When a sample of material is struck, it vibrates, the frequencies at which it vibrates is dependent on its dimensions, its stiffness (Young's modulus), density and how much it deforms when compressed (Poisson ratio).
How long it vibrates for is a function of its damping factor. Have a look at the trace below. My three axis accelerometer was taped to this material, and struck. The sound was captured on a computer, and recorded using Audacity software. You will notice the rate of decay is quite well defined, fast at first, and becoming asymptotic towards the end. The damping factor is calculated by looking at successive peaks, and measuring their heights from the zero line. This gives the 'log decrement', and from this the damping factor is calculated.
The damping factor, Greek letter eta, n , is a dimensionless number which represents the amount of intrinsic damping a material has. The property is like density, it is irrespective of other properties, including dimensions.
A material with a damping factor of 0.07, or above, is said to be damping, above 0.1 and it is a good damping material, and as the damping factor is twice the ratio of its damping compared to critical damping, a value of 2 represents critical damping. Most materials encountered in hifi products have a damping factor between 0.01 (or below) and about 0.2, so they are well below the critical damping figure.
The trace below shows how a slate tile rings for about a second, when struck, (and they say it makes a good plinth material). Its damping factor is 0.017, very poor! Amplitude up the y axis, time (seconds) on the x axis.
An experiment I ran to determine the effect of a platter mat on the damping of a record produced interesting results. When I measured the damping of a vinyl record, I found a figure of 0.05 when I placed it directly onto the metal platter of a Goldring GL75 transcription deck. When placed on a platter mat (bought from Ikea as a place mat) a doubling of the damping factor was observed, but when a weight was placed over the centre of the record, a slight decrease in damping factor resulted. Clearly, record clamps do nothing for damping the vinyl.
However, when the same experiment was done on a '78' shellac, the damping factor increased from a lowly 0.014 (in free air) to a massive 0.26! Obviously some further investigation is necessary.
A note on adding damping material.
When constructing plinths, it is far better to use materials that have high intrinsic damping factors, than to attempt to apply damping to materials that have low intrinsic damping. I ran an experiment where I added increments of mass of a damping material ('Newplast') to a known mass of perspex (acrylic). The acrylic had a damping factor of 0.0565, and that of the 'Newplast' was 0.398. The mass of the acrylic was 16.5g, and I added aliquots of 16.5g of 'Newplast' to it, and measured the damping factor of the composite after each additional layer was added.
I found a relationship between the number of layers added (that is, the mass added) and the damping factor of the composite formed. It is graphed below:
the relationship was described by the equation F(x) = 0.05 . 1.26^x with a correlation coefficient of 0.98. Whether this can be repeated for different materials will need further experimentation.
to be continued....