Ethan, size vs tuned freq of bass traps

Posted: Sat Mar 01, 2003 7:58 am

I need to limit one of the traps in both of my front wall corners to approximately 12 inches wide. height can be 6 to 8 feet. First, which should I adjust in size the low or high bass trap Second, is there an easy rule to determine the typical frequency response a trap is tuned to based on size.
Also Ethan do you sell your trap hanging devices seperate?

Thanks.

Posted: Sat Mar 01, 2003 9:08 am

Al,

> need to limit one of the traps in both of my front wall corners to approximately 12 inches wide. <

If you make a trap half as wide you need to make it twice as tall, or twice as deep, or a combination that results in the same internal volume [cubic feet]. But I think one foot is probably too narrow because the front panel won't be able to flex sufficiently.

> is there an easy rule to determine the typical frequency response a trap is tuned to based on size. <

According to Everest's book "The Master Handbook of Acoustics," if you use 1/4 inch plywood spaced with 4 inch wood [which is really closer to 3-3/4 inches] the formula is:

Center Frequency = 170 / sqr[0.74 * 3.75]

Where 0.74 is the surface density of 1/4 inch plywood and 3.75 is the depth of the chamber in inches. But I am suspicious of this formula because it does not include the height of the trap which affects its internal volume.

> do you sell your trap hanging devices seperate? <

No, but it's just a piece of wood cut at an angle!

--Ethan

Posted: Sat Mar 01, 2003 10:33 am

I take it that 1 foot is also to narrow even for the 1/8 inch used for the mid bass traps?. If so I will just put traps ont the front wall and the small side sections either leave blank or perhaps some corner foam or firber glass 4 or more inches thick?

Posted: Sun Mar 02, 2003 6:35 am

Al,

> I take it that 1 foot is also to narrow even for the 1/8 inch used for the mid bass traps? <

Probably, but I've never tested this so I don't really know for sure.

> I will just put traps ont the front wall and the small side sections either leave blank or perhaps some corner foam or firber glass 4 or more inches thick? <

You can't fit any in the corners? Or horizontally on the walls near the floor or ceiling? If not, then just use the thickest fiberglass you can.

--Ethan

Posted: Sun Mar 02, 2003 1:14 pm

Ethan,

I always thought the the frequency that a trap will absorb is based on the depth, not the volume. The volume determines how much sound it will absorb at that frequency.

I was looking at that formula and I am pretty sure it is correct. I am reaching this conclusion because both Everest and John Sayers have designs that are broadband absorbers because they vary the depth.

John also has a broadband helmholtz resonator that is broadband because it varys the width of the slots and the depth of the of the cavity behind the panel.

This is from the SAE website written by John

"The two factors determining the frequency of absorption are:

The mass or density of the panel.
The depth of the air cavity, i.e. depth of the sealed timber frame.

You can create a broadband low frequency absorption wall by building a series of sealed boxes with different depths with each box being only 1m x 1m (3' x 3'). With a variety of different thickness of plywood you can cover the whole low frequency range."

Posted: Mon Mar 03, 2003 6:35 am

Eric,

> I always thought the the frequency that a trap will absorb is based on the depth, not the volume. <

I have to admit that I'm not enough of a physicist to say for sure. But for the panels traps I've built at different heights, with all else the same, the resonance definitely changes between the taller and shorter types.

I'd be interested to hear what John Sayers has to say.

--Ethan

Posted: Mon Mar 03, 2003 1:31 pm

Eric,

> I always thought the the frequency that a trap will absorb is based on the depth, not the volume. <

I just heard from my guru, Matt Nobile at IBM, and he said the depth is indeed the main factor. But he also said he thinks the height, and thus the total volume, is at least somewhat of a factor too because the panel's total size affects its compliance.

--Ethan

Posted: Tue Mar 04, 2003 11:11 am

Could it be that the size of a panel affects how much it vibrates. A panel that is smaller wont have as much freedom to vibrate, therefor it won't vibrate at the lower frequencies that a larger panel would?

Posted: Tue Mar 04, 2003 12:26 pm

Eric,

> A panel that is smaller wont have as much freedom to vibrate, therefor it won't vibrate at the lower frequencies that a larger panel would? <

Yeah, probably that too.

--Ethan

This topic sponsored by: