Enquiring Ear

Field recording and found sounds

Category: electronics

  • Soundman OKMII repair

    Soundman OKMII repair

    I’ve had my Soundman OKMII binaural microphones for over ten years, and they are my favourite mics for urban field recordings. DACS is the UK supplier.

    Soundman OKMII binaural microphones
    Soundman OKMII binaural microphones

    They don’t really work in wind, and they aren’t the quietest, but they are as stealthy as you can get, looking just like earbuds. A decade ago you still looked a little bit of a geek using them and monitoring on a recorder, but nowadays most people are looking at their smartphones, rather than the lamp-post/road they are about to encounter. On the off-chance that they do look up, they will assume you are just another human trying to escape the real world for the virtual one inside your phone, which happens to be a field recorder.

    I was going for a general recce on the Somerset levels, looking for interesting sounds. I heard a lot of birds congregating in some trees, and started the OKMII. There was a lovely little flurry of about 100 starlings flying overhead at about 2 and 4 secs against a background of other starlings gathering in the trees

    The OKMII isn’t really a birding mic, but it picked up some of the essence of these guys

    and then I encountered this ghastly full-scale 0dBFS noise on the left

    a bad contact on the left channel. After 10 years these mics don’t owe me anything, but I figured it’s worth a look if a fix is possible. Skinning the foam earpads shows this

    OKMII Two plastic shells melted together
    OKMII Two plastic shells melted together

    It’s possible to separate these with a craft knife along the obvious seam, concentrating on the melted bits. The microphone is glued to the shell with a hole, try and keep this together to expose the contacts on the rear of the capsule.

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  • Resonant Bridge

    Resonant Bridge, contact mic

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    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.