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
 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. :)

@erik_squires wrote:

" 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. "

It sounds like you are still looking at small rooms as if they were large ones, where 20% wall coverage has the same effect no matter where that 20% is located. And I disagree, because in a small room the earliest reflections will not only be the loudest but also the most likely to be detrimental, therefore THOSE are the ones we should pay the most attention to.

And I think that in general you lean more towards absorption, whereas in general I lean more towards diffusion, because I want to preserve the spectral balance of those reflections. If their spectral balance is inherently bad (because of poor radiation pattern control), that’s a different situation - then we would be using room treatment in an attempt to FIX a problem which ORIGINATES with the loudspeakers. And that is not easy to do well because room treatments are generally not frequency-region-specific enough in the RIGHT regions; room treatments generally paint with broad brushes, so to speak.

I’ll go along with the "AS NEEDED" part, with each of us obviously having a different idea of what that means.

Duke
If I understand correctly, you are still looking at small rooms as if they were large ones, where 20% wall coverage has the same effect no matter where that 20% is located.


My gods we are being so literal.  I'm saying that if best acoustic principles call for 20% absorption on a wall surface near the speakers, whether those panels EXACTLY cover a reflection point which works for exactly one seated position is irrelevant.

You can be off that singular, zero size point by a couple of feet and it will still sound good.

Conversly, ONLY covering that zero area point with a 1'x1' panel will be negligible.