An Enticing Marketing Story, Theory Without Measurement?

[DWPress]

Other way around will do the most to improve your sound. EQ can be used of course with beneficial results but taking care of issues that DSP and EQ cannot compensate for is even better - then the process you intend to use for room EQ/correction/convolusion will have an easier time of it and the results should sound more "musical".

 

[andreasmaaan]

If I'm going to do physical room treatments...not rebuilding any structures, but perhaps some diffuser panels or bass traps, if warranted....should I try to do room correction EQ first, then do it, or the other way around?

The other way around 100%.

 

[Floyd Toole]

What about the special microphone used by Trinnov? It looks as if it has 4 channels and it should be possible to separate direct and reflected sound.

It is used to locate sound sources using relatively high frequencies (it is not large compared to most wavelengths). Measuring the broadband spectrum of sounds from different directions is a much more demanding task, given that multiple reflections will be arriving within very short time intervals of each other. So, one has spatial resolution and time resolution issues to deal with simultaneously, and both can affect frequency resolution of the spectral measurement. Not trivial, if useful frequency resolution is expected.

It is for reasons of this kind that estimating early reflections from anechoic data on the speakers was attractive - and experience has shown that it works more than reasonably well in anticipating perceived sound quality in normally reflective rooms. The interesting issue seems to come down to the fact that loudspeakers delivering flat and smooth direct sound, followed by reflected sounds having similarly smooth, but not necessarily flat responses, are consistently highly rated. Part of that may relate to the fact that the precedence effect works best when the direct and delayed sounds are similar. Some amount of reflected sound appears to be preferred by many listeners.

If a loudspeaker is not well behaved off axis, the only practical option is to acoustically absorb the offensive early reflections, which if done well results in an uncomfortably dead listening space. As explained in my book, this was part of the motivation for recording control rooms having "dead" ends. Some pro monitors behaved very badly off axis.

 

[FrantzM]

Question to Doctor Toole

From the top of my memory, I read somewhere perhaps from Earl Geddes that it takes up to 50 ms to properly perceive a 50 Hz tone. It this is the case that woudl suggest that in the bass say below 100 Hz we are dealing with a quasi-steady state phenomenon, thus Linearization in the Frequency domain with steady state approximation should work..
Right? Wrong?

 

[Floyd Toole]

Question to Doctor Toole

From the top of my memory, I read somewhere perhaps from Earl Geddes that it takes up to 50 ms to properly perceive a 50 Hz tone. It this is the case that woudl suggest that in the bass say below 100 Hz we are dealing with a quasi-steady state phenomenon, thus Linearization in the Frequency domain with steady state approximation should work..
Right? Wrong?

Both. Perceiving a tone is one thing, perceiving a bass transient is another. In my experience most complaints about the bass in small rooms is that it is "boomy" implying a time-domain problem. Fortunately prominent low frequency room resonances behave as minimum-phase phenomena, meaning that parametric equalization addressing those peaks will tame the ringing as well. But there is more: In Section 8.3 in my book "Do We Hear the Spectral Bump, the Temporal Ringing or Both?" addresses the fundamental issue. It turns out, thanks to serious research by some serious Europeans, that we respond to the bump, not the ringing - in spite of what we think we hear. Years before, Sean Olive and I published a paper showing that this was true at frequencies above about 200 Hz as well.

 

[amirm]

Look at Section 7.4.7 "Floor Reflections: a Special Case?". There is evidence that humans evolved with something always below their feet and when acoustical evidence of it is removed they may not approve.

Indeed. FHG presented a paper on the design of their listening room a few years back and ran into exactly this:

Vision and Technique behind the New Studios and Listening Rooms of the Fraunhofer IIS Audio Laboratory
Andreas Silzle, Stefan Geyersberger, Gerd Brohasga, Dieter Weninger and Michael Leistner
Fraunhofer Institute for Integrated Circuits IIS, Am Wolfsmantel 33, D-91058 Erlangen, Germany
Innovationszentrum für Telekommunikationstechnik GmbH IZT, Am Weichselgarten 5, D-91058 Erlangen, Germany
Fraunhofer Institute for Building Physics IBP, Nobelstraße 12, D-70569 Stuttgart, Germany

