Tweakers' Asylum

An excerpt from the DJ Society

147.240.236.8

Jay Buridan

In Reply to: RE: Crushed granite sand or posted by unclestu52 on October 29, 2007 at 20:18:49

Auditioning Speakers

Rap your knuckles on the speaker cabinet. If it sounds like you are rapping against a piece of granite, this means the cabinet is well damped from vibrating on its own (the inside of the cabinet contains wool, fiberglass, sometimes lead or sand to dampen vibrations of the cabinet). If rapping your knuckles against the cabinet results in a drum-like sound, called resonance, the cabinet may not be well damped. Resonance can add artifacts to the sound (the vibrating cabinet acts like a speaker itself). Click here to listen to a wav file that was recorded when rapping on a speaker that has excellent damping (the speakers shown on the left).

Follow Ups:

RE: An excerpt from the DJ Society - unclestu52 11:16:08 10/30/07 (2)
The idea is to move the resonant frequency above the critical midrange bandwidth. That being said, examine the philosophy of the critically acclaimed Harbeth speakers. They use a simple 9 mm birch type ply without bracing for their cabinets which actually rings quite a bit. They claim that the resonant frequency is part of the design, but is high enough to be situated outside the critical mid band.

Most so called 'non resonant' cabinets are simply well braced, some with a double wall construction which usually raises the frequency either well above or well below audibility. They still resonate, hence the ability of devices like the Tekna Sonic to 'work'. As a matter of fact, that's why even cones work on speaker cabinets.

The 'rap' test is more a measure of cabinet bracing and strength of construction. The idea is not to eliminate resonance, which is impossible, but to control it and to transform it into a more benign form.

Stu
- Interesting. What exactly is "above the critical bandwidth"?? (nt) - cheap-Jack 12:16:42 10/30/07 (1)
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  - RE: Interesting. What exactly is "above the critical bandwidth"?? (nt) - unclestu52 19:14:21 11/05/07 (0)
    Generally, it s regarded as the range where the human ear is most sensitive and you can examine the loudness curves to get the approximate frequency ranges. Bear in mind, individual sensitivities will vary, but similarly, as with setting crossover points for a tweeter, most designers try to either move the resonant points beyond the mid range bandwidth. Most opt for lower frequency bandwidths, but there are speakers with granite cabinets, aluminum cabinets, even steel cabinets, but, of course, resonance is not only determined by material, but also by size and other details of assembly.
    
    Stu
RE: An excerpt from the DJ Society - Ozzy 09:48:14 10/30/07 (0)
> Click here to listen to a wav file that was recorded when rapping on a speaker that has excellent damping (the speakers shown on the left). <

I would REALLY like to "click here", but there is no here here.

Oz

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