  We investigate the acoustical properties of uncompressed and compressed
open-celled aluminum metal foams fabricated using a directional solidification
foaming process. We compressed the fabricated foams using a hydraulic press to
different compression ratios and characterized the effect of compression on the
cellular microstructure using microtomography and scanning electron microscopy.
The static airflow resistances of the samples are measured and related to the
observed microstructural changes. We measured the normal incidence acoustical
properties using two- and four-microphone impedance tube methods and show that
the compression substantially improves their sound absorption and transmission
loss performance. We then stack individual disks with different compression
ratios to create various stepwise relative density gradient configurations and
show that stepwise gradients provide a significant improvement in properties as
compared to the uncompressed sample. The effect of increasing and decreasing
relative density gradients on the overall absorption and transmission loss
behavior is characterized. Finally, we use an experimentally informed and
validated transfer matrix method to predict the effect of various layer
thicknesses and stacking sequences on the global acoustical properties. Our
results show that open-celled metal foams with stepwise relative density
gradients can be designed to provide tailored acoustic absorption performance
while reducing the overall weight of the noise reduction package.
