If one wants to lower the resonant frequency of a metal speaker enclosure, there used to be a product out that was available in a spray can, and it would form a thick, tar-like surface. It was sold in auto supply houses for coating wheel wells and car frames.
73, Evan, K9SQG -----Original Message----- From: Richard Knoppow <[email protected]> To: Bob Loving <[email protected]>; K9sqg <[email protected]> Cc: Drakelist <[email protected]> Sent: Wed, Apr 17, 2013 1:38 pm Subject: Re: [Drakelist] MS-4 Speaker ----- Original Message ----- From: "Bob Loving" <[email protected]> To: "K9sqg" <[email protected]> Cc: "Drakelist" <[email protected]> Sent: Wednesday, April 17, 2013 9:43 AM Subject: [Drakelist] MS-4 Speaker > Evan, what thickness of the acoustic foam did you use? Is > it placed directly in back of the speaker but not > touching? Is the foam available at RadioShack and the > like? Thank you. > > Bob k9JU > The difference between acoustic foam and ordinary foam is that the acoustic stuff is porous. Ordinary foam will just block the sound without absorbing it. For enclosed cabinets Fiberglas is superior to foam but for open back boxes foam is less hazardous. Many small speaker boxes of the sort made for communications receivers are made of thin material that can produce spurious resonances. One cure for this is to use a damping material on the insides. Automobile undercoat works fairly well. Long ago you could buy acoustic damping paint from Altec-Lansing, who coated their high frequency horns with it but it was expensive and I don't know if anyone makes it any more. The purpose of filling the cavity with an absorbent like foam or Fiberglas is to absorb the standing waves and reduce the resonance cause by the open pipe effect of the cabinet. In small, sealed, cabinets the absorbent changes the acoustic impedance of the box lowering the resonance frequency plus also reducing resonances caused by standing waves due to reflection from the sides of the box. Some of the small speakers used in these receivers will astonish you if put into larger, well made, boxes or even just a large flat baffle. -- Richard Knoppow Los Angeles WB6KBL [email protected]
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