Its too hard to tell from the description, but I have used curtains before and they worked well. The fabric should be thick and heavy and if need be use two rows.
Dr Geddes,
Is it better the entire wall absorptive or is it sufficent only the area
between the speakers ?
Cheers,
Paolo
Is it better the entire wall absorptive or is it sufficent only the area
between the speakers ?
Cheers,
Paolo
I would say that the more absorption there is behind the speakers the better. I do the whole wall.
Thanks Dr Geddes, very interesting topic/discussion . Room acoustics is so important imho .
Cheers,
Paolo
Cheers,
Paolo
Sorry Markus, I don't. It comes from my readings here...
You will have to ask Earl for more details!
/Etienne
You will have to ask Earl for more details!
/Etienne
markus76 said:
Most often the front wall is very close to the speakers and there will be a very early reflection (VER) off of it. This will tend to be LF limited in my designs of course, but it does help to minimize the VERs. I have measured these in many rooms and found that the absorption is very effective. As we have often discussed, I am trying to get that first 10 ms. reflection free. Directivity helps, but sometimes some absorption works well.
Subjectively my partner in Thailand experimented with this is a very bad room that we had. We found that the more we absorbed the wall behind the speakers the better. The same absorption elsewhere in the room did not have the same positive effect.
Floyd also supports this.
front reflections
{oops - I'm actually replying mostly to Etienne88 re: front reflections are negative}
It really all depends on the distance from the front wall, which determines the DELAY of the reflections. It also depends somewhat on the nature of the front wall surface - Diffusive is great, IF sufficient wall distance=delay is incorporated.
Short delays (single digit milliseconds) are bad. The brain won't seperate them from the original source (speaker) itself, especially when occuring on the same side of the head and room as the source, resulting in smeared imaging, comb filtering, etc.
Most home theater setups are close to the front wall (screen) . . .
Longer delays are useful, as the brain is able to sort-out the information to properly localize original sounds, while the reflections reinforce volume and keep things from sounding like headphones - as you move your head you need cues from a real space. Earls use of opposing sidewall reflections achieves this nicely.
Much longer reflections (like 30+ milliseconds) are bad (for recording playback). Fortunately, delays of this length are very rare in domestic settings - room would have to be live and sized like a small auditorium. Leads to discernible delay (echoes) and a lopsided ratio of reflected-to-direct sound.
In Earl's application (home theater) mains are too close to front wall for beneficial (that is, sufficiently delayed, diffused) front reflections. He uses opposing sidewalls to intentionally create delayed reflections. If one lacks (useful, delayed) sidewall reflections for some reason, the front wall can be very helpful in restoring useful reflections -- IF given sufficient delay. The use of diffusion or angled corners can also help redirect reflections to the opposing side, for less confusion with the original source. This means parking speakers a meter (and ideally more) from the front wall.
Of course, having highly directional speakers also changes the spectrum of front wall reflections, whereas sidewall reflections (depending on surface) can be closer to full-spectrum, dependant on directivity, toe-in, & listener location. This is what makes Earl's approach superior.
Earl's got it right - if setup as he prescribes, no short reflections occur, and plenty of well-delayed (like above 10 ms) reflections occur, and occur at the opposite ear from the source.
Sources with lesser directivity will either require in-wall, far away from sidewall, or absorptive wall approaches if one wants a similar result. Earl's systems would probably thrive in an in-wall installation, if given the right room geometry.
Markus, studies abound, most very technical. Earl's free chapters from his book on his site give a REALLY good primer, without getting overly technical. He also nicely covers some things about reflections in the Summa white paper. All these informal writings are founded on studies.
Sorry so long, hope this helps.
EDIT: Earl posted while I was typing . . . still good info though.
--TubaMark
markus76 said:
{oops - I'm actually replying mostly to Etienne88 re: front reflections are negative}
It really all depends on the distance from the front wall, which determines the DELAY of the reflections. It also depends somewhat on the nature of the front wall surface - Diffusive is great, IF sufficient wall distance=delay is incorporated.
Short delays (single digit milliseconds) are bad. The brain won't seperate them from the original source (speaker) itself, especially when occuring on the same side of the head and room as the source, resulting in smeared imaging, comb filtering, etc.
Most home theater setups are close to the front wall (screen) . . .
Longer delays are useful, as the brain is able to sort-out the information to properly localize original sounds, while the reflections reinforce volume and keep things from sounding like headphones - as you move your head you need cues from a real space. Earls use of opposing sidewall reflections achieves this nicely.
Much longer reflections (like 30+ milliseconds) are bad (for recording playback). Fortunately, delays of this length are very rare in domestic settings - room would have to be live and sized like a small auditorium. Leads to discernible delay (echoes) and a lopsided ratio of reflected-to-direct sound.
In Earl's application (home theater) mains are too close to front wall for beneficial (that is, sufficiently delayed, diffused) front reflections. He uses opposing sidewalls to intentionally create delayed reflections. If one lacks (useful, delayed) sidewall reflections for some reason, the front wall can be very helpful in restoring useful reflections -- IF given sufficient delay. The use of diffusion or angled corners can also help redirect reflections to the opposing side, for less confusion with the original source. This means parking speakers a meter (and ideally more) from the front wall.