Figure 11 shows sections of the first 30 ms of two impulse response measurements, band pass filtered from 1 kHz to 8 kHz, both from the center loudspeaker to the reference listening position. The graphs are almost identical with each other, except around 13 ms. The first 10 ms with zero signals represent the direct sound travel time, the strong peaks at 10 ms originate from the direct sound. In the measurement of the untreated room with the regular carpet covered floor (black graph), a second peak at 13 ms with 0.6 amplitude relative to the direct sound, which is equivalent to 4.5 dB attenuation, represents the reflection caused by the floor. The time delay of 3 ms corresponds exactly to the 1.7 m path length difference between direct and reflected sound. This strong reflection violates the requirements of the standard. It can however be attenuated to a negligible level with little effort by placing a piece of moderately absorbing material (e.g. 50 mm thick porous material, about 1.5 m x 1 m) on the floor at the mirror point, i.e. the point, where the sound is reflected on the floor. The measurement result for this case is presented with the red graph in Figure 11. None of the reflections exceeds 0.3 amplitude relative to the direct sound, which is equivalent to more than 10 dB attenuation."

This is Fig. 11:

Later they perform listening tests and find that was a bad idea:

"3.1.4. Subjective room assessment"

[....]

Regarding the floor reflection, the audible influence by removing this with absorbers around the listener is negative – unnatural sounding. No normal room has an absorbent floor. The human brain seems to be used to this."

 

[amirm]

If I'm going to do physical room treatments...not rebuilding any structures, but perhaps some diffuser panels or bass traps, if warranted....should I try to do room correction EQ first, then do it, or the other way around?

Every room is broken below a few hundred hertz. Above that, there may not be anything wrong unless it is an empty box (dedicated room). So my answer is to get an EQ system going for low frequencies. That, you need. Diffusers and such are for higher frequencies.

 

[Music1969]

Later they perform listening tests and find that was a bad idea:

So @amirm @Floyd Toole is it better to use an absorber at the mirror point on the ceiling instead? And leave the flooring 'normal'?

A bit like recording studio setups do?

 

[DDF]

I think context is key. A room without any floor reflection would certainly sound confusing and uncomfortable for normal conversation, as ingrained expectations aren't met. I worked for years in BNR's anechoic chamber and know the feeling well.

Listening to reproduced orchestra is a different audio scene than this.

We do know for example from Bech's work (Perception of Reproduced Sound: Audibility of Individual reflections in a Complete Sound Field II, AES preprint 4093) that the floor reflection has an impact on timbre, and that timbre effects are the most sensitive to be detectable as reflection strength drops (the other effects being perceived position and loudness). So the floor reflection is audible and if it's not meant to be there "live", it's a colouration.

While an orchestra plays on a stage, the effect of the stage bounce under them is to increase level in a relatively broadband way. The stage floor heard in a concert venue would have a different audible effect than a living room floor reflection on playback over speakers: the time delay of the reflection in room is significantly greater than in hall due to the much greater listening distance live; the stage floor is also not between the listener and orchestra (any first reflection live might be from all the bald heads of the aged audience). The two can't be equated. I agree more research is needed and I'm surprised at the lack of exploration of this effect.

You'll probably disagree with the aim, but companies like PSB attempt to ameliorate the floor bounce effect by using multiple vertically spread drivers operating below ~ 400Hz, to add some chaos through multiple reflections, spreading out the notch and lowering its absolute depth.

Chasing this issue to ground will require some real research. In the meantime, evidence that humans adapt to listening spaces is encouraging.

No, no, no, that's the room "breaking in".

 

[amirm]

So @amirm @Floyd Toole is it better to use an absorber at the mirror point on the ceiling instead? And leave the flooring 'normal'?

A bit like recording studio setups do?

If we are talking about a dedicated room, the floor is a good place to put some carpet and reduce how live the room is. There is nothing beneficial about room reflections to want to keep them there.

 

[andreasmaaan]

The time delay of 3 ms corresponds exactly to the 1.7 m path length difference between direct and reflected sound. This strong reflection violates the requirements of the standard.

That's strange, a 1.7m path length difference should result in a (roughly) 5ms delay.

