the power response and directivity index at low frequencies ? By putting the woofer closer to the floor we have less destructive interference and greater response at the listening position for the same power level, but have we increased the effective efficiency or have we really just increased the directivity index ?

How about if we also have the baffle width such that the baffle step half way frequency occurs at the crossover frequency, then what will we find in directivity index shift between bass and treble vs a speaker whose drivers are all a long way from the floor ?

Seems like a very successful approach would be - (1) crossover frequency at the Schroeder frequency, (2) baffle width to put the baffle step frequency also at the Schroeder frequency (3) woofer very close to the floor, (4) midrange high up, (5) network attenuation levels adjusted to give the midrange and tweeter a bit less than 6dB of baffle step correction to allow for the floor gain of the low mounted woofer.

The constraints of the different factors I've discussed would lead to the crossover frequency being somewhere between 200-300Hz.
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Old 23rd August 2011, 09:47 PM   #49
graaf is offline graaf  Poland
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Quote:
Originally Posted by DBMandrake View Post
What makes you think room reflections are necessary for the formation of a centre phantom channel in a stereo playback system ? Is this just a hypothesis of yours, or have you actually tested it ?

....
The human brain is perfectly able to localize images without room reflections - can you figure out which direction a noise is coming from outdoors ? Of course you can.
Quote:
Originally Posted by 5th element View Post
Lets not forget that we also determine the position of sound sources by the difference in sound between the left and right of the head principally because the shape of the head and then the ear alters the frequency response of the incoming sound so that what the left and right ear hear are quite different. The brain then works its magic and we say - it's over there.

If there happened to be a single loudspeaker replaying sound in a reflection-less environment you'd still be able to determine where it is because of these differences. You don't need reflections to know that it is over there. If you then brought in a second loudspeaker, say in a typical triangle configuration, as would be used in stereo listening, you'd know that the left speaker is over there and the right one were over there. The sound would not be 'in your head' for each individual loudspeaker, so why would it suddenly jump inside your head if you turned both on at once?
Guys, You should make the distinction between lateralization and localization - two different things

You should also check this one out:
Sound reproduction: loudspeakers and ... - Google Books

let me quote:
Quote:
Interestingly, a demonstration of four-loudspeaker Ambisonic recordings played in an anechoic chamber yielded an auditory impression that was almost totally within the head. This was a great disappointment to the gathered enthusiasts, all of whom anticipated an approximation of perfection. It suggested that, psychoacoustically, something fundamentally important was not being captured or communicated to the ears.
An identical setup in a normally refl ective room sounded far more realistic, even though the room refl ections were a substantial corruption of the encoded sounds arriving at the ears
bolds are mine
the reality of an anechoic chamber is just as ra7 describes it - a disaster, sound in the head

it is not just hypothesis, it was rather painfully tested, what a disappointment...
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Old 23rd August 2011, 09:54 PM   #50
dantheman is offline dantheman  United States
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The illustration below from Spatial Hearing by Jens Blauert demonstrates the principle:
Click the image to open in full size.

The picture tells us the ear has trouble distinguishing what spectral information is coming from the source and what is caused by the Pinna and ear canal.

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