| I thought that the positioning of the foam relative to foam peaks and valleys might alter the sonic filtration, but as is shown in both the MLS and Sinewave frequency responses below, such is not the case. Below 500 Hz, the insertion of this foam increases measured bass response. I suspect this results from an acoustic lens effect, since severely off axis bass waves will travel through less foam (the valleys) and will travel faster causing the wave to bend around toward the microphone. This anomaly shouldn't apply to the effects of foam lining the inside walls of a loudspeaker enclosure. That the foam does decrease the speed of the sound waves is apparent in the waterfall decay plots shown at the bottom of this page. Beginning at about 800 Hz and above, the foam also attenuates the sound energy. The higher the frequency, the greater this attenuation. Clearly, this effect is desirable so that mid frequencies (and higher) are less likely to cause wall vibration or escape back through the cone or any ports into the listening environment. Because the wave velocity is reduced by the foam, it is clear that lining the inside of a loudspeaker enclosure will increase the box apparent volume. I'm not clever enough to see from these measurements by how much the box volume would appear larger. Therefore, I shall next compare the results in the same setup interposing other damping materials whose effects on apparent box volume are better understood. (R19 fiberglass, for example) |
