Infrasonicon icon. Click here for home page.INFRASONICON: Time-Compressing Infrasonic Recordings to Discover New Sounds, by Clark Huckaby

Overview, Part 3 (of 6):

Optical Probe

Many infrasonic phenomena with periodic changes in emission, absorption, or reflection of light can be recorded using an optical pickup. Pictured in Figure 5, my version uses a TSL230 light intensity-to-frequency converter IC from Texas Advanced Optoelectronic Solutions (TAOS; Ref. 5). With programmable sensitivity and output frequency scaling, this chip's response is linear across a very wide light intensity range. The Optical Probe's technical page gives the schematic diagram and circuit description.

Figure 5: Photo of optical probeFigure 5: Optical Probe.

An optical probe simply measures intensity of light hitting it, regardless of source. Infrasonic recording sessions need both ingenuity and trial-and-error to isolate phenomena while staying within the Infrasonicon channel's linear range. For example, light reflected from a rotating object that is spot-illuminated with a laser pointer sensitizes the pickup to the motion. If signal amplitude is too high, the TSL230 chip can be photobiased with an LED operating at constant current (see probe's tech page; household lighting offers poor bias due to AC components.) Example 3 and Example 4 in the Sound Example Page feature laser-illuminated rotating objects. Figure 6 is a low-resolution frequency analysis of Example 4; a higher resolution spectrogram and discussion is at this link.

Figure 6
Figure 6: Time-evolving frequency analysis (spectrogram) of coasting flywheel, using Digidesign's Sound Designer II software. Corresponds to  Sound Example 4. My page on Analysis of Flywheel Recording includes a higher-resolution spectrogram and more details about the experiment. 

I've already mentioned an example of direct pick-up of emitted light (the candle in Figure 1). For an example using light absorption, I transmitted light through my finger (Figure 7) to record my pulse (Figure 8). According to Wikipedia, this is called a photoplethysmograph (PPG; Ref. 6). Pulses change the finger's volume and (thus) optical absorption in time with heartbeats. My 165-fold time-compressed PPG sounds like a buzzing insect (Sound Example 2). This reminded me of "The Fly," a film in which another mad scientist experimented on himself.

Figures 7 (left) and 8 (right)
Figure 7 (left): Set-up for recording PPG. Laser module transmits light through finger to optical probe. Hand and finger are taped down for stability.
Figure 8 (right): A portion of PPG waveform; corresponds to Sound Example 2. The time scale shows original (not compressed) time. The amplitude scale is modulation relative to 1-KHz carrier. The "hash" is mostly 60-Hz noise, and is about 2 LSBs peak-to-peak (50 dB less than the PPG signal). At 20.3 LSB/Hz, Data Converter gain was near maximum. Laser current was adjusted slightly three times during the 30-minute recording to compensate for drift (I was monitoring the carrier in real time with a frequency counter).

I also used the Optical Probe and Infrasonicon channel to investigate neon lamp pulse shape in vintage Fender guitar amplifier optical tremolo circuits