Of course, having highly directional speakers also changes the spectrum of front wall reflections, whereas sidewall reflections (depending on surface) can be closer to full-spectrum, dependant on directivity, toe-in, & listener location. This is what makes Earl's approach superior.
Earl's got it right - if setup as he prescribes, no short reflections occur, and plenty of well-delayed (like above 10 ms) reflections occur, and occur at the opposite ear from the source.
Sources with lesser directivity will either require in-wall, far away from sidewall, or absorptive wall approaches if one wants a similar result. Earl's systems would probably thrive in an in-wall installation, if given the right room geometry.
Markus, studies abound, most very technical. Earl's free chapters from his book on his site give a REALLY good primer, without getting overly technical. He also nicely covers some things about reflections in the Summa white paper. All these informal writings are founded on studies.
Sorry so long, hope this helps.
EDIT: Earl posted while I was typing . . . still good info though.
--TubaMark
Thanks Earl. Of course there are two circumstances that make front wall absorption mandatory:
- speakers are too close to wall (which would lead to summing localization errors due to reflections < 2 ms)
- speaker radiates low and mid frequencies from its back (due to nonlinear directivity pattern which would lead to colorization of reflections)
Other than that there's very little data that supports attenuating front wall reflections. Maybe it helps because it makes the listening room more like the typical RFZ control room. On the other hand there might be detrimental effects of VERs (don't know if I like the word) < 10 ms. I guess we have to wait for data on that.
- speakers are too close to wall (which would lead to summing localization errors due to reflections < 2 ms)
- speaker radiates low and mid frequencies from its back (due to nonlinear directivity pattern which would lead to colorization of reflections)
Other than that there's very little data that supports attenuating front wall reflections. Maybe it helps because it makes the listening room more like the typical RFZ control room. On the other hand there might be detrimental effects of VERs (don't know if I like the word) < 10 ms. I guess we have to wait for data on that.
All quite correct. When responding I tend to think of my own situation exclusively, and it is the most typical, but there are situations where the front wall could be reflective and this may be benificial. The trouble with Room Acoustics is that rooms vary so widely that it is very difficult to give "cook book" answers. Then someone takes your answer out of context, which means in ANY other room, and claims your wrong.
John K is partly correct when he says that there is no one right answer for an arbitrary room, that is correct. But there may well be one right answer when the room is not an unchangable given but can be designed for the best playback - as I have done. But then let's NOT get *this* wrong either. That doesn't mean that the Summa designs won't work very well in an arbitrary room, they may well be ideal in that situation too.
John K is partly correct when he says that there is no one right answer for an arbitrary room, that is correct. But there may well be one right answer when the room is not an unchangable given but can be designed for the best playback - as I have done. But then let's NOT get *this* wrong either. That doesn't mean that the Summa designs won't work very well in an arbitrary room, they may well be ideal in that situation too.
gedlee said:
Quite right, but I think important data is missing that would prove what the right answer really is.
Hi all,
I just found this thread and have been reading through it. I found the parts about setting up subs and integrating with mains interesting since I am always doing this with my own systems in small rooms. I'm also working on setting up a main system in my small, non-optimal living room at the moment. I had been planning on doing a multiple sub arrangement to try to smooth out the low frequency response. In the past, I've thought about doing something more advanced for this - lining both side walls with subwoofers (spaced close enough to create a plane wave) and using DSP to cancel the reflections with the opposite wall. John Hancock suggested this on this forum several years ago. I'm not sure if anyone else came up with it prior to that. At the time, I had about 50-60 8" woofers that I could have done this with, but I gave them all away when I moved about 8 months ago (they were free to me). Today I got the November issue of the JAES and they had an article using the same technique, except they only use 4 subwoofers. Two on the front wall, two on the back wall mounted at 1/4 and 3/4 of the width of the room, and centered vertically. They showed scans of the pressure in the room at different times when excited by the system and you could see the plane wave develop and travel through the room. I've done that before in a horn, and it was cool to see. Anyway, they achieved very nice looking response curves - very smooth, in the range of +-1 to +-2dB below 100hz (although that would depend on room size). Also, the effect occurred though most of the room. I haven't had time to read it in detail, but looking at their pressure scans, the plane wave takes a little bit of distance to develop - right at the subs it's not a plane wave. Subjectively, they said it eliminated bass boom and you could not hear the rear subs.
Since I was going to do 4 subs and they were already going to be tall, narrow ones (tapped horns) with DSP, I will probably give this a try in the future. If it doesn't work I can always go back to my previous plan.
Earl, if you are reading, any thoughts on this versus the multiple sub approach you have described?
