One of the great things about syntactic foam is that one can make an almost infinite amount of changes to the types and levels of the constituent materials in order to impart particular properties to the final part. The types of matrix materials, hollow spheres, additives, and combinations thereof offer materials engineers a wide range of tools to determine end-properties. Unfortunately, because the possibilities are nearly endless, it is difficult to stop making changes in order to perfect a particular property. This holds especially true in the area of acoustic properties. As part of our investigation into the development of products for industries interested in the acoustic properties of syntactic foams, we have discovered a number of interesting areas for exploration.
When we think about acoustic properties, we generally think about whether a material absorbs or reflects sound. For this discussion, we are thinking about syntactic foam as more of a window that sound waves can pass through. In very general terms, the degree to which sound is reflected or absorbed by a material is dependent upon the acoustic impedance of the material and the media through which the sound is traveling (in the case of buoyancy materials, this is mainly water). Other factors, such as wave frequency and incident angle can also play a role, but these are not within our control. In most cases we are simply trying to match the acoustic impedance of water. The acoustic impedance of seawater is ~1.50 MRayls[i] and is defined as the speed that sound can travel through the media (~1,450 m/sec) times the density (~1025 kg/m3). These values will vary depending on things like temperature, water salinity and depth. Variations can be as great as 10% or more.
Using 1.50 as an initial target, we set out to understand how close we could come with our standard syntactic products. Measurements were performed on the different types of syntactic foams with a 1 MHz transducer. Sample densities were also recorded. Speed of sound and the calculated acoustic impedance in MRayls were reported for each material. Overall speed of sound was found to be mainly dependent on density but was not independent of the modulus of the matrix material used. Also, the variation in density through the thickness of the material was found to be very important.
In the next installment, we discuss comparative testing and data for multiple syntactic materials.
[i] MRayl, or Rayleigh, is a unit of measure used to describe characteristic acoustic impedance.