The amazing theatre-maker Roslyn Oades has lent me a contact mic for the purposes of experimentation. Listening to sound travel through solid objects has given me a new perspective on the world around me.
Sound travels through solid objects very differently to the way it travels through air. The main consideration is that there is not much reverberation. This Music of Sound blog has an example where they showed that if you were to record a cymbal you would mostly be recording the sound of the direct hits.
Armed with this knowledge, my friend Maya and I set out to find interesting sound textures. Maya had the thought that tram tracks might be able to harvest sounds from a great distance, which I think was correct. I have uploaded some recordings of tram tracks to my Freesound account and you can find one here.
This is a spectrogram of the above recording, and you can see that a great deal of 50Hz signal was coming from the tram tracks, so part of the recording would probably have been electricity. However there is another prominent frequency band at 300 Hz, which is not a harmonic of 50Hz, and there is noise across the frequency spectrum. In this spectrogram no noise would be depicted as black, whereas orange shows the presence of a fair amount of noise.
The recording is interesting, but it didn't capture the full effect. While I was standing next to the tram tracks it felt like there was another layer of sound coming through the air. This made the tram tracks feel epic. I tried to replicate this effect by adding reverb to the recording, and you can find that here. In the future I would like to go back to the tram tracks and simultaneously record through a contact mic and a regular microphone to see if I can capture what I was hearing.
Maya and I made a number of recordings of trees, which gave me a new appreciation for them as complex ecosystems. The most intriguing recording was one which sounds like heart beats. It is true that a tree has a vascular system but I was under the impression that liquids moved through trees through transpiration rather than a pump. Another explanation might be that the tree was picking up sounds from distant rail works that were not audible through the air. I did hear a series of regular bangs coming from the rail works the next day. The original recording is very low frequency, so if you'd like to listen you will need playback equipment with good bass response. You can find the original recording here. I also made a version that would give at least some response to regular headphones and you can find that here.
If you're interested in recording with a contact mic I would recommend the entire article from the Music of Sound blog called The First Rule of Contact Mic Club.
If you'd like to hear more of my contact mic recordings you can find them on my Freesound account at https://freesound.org/people/AppleCorey/