engineered syntactic products

Acoustic Properties of Syntactic Foam – Part 3

Enhanced Density Products

The following measurements were made on our low density syntactic foams:

Depth Rating (meters) Average      Density (Kg/m3) Average

Speed of Sound (m/s)



1000 385 2,186 0.84
2000 400 2,461 0.99
3000 432 2,698 1.17
4000 457 2,767 1.26
4000 515 2,675 1.38
4500 490 2,922 1.43
5000 496 2,968 1.47
6000 552 2,952 1.63
7000 545 3,100 1.69

As noted, even though the hollow glass spheres and resin systems were different, the speed of sound and impedance measurements tracked well with depth rating.

It should be noted that these measurements were made on what we consider to be some of our standard buoyancy products, not materials tailored specifically to provide specific or targeted acoustic properties.  For example, formulations made to Navy specifications have the following properties:

Formulation 1

Density: 690 kg/m3 ± 16

Speed of Sound: 2,850 m/s ± 100

Acoustic Impedance: not specified

Formulation 2

Density: 380 kg/m3 ± 32

Speed of Sound: 2,595 m/s

Acoustic Impedance: 0.940 MRayls

In these instances, specific combinations of resins and microspheres were used to attain the desired properties.  This approach is always an option, though it does require some development and input from the end user.

Properties of CMT Materials Standard Tooling Products

The following measurements were made on our standard plug-assist tooling material (HYTAC®)

Product Name Avg Density (Kg/m3) Avg Speed of Sound (m/s) Impedance


HYTAC-W 690 2,151 1.48
HYTAC-B1X 708 2,568 1.82
HYTAC-C1R 747 2,579 1.93
HYTAC-XTL 755 2,697 2.04
HYTAC-FLX 838 2,749 2.30
HYTAC-WF 838 2,788 2.34
HYTAC-FLXT 897 2,647 2.38
HYTAC-WFT 968 2,700 2.61


While HYTAC materials are not optimized in any way around acoustic properties, the information is interesting nonetheless.  HYTAC-W, for example, is very close to a match for seawater with minimal inconsistencies in terms of density stratification.  As with all of the plug assist tooling products, HYTAC-W is available in a wide range of sizes and shapes.  For example, 2” diameter rods are maintained as a stock item.  This would yield an immediate ~22% savings on material for a given diameter compared to the same amount of material taken from cut blocks if the application required cylindrical shapes.  Sheet sizes starting at one-inch thick up to six-inch thick are also standard, at 0.5-inch increments.  Again, this could mean a significant material savings.

There are several additional observations of note from these test results.  Two sets of the products have derivatives that include polytetrafluoroethylene (PTFE) in syntactic.  (HYTAC-FLX => HYTAC-FLXT and HYTAC-WF => HYTAC-WFT). The PTFE is added to improve the slip at the surface of the material during the thermoforming process and to prevent sticking and material build-up (for more information on thermoforming, visit  The addition of PTFE increases the density but results in a decrease in the speed of sound.  Correspondingly, each material also shows a slight decrease in modulus.  It is also interesting to note the difference we see when making a dramatic change in the matrix material.  There is only a very subtle difference in density between HYTAC-W and HYTAC-B1X, yet the average speed of sound is significantly different due to the change from an epoxy matrix (W) to a thermoplastic matrix (B1X).

In summary, these simple observations illustrate the flexibility available to us as formulators.  The wide range of end-products allows the user to select an off-the-shelf product that may just as easily meet their needs as a complex, unique formulation.