REW'S GRAPHS. how to. - Home Theater Forum and Systems - HomeTheaterShack.com

Old 11-04-10, 10:41 AM Thread Starter
Shackster

irombeach

Join Date: Sep 2010
Posts: 76
REW'S GRAPHS. how to.

Hi guys, hi Wayne,
i'm here today to ask if i have understood something.....

let us suppose our goal is to optimize our listening session, without using equalization,
then, suppose we are using REW software.....

suppose we cannot move our listening position....
suppose we have the ownership of the listening room and no wife can limit room treatments

and finally suppose we have two beautiful graphs.

Impulse response and SPL.

Now, we want to "minimize" measurements to know where to place acoustic panels, so we'd like to know
if graphs can tell something.

this is what i "know" about IR graph : ir graph shows what the spl/mic hears against time.
peaks and dips are expressed against time 0 and they represent an "amount" of "noise", regardless to frequencies.
knowing sound speed in the air, seeing a peak after xx millisecond, we can evaluate how many additional meters sound
"runs" before reaching listening position.
these additional paths are due to reflections.
am i correct?
now how can i simulate sound's path to know where the wave "knocks" to a wall or to the ceiling?

OK, i might look like a lazy man but why doing hundred of measurements to know where to place a panel?
it's quite simple to try position against walls but WHAT ABOUT the ceiling?
in which way can i position a panel without drilling ?

kisses
Alessandro
irombeach is offline

Old 11-04-10, 12:21 PM
Senior Shackster
Bill

Join Date: Sep 2009
Location: Austin, TX
Posts: 676
Re: REW'S GRAPHS. how to.

Quote:
irombeach wrote: View Post
... this is what i "know" about IR graph : ir graph shows what the spl/mic hears against time.
peaks and dips are expressed against time 0 and they represent an "amount" of "noise", regardless to frequencies.
knowing sound speed in the air, seeing a peak after xx millisecond, we can evaluate how many additional meters sound
"runs" before reaching listening position.
these additional paths are due to reflections.
am i correct?
now how can i simulate sound's path to know where the wave "knocks" to a wall or to the ceiling? ...
Hello Alessandro,

Yes you are correct. I've tried to look at the data the way you describe; it gave me many hours of fun with Excel.

It's straightforward to look at the problem if one considers only single first reflection points.

One approach I took was to look at each peak in the ETC graph, convert the time to that peak into distance, and for each such distance, calculate the corresponding frequency for a 1/2 wave, full wave, 3/2 wave reflection. I then compared these frequencies to the response curve to see which 1/2 wave and 3/2 wave lengths matched dips, and which full wave lengths matched peaks. From these, I could see which distances explained observed dips at one frequency and peaks at another, and I could then look for room reflections that matched these distances.

A second approach I took was to start with the room geography. I carefully measured the distances to the woofer of each speaker and to the listening position, and the distances between each of the walls and ceiling, and calculated the difference in distance between the direct path from speaker to listener and the reflected path off of each surface. For each such difference in distance, I calculated the 1/2 wave, full wave, 3/2 wave, 4/2 wave, 5/2 wave, 6/2 wave, and 7/2 wave frequencies. I could then look at the frequency response curve and see which of the distances explained an observed dip from an odd half wave reflection or an observed peak from an even half wave reflection.

Obviously, this single reflection point analysis explains only some of the variation, as two reflections each 120° out of phase might add to give a signal 180° out of phase with the direct sound.

What I learned from this exercise was that I had not been adequately considering the reflections from the opposite walls -- before I did this, I had only considered the walls near each speaker. In my room, the opposite are the most significant issue. I also learned that the ceiling reflection, which had worried me, explained very little of the variation in the frequency response. This is probably why Brian, in the audio acoustics forum, suggests attacking the ceiling with treatments last.

Some day I might even use this information to install some treatments, when budgetary considerations permit.

Have fun,
Bill
laser188139 is offline

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