Wanted: Proof of multiple subs and sub EQ

[Ethan Winer]

So it would be natural to accept that Ethan hears the reflections as artifacts that we don't. He does not need to provide proof of this.

Thank you Amir. I'm sick of arguing with people who need to pump themselves up by tearing others down, and I won't engage those types further here or elsewhere. I'm always up for a good technical discussion, even when people disagree. But I've had it with the insults and accusations, especially from people who clearly have no real experience beyond what they've read.

To that end, I have little doubt that he hears the effect of an absorber. But I am very doubtful that many would. Instead, they see the reflector and imagine an improvement in sound whether there or not.

I'm quite certain that many untrained people appreciate the improved clarity when early reflections are absorbed because I've seen it in person many times in my own home theater when non-recording type friends visit. (Though many are musicians.)

One hugely important point that's ignored even though I've repeated it often is that the effect of side-wall (and ceiling) reflections depends directly on the size of the room. In a typical home-size room 15 feet wide the reflections are strong and early. In a much larger room, still within the scope of a "living room" maybe 30 feet wide, the effect of those reflections is totally different and also greatly reduced. I'm equally tired of having to make these same points over and over again. In your "Fig 11 box plots" graph above, do we know the size of the room and where the listeners were in relation to the boundaries?

--Ethan

 

[h.g.]

I guess I read too little too quickly. I saw "active room absorption" and assumed it's an active bass traps. I looked more carefully now but couldn't figure out exactly what it is or what they claim it does.

It is a subwoofer and subwoofers can absorb the sound of other subwoofers if they are setup to do so. You can see this for yourself in your REW room simulation by perhaps following something like the example in the manual. The physics is that work (rate of per unit area) is force (per unit area) times distance (rate of) move in the direction of the force. That is, the rate of flow of work is the product of the air pressure and cone velocity. Like pushing or pulling a child on a swing when a push or pull is timed can put work into the motion or take it out.

 

[amirm]

One hugely important point that's ignored even though I've repeated it often is that the effect of side-wall (and ceiling) reflections depends directly on the size of the room. In a typical home-size room 15 feet wide the reflections are strong and early. In a much larger room, still within the scope of a "living room" maybe 30 feet wide, the effect of those reflections is totally different and also greatly reduced. I'm equally tired of having to make these same points over and over again. In your "Fig 11 box plots" graph above, do we know the size of the room and where the listeners were in relation to the boundaries?

Sure, he is the one room they used at McGill University.

This was the second room:

"The second testing location was the control room attached to Seiji Ozawa Hall at the Tanglewood Music Center in Lenox, Massachusetts. This facility serves as primary control room for all recordings conducted in Ozawa Hall, as well as a critical listening and mixing environment for the center’s audio staff. The room was slightly smaller than the first testing environment, with a commensurately shorter decay time of approximately 175 ms. The room was equipped with a similar three-way loudspeaker setup for full-bandwidth playback."

Note that the purpose of this paper was to examine whether recording engineers while doing their work prefer absorbing, reflecting or scattering side reflections.

 

[Ethan Winer]

Thanks Amir. I wonder why they didn't put the absorption on the walls, farther away, where they'd be in the real world? Putting it on solid walls surely affects more than just the reflections. Do they say how thick the temporary walls were? I wonder what effect the diffusers on the real wall have and how they interact with the side walls? Also, it's a little hard to tell in a room this shape, but the panels seem not for enough forward to absorb all the reflections. Where is this from? Toole's book? I have his book but don't recall this specifically.

 

[amirm]

No, it is not from Toole's book. It is from an AES paper, The Practical Effects of Lateral Energy in Critical Listening Environments
RICHARD KING,1,2 AES Member
([email protected])
, BRETT LEONARD,1,2 AES Student Member
([email protected])
, AND
GRZEGORZ SIKORA,3 AES Member
([email protected])

They actually set out to prove or disprove Dr. Toole's theory with the listening tests. And here is the specs for the first room:

"The first of the three modules was a two-dimensional
primitive root diffusor varying in depth up to nine
inches (22.9 cm). This particular design was formed from
polystyrene and is a rather common fixture in recording
studio control rooms (Fig. 4). The second module
was a simple porous absorber, approximately six inches
(15.25 cm) in thickness, allowing for respectable lowermid
frequency absorption. The absorber was built into
a wooden frame and covered with acoustic fabric. The
third and arguably most uncommon treatment module was
painted 1/2-inch thick (1.27 cm) drywall. The raw drywall
was primed and painted with three heavy coats of
latex paint to increase reflectivity. This treatment most
closely resembled a typical residential wall, such as found
next to numerous loudspeakers in living rooms around the
world."

