New system measuring tool

[User211]

I spent sometime today looking through old Apogee forum archives concerning rear wall treatment. I came across the following quote which I thought interesting:

"I have had dipoles since 1976 and nothing but. I have tried every permutation of deadening the rear wall and have always wound up back to a fully reflective wall.

In deadening the rear wall you cut the systems output and dynamics drop. For a while I might think the imaging was more accurate but you loose the air and sence of depth same as pushing the speakers to close to the rear wall. In most cases I have found that as long as the floor is really dead, heavy padding and carpet everything else takes care of itself."

I've now seen elaborate diffusers constructed at considerable expense - only to be rejected by the man who went to the trouble of doing so. Not his "cup of tea". The preference seems to be to use diffusers as opposed to absorbers.

I have to admit I am not encouraged by what I have read overall. Though I have read both positive and negative views on the matter.

With regards to the GIK bass panel, I feel this is simply not a great idea. If you want to kill the back wave, you really want to kill or diffuse it it all and not just part of it - otherwise I feel you really are going to end up with strange results. Looking at the in-room FRs I've taken, I'm pretty sure I don't wan't to go absorbing bass output anyway - it simply isn't excessive.

Another point made is that you may have a 20DB peak to peak variance in your room and acoustic panels may make a 2DB difference to that at best. Mine's is about 12DB but that is absolute worst case. The vast majority is within about 8DB. That's a pretty damn fine in-room response, IMHO.

 

[Rich]

I spent sometime today looking through old Apogee forum archives concerning rear wall treatment. I came across the following quote which I thought interesting:

"I have had dipoles since 1976 and nothing but. I have tried every permutation of deadening the rear wall and have always wound up back to a fully reflective wall.

In deadening the rear wall you cut the systems output and dynamics drop. For a while I might think the imaging was more accurate but you loose the air and sence of depth same as pushing the speakers to close to the rear wall. In most cases I have found that as long as the floor is really dead, heavy padding and carpet everything else takes care of itself."

I've now seen elaborate diffusers constructed at considerable expense - only to be rejected by the man who went to the trouble of doing so. Not his "cup of tea". The preference seems to be to use diffusers as opposed to absorbers.

I have to admit I am not encouraged by what I have read overall. Though I have read both positive and negative views on the matter.

With regards to the GIK bass panel, I feel this is simply not a great idea. If you want to kill the back wave, you really want to kill or diffuse it it all and not just part of it - otherwise I feel you really are going to end up with strange results. Looking at the in-room FRs I've taken, I'm pretty sure I don't wan't to go absorbing bass output anyway - it simply isn't excessive.

Another point made is that you may have a 20DB peak to peak variance in your room and acoustic panels may make a 2DB difference to that at best. Mine's is about 12DB but that is absolute worst case. The vast majority is within about 8DB. That's a pretty damn fine in-room response, IMHO.

Justin, I mean this with all due respect and no offense intended, but your post shows a fundamental lack of understanding about the science of acoustics and the reasons why absorption is necessary.

Three points. First of all, a little bass trapping will not and can not suck the life out of bass response. You would have to cover every wall, ceiling and floor in the room to even begin to have that effect on a bass wave. And bass trapping isn't really about absorbing "excess" bass, or trying to achieve an absolutely flat frequency response curve. It is about taming modal ringing from room modes. These are the fundamental frequencies where the dimensions of your particular room setup cause an overload resonance at a particular frequency (and at harmonics of that frequency). The frequency varies with the exact dimensions of the room. Unless your room was designed by an acoustical engineer to exacting specs, then it has room modes that will be improved by some bass trapping. Bass trapping will result in cleaner, clearer bass overall, and especially at higher volumes. This is true in just about any room in any house in existence. There is almost zero chance your sound wouldn't be improved by some bass trapping in the corners of your room.

Second, the quote you quoted states that absorption on the rear wall "cuts the system's output and dynamics drop." Again, this shows a fundamental misunderstanding. The front wave of the speaker is the sound of the original recording and represents the entire "output" of the speaker as far as the recording is concerned. The rear wave is a byproduct of the design and is basically "excess sound" that is then reflected around the room. It gives the illusion that the system is producing higher output, but that is actually just because there is more noise in the room. This reflected sound energy requires you to listen at a lower volume level and muddies your system's clarity and its ability to image.