 

[DDF]

Indeed. FHG presented a paper on the design of their listening room a few years back

Taking a look at their graph, I don't see any evidence that the floor bounce was attenuated in the typical well ~ 200 Hz, but only that the higher frequency content was absorbed (looking at the time scales). I can see that this might sound strange.

I've made hundreds if not thousands of loudspeaker measurements in-home (MLSSA). In an attempt to absorb the floor bounce and extend the reflection free window for FFT, I've tried multiple types of absorption from layers of sonex to 3 foot thick Roxul in 6'x3' wide plots. None come close to reducing the strength of the reflection around 200 Hz to any useful degree for characterization purposes, unfortunately. From my experience, their thin absorber should have no significant impact on the lowest frequency floor notch. This makes sense when considering the long wavelengths involved.

A DSP used to cancel the floor bounce only in the modal region might yield significant positive benefits for orchestral works (especially) or other music meant to replicate listening in a hall.

 

[Music1969]

There is nothing beneficial about room reflections to want to keep them there.

So if we have the option (in a dedicated) room to reduce ALL 1st reflections, we should?

Floor, ceiling, back wall and side wall 1st reflections?

 

[amirm]

So if we have the option (in a dedicated) room to reduce ALL 1st reflections, we should?

Definitely not. That's the path to pretty graphs that sound bad. I would leave side reflections alone. Ceiling is optional.

 

[Guermantes]

If I'm going to do physical room treatments...not rebuilding any structures, but perhaps some diffuser panels or bass traps, if warranted....should I try to do room correction EQ first, then do it, or the other way around?

I'll say do both. Do the EQ to measure the room response first, if you can. That will give you an indication of where in the FR you need to focus. Diffusers and absorbers come in a variety of types to target different parts of the spectrum -- if your room has specific problems then measurement will help to identify those areas that need to be addressed.

Then re-do EQ again afterwards. You may even find that you prefer your new room un-processed.

 

[Bjorn]

@Keith, nobody (or very few?) questions DRC under Schroeder. So using DRC to kill boom (more precise: take away peaks) seems to be uncontroversial.

Everything under the Schroeder frequency isn't minimum phase behavior, thus you may introduce time anomalies here as well if you correct it with amplitude equalization. Minimum phase areas are mostly in the very lowest region (sub frequencies), but you can't make any generalizations and need to measure in each case.

One could perhaps assume that to correct what's non-minimum phase below the Schroeder frequency doesn't lead to definitive audible aberations because we are less sensitive in that region. But another way to look at it is to compare it to acoustic treatment. And anyone who has experienced a serious improvement in the 100-250 Hz region with acoustic treatment knows how much of change that is. Meaning that you will not get the same result with EQ here, despite that introducing anomalies with weird phase wrappings may not be that audible.

Said in another way: The EQ of what's non-minimum phase behavior below Schroeder may not to much damage but it will not improve much either, like acoustic treatment will. It's vital to minimize the resonances and shorten the decay for a great result, something that will not happen if you EQ what's non minimum phase below the Schroeder frequency.

 

[Bjorn]

Perhaps the most persuasive evidence relating to the audible consequences of the floor bounce is in:
Silzle, A., Geyersberger, S. Brohasga, G., Weninger, D., and Leistner, M. (2009). “Vision and Technique behind the New Studios and Listening Rooms of the Fraunhofer IIS Audio Laboratory”, Audio Eng. Soc. 126thConvention, Preprint 7672.
It is said:
“Regarding the floor reflection, the audible influence by removing this with absorbers around the listener is negative—unnatural sounding. No normal room has an absorbent floor. The human brain seems to be used to this.”
Other anecdotal experiences appear to agree. Is it absolutely definitive? The Fraunhofer room is an impressive facility, so it comes down to what program they were listening to. I have not gone back to look.

As for floor bounces in symphony orchestras, there is a floor below the musicians so a floor bounce is included in the "direct" sound arriving at all listeners - different for different elevations, obviously. I recall reading about investigations into the audible consequence of this floor bounce, but have no references to offer. I sit in front balcony/terrace seats to avoid the acoustical interference of audience and the seat-dip effect, both of which add to the floor bounce in the main "orchestra" seats.

