An Enticing Marketing Story, Theory Without Measurement?

[Bjorn]

I believe that it is practically impossible to eliminate floor reflection in a normal domestic room, like they did in Denmark study.

Actually, it's fairly easy (though unpractical) to deal with the specular reflection of the floor. However, treating the vertical mode is much more difficult. Two different things and let's not mix these.

Here's a measurement without floor treatment with a horn speaker (that already minimizes the floor reflection). Take note about the strong reflection at around 2 ms.

And here with two absorbers on the floor:

The 2 ms reflection is now effectively anechoic.

Frequency response is of less interest here but I've still added it.

Before (no floor treatment):

After (with floor treatment):

 

[Cosmik]

It happens typically around 200Hz, depending on speaker height and listening distance.

Isn't that missing the point of the discussion so far? A reflection isn't represented by a frequency. A reflection is a delay, and can be represented by a time.

On a continuous tone, the reflection causes peaks and troughs in what a mic and laptop will pick up and analyse with an FFT - which is what you're talking about. But our hearing is tuned to detect the delayed reflection, and separate it from the direct sound based on the multiple cues that transients provide.

This is why floor bounce, basically, is not audible, as has been mentioned earlier. It isn't because the brain expects it and is ignoring it; it is that the brain has removed it on-the-fly from its perception of the direct sound. There is literally no point in trying to remove it yourself electronically because, to the brain, it is not there in the first place (except as has been mentioned earlier as a sense of 'spaciousness' that is separate from the direct sound). If you do try to remove it by altering the signal, the brain will just hear an altered direct sound.

If the recording incorporates floor bounce (having been recorded using a mic in the audience for example), the brain can't separate it from the direct sound because the recording doesn't capture the complete sound field and allow the brain and ears to do their thing. In this case, the floor bounce will simply be heard as colouration - hence the reason for close-mic'ing of instruments and separate addition of 'ambience' in professional recordings.

Everyone surely knows that a tape recording of a school concert does not sound like it did when they were there in the audience. Now we know why

 

[Cosmik]

Peter Lyngdorf also said the following:
"The second worse reflection is the ceiling reflection...."
Personally I think this depends on how well the speakers measures vertically and horizontally combined with the room layout.

Yes. The quote you mention is making the common mistake of try-it-and-see methodologies: thinking that the variable that the experimenter is changing must be the sole reason for what changes in the results. (There must be some better words, probably in Latin that describes this).

So, if the experimenter attenuates the ceiling reflections and it sounds better, and if they attenuate wall reflections and it sounds worse, they conclude that ceiling reflections are bad and wall reflections are good.

In fact, what they are hearing is their particular speakers' non-neutral directivity characteristics and EQ interacting with the walls and ceiling in different ways. Despite what their results say, they have not shown that there is anything good or bad per se about floor, wall and ceiling reflections.

 

[solderdude]

One can debate and measure all day long and think one is being very objective and scientific....

Put the measurement microphone to a specific position and measure and see what the correction made of it.
Move it a meter away or to a very different position and repeat... different results..

So you don't 'correct' for a room, one attempts to 'correct' for a 'listening position' of a 'dumb' microphone.
It may give improvements and one may introduce unwanted 'corrections' as well.

Similar discussion in headphones and headphone measurements.

Treat your room (if possible) and apply some targetted EQ if still needed.

 

[Juhazi]

Nulling seen in SPL response is consequence of the floor reflection in my post. Another way to look at it.

What cosmic said, applies to eg programs that mostly look at SPL only and try to correct that.

 

[andreasmaaan]

So, if the experimenter attenuates the ceiling reflections and it sounds better, and if they attenuate wall reflections and it sounds worse, they conclude that ceiling reflections are bad and wall reflections are good.

I’m not sure what was done in the case of the experiments @Bjorn referenced, but the better experiments concerning room reflection perception don’t actually use real room reflections, but rather use simulated reflections from secondary loudspeakers in anechoic chambers. I’m not suggesting this removes absolutely all room for doubt or interpretation from the results, but it certainly does neutralise the specific doubt you’ve raised here.