By absorbing the rear wave, you can turn the volume up much higher (because you don't have all that excess sound energy bouncing around the room) and you are only listening to the direct front wave of the speaker, which is exactly what the recording engineer was listening to when he mastered the recording. The output of the speaker is no less than it was before, nor is the dynamics. It just takes a little more amp power to reach the same levels, because you are now only listening to the front wave of the speaker and don't have excess reflected sound energy. The front wave is the sound you were intended to hear, without the noisy byproducts caused by the dipole design. The "air and sense of depth" in a stereophonic image is supposed to be created by the mastering engineer, not from some byproduct of speaker design and sound being reflected around the room willy-nilly.

Would you take a box speaker and buy two pairs so that you could turn one around backwards and run it out of phase so that you could have "more air and sense of depth" and "higher output and dynamics"? Of course not. That would be ridiculous. Why would panel speakers be any different?

Finally, the purpose of absorbing the rear wave of the speaker is not to try to create an absolute flat frequency response, as you imply in your final paragraph. The purpose is to reduce the effects of reflected sound, comb filtering and ringing, which is what hampers imaging, clarity and soundstaging. Take your time and read through Jonathan's posts on the subject and try to understand his measurements and what they represent, and then you may begin to understand what I am talking about.

Ultimately, you may be right. It may not work for you. You may be so used to and for that matter, dedicated to, the idea that the back wave is a necessary part of the sound you expect out of your speakers that you will never be convinced otherwise. Just as that hypothetical person I described earlier who is absolutely wedded to the euphonic sound of certain tube amps and doesn't care about their limitations. And if so, that's ok. As long as you are happy with your sound, it doesn't really matter what I think.

But I do think if you were open-minded and gave it a serious try and enough time to get used to the difference in sound before you formed strong opinion, then you might just convince yourself otherwise. And you may find that your system is capable of a lot higher sonic performance than you realize currently.

One final note, unless you have an extremely large room, diffusion of the back wave isn't going to help at all. Absorption is necessary in most normal size rooms.

 

[User211]

Good post Rich - and I appreciate it must have taken more than a minute or two to compose.

I've read it all and considered what you have said. I'm not sure that analysing mind of yours is reading too deeply between the lines of what I have said and come up with the wrong conclusions as to what I am actuallly thinking.

1) Bass absorbtion - the point is that they will absorb some bass energy that is otherwise reflected off the rear wall. Whether that causes phase anomolies with the "forward output" of the dipole bass driver, or just the fact that it arrives at my ear "out of time" with the forward wave is NOT the point here. The point is simply that if you are going to do this for the bass frequencies, then you also need to do it for the mid/treble. Doing it for a sub-section of the frequency range is "odd". You need to do it for all of it, and by the same level. I'll wager there is NO realistic chance of achieving that - just a part way bodge.

2) I'm pretty familiar with standing waves created by my sub. Just place it mid-way between the two side walls for disasterous bass. So I understand the practical results of really bad soundwave management.

3) The quote was simply a quote to highlight what one chap felt about it all and not to illustrate any technical points.

4) There is no doubt in my mind that using these traps will affect frequency response - so it IS a consideration. Whilst I confess I have thrown a lot of angle on this aspect, mainly because I've been doing lots of FR tests recently, I don't think I'd try to use them to do so as they just aren't very good at targeting specific troublesome frequencies.

Hope that helps, Rich. Wife wants to watch the telly so I'll edit to add more later

Star gazing telly program over with...

5) The point of diffusion as I understood it from the Apogee forum post was to create a wider soundstage, not to absorb the rear wave.

6) I record myself playing the guitar and am intensely aware of mic positioning and the effect it has on the results. I do this for both acoustic and electric guitars. I am also very aware of the difference recording in different rooms in my house has. Hell I've even recorded hi-fi show rooms for this site!!! Mics pick up room acoustics far more than the human ear does, so I totally understand a studio's needs for products like Ethan's.

7) The assertion that rear wall reflection is noise is, I submit to you, actually really pretentious - possibly totally undesirable. Here's a few points:

a) most instruments radiate omni-directionally to a greater or lesser extent.
b) MBL must be a complete bunch of morons to want to create omni-directional speakers, no?
c) all dipole speaker makers must be mad not to include rear wave absorbtion in their product.
d) bands play in rooms using real instruments and voices and that sound goes all over the shot.