I just found this thread and have been reading through it. I found the parts about setting up subs and integrating with mains interesting since I am always doing this with my own systems in small rooms. I'm also working on setting up a main system in my small, non-optimal living room at the moment. I had been planning on doing a multiple sub arrangement to try to smooth out the low frequency response. In the past, I've thought about doing something more advanced for this - lining both side walls with subwoofers (spaced close enough to create a plane wave) and using DSP to cancel the reflections with the opposite wall. John Hancock suggested this on this forum several years ago. I'm not sure if anyone else came up with it prior to that. At the time, I had about 50-60 8" woofers that I could have done this with, but I gave them all away when I moved about 8 months ago (they were free to me). Today I got the November issue of the JAES and they had an article using the same technique, except they only use 4 subwoofers. Two on the front wall, two on the back wall mounted at 1/4 and 3/4 of the width of the room, and centered vertically. They showed scans of the pressure in the room at different times when excited by the system and you could see the plane wave develop and travel through the room. I've done that before in a horn, and it was cool to see. Anyway, they achieved very nice looking response curves - very smooth, in the range of +-1 to +-2dB below 100hz (although that would depend on room size). Also, the effect occurred though most of the room. I haven't had time to read it in detail, but looking at their pressure scans, the plane wave takes a little bit of distance to develop - right at the subs it's not a plane wave. Subjectively, they said it eliminated bass boom and you could not hear the rear subs.
Since I was going to do 4 subs and they were already going to be tall, narrow ones (tapped horns) with DSP, I will probably give this a try in the future. If it doesn't work I can always go back to my previous plan.
Earl, if you are reading, any thoughts on this versus the multiple sub approach you have described?
It would work fine, its just more complex than necessary.
A pure plane wave isn't necesarily what one wants - that simulates free space, outdoors, and indoor bass is better in a larger room. So, for me, just getting the response smooth and uniform is the better goal and all that signal processing isn't necessary. In fact, in my ideal, I would add reverberation to the LF signal to better approach the bass in a large room - not outdoors.
A pure plane wave isn't necesarily what one wants - that simulates free space, outdoors, and indoor bass is better in a larger room. So, for me, just getting the response smooth and uniform is the better goal and all that signal processing isn't necessary. In fact, in my ideal, I would add reverberation to the LF signal to better approach the bass in a large room - not outdoors.
terry j said:
markus76 said:
terry j said:
Just found this interesting tidbit from this link..
http://www.mastersonaudio.com/features/20040401.htm
By the same token, some types of audio equipment tend to delay certain frequencies with respect to others. This is known as "phase shift" or "group delay," and is most often a physical attribute of speakers, although it has always been a measurable factor in the performance of filters, crossover networks, equalizers, and the like. To try to find out what we can and can't hear, David Clark of DLC Designs in Michigan conducted a series of listening tests some years ago. As listening subjects, Clark called on the worthy members of the Southwest Michigan Woofer and Tweeter Marching Society -- SWMWTMS (pronounced "smootums") -- one of the United States' most active and enthusiastic audio clubs. I participated as an observer.
Clark decided to use one type of nonlinear distortion -- total harmonic distortion (or "grunge") -- and two forms of linear distortion: phase shifting at several levels of severity and a frequency-response peak in the sensitive midrange.
Phase shift turned out to be the most difficult form of distortion to hear. Four amounts of phase shift were used. This was progressively reduced to 2150°, 1620° and 1080°. The greatest was some 2700°; if this were to be encountered in a speaker, the woofer would have to be seven feet behind the rest of the speaker. None of these would ever be encountered in a normal audio component. The order of tests had to be altered in this case, with the test signal being auditioned first, just so the subjects would know what to listen for.
Once educated, however, the majority of listeners had little difficulty hearing phase shift at all levels, as long as the test signal was used. Only with the least amount of shift did responses drop to the 73%-correct level, but this amount was probably still audible. With the optimal music, which was not dissimilar to the test pulse, but was masked to some extent by reverberation on the disc, the scores dropped, only reaching some certainty at 1600°; above and below this amount, the scores were close to chance. With the "natural" music, even the greatest amount was not audible.
To my uneducated mind, this seems to be what I found with my delay settings on my subs. Also, in my case, I was only using the subs under 70 hz or so, which perhaps increased the non detection effect.
Of course, I was listening to music, not test signals which seems to allow a greater level of detection, once music is played then the difficulty greatly increases.
So, plonk the subs anywhere I reckon!!
454Casull said:
Nils posted more recent data here (German but the diagrams speak for themselves):
http://www.beisammen.de/thread.php?threadid=93411
Pros and cons for multiple subwoofers
+ only 3 cheap subwoofers necessary
+ no additional equipment needed
+ works in rooms with restricted setup locations
+ works with any room layout
+ smooths the frequency response in the modal region up to the transitional region
- measurment software necessary
-- additional absorbers necessary to shorten low frequency decay time
...and DBA (Double Bass Array) / CABS
- at least 4 subwoofers necessary
- delay and phase inverter for array on rear wall necessary
- subwoofers have to be installed at predefined locations on front and rear wall
- works only in rooms with rectangular floor plans
- needs a lot more subwoofers to work beyond 100 Hz or when wall size increases
+/- measurement software only necessary to verify setup
++ no additional low frequency absorption necessary
Best, Markus
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