Here is a picture of the above:

I think the reason they put them in the room is because they wanted to change them over quickly by rolling one in and the other out.

I will do a summary digest on it when i get some time.

 

[Ethan Winer]

They actually set out to prove or disprove Dr. Toole's theory with the listening tests.

How did they do?

I think the reason they put them in the room is because they wanted to change them over quickly by rolling one in and the other out.

It seems to me it would have been more sensible to use a normal (size and shape) room, then have people listen with and without absorbers at the correct place on the walls. When I tested this in my own living room that's 16 feet wide, I replaced the absorbers on the side walls with diffusers in the exact same location. My wife and I both thought the absorbers sounded great and the diffusers sounded awful, not much different than a bare wall.

Thanks Amir.

--Ethan

 

[h.g.]

How did they do?

I think you may have got your quoting mixed up. I am not sure who said what you quoted but it wasn't me. I am the one prodding you about subwoofers absorbing sound.

 

[Ethan Winer]

LOL, I have no idea how that happened. Hey Amir, that was for you.

 

[Ethan Winer]

I am the one prodding you about subwoofers absorbing sound.

I know, but I don't have much to offer. If the Neumann literature shows a graph I'd believe them. As opposed to some of the BS companies that we all know fabricate their data.

 

[h.g.]

I know, but I don't have much to offer. If the Neumann literature shows a graph I'd believe them. As opposed to some of the BS companies that we all know fabricate their data.

Read the manual. Your graph is near the bottom.

 

[Ethan Winer]

I'm talking about a waterfall graph showing a reduction in ringing. We already know that multiple subs flatten the response.

 

[h.g.]

I'm talking about a waterfall graph showing a reduction in ringing. We already know that multiple subs flatten the response.

You would not consider a frequency response graph to be an acceptable alternative to a waterfall graph?

 

[Ethan Winer]

No, a response graph shows only the steady state response at that one location in the room. A waterfall shows the response and the decay times for all the "peak" frequencies. If you care:

Room Measuring Primer

This is sort of the whole point of this thread. We know that bass traps improve peaks, nulls, and ringing. The question is how much multiple subwoofers and EQ can reduce ringing. And not just at the one place the microphone was put when the sub volume levels and phase, or the EQ, were calibrated. But also if any improvements disappear six inches away, two feet away, and so forth.

 

[h.g.]

No, a response graph shows only the steady state response at that one location in the room. A waterfall shows the response and the decay times for all the "peak" frequencies. If you care:

Room Measuring Primer

Interesting. Have you noticed a correlation between the shape of the steady response and the decay? Or perhaps wondered why the more "technical" publications rarely show waterfall plots?

This is sort of the whole point of this thread. We know that bass traps improve peaks, nulls, and ringing. The question is how much multiple subwoofers and EQ can reduce ringing. And not just at the one place the microphone was put when the sub volume levels and phase, or the EQ, were calibrated. But also if any improvements disappear six inches away, two feet away, and so forth.

The point can indeed missed when people are not building on the same set of basic knowledge they consider to be true.

 

[Ethan Winer]

Have you noticed a correlation between the shape of the steady response and the decay?

No, those are two different things measured in different ways.

Or perhaps wondered why the more "technical" publications rarely show waterfall plots?

Say what? Waterfall plots are useful when you need to display what they can show. I don't see what that has to do with technical publications. My own article in the "highly technical" Sound & Vibration magazine uses waterfalls to show how bass traps reduce modal ringing:

Test Methods for Acoustic Treatment Products

How else would you show modal ringing?

 

[AJ Soundfield]

No, those are two different things measured in different ways.

They are both convolved from the same impulse.

 

[Wayne A. Pflughaupt]

Late to the game, but:

1) I'm looking for proof that using multiple subwoofers can reduce modal ringing. I've heard proponents claims you can place a sub "where it won't excite room modes," but that makes no sense to me. I've asked several multi-sub proponents for Before/After waterfalls, but they never had any actual proof of this claim.

2) Likewise, I'm looking for proof that subwoofer EQ can reduce modal ringing more than a tiny amount.

