Science Buddies: "Ask an Expert"

Hi,

I am doing a science fair project, and I've run into a problem that I can't explain with the background research I've done. I am testing attenuation of sound waves with different materials. So far, every material reduced the decibels of the orginal wave, like expected. However, when I put an open celled polyurethane foam infront of the sine wave, it amplified the wave. HUH? I'm very sure it's not my testing issue, because when I removed the polyurethane foam, the meter read back at 93 dB, which is what I was orginally testing at. I put the polyurethane foam in front a couple times, and each time it jumped to 94 dB.

Does anyone know what is going on here? Also, for lower frequencies, do denser objects block more sound?

Hi avocadofries19,

I'm assuming that you're working on the project described here:

https://www.sciencebuddies.org/science- ... on#summary

You are testing with a fairly high sound level. My guess is that your 94 dB reading with the polyurethane foam in place is due to a reflection or resonance of your sound wave. You might try repeating your test with a lower sound level (say 65 dB), making sure that your foam material is very close to the front of the speaker. These steps should reduce sound wave interactions with the walls and furnishings in the room.

I hope this helps. Please post again if you have more questions.

A. Norman