Does the first reflection point actually matter??


Hello my friends,

So please read the whole post before commenting. The question is nuanced.

First, as you probably know I’m a huge fan of the well treated room, and a fan boy of GIK acoustics as a result, so what I am _not_ arguing is against proper room treatment. I remember many years ago, perhaps in Audio magazine (dating myself?) the concept of treating the first reflection points came up, and it seems really logical, and quickly adopted. Mirrors, flashlights and lasers and paying the neighbor’s kid (because we don’t have real friends) to come and hold them while marking the wall became common.

However!! In my experience, I have not actually been able to tell the difference between panels on and off that first reflection point. Of course, I can hear the difference between panels and not, but after all these years, I want to ask if any of you personally know that the first reflection point really matters more than other similar locations. Were we scammed? By knowing I mean, did you experiment? Did you find it the night and day difference that was uttered, or was it a subtle thing, and if those panels were moved 6" off, would you hear it?


Best,


Erik
erik_squires
Erik, first reflection "point" is probably an unfortunate term, which probably derives from conventional techniques used to determine the area in question.   First reflection zone would probably be much more accurate. Look at your REW impulse graphs.  You don't see a single intense reflection at, as example, 9.6 ms.  That is what you would see if the side wall early reflection came from a single point.  Instead, you see a cluster of reflections in a band that may be 3 ms wide or more.   All of those early reflections, or at least the higher frequency component of those reflections, are going to compromise image. 

An interesting experiment is to look at REW impulse graphs before and after adding conventional absorbing panels to those first reflection zones.  Even a GIK 6" full range Monster trap won't attenuate side wall first reflections anywhere close to the -20dB (compared to impulse) level necessary to mitigate 1st reflection erosion of spacial clues in most rooms.   An angled barrier that reflects sound back to the front of the room is a much better approach.  Depending upon the size of the barrier, some of the sound (low frequencies) will just wrap around the barrier as if it weren't there.  But the shorter wavelength (higher frequency) sound that is responsible for localization, won't wrap around the barrier but instead is reflected back towards the source.  The sound isn't turned into heat by absorption, it is just turned from an early reflection into a later reflection that adds to the perception of spaciousness. 




@erik_squires wrote:

"I believe most audiophiles would be unable to tell if those treatments were at the reflection points or not, and that in many cases 4 panels of 2’x2’, no matter how well placed, would be unable to effect an audible improvement."

and

"By ["treat"] I mean to alter the [room] surfaces by increasing the absorption and decreasing the ability of those surfaces to throw a coherent reflection by both absorption and diffusion."


Thank you Erik.


My reservations about using absorption on the entire surface, rather than just on the places where it has the most beneficial effect, are twofold.


First, to the extent that absorption is more effective at short wavelengths than at long ones, it will change (darken) the spectral balance of those first reflections. That may still be a worthwhile net improvement if the room is overly reflective, but in general it is desirable for the reflections to have approximately the same spectral balance as the direct sound.


Second, absorption continues to be effective long after the first reflections... ALL subsequent reflections which strike the absorptive material have their spectrum and overall loudness altered accordingly. So treating entire room surfaces can result in an overly dead room.


Reverberation time is seldom an issue in small rooms unless they have a slap-echo issue, and even then treating the entire wall with absorption is probably unnecessary. (Some argue that the term “reverberation” is actually inappropriate for small rooms because discrete reflections dominate, but I think it conveys a useful concept.)


As has been mentioned, loudspeakers are not true point sources, nor do we normally sit with our heads sufficiently in a vice that a 1 foot square treatment panel is what anybody is advocating. So I think "first reflection zones" is a more useful concept than "first reflection points."


I think you and I disagree on whether the timing and magnitude of reflections matters. If not, then neither does it matter where your absorptive panels go. If it does, then where they go also matters.


As I stated before, imo your assumptions are valid for large rooms (wherein the reverberant field is uniform enough that the reflections average identically at any given location) but not for small ones (wherein we have discrete reflections at any given location).


It sounds to me like you want to use enough absorptive acoustic treatment panels to make a significant difference throughout the room. Imo that would make sense ONLY if the speaker’s off-axis response is so bad that the reflections are generally detrimental. If the reflections are beneficial, weakening all of them and degrading their tonal balance with absorption would be detrimental. I’m not saying absorption has no place in home audio, but I am saying that the less of it we "need", the better.


Imo there is an alternative approach which starts out with the design of the loudspeakers themselves, and which does not call for anything remotely approaching treatment of entire room surfaces in order to get good results. Briefly, the loudspeaker sends spectrally-correct energy in directions which minimize early sidewall reflections, and the reverberant energy is allowed to decay more or less naturally, perhaps using diffusion, as opposed to being rapidly absorbed. If anyone is interested I’ll go into detail.


Duke


 Great dissertation Duke.

 The rooms most of us listen in usually do not reverberate, they are to small. A 10,000 seat indoor venue reverberates.

The reflections in our rooms occur relatively early and die off quickly. The early ones that reach your ears are interpreted as part of the music the effect being that of a blurred picture which in audiophile terms is a lack of detail and a blurred image. Preventing the earliest reflections from getting to your ears is always worth while including those of us with full spectrum room control. Zones, points or whatever the goal is to block reflection by absorption. We have discussed how to locate these areas. Absorbing sound below 200 Hz is difficult. The lower you go the more difficult it becomes.

Over doing it is just as bad as not doing it at all. It is just a waste of money and cosmetically unacceptable not to mention that it sounds as if your head is stuffed full of cotton.

Duke alludes to a very important point. Certain speaker designs by virtue of the way they radiate sound create fewer and/or less powerful  reflections. Horns, dipoles and line sources are examples. You don't have to worry about absorption below 200 Hz if there is no reflection.
My reservations about using absorption on the entire surface,

Also not what I meant, sorry.  I meant treating the surface appropriately.  That rarely means covering the entire surface but considering the entire surface.

You have this 1" by 1" reflection point.  That is irrelevant. What matters is that portion of the wall, and the overall, average results.  For instance, covering 20% of the wall with absorption and 5% of it with diffusion, as needed. 

First, to the extent that absorption is more effective at short wavelengths than at long ones, it will change (darken) the spectral balance of those first reflections. That may still be a worthwhile net improvement if the room is overly reflective, but in general it is desirable for the reflections to have approximately the same spectral balance as the direct sound.
Yes, that is a problem higher frequencies is easier to absorb than lower ones. And will result in darken and we loose the "spectral balance".

It is good to always keep that in mind when we choose asorbent treatment. When we known that the lower frequency gets and the wave lengths get longer the thicker the panels need to be. For them to to have a high absorption coficient lower down in frequency.

Yes in practice the depth will be unreasonably thick.. So you can never have to thick panels so no worry there.

But if we always go as tick that we can or care for is desirable. 

To lessen the risk to end up with a dark/lifeless acoustic sounding room. 

We see that it is not only coverage area that matters. It is the thickness also. 

I would chose fewer and thicker absorbers than many more of them and thinner ones. To not get the issue that is in citation above.

Remember it is in bass their is the most energy and the most difficult frequency range to treat and therefore the most problematic. And will not get solved easily.

I have big few panels that are 19 cm thick (I wish they were thicker) on first reflection points (and two in the corners their bass builds up) I always think "bass first". When we put up absorbers we get a reduction of high frequencies "automatically". 

Another "trick" to get the absorbers you have to get better asorbtion coefficient at lower frequencies is to distance your panels from the wall. They will act as they are thicker than they are. All to just try to optimize bass absorption that we are in disadvantage at the get go.

I hope this helped someone. :)