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
Let me try this another way. Imagine a highly reflective room with a pair of traditional 2 way speakers and 1 listening chair in a fixed location. The room is 20’x20’x15’ tall.

There are at least six (eight if we include the ceiling) first reflection points. Being points, they are infinitely small. The audio legend is that these specific points are more important, by far, than any other place in the room. In this room we may place 1’x1’ absorptive panels.

My argument is that the legend/myth is wrong. The first six reflection points are not going to be noticeably better than any other place to put an absorber or diffusor.

Lets go through a bit of a mental exercise. We’ll consider two sides to this.

First, the very sparse case in which we treat six points and only those. At six absorbers the room remains too lively to make much of a difference.

Let’s consider the opposite situation. We have 40 panels of the same 1’x1’ dimension. Now we can make enough of a difference in the reverberant field to affect the sound quality. We put 20 or uniformly spread across the wall behind the speaker, and 10 on each side on the speaker end.

Let’s say by chance, four of these panels (2 on the rear, and 1 on each side) are exactly on the first reflection points in terms of the listening chair. In this case, removing those four panels and randomly relocating them will make a very small, if any, noticeable difference.

And this illustrates my point. Treating the initial reflection points is actually not as important as treating the room. The audiophile using a mirror to place a panel exactly on that spot is wasting his/her time.  What is more important, by far, is getting a critical mass of room treatment so that the reverberant field becomes well controlled.
@erik_squires , 

Floors are a different thing, but we don't spot treat floors. We treat the entire area in front of a speaker. I've never seen anyone put down a 2'x2' carpet exactly in the first reflection point, and this is kind of what i mean. 

Eric, I have a dedicated room.  I made 2'x4'x4" OC 703 panels that cover first reflection points on both the floor and ceiling.  The rest of my floor is untreated.  I realize this is not common, but I have a dedicated room and I am the only listener, so I can do what I want.   Both ceiling and floor "treatments" have very little influence on imaging in comparison to my side wall "treatment."    They certainly offer other benefits. which more than justify their use.  I have considered modifying my ceiling panels in order to add forward facing reflection to the existing absorption, but have not done so yet. 

As I recall, my measurements show that the floor and ceiling reflections are delayed about 4-8 ms, whereas the side wall reflections are more in the 6-12 ms range.   Without treatment, the ceiling and floor reflections are louder than those from the side walls.   With treatment, the side wall reflections are substantially reduced, whereas the ceiling and floor reflections remain relatively high.  To a certain extent, I think this is indicative of how much more effective reflection is compared to absorption.  

From my perspective, sidewall 1st reflection point treatment is conceptually sound, and in my room, theory has been supported empirically.  Imaging and localization in my room is superb.  It is the best by far that I have ever heard in a narrow room.  If you have experienced something different, perhaps that is because you are attempting to "treat" your sidewalls using panels that are not up to the task.  Again, I'm going to suggest that before you discount the concept, you evaluate the approach articulated at least twice by Duke and several times by me.   Rip a 4x8 sheet of half inch plywood into 2 x 8 sheets and place them along the side wall behind the first reflection points angled so as to reflect sound back to the front of the room and away from the MLP.  This is enough to reflect the wavelengths that are important to localization clues, and it turns early reflections into late ones.  It is a cheap and easy experiment.  Keep in mind that the first reflection "point" is far from a point, because the sound coming from your tweeter and midrange will spread out as it travels rather than proceed as a laser like ray.  So you will need to experiment with placement and angle.
Of course quantity trumps.

If you have 6 or 40 panels. 40 is 6.66 times more area than only 6 panels.
And when a reflection is more than reflecting one time. Let say it bounces 3 times before it is so week that it is -60 dB.

Then we have 6.66 times x 3 bounces gives us 20 time more treatment... 

So that is why we do not react or find any big difference on if you treated the first reflection points or not when I'm comparison to a room that have 20 times more treatment than the other!

I still say not all have of different reasons (time, founds or ascetics) not willing to get 40 pieces of treatment all over the room.

In most cases maybe you only buy 2/4/6 panels maybe bigger and thicker 1m x 0.5m each.

And we known for each reflection sound does it get weaker and weaker. So if/when you only got a few panels (less of a coverage area) then we start to treat the strongest reflection that is the first one. Of course absorbers will not take all reflection away. And of course the few panels will not contribute as much as if you had 6.66 times more panels and coverage area.

But it is for most of us a starting point and we can always add more panels along the way.
Maybe the only real problem is how I interpreted this information early on.  Maybe it was never meant to be a point thing but a starting place?
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.