For about 2 weeks now I have been struggling to find a test that I could perform to ascertain the absorption coefficient in a small tank. The problem is that this tank is only 0.5 m x 1.5 m x 0.3 m and most tests are producing spurious reflections that are masking the reflection that I want. For example, if I were to lay the material under test at one end of the tank and transmit, most of the returning signal will be 1) masked by the reflections from the bottom, surface and side resonances and 2) spread due to the path length difference in the returning waves. These two problems effectively ruin a clean experiment and the only solution is to use a pipe.
The transmission signal being used is pulse-like since we did not have the capabilities to transmit a burst of a sinusoid at a predetermined frequency (which would have been nice) and set the pulse length to something like the impact pulse of a raindrop (50 uS).
Unfortunately, the hydrophones I have are hardly directional, so a lot of energy is transmitted to the side walls of the tube (perpendicular to the hydrophone direction) and bounce around there for ever. To attempt to eliminate this I wrapped the Rx in rubber to eliminate (absorb) some of the sound energy resonating in the pipe. However, this damped the Tx slightly and reduced the receiving area on the hydrophone by an order of magnitude; the reflected pulse was therefore ridiculously small in amplitude.
Hence another experiment was devised to exploit the fact that we cannot remove these resonances. The material under test was place at the bottom of the tank stuck to a metal plate (aluminium to produce a perfect reflection) with the hydrophones placed on top. A coil of rubber was then placed around the test bed to attempt to reduce some of the reflections from the sides of the tank (this seemed to work fairly well). A pulse was then transmitted and allowed to reverberate from the material to the surface and back again. With just metal, the reverberations lasted a long time (about 2.5 mS/3.6 m), but with the absorbing materials the reverberations quickly petered out.
This effect can be clearly seen in the graph, although for a quantitative result I averaged the entire pulse-reverberations over approximately 100 transmissions, squared the data to get the power of the signal and then took the mean. As some example values:
- Metal = 0.3477
- White Absorber = 0.1628
- Egg Foam = 0.0774
- Flat Foam = 0.0723
Surprisingly some open cell foam performed better than the specifically designed acoustic absorption material, however most of this could be due to scattering. According to these results the acoustic absorption material had an absorption coefficient of approximately 0.5, but the foam had a coefficient of 0.8. Considering the foam is about 100th of the price of the absorber, I don’t think that is too bad!
It must be noted that these experiments were designed and performed under our own criteria, namely the droplet-like pulse shape, and they were entirely non-standardised. It does however illustrate that similar, if not better performance can be obtained from much cheaper alternatives.
