Ribbon Microphone

The corrugations are what let a stiff strip behave like a limp one.

1. Cut a strip of the thinnest aluminium foil you can obtain, about 4 mm wide and 50 mm long.
2. Lay it between two coarse-toothed gears or a corrugating jig and press gently to form transverse ripples.
3. Handle it only with tweezers and a light touch — a fingerprint will tear it.
4. Inspect for cracks along the corrugation crests.

A flat strip pulled taut is stiff and resonant; a corrugated one is compliant along its length. The ripples let the ribbon stretch and move as a whole without needing tension to hold it flat, so its resonance can be placed below the audible band. That is the trick that makes the ribbon work, and it is why every ribbon ever made is corrugated.

The ribbon is a single-turn conductor, so it needs all the flux it can get.

1. Mount two magnets facing each other with a gap of about 5 mm, poles opposed.
2. Clamp the ribbon at top and bottom so it hangs centrally in the gap, with just enough slack to be limp.
3. Ensure both faces are open to the air.
4. Connect a lead to each clamp.

The output impedance here is a fraction of an ohm, because the ribbon is one turn of aluminium. The voltage generated is correspondingly tiny — far smaller than a moving coil with forty turns. This is why every ribbon microphone has a step-up transformer built into its body, and why an early ribbon connected directly to an input produced almost nothing.

Without it the signal is unusable; with it the ribbon becomes a practical microphone.

1. Wind or obtain a transformer with a very low impedance primary and a high impedance secondary.
2. Connect the ribbon to the primary and take the output from the secondary.
3. Measure the signal before and after the transformer with the oscilloscope.
4. Note the ratio.

The transformer is doing impedance matching, not just amplification. It converts a very low voltage at very low impedance into a usable voltage at an impedance a preamplifier can work with. No energy is created — the transformer trades current for voltage — but without that trade the ribbon's output is buried in the noise of whatever follows it.

The pattern comes free with the physics — verify it and then use it.

1. Set a steady tone playing and walk it in a circle around the microphone at constant distance.
2. Record the output every 15 degrees and plot it as a polar diagram.
3. Find the angles of deepest rejection.
4. Now place an unwanted noise source exactly at a null and a wanted source at the front.

The ribbon microphone was developed in the early 1920s — Walter Schottky and Erwin Gerlach in Germany, and independently Harry Olson at RCA, whose 44 and 77 series became the defining broadcast microphones of the 1930s and 40s. The RCA 44 hanging in front of a big band is one of the most recognisable objects of the era.

Its pattern turned out to matter enormously. Alan Blumlein, whose stereo work is next in this batch, used crossed figure-of-eight microphones to capture a stereo image — a technique impossible without a transducer that naturally produces that pattern. So the ribbon is not merely a microphone in this chain; it is the enabling component for stereo recording.

Its acoustic character has a physical explanation. The ribbon's moving mass is a fraction of a moving coil's, so it follows sudden transients without overshoot, and its high-frequency response falls away gently rather than peaking. Both together produce the smooth, unaggressive sound ribbons are prized for — and both are consequences of a two-micron foil rather than of any deliberate voicing.

Its honest limits are severe: the ribbon can be destroyed by a gust of wind, by phantom power applied to the wrong pins, or by being carried face-first into a draught. Output is very low. And the figure-of-eight picks up the room behind it as much as the source in front, which is wonderful in a good room and unusable in a bad one.