Material density and sound transmission
The hypothesis that the higher density material will transmit less sound is proven to be true. Higher density materials are very good sound barriers.
Sound barriers are used to protect homes that are built close to the airports, highway and train lines from noise pollution. It helps to improve the quality of life for the people who have been affected by these noise pollutions.
The experiment can also be done comparing different elastic material like rubber, metal, etc.
Try and repeat the experiment with different source of sound like a portable CD player.
Noise barrier - http://en.wikipedia.org/wiki/Noise_barrier
The speed of sound in other materials - http://www.ndt-ed.org/EducationResources/HighSchool/Sound/speedinmaterials.htm