TABLE OF CONTENTS A doorbell, as shown at the right on the facing page, can be used to demonstrate the basic principles of sound isolation. With no isolation, the doorbell produces 70 dB at a few inches away. When the doorbell is surrounded by a 3/4-in-thick enclosure of low-density, porous glass fiberboard (called fuzz), the transmitted noise is reduced by only 3 dB. Porous sound absorbers are very poor isolators because air molecules can readily pass through them. By themselves they act as sponges; they absorb sound but do not prevent its transmission!
When the doorbell is surrounded by a 1/2-in-thick plywood enclosure with an airtight seal around its edges, the noise is reduced by 28 dB (from 78 dB within the enclosure to 50 dB outside). This is a tremendous change in noise level and would be perceived by most observers as about one-fourth as loud. The plywood enclosure is an effective barrier because it is solid, has sufficient mass, and is sealed airtight around the edges. The seal is essential because even a very small opening can noticeably increase the transmitted sound.
When the doorbell is surrounded by a 1/2-in-thick plywood enclosure fully lined with 3/4-in-thick fuzz, the noise is reduced by 29 dB. However, the noise outside is further reduced to 43 dB because the sound-absorbing lining reduces the buildup of reflected sound energy within the enclosure by 6 dB.
The illustrations on the left show the effect that nearby reflective surfaces...
