Resonant Bridge

Resonant Bridge, contact mic


Resonant Bridge, contact mic with no mass coupling to back of element

Resonant Bridge, contact mic with mass coupling to back of element

To a man with a contact mic mounted on a magnet, every sound source looks like a large lump of steel, same way as the guy with a hammer sees nothing but nails…

My luck was in when I came across this concrete footbridge across a dual carriageway. So I set up, carefully placing myself over the central reservation so drivers don’t get paranoid I’m about to zap ’em with a speed camera or throw stones or drop magnets on them. You get some funny looks attaching magnets and a piece of wire to a bridge by the other people, and the regional Police HQ is only 1/2 mile away. This unpreposessing bridge has a decent tone to it, presumably ringing to the vibration of the traffic passing by. Which was awesomely noisy – you don’t realise just how loud the tyre noise is on a road until you try and cycle a stretch like this and find your ears ringing afterwards. The contact mic worked its usual magic in getting rid of all this racket.

Resonant Bridge, traffic noise, OKMII binaurals w battery box

The two recordings with the contact mic were made, one with the mic held to the bridge with a strong magnet, the other with the same mic held to the bridge in the same way, but with an approx 0.5kg weight acting as a reaction mass on the other side of the piezo, to see if this picked up low frequencies better. You can hear the difference – or not, for yourself!

the traffic seen from the footbridge
the traffic seen from the footbridge

Piezo contact mics seem to cause a lot of pain for people losing low frequencies, but I do not feel I am short of low frequencies here, using a basic FET buffer. The peaks of the bridge resonance are 120Hz, 240Hz, 380Hz, 760Hz. There is, however, energy at 22 and 43 Hz at -36dB on the resonant peak, at a similar level to the broad peak around 3kHz which is probably the percussive sounds amplified by the piezo self resonance. The piezo was a Maplin YU87U 27mm 1.8kHz item terminated in 3.9Mohms, but the circuit could be improved to work better with a HiMD recorder mic input of 5k.