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Find me! Call DAP at 214.350.7678 or email rene@dallasaudiopost.com. Also check out echocollectivefx.com for custom sfx, and tonebenders.net for my podcast.

Thursday, March 29

contact mics and guy wires

Everyone knows that Ben Burtt made the blaster sounds in Star Wars by striking guy wires and recording them with contact mics.

Countless others have done this as well, but I needed to test out some new gear so I figured I'd give it a go.

Gear included my recently acquired Jez Riley French contact mics and my new Sound Devices mixpre that I bought to use as a front end for my PCM D50.

The entire signal path was:  doublesided carpet tape --> JRF contact mics --> Hosa MIT-129 impedance transformer --> mixpre mic ins --> mix pre tape out --> PCM D50 line in.  These recordings are 100% unedited (including gain changes - this is the level I cut them at)

re: contact mics, Tim Prebble has an excellent post outlining the entire process, but the long and the short of my setup was that the impedance transformers were absolutely required to get all of that low end out of my rig.  This is because contact mics are high-z sources and the mixpre is a low-z preamp.  In the past I've also had good luck running straight into an H4 with other contact mics, but I suspect that's because the H4 takes hi-z inputs by design.  With that said, I've never quite recorded low end like this with contact mics to date, so I was very happy with the result.

Also, I laughed a little when I saw that the BBC went into an anechoic chamber and recorded a few very tiny insects using these exact contact mics and preamps.  It's quite the testament to what they're capable of to hear them seamlessly running from centipede feet to the huge guy wire hits I got.

Here's that BBC vid (check out the contact mics visually at 2:00 and then the recording at 3:40):





Now onto my vids. First up are the guy wires.  The single most interesting thing that I discovered was that I could resonate one wire by striking the other, probably through some connection that they were making underground.  You'll see in the vid that they're not buried right next to each other though, so it's possible that the contact mics are actually just picking up sympathetic vibrations.  It's all very cool though, and there is TONS of low end, so crank up the speakers.

Again, this audio is 100% unaltered - not even gain changes.




Next up is the metal fence that was nearby.  I did some similar stuff where I was striking the surrounding objects and getting indirect vibrations, which was pretty cool.  The distortion sound in one of the channels isn't clipping, its the effect of the sticky tape losing its grip.


And for fun, here's a 96k downloadable soundcloud vers of the guy wires.  Enjoy!


6 comments:

Andy Lewis said...

Damnit Rene! After reading your post and listening to the examples I had no choice but to go and order a pair of these contact mics right away! lol I was hoping not to spend anymore on audio things this month ;)

Cool post as always! Thanks for sharing :)

Rene said...

Ha, that's how I felt after listening to JRF's soundcloud feed. :)

Anonymous said...

Correct me if I'm wrong but aren't they attached upside down?

The white circle is used to protect the mic when in contact with the surface

Rene said...

Contact mics are different from traditional mics in that they don't actually transduce air movement into electricity. They work by transducing mechanical vibrations into electricity instead.

As such, there is no mic element in therer to be protected. You just mechanically attach the thing to whatever you want to record with double sided tape and what you hear is a completely different thing than you would if you close miked the the same source.

Anonymous said...

From Jez Riley French....

http://hydrophones.blogspot.co.uk/2009/06/frequently-asked-questions-q-do-you-use.html

Q: what is the foam / soft circular dot for on the c-series contact microphones ?
A: this is simply to provide some additional protection to the element when pressing the
microphone onto the surface to be recorded

Rene said...

interesting. the fact remains that contact mics work by coming into contact with physical objects and not with air, and in my experience they sound the same whether you mount them from the front or the back. in the case of the guy wires it was far simpler to mount them with the flat side than the non flat side.