 

[andreasmaaan]

FWIW, a number of posts here are assuming that the primary floor null is typically below 200Hz. I don’t believe that assumption is correct: for a dynamic speaker with its midwoofer or midrange around 90-100cm from the floor and a crossover over at around 200Hz (if the speaker is 3+ way), the primary null will typically be in the 200-500Hz range for seated listeners at distances greater than 2m; IME this is most speakers (ie both 2-way standmounts and 3+ way floorstanders) and most domestic listening room layouts.

 

[Cosmik]

I’m not sure what was done in the case of the experiments @Bjorn referenced, but the better experiments concerning room reflection perception don’t actually use real room reflections, but rather use simulated reflections from secondary loudspeakers in anechoic chambers. I’m not suggesting this removes absolutely all room for doubt or interpretation from the results, but it certainly does neutralise the specific doubt you’ve raised here.

Gulp, that would seem to open a whole different can of worms. Layer upon layer of deviation from the experience of a true acoustic source in a physical space that responds to the orientation of a listener's head with two ears in a consistent, systematic way.

 

[Bjorn]

200 Hz is also below the Schroeder frequency in many small rooms, thus it's not a specular reflection but a standing wave. The danish study was on reflections and not on standing waves to my knowledge.

 

[Kal Rubinson]

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

Memorable!

 

[Juhazi]

^Björn, don't you think that boundary reflection-related boosting and nulling of spl and standing waves can appear simultaneously, typically between 100-500Hz? I see reflections and modes as two different phenomena, of which the other is dominant for detection/hearing, I don't see "Schröder frequency" as a solid border between these. A paper about this http://www.akutek.info/Papers/MS_Schroeder_Revisited.pdf
"5. Conclusion From the discussion above, it is concluded that it remains to determine a measurable cross-over frequency or cross-over region between the high frequency Schroeder region and the lower frequency Modal Region. Several possible candidates are presented, together with suggested criteria for verification or falsification. "

Earl can explain better how these get born and how they are heard. We must combine "laws" of timbral and spectral hearing resolution, D/I ratio, precedence, Schröder, room modes, spatial localization etc.

With REW it is easy to change IR gating for spectral analysis, and we easily can detect eg. floor reflection nulling with 5-6ms IR gating. Impulse response graph shows timing of reflections, but at least I cant see how it affects spl amplitude response through spectrum. And yes, a reflection reflects all frequencies, but with different absorption, diffraction and different consequencies.

 

[JJB70]

This thread vindicates my decision to spend £600000 demolishing my house so I could rebuild it on audiophile grade foundations.

 

[KSTR]

This is why floor bounce, basically, is not audible, as has been mentioned earlier. It isn't because the brain expects it and is ignoring it; it is that the brain has removed it on-the-fly from its perception of the direct sound. There is literally no point in trying to remove it yourself electronically because, to the brain, it is not there in the first place (except as has been mentioned earlier as a sense of 'spaciousness' that is separate from the direct sound). If you do try to remove it by altering the signal, the brain will just hear an altered direct sound.

While it is true that the brain easily adapts to the floor bounce there still is an audible effect whether it's low/absent (as are other specular reflections) or not: It alters the minimum preceived distance of phantom sources (including hard L/R pans). With strong floor bounce present, the whole soundstage starts at the speaker baseline and extends to the back from there because the floor bounce gives clues about the speaker's location/distance. Without specular reflections, we only can make out the direction of the sound source but not the distance and this way very dry signals can project closer than the distance to the baseline, up to about roughly 1m of apparant distance (closer apparent distances require 1/r SPL differences to be factored in, that is, crosstalk cancelling must be applied). In the extreme case, listening in an anechoic chamber, it actually sound very similar to headphone playback with crossfeed applied.

 

[andreasmaaan]

Gulp, that would seem to open a whole different can of worms. Layer upon layer of deviation from the experience of a true acoustic source in a physical space that responds to the orientation of a listener's head with two ears in a consistent, systematic way.

Yes, definitely a valid criticism (although I only see one "layer" in what you've said).

Nevertheless, this avenue of experimentation does level the playing field between specular reflections from all angles.

 

[Cosmik]

Yes, definitely a valid criticism (although I only see one "layer" in what you've said).

It's a simulation of a reflection only if we agree that a reflection resembles something that emanates from a point(-like) source that has its own directivity characteristics, beaming, etc. And this assumes that the experimenters aren't being clever and representing different materials, sizes and distances with DSP e.g. a reflection from 20 feet away coming from a speaker mounted 10 feet up, etc. The anechoic chamber is a very odd place to start with. Our own voices won't be consistent with the 'reflections'.

