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Archive for June 6th, 2010

Bike Helmet Mic Boom

Drilling mic enclosure

Drilling mic enclosure

This is my latest attempt to come up with a robust electret mic capsule mount for our bike helmets.

The general idea is to put the capsule in a small brass tube (from my box o’ random cutoffs) soldered to the end of a copper-wire boom lashed to the helmet. The tube provides alignment and physical protection, the boom doesn’t pose a poking hazard, and some decent electrical tape secures the mic cable to the boom.

The mic capsule has back vents that allegedly provide ambient noise reduction, so the brass tube must be open on both ends. This does not implement the “waterproof” part of the spec; I still haven’t figured that out yet.

I annealed a length of 12 AWG copper wire to make it easy to bend around the helmet’s contours; two passes with a propane torch to red heat does the deed. It will work-harden quickly and maintain its shape after that.

AWG 12 wire is 0.080 inches in diameter, close enough to 2 mm that I poked a hole in the brass tubing with a 2 mm end mill. Filed the end of the wire flat, stuffed it in the hole, fluxed the joint, applied the big soldering gun to the wire, flowed some silver solder, and it’s all good. Fairly obviously, this meets my “the bigger the blob, the better the job” soldering criterion…

Mic rear

Mic rear

The capsule has two layers of Kapton tape wrapped around it to snug up the fit, although I doubt that insulating it from the brass tube makes any difference.

Mic front

Mic front

The windscreen is a ball snipped from an open-cell acoustic foam sound deadening panel that has contributed myriad mic windscreens over the years. The mic fits into a slit cut with an X-acto knife; no finesse required. The nylon cable tie will disintegrate from sun exposure at about the same time the foam rots away, which takes about two years.

Mic foam windscreen ball

Mic foam windscreen ball

Despite what you might think, the helmet attachment is dramatically less butt-ugly than in years gone by…

Boom-to-helmet detail

Boom-to-helmet detail

The trick is lashing the bent portion of the boom to the helmet, which prevents the entire boom from rotating around its long axis. That keeps the mic aimed directly at your mouth, regardless of how you bend the boom.

The earbud wire loops around the mic boom a few times, with the first loop over the boom to take advantage of its rounded surface. With any luck, that will delay the inevitable fatigue failure. Mary favors old-style cylindrical earbuds, rather than newer flat or round ones.

The USB cable (this is not, repeat not a USB headset) gets lashed to various parts of the helmet foam and routed out to the middle of the back, with the male connector a few inches below the helmet. That puts the cable over the back of the Tour Easy’s seat frame, leaving the bulk of the cable hanging behind the seat. The cable length from the female connector to the radio interface is a delicate trade off between being

  • Long enough to let you stand up and
  • Short enough to stay out of the rear wheel.

This vertiginous shot looks down at the helmet hanging on the seat of Mary’s bike. Yup, that’s her bright new homebrew seat cover to the upper left…

Helmet overview

Helmet overview

Now, for some power-on hours!

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