Chasing this issue to ground will require some real research. In the meantime, evidence that humans adapt to listening spaces is encouraging.

There was a danish study that came to another conclusion. It was a joint project of Danmarks Tekniske Universitet, Bang & Olufsen and KEF led by Prof. Soren Bech.

Peter Lyngdorf who participated in it said the following:
"I’ve done a lot of testing on the effects of reflections in rooms, and there was a big, big project in Denmark about twelve years ago, with a lot of companies involved in investigating effects of reflections in rooms. I had the pleasure of being a test person, where we could actually simulate the audible effect of the floor reflection, sidewall reflection, ceiling reflection, and so on independently. The single most disturbing reflection in the room is the floor reflection. That is what makes the speaker sound like a radio and not like the actual event. ... The floor reflection absolutely must be handled"

I believe they found the ceiling reflection to be the second most disturbing reflection, quite contrary to the Harman studies.

 

[andreasmaaan]

There was a danish study that came to another conclusion. It was a joint project of Danmarks Tekniske Universitet, Bang & Olufsen and KEF led by Prof. Soren Bech.

Peter Lyngdorf who participated in it said the following:
"I’ve done a lot of testing on the effects of reflections in rooms, and there was a big, big project in Denmark about twelve years ago, with a lot of companies involved in investigating effects of reflections in rooms. I had the pleasure of being a test person, where we could actually simulate the audible effect of the floor reflection, sidewall reflection, ceiling reflection, and so on independently. The single most disturbing reflection in the room is the floor reflection. That is what makes the speaker sound like a radio and not like the actual event. ... The floor reflection absolutely must be handled"

I believe they found the ceiling reflection to be the second most disturbing reflection, quite contrary to the Harman studies.

Interesting. Do you know the name of this study or have a source for the quote, please?

I've certainly experienced better than radio-quality sound in rooms in which the first floor reflection null isn't handled; obviously exaggerated statements like this don't inspire much confidence in the research.

 

[Bjorn]

Interesting. Do you know the name of this study or have a source for the quote, please?

I've certainly experienced better than radio-quality sound in rooms in which the first floor reflection null isn't handled; obviously exaggerated statements like this don't inspire much confidence in the research.

Can't remember for certain as its' been a while since I read them, but could be these:
http://www.aes.org/e-lib/browse.cfm?elib=6383
http://www.aes.org/e-lib/browse.cfm?elib=7673
http://www.aes.org/e-lib/browse.cfm?elib=7577

Peter Lyngdorf also said the following:
"The second worse reflection is the ceiling reflection. Sidewall reflections, if they are sufficiently delayed (more than about five milliseconds) and are left/right symmetrical, can be actually beneficial to the sound. But if your speakers are very close to the sidewalls, you have to kill the side reflections. But do not be too concerned about the sidewall reflections. The floor reflection absolutely must be handled, followed by the ceiling reflection, either by absorption or diffusion."

Personally I think this depends on how well the speakers measures vertically and horizontally combined with the room layout. But I've found that most who does extensive treatment often experience ceiling treatment as the greatest advantage. Something that makes sense considering that most speakers measure the worse vertically due to phase anomalies, the ceiling reflection in most small rooms arrives very early, and the fact that vertical arrival reflections don't really do anything good psycho acoustically.

Very few experiment with treatment of the floor reflection because it's so unpractical. So here we have a lot less feedback. After all, who wants to have large absorbers on the floor?

 

[Juhazi]

I believe that it is practically impossible to eliminate floor reflection in a normal domestic room, like they did in Denmark study. What we can do is to attenuate it in several ways. It happens typically around 200Hz, depending on speaker height and listening distance. In worst scenario also front wall (behind the speaker) and back wall (behind the listener) make this same 1st order nulling around the same frequncy! With longitudinal listening axis in small room, also sidewall nulling comes in same range.

Thing to do is better postioning of the listening triangle in the room. It helps for low F modes too. Floor and ceiling reflection are hardest to handle. A tall wmtmw speaker helps too, like cardioid pattern. Dipole might help, but gives other issues to be handled or enjoyed...

A very easy thing to do in every room is to drag the listening chair closer to the speakers! This simple procedure helps in many ways and is the cheapest trick! Can I get this procedure patented and be a millionaire?