OCM-2 Stereo Photophone

The Rare Waves OCM-2 Stereo Photophone is a new type of audio transducer that uses light to pick up detailed stereo sound from vibrating surfaces. It’s made for sound designers and audio producers who prefer creative tools with hands-on playability for generating audio source material. This is a true analog sensor without DSP embellishment. In some cases it can serve as an optical contact microphone, picking up natural-sounding full frequency range audio from unusual places.

Demonstrations

OCM stands for optical contact microphone, and that is a hint toward the jobs it can perform. Check out our demo videos for examples of audio from different subjects.

Applicability

This is not a drop-in replacement for ordinary microphones. It works in a fundamentally different way. OCM-2 responds to fluctuations in reflectivity. It does not directly sense vibrational displacement, acceleration, or air pressure variations.

Unlike ordinary contact microphones, it is not suited for picking up faint vibrations in rigid flat surfaces. It is not a 100% drop-in replacement for piezo disc contact mics, hydrophones, or geophones. It doesn’t give good results as an acoustic soundboard pickup.

It does not pick up sound waves traveling through air. That means it won’t directly pick up vocals or speech, you can’t use it as a room mic, and it won’t work as an acoustic sound hole pickup. Nevertheless, it could find applications with drum and percussion instruments.

Note, this is not a laser microphone, and it would not be useful for covert listening (eavesdropping).

When considering where to apply the OCM-2, remember this rule of thumb: If you can see a motion blur from the vibrating subject, then it is likely to give a useful signal.

Being mindful of its limitations, and depending on what vibrating subject you choose, the OCM-2 Stereo Photophone can be very playable with plenty of hands-on control over the sounds that you get from it.

What can it do that a microphone cannot?

The OCM-2 Stereo Photophone can pick up full-range audio from vibrating sources that are too small to radiate low frequencies effectively. In some cases, dramatic sounds can come from little things that ordinarily sound trebly and thin through a microphone.

It can capture audio from places that are off limits to ordinary contact microphones, such as small sized and lightweight subjects whose behavior would be affected by mass loading.

It can isolate the sound coming from specific regions of a vibrating subject, while rejecting sounds that are coming from nearby.

There are also unique applications that are only possible because light is the medium. It can pick up audio waveforms from ripples on liquid surfaces. Additionally, it can pick up audio waveforms from shape and texture of moving or rotating surfaces.

Audible ripples (Capillary Waves)

Waves on liquid surfaces are a unique application for the photophone. Water ripples may astonish you when you hear them for the first time, because what you hear through the photophone is unlike the familiar sounds of water. Capillary waves (ripples) are driven by the dynamics of surface tension. Compared to sound waves, they travel much slower and exhibit nonlinear acoustic properties like frequency dispersion and extreme pitch modulations.  A tabletop container of water becomes a touch-responsive source of trippy audio sounds that can spark your creativity.

Setup tips

The OCM-2 Stereo Photophone is straightforward to set up, without any settings or parameters to tweak. Sound quality is entirely a matter of your choice of the vibrating subject and how you position the photophone near it. In that regard it’s similar to using a microphone. However, varying its position and angle have a larger effect on sound than what you may be used to from your experience with microphones.

OCM-2 fits in many standard mic clips used for handheld vocal mics (clip not included). We suggest supporting it on a boom stand so it can be angled precisely where you need it.

The pickup pattern is directional and provides a L-R stereo image over a field of about 20 degrees. It gives strongest signal at a distance range of 2 – 12” (3 – 30 cm) from a vibrating subject. It cannot pick up audio at long distances, no matter how loud the source is.

Sound engineers know how to find the sweet spot for a microphone’s position by listening carefully while adjusting it. This is even more important when using the photophone. Compared to a microphone, the photophone is less forgiving for variations in the angle it is held at relative to the subject. That’s because it works using light, and light tends to travel in a straight line whereas sound waves diffract around barriers. This means with the photophone, the sound will cut out completely if the subject is at the wrong angle or if something blocks the light path. For creative purposes, this factor makes it possible to produce tremolo effects by manipulating a perforated mask to modulate the optical path.

Since it does not pick up sound waves traveling in air, it is immune to room tone and background noises. This means you don’t need to have a quiet recording studio environment to produce recordings free of interfering noises like traffic, HVAC, computer fans, conversations and footsteps, etc.  Also, it is insensitive to handling noise, so accidentally bumping or jarring it during a session is unlikely to spoil your recording.

Connectivity

OCM-2 has two stereo audio output ports. They take standard 3.5mm stereo mini plugs. These are analog unbalanced signals with -10 dBv nominal level.

For recording purposes, patch the Main Out jack to your recording device’s analog stereo mic/line input, using a 3.5mm stereo aux cable (not included). The Main Out has greater dynamic range and lower noise floor than Aux Out.

OCM-2’s Aux Out jack is intended for connecting headphones or monitor speakers when you need an easy to reach volume control. The rotary knob adjusts the signal level at the Aux Out jack.

OCM-2 is powered by AC adapter (included). There is no internal battery. Be advised, it dissipates 5 watts of power, so it is normal for the case to feel warm or almost hot to the touch while in operation.

If you need to connect it to XLR inputs and long cables are not required, simple audio adapters can be used. It is not necessary to add an active preamp or DI unless a long cable run is involved. The graphic below gives an example of a common solution with audio adapters.

Serial numbering

Each unit is permanently engraved with a unique identifying serial number.

Documentation

OCM-2 Stereo Photophone – Application Note

OCM-2 Stereo Photophone – User Guide

Warranty

OCM-2 comes with a 5-year limited warranty against defective materials and workmanship. The warranty does not cover cosmetic appearance or damage from misuse. Warranty void if case opened.

Background

Developed for Rare Waves by Eric Archer in 2024, the OCM-2 Stereo Photophone works on a different principle than our Lite2Sound devices. In the past, we heard that users of Lite2Sound PX had found a way to pick up water noises by combining it with a flashlight. The OCM-2 Stereo Photophone began as an elaboration of that technique. OCM-2 gives far greater sensitivity, wider dynamic range, and much less distortion than using a Lite2Sound device in this manner.

A comment on our terminology: The term photophone has historical roots in nineteenth-century wireless voice communication technology. However, we are using it in a different sense. This device is designated a photophone to fit the naming scheme for general families of audio transducers such as microphones, hydrophones, and geophones.