As we move our heads, what reaches our ears will deviate from what would reach them if the reflection was a large flat surface (geometry and so on). It's only a simulation of a reflection if we really want it to be one..!

If we hear 'something' that we work out isn't a reflection from above us, we don't know that humans don't have an instinctive fear of predators (pterodactyls?!) carried through from when we were further back in evolution!

 

[svart-hvitt]

@Cosmik , curious (and I may have asked you the same question before...), but do you prefer an «unadjusted» bass response in your reproduction of sound?

And is your playback system based on a psychoacoustic model where bass below a certain low frequency is limited and high frequencies are limited at a certain limit - not so much as to reflect what’s real but what’s considered psychoacoustically adequate?

 

[andreasmaaan]

It's a simulation of a reflection only if we agree that a reflection resembles something that emanates from a point(-like) source that has its own directivity characteristics, beaming, etc.

The experiment is conducted in an anechoic chamber, so directivity characteristics of the simulated reflection aren’t important.

And this assumes that the experimenters aren't being clever and representing different materials, sizes and distances with DSP e.g. a reflection from 20 feet away coming from a speaker mounted 10 feet up, etc.

These factors are accounted for in the studies I’ve read.

The anechoic chamber is a very odd place to start with. Our own voices won't be consistent with the 'reflections'.

The same basic test setup is the basis of our understanding of the precedence effect. Would you say that our understanding of this effect is wrong because the anechoic chamber is an odd environment?

If we hear 'something' that we work out isn't a reflection from above us, we don't know that humans don't have an instinctive fear of predators (pterodactyls?!) carried through from when we were further back in evolution!

Actually I didn’t quite get this one! Could you explain a bit more?

 

[Juhazi]

^I guess it is this view https://mue.music.miami.edu/thesis/jwest/Chap_2/Chap_2_Spatial_Hearing.html
"
Spatial Hearing Theory

Evolutionary Views on Hearing

Spatial hearing should be examined in the context of biological evolution. "The most important higher cortical functions of an animal brain are environmental representation and prediction, and the planning of behavioral responsewith the goal of maximizing the chances of survival and perpetuation of the species [12]." Our sensory systems, necessary for environmental representation, evolved based on their usefulness in picking out the information that is most useful to our survival from the sea of energy around us [13].

McEachern [17] argues that signal detection, identification, and location (localization) were the most critical signal processing tasks for the eyes and ears of early humans. (I recall an unknown reference that observed that an object must emit or disturb energy to be detected by a biological or man-made sensor.) To hunt and avoid being eaten, our ancestors had to detect nearby movement, identify it as prey, predator, or human, and determine its location so they could run towards or away from it as necessary. While his ideas are reasonable, they are more useful in engineering applications than in describing actual perceptual processes because identification and localization are not independent.

All of these views point to the evolutionary advantage of environmental representation through perception of spatial object-person relationships. Spatial hearing exists because it is advantageous to humankind's survival.

 

[DWPress]

Let us pause then and reflect on the wonder that is man. From the blink of our existence to the past 100 years we have "evolved" into an animal that creates "music" and has also created the means of reproducing it with our brilliant mechanical tools so that we may debate objectively on how our early ancestors might respond to such unnaturalness using science.

Aside from our natural tendency to exploit and destroy most of what we can we also come up with the good stuff. We may have evolved too fast for our own britches but ahhhhh, the ecstasy of sound done well.... Which makes me wonder how and with what tools we will be listening in another 50 to 100 years.

 

[Cosmik]

The experiment is conducted in an anechoic chamber, so directivity characteristics of the simulated reflection aren’t important.

But the simulated 'reflection' is coming from a point-like source with its own directivity characteristics..? The discrepancies between what reaches the two ears will be different from a real reflection, and as the listener moves their head, more information about the source of the sound will become available.

Actually I didn’t quite get this one! Could you explain a bit more?

I'm saying that because the listener is most likely not fooled by the simulated reflection, they're going to register it as a source of sound. And then the experiment is really about what locations for sound sources humans have evolved to find the least/most threatening. Sounds from above might be especially threatening, so people registering ceiling 'reflections' as detrimental to the sound are really just indicating some evolutionary mechanism. If it was a genuine reflection they wouldn't register it in the same way.