While reflections can create havoc with microphones, recording an instrument using an acoustically treated studio and then playing it back using an omni or dipole speaker actually makes for a lot of sense - as a guitar, say, will radiate in an omni type manner in the real world anyway. Not perhaps the best example as most of the sound will emanate out through the sound hole, but there's a lot emitted by the chassis too in all directions.

Rich - there's no right and wrong here. Accept it. I accept that treating the rear wave will make a difference. I don't think it is necessarily a desirable one.

 

[Rich]

Thanks for the response, Justin. A few thoughts in reply below.

The point is simply that if you are going to do this for the bass frequencies, then you also need to do it for the mid/treble. Doing it for a sub-section of the frequency range is "odd". You need to do it for all of it, and by the same level.

Totally agree. Which is why I recommended a panel that evenly absorbs full-range originally. Absolutely there is a way to achieve that. But you will need to discuss it with the guys selling the room treatments to see which panel will work best for your needs.

The truth is, absorption above 100 hz. is really a lot more important for taming the back wave of the speaker than absorption below 100 hz., because of the difference in how a bass wave reflects and reacts with the room compared to the higher wavelengths. It is the reflection of the mids and highs in the rear wave that you should be most concerned with as far as the back wave is concerned, because they will cause the comb filtering that will smear your imaging. The bass can and should be best treated with corner bass traps. With the 244 bass panel recommendation, I was just trying to come up with a compromise that, if it didn't work for you behind your speakers, could still be used in the corners as a bass trap so your money was not spent in vain. It may still be your best bet, but I would talk to the folks at GIK about your intended use before buying to make sure it is the best panel for that application.

The point of diffusion as I understood it from the Apogee forum post was to create a wider soundstage, not to absorb the rear wave.

Understand, but again, I disagree that this is a desirable way to try to do so. There is simply not enough distance in most normal-sized listening rooms to adequately diffuse sound to the point where it will not cause harmful side effects. Again, I would make the point that soundstage is supposed to be created by the stereophonic effects of the master recording and the sound wave coming from two speakers a certain distance apart, not from reflections off the walls.

6) I record myself playing the guitar and am intensely aware of mic positioning and the effect it has on the results.

Right, but do you understand how to record with two mics in order to achieve a stereophonic image that places instruments in exactly the right spot? This can be done with several different miking techniques, or it can be done with the studio mixing board. (See the link I provided below for a basic explanation). The point I am trying to make is that it is all done in the recording phase, and the information is contained on that little disc, or on that magnetic tape, or whatever. Point source speakers can reproduce that sound stage and imaging perfectly. Anything added by a reflected, out of phase back wave is not an intended part of the reproduction of that musical event. And it can hamper your ear's ability to hear that reproduction, and your brain's ability to decipher what your ears are hearing. Therefore, it is noise.

The assertion that rear wall reflection is noise is, I submit to you, actually really pretentious - possibly totally undesirable.

(See explanation above for reason why I think it is rightfully called noise). Planar speakers using ribbons or electrostatic membranes are the only speakers that have an out of phase sound wave pointed in the opposite direction of the main sound wave and then reflected off the front wall. Box speakers don't do this. Horn speakers don't do this. If this was as desirable as you say it is, then why haven't box speaker manufacturers designed their speakers to do the same thing? It is merely a byproduct of the design of most planar speakers and rather than dealing with it in an effective manner, manufacturers have tried to push it as a benefit to the sound. From personal experience, I know that is simply not true.

Why is it recommended to treat the side walls with box speakers at the first reflection point? Because it is common knowledge that reflected sound smears imaging. Why would you think the back wave of a dipole speaker would be any different in this respect?

Here's a few points:

a) most instruments radiate omni-directionally to a greater or lesser extent.
b) MBL must be a complete bunch of morons to want to create omni-directional speakers, no?
c) all dipole speaker makers must be mad not to include rear wave absorbtion in their product.
d) bands play in rooms using real instruments and voices and that sound goes all over the shot.

I don't think you can conflate a live performance with a recording. Musical instruments are omnidirectional, yes. But a recording was never meant to be played omnidirectionally. A two-channel recording is mastered by an engineer with the intention that it be played from unidirectional speakers set at a certain angle from the listener in order to try to reproduce a stereophonic image of the original recording. Please read the section entitled "Background on Stereo Imaging" (page 5) in this link to understand what I am talking about.

