syntactic foam

Acoustic Properties of Syntactic Foam – Part 2

Comparative Testing Procedures

Tests were done on two formulations with densities of 24 lb/ft3, the lowest density that we can currently provide for a microsphere syntactic foam. One formulation was rated for 1000 meters (MZ grade) and one was rated for 2000 meters (BZ grade). The formulations were designed for the specific depths using different resin systems and glass bubbles, so we expected to see some significant differences. The lower strength and stiffness MZ material had a speed of sound of 2,186 m/s while the BZ sample had a speed of sound of 2,489 m/s. This is interesting because the values nearly mimic the average modulus difference between the two products. The compressive modulus of BZ is about 1.15 times that of MZ. It is impossible, however, to separate whether the stiffness difference is due solely to the higher modulus resin or the different glass bubbles.

Overall we can report the following general information from the test:

Sample Density (Kg/m3) Speed of Sound (m/s) Impedance (MRayls)
MZ 385 2,186 0.84
BZ 385 2,489 0.96

The process for manufacturing low density syntactic foam does not lend itself to producing a product that is perfectly consistent in terms of density. This density difference can appear from block-to-block as well as within a block. The natural size and density variation of the hollow glass spheres that make up ~ 70% of the syntactic structure are the main reason for these differences. As made, the hollow glass spheres can vary by ±15 % in density in a single batch. This results in a density acceptance standard of ± 2 lb/ft3 (± 32 Kg/m3) for each block. While the speed of sound did not change dramatically with the density variations, the overall impedance could be greatly varied due to the wide density swing. For example, this could translate to rough impedance differences of between 0.80 and 0.91 on the MZ product. Due to the manner in which these enhanced density materials are made, the stratification between the top and bottom of a block can be even greater than the 2 lb/ft3 difference between individual blocks. Material cut from the bottom or top of a block may show these dramatic differences while the average block density is within specification. For MZ this may mean a 10 – 20% difference in both the speed of sound and impedance through the block thickness. Therefore it is crucial that the designer understands these variations while making material choices. There are ways to preselect and/or classify the products to minimize these variations if a very narrow impedance range is needed and we invite designers to have discussion as early as possible in their process.

Given these differences, we continued to explore the acoustic properties, but now we looked at them from two views. Our first goal was to measure the average properties of our high performance materials (lowest density per depth) over the established density and depth range. In this way we could determine the lowest attenuation levels for a given operating depth. Our second area of interest was in the measurement of our tooling materials (HYTAC®). We took this unusual step because the driving force behind the design of the tooling product line was not to achieve the lowest density possible. This allowed us much more flexibility in the manufacturing process allowing for a product that can be directly cast to different shapes and sizes, such as small (2” / 51mm) diameter rods. The cost advantage in utilizing a wider range of starting shapes is significant. Lower machining and material yield losses directly result in lower production costs for our customers. Further, these products do not have the same wide density variation as the enhanced density products offering better consistency in acoustic performance.

In the third and final installment, we will review material properties of our HYTAC line of syntactic foams to illustrate differences in matrix material.