Phil Winder’s Blog

I have now completed the anechoic lining tests and reached an acceptable combination of materials.

To recap, over the last couple of weeks I have used an audio frequency amplifier to increase the transmit power of the pulsed tx signal.  I expected this to provide poor results since the pulse was 6.6 uS long, hence providing a spectrum containing a sinc function with the first zero at about 150kHz.  However, despite the amplifiers (probable) low pass filters it produced a pulsed signal that could be used and most importantly, at a higher power.

With this power I could now repeat the metal tube tests and actually see the reflection and after about 6 repetitions came to a descision on which material combination to use: a single sheet of heavy rubber backed with foam.  The rubber provided some absorbive attenuation, and what doesn’t get absorbed is scattered by the foam.

Armed with the new material I constructed a test tank from a oversized plastic A4 box, typically used for storage, and lined it with the rubber-foam combination.  Below are the results from a pulsed tx test from within the tank.  Note that the absorption coefficients are not normalised to an area and therefore cannot be directly associated with a sabin coefficient, or equivilent.

From the two tests (these results were very stable) an average absorption coefficient of 0.46 was attained.  Just to make sure, I did swap the anechoic around (so that the rubber was on the inside, next to the box wall) and results were poor, indicating that the foam was scattering before the rubber has had time to absorb.

Next time… Results from the (un)infamous stairs test.