My only requirement for graphs is that the Before and After measurements must have been made with identical microphone placements. If the mic moves even an inch or two between sweeps, that alone will create differences large enough to invalidate the data.

I'm talking about a waterfall graph showing a reduction in ringing. We already know that multiple subs flatten the response.

I’m not aware that anyone has ever claimed that multiple subs reduce ringing. Can you or someone point me to that?

As for #2, I doubt you’ll be convinced, but here ya go. Both measurements were taken back to back, so the mic was not moved. In this comparison, a parametric filter was set for a mode at 41.9 Hz. In the second graph, the level of the signal was raised after parametric equalization to match the SPL reading the mode was displaying before being equalized. In other words, 41.9 Hz are at the same SPL in both graphs. We can see that after EQ the rate of decay for the mode improved to that of the room average, which is all EQ can be expected to accomplish. Anyone desiring or requiring a reduction in ringing below that point needs absorption – i.e. bass traps. It should be self evident that an equalizer can only make adjustments in gain levels to problematic frequencies; it cannot absorb acoustical energy.

Regards,
Wayne A. Pflughaupt

 

[NorthSky]

Late to the game, but:
I’m not aware that anyone has ever claimed that multiple subs reduce ringing. Can you or someone point me to that?
Regards,
Wayne A. Pflughaupt

Bonsoir Mr. Wayne

• http://www.acousticfrontiers.com/20...ow-to-use-parametric-eq-to-flatten-your-html/

"The results of using multiple subs can be spectacular – near flat frequency response, little modal ringing and low seat-to-seat bass quality variability in multi-seat theaters."

• http://www.acousticfrontiers.com/20...improve-sound-quality-part-2-room-modes-html/

"The effect of a subwoofer on the level of a room mode depends upon where you put the subwoofer. If you put it in the center of the room at the null there would be no energy coupling between the subwoofer and the mode. The mode would not be excited and there would be no measureable peak in the frequency response or ringing in the time domain.

The other way to achieve the same effect is to use two subwoofers, each of which is positioned in a lobe of different acoustic polarity. This means that the positive sound wave created by the subwoofer in the left, positive lobe is met by a sound wave of equal level but negative polarity created by the subwoofer in the other lobe. If you have two sound waves that are identical in frequency and level but the polarities are opposite then there will be perfect cancellation. This means that there will again not be any measurable frequency response peak or time domain ringing.

There is so much that can be achieved by listening to a system to identify a sound quality issue, using measurements to find the cause and then using appropriate solutions. In this case I hope you can see how subwoofers can be used to effectively solve some room mode related problems."
________

And somehow an opposing view, or not?

• https://books.google.ca/books?id=TIfOAwAAQBAJ&pg=PA416&lpg=PA416&dq=multiple+subs+reduce+ringing&source=bl&ots=5nRTa2oHQa&sig=ULCv_bdUuBK-XfRV0cpyWv1vloY&hl=en&sa=X&ved=0ahUKEwiPiafl4_HLAhXnloMKHVfTCRAQ6AEIODAE#v=onepage&q=multiple subs reduce ringing&f=false

Page 416 (Chapter 16), last paragraph.

 

[Don Hills]

Room resonances at low frequencies behave as “minimum phase” phenomena, and so, if the amplitude vs. frequency characteristic is corrected, so also will the phase vs. frequency characteristic. If both amplitude and phase responses are fixed, then it must be true that the transient response must be fixed – i.e. the ringing, or overhang, must be eliminated. - (Toole, The Acoustical Design Of Home Theaters, 1999)

There are dissenting opinions, arguing that "room resonances at low frequencies" are rarely minimum phase.

 

[h.g.]

No, those are two different things measured in different ways.

Is that an assumption on your part because they show different things or do you know how they are measured differently?

Say what? Waterfall plots are useful when you need to display what they can show. I don't see what that has to do with technical publications. My own article in the "highly technical" Sound & Vibration magazine uses waterfalls to show how bass traps reduce modal ringing:

Test Methods for Acoustic Treatment Products

If you want answers to technical questions or to learn about a technical subject a common and effective way to do it is by looking at technical publications written by people that understand the subject area. Assuming you had done this in your area, I was asking if you had observed a correlation between the type of person/publication that tends to use waterfall plots and those that don't. It looks like the answer is probably no.

How else would you show modal ringing?

As mentioned earlier, unless we know what "modal ringing" means to you it will involve guesswork. I have already guessed once and got it wrong by referring to graphs that would have answered your questions for many.