Exploring the ambient

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Put on your headphones and listen while pointing a photodiode amplifier around your environment.  Start indoors and you’ll find many of your electronic gizmos are making frequencies, hums, buzzing, and clicks. But don’t stop there! Take it outside at night in the city, anywhere there is high-tech illumination and visual activity. Street scenes, commercial areas, LED art exhibits, video arcades, high streets, casinos, midway at the fair – these are just a beginning of the list of places where you’ll find an ever changing sound palette.

Your ears are very good at discriminating between tones, and you’ll quickly discover that many types of technology have a distinct optical sound signature.  Invisible infrared sources are revealed – what are they doing, and why?  Where can you find the most musical sounds with rhythm and harmony?  An exclusive world of audio is waiting to be explored, and you can be the first to discover how it sounds.

Recording audio

Patch the output of Lite2Sound to the line input or mic input of a portable audio recorder.

Recording video + audio

Patch the output of Lite2Sound to the line input or mic input of a camcorder. Mount the Lite2Sound on the body of the camcorder so the sensor is pointing in the direction of the camera lens. Now when you record, the optical sounds will be synchronized with the video recording.

Adding a lens

The performance of a photodiode amplifier can be increased significantly if you add a lens to focus light onto the sensor.  For the DIY experimenter, an efficient solution is to re-purpose a junk film camera.  Basically you must disable the shutter, and mount the photodiode where the film was supposed to be.  Then it becomes possible to pick up light sources much farther away, and isolate individual sources. 

8mm film camera modified with a photodiode amplifier inside

There are different types of junk cameras you could use, which is best?  A 35mm SLR (single lens reflex) camera will work, but since its viewfinder shares the lens with the film, the viewfinder will be useless after you mount the photodiode and disable the shutter.  Its more fun if you have a viewfinder to look through while listening to the scene.

A twin-lens reflex camera makes a better optical system for use with a photodiode amplifier. That is because its viewfinder has a separate optical path. You can modify the shutter and put a photodiode where the film used to be, and the viewfinder will still work. The result is you will be able to look through the viewfinder while you listen. You can easily pick up optical sound sources in your environment from a distance, and identify where they are coming from.

Twin-lens reflex cameras come in many kinds. You should choose a cheap one that doesn’t have much value to collectors. The old 110 film format is suitable, or if you find a junk 8mm movie camera, that works too.

As a bonus, the metal case of the camera acts as a shield to reduce electromagnetic interference. This can improve the sound quality of the audio from your photodiode amplifier.