Panel absorber and SPL detector

[Richard Daigle]

I'm in the process of testing my small studio, and I have a few questions.

Can a Real time analyser be used instead of a digital SPL detector?
I am only interested in comparing one frequency relative to another.
Using Real time analyser software, I found variations of up to 6.5 dB between 140 and 160 Hz in my studio (11'X13'X8'). The rest of the frequencies seemed to be OK (50 to 300 Hz)
I'm planning to build a panel absorber to absorb at 148 Hz, using 1/8" plywood over 4" deep cavity, with 2" rigid insulation inside the cavity.
The calculation is for a 2' x 4' panel trap. But a change of frame dimension size changes the center frequency of the trap.
My panel trap is 2' X 6' 8", does anyone know how much my center frequency will vary?
Also, many thanks to Ethan Winer for the many articles he has written.

Richard

[Ethan Winer]

Hi Richard,

> Can a Real time analyser be used instead of a digital SPL detector? I am only interested in comparing one frequency relative to another. <

Frequency response is only half the story. You should also be concerned about time-related issues like flutter echo and modal ringing. As for how best to measure the response, for low frequences below, say, 300 Hz, you need high resolution. I prefer 1 Hz intervals, but 1/24 octave or better is adequate. From the low mids and up the lower resolution of 1/3 octave is actually better. But you still need to assess ringing and individual reflections and their decay times.

> Using Real time analyser software, I found variations of up to 6.5 dB between 140 and 160 Hz in my studio (11'X13'X8'). The rest of the frequencies seemed to be OK (50 to 300 Hz) <

That's why you need to measure at 1 Hz resolution. In fact, the spans from peaks to nulls are far worse than the 6.5 dB you measured, but the resolution was too low to be able to see that. I'll bet you $100 the response in your room varies at least 20 dB several times below 300 Hz, if not 30 dB or even more.

> I'm planning to build a panel absorber to absorb at 148 Hz <

For a 13 by 11 room you'll do better with broadband absorption in the form of thick rigid fiberglass. Again, this is why you need to measure bass frequencies at high resolution. Had you built tuned traps you'd fix only a small part of the problem.

--Ethan

[Richard Daigle]

I probably should have given you more information:
I have a small room around (11' X 13' X 8')
For each of the corners of my room; I built a 80 Hz panel absorbers 2' X 6' (1/2" plywood over 3 1/2" cavity), and another one which absorbs at 90 Hz (3/8" over 3 1/2").
I then put fibrex triangles , rigid insulation 4" thick, 6 pcf (2' X 2' X 2'10") in front of panel absorbers reaching from the floor to the ceiling, 20 pieces for each corner.
Know as "Super Chunks", Building the StudioTips “Super Chunk” Bass Traps
see: http://forum.studiotips.com/viewtopic.php?t=535)
I have a drop ceiling of 12". I used 4" to 6" of fibrex insulation to make drop ceiling. I also put 3 aborbing panels behind drop ceiling to aborb at 90 Hz and 2 to absorb at 80 Hz.
Inside my panel absorbers, I used 2" rigid insulation spaced 1/2" from the plywood, to give me a wider range of frequency.
The 2" insulation changes the "Q", I was hoping to attenuate frequencies from around 60 Hz and 120 Hz, and then to attenuate higher frequencies of 120 Hz to 18 KHz with the fibrex triangles in front of the panels.
I also made 2 2'X6' panels of rigid insulation 4" thick to put on walls, leaving a gap between insulation and wall.
I then played frequencies using your CD "using Ethan Winer's RealTraps Test Tone CD test frequencies from 20Hz to 300 Hz."
I noticed that the amplitudes varied the most for the individual frequency measured between 140 and 160 Hz (up to 6.5 dB difference), all the other frequencies were between 1 and 2.5 dB.
When I measured, I think I used a fairly narrow bandwidth.
This is why I wanted to build a panel absorber to absorb at 150 Hz.
According to my room size (11' X 13' X 8'), when calculating the room modes, I think it makes sense that I would have a problem between 140 and 160 Hz. (The panel absorbers make the room a little smaller and there's a place in the corner of the room where the ceiling is only 7 feet high, 25 sq ft of ceiling)
But because I have so many panel absorbers of 80 Hz and 90 Hz, and because of the super chuncks the lower frequencies of 60 to 140 Hz are good.
What do you think?
I'm planning to build a panel absorber to absorb at 148 Hz, using 1/8" plywood over 4" deep cavity, with 2" rigid insulation inside the cavity.
The calculation is for a 2' x 4' panel trap. But a change of frame dimension size changes the center frequency of the trap.
My panel trap is 2' X 6' 8", do you know how much my center frequency will vary?

Have a good day!
Thanks again for your help!
Richard

[Ethan Winer]

Richard,

... snip ...

Okay.

> The calculation is for a 2' x 4' panel trap. But a change of frame dimension size changes the center frequency of the trap. My panel trap is 2' X 6' 8", do you know how much my center frequency will vary? <

The center frequency is determined by the panel mass and cavity depth only. Here's a thread that shows the formula:

http://www.musicplayer.com/ubb/ultim.../t/000766.html

--Ethan

[Richard Daigle]

Thank you for answering my question, I look forward to reading more of your articles.

Richard