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Re: Attn. Ethan Winer re bass traps

Posted by Ethan Winer (M) on February 7, 2004 at 06:56:03

In Reply to: Re: Attn. Ethan Winer re bass traps posted by Jon Risch on February 5, 2004 at 17:58:14:

Jon,

> No, I said it acts as a resistor. I meant resistor. <

My comment that a membrane is akin to an inductor applies to panel traps only. I agree that rigid fiberglass acts like a resistor. But in a classic membrane trap the membrane is an inductor, the air inside the sealed cavity is the capacitor, and the damping fiberglass also inside the box is a resistor that lowers the circuit Q. This is easily proven by simply looking at the absorption curve of a membrane trap. There's a center frequency where absorption is greatest, and it falls off on either side. The panel traps I've built appear to have a Q of about 1 or maybe a little less. An RC circuit behaves very differently, having a shelf curve that falls off continually in one direction. Just like the flat response and LF rolloff you get with a piece of rigid fiberglass.

> You sem to be laboring under the impression that either my or ASC traps use 703 or 705 rigid fiberglas boards <

I've seen the inside of an ASC tube trap and it sure looked like 1-inch thick 703 to me!

> I call out a simplified version of a bass trap in my DIY notes that uses 705 or 705 panels glued together with end caps, but these are not as effective as the ASC traps or my original clone design. <

I've asked you this before: On what do you base that claim? If you have any test data, now is the time to share it.

> See: http://www.acousticsciences.com/tubetrap.pdf for the acoustical circuit of the ASC type trap. <

Yes, I've seen that. In fact, it confirms that a membrane serves as an inductor! However, I remain unconvinced that the fiberglass inside a tube trap really acts like a membrane.

> the air volume is the C, not the depth <

The formula to calculate the center frequency of a panel trap, from Everest's "Master Handbook of Acoustics," Fourth Edition, page 206, uses only two variables: The density of the membrane in pounds per square foot, and the depth of the air gap. No mention is made of the air volume.

> the free-standing velecity type of absorber is at the worst disadvantage when placed in the corner <

I'm sorry but that is simply wrong. Look at the absorption data on the MiniTraps page of my company's site www.realtraps.com. You can see a side by side comparison of MiniTraps, which are velocity absorbers, in the room corners versus not in the corners. The same traps absorb 4.5 times more in the corners (21.54 Sabins versus 4.83 Sabins).

--Ethan

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