----- 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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