ML's give you ambiance. MBLs give you a super-wide sweet spot. Both can give you an unnaturally wide soundstage. And both do so at the expense of clarity, imaging and accurate soundstaging. As with anything audio, it is a tradeoff. You are gaining ambiance. You are losing accuracy of reproduction.

Rich - there's no right and wrong here. Accept it. I accept that treating the rear wave will make a difference. I don't think it is necessarily a desirable one.

I agree that there is no right or wrong because ultimately sound quality is a subjective thing. I do believe that you are willing to accept a limitation in the accuracy, clarity, imaging and soundstaging of your speaker system in order to have a greater sense of ambiance and perhaps an unnaturally wide soundstage. Personally, I prefer greater clarity and a much more accurate soundstage, along with pinpoint imaging. In the end, it is all subjective and personal preference.

 

[RUR]

The truth is, absorption above 100 hz. is really a lot more important for taming the back wave of the speaker than absorption below 100 hz., because of the difference in how a bass wave reflects and reacts with the room compared to the higher wavelengths.

Rich, in the typical size room we're discussing, Schroeder will be around 200Hz.

 

[Rich]

Rich, in the typical size room we're discussing, Schroeder will be around 200Hz.

Thanks. I was just guesstimating off the top of my head so I was being a little conservative.

 

[Sambob2]

We frequently talk about how one can accurately measure the system response (in-room) to help provide placement and system tuning feedback.

There is a new affordable ($300) kit coming out that looks pretty nice. The folks involved definitely have the right pedigrees.


Dayton OmniMic Precision Measurement System

As I always say, an audiophile without a measurement system is like a carpenter without a tape measure.

This is an interesting system Jon. I've been looking around for some sort of RTA also and was thinking of going with a PC based system rather than a stand alone unit which can be much more expensive. I've seen similar systems, some on eBay like the RTA-168 (or something like that - the model slips my mind right now but I think that's it). I received a recent flyer from Parts Express where I have shopped before for electronic paraphernalia. They have the Dayton Audio system right on their front page and then details inside. It's looking like I may make a purchase but I want to wait until next months billing cycle to clear the Christmas splurge first! For people who know how to read the data it is an invaluable tool to have. I love all the features it has including RT60 and also the waterfall plot.

 

[Len44]

Well, this has certainly turned out to be a most interesting discussion. Interesting also that some company...Oh yeah -- Martin Logan, under the heading "ESL 101: Electrostatic Theory" includes the following:

"Controlled Dispersion. Another key attribute of an electrostatic speaker is its naturally dipolar radiation pattern. A true dipole transducer radiates with equal intensity from the front and back of its diaphragm, but the outputs are in opposite phase. As a result, sound waves rippling out toward the sides meet at the speaker's edge and cancel. That and the relatively large size of a typical electrostatic panel cause output at the sides to be very low relative to that of a conventional loudspeaker, which in turn minimizes side-wall reflections that tend to muddle sonic detail and stereo imaging. While the reduction in output to the sides contributes to the astonishing clarity for which electrostats are revered, the energy reflected off the wall behind the speaker opens up and deepens the sound." [bold emphasis added]

Comments are invited.

 

[RUR]

Well, we could always listen to Gayle Sanders:

"In your room, we tried something startling for a dipolar design—we damped most of the backwave. Then we opened up the dispersion a bit by putting the reflective side of the Studio Traps near either side of the SL3s. We emphasized that energy, but we had enough absorptive material in the room so that the energy died rapidly and was quite evenly distributed. That gave us better imaging, better focus, and a better sense of space.

"That whole experience has altered the way I think. I used to depend upon the backwave, but absorbing it showed me that we can free ourselves from it. We can start to work closer to that back wall as long as we can emphasize that midfield energy."

http://stereophile.com/content/martinlogan-sl3-loudspeaker-california-brisson-and-soundroom-doom

 

[Len44]

Well, we could always listen to Gayle Sanders:

http://stereophile.com/content/martinlogan-sl3-loudspeaker-california-brisson-and-soundroom-doom

Dammit! More conflicting information / advice -- and this from some fella with a dash of cred...

 

[Rich]

Comments are invited.

Marketing 101. Take the disadvantage of your engineering design and make it sound like an advantage.

If side-wall reflections muddle sonic detail and imaging, why would front wall reflections not do the same? After all, the way we toe-in the speakers, those reflections off the front wall bounce off that wall onto the side walls and then onto our ears, which are trying to listen to and interpret the stereoscopic image created by the front wave off the speaker.

The rear wave of an esl speaker adds appx. 3 db to the overall sound level at the listening position, but this additional sound is out of time and out of phase with the front wave of the speaker. How could this additional sound not muddy imaging, clarity and soundstaging? The back wave bounces off the back and side walls, and recombines with the front wave of the speaker, causing comb filtering (essentially meaning that at some frequencies the waves destroy each other, creating nulls, and at other frequencies the waves reinforce each other, creating lobes. These are direct wave interactions that affect the frequencies your ears are perceiving. Again, how can this possibly be an advantage? How can it not affect clarity and accurate reproduction of a stereophonic image?

And what exactly is meant by "opens up and deepens the sound"? Sounds like very subjective marketing terms rather than objective criteria to describe the sonic impact. The real question for most to decide is whether they want sound that is "more open and deep" or sound that is more accurate in its clarity, soundstage and imaging? There really is no "right" answer to that. It will vary with each individual.

 

[Len44]

Good points, Rich.

I was just pointing out the oddity of the traditional (and, evidently, still current) ML advice. Couldn't resist...:devil:

 

[RUR]

Rich, for an interesting discussion of FRP absorbtion vs. diffusion, see here, esp. the links to Toole. My ASC tube traps have hemispherical HF diffusors on half the diameter, permitting me to experiment between absorbtion/diffusion simply by rotating. It's a compromise between envelopment and imaging and I'm still tweaking.

Backwave's been much more straightforward: no matter how many experiments I've tried, I've always returned to essentially full absorbtion.

/Ken

 

[Rich]

Good points, Rich.

I was just pointing out the oddity of the traditional (and, evidently, still current) ML advice. Couldn't resist...:devil:

I think that's great to add all that to the discussion. A lot of people buy into this hype. We just have to realize that a lot of what you get from ML on these issues is indeed marketing hype. Reminds me of a couple of years ago when they were stating on one of their subwoofer marketing page how bad it was sonically to have ports in the subwoofers, and how that language magically disappeared from that page once they decided to introduce a cheaper sub with ports. Now they just say that their ports are "carefully designed to provide superb low-end efficiency while minimizing port induced distortion." Gee, ports went from awful things you wouldn't want in any subwoofer to not so bad if designed properly, all with a little marketing hyperbole.

To be fair though, they do say on the Dynamo 700 page:

"Although ports are a convenient and cost-effective way of increasing low-frequency output, they rely on resonant energy in a way that impairs bass quality. A good sealed system will exhibit less transient-blurring group delay while maintaining smooth, consistent response regardless of music or movie source material. Listen carefully to the sound of a bass drum and you'll hear the difference." But this is much gentler way of saying ports suck than they were saying before they introduced a cheaper sub with ports.

Does anyone remember that Gayle worked with one of the acoustic treatment companies (might have been ASC, I can't remember for sure) to develop a back wave absorber that fit on the back of an Aerius panel? I recall seeing that product at one time. Also, Soundlabs offers this accessory: Sallie

 

[JonFo]

...

Does anyone remember that Gayle worked with one of the acoustic treatment companies (might have been ASC, I can't remember for sure) to develop a back wave absorber that fit on the back of an Aerius panel? I recall seeing that product at one time.

Rich, the product was the ASC-BackBox. See pic below.

There are some for sale here: http://www.acousticsciences.com/clearance/clearance.htm

 

[JonFo]

The problem with closely spaced rear-wave absorbers is cavity resonances and reflections.

My favorite method of dealing with back-wave would be to mount the panels in an infinite baffle alignment.

Basically, divide the room up with a wall and mount the panels in the wall, then treat the area behind the wall with as much absorption as possible.

One of these days I'll draw this up and we can debate it. My room would accommodate such a layout, and I'm very tempted to go that way.

 

[churchill]

Backwave's been much more straightforward: no matter how many experiments I've tried, I've always returned to essentially full absorbtion.

/Ken

I'm in agreement with Ken and also use ASC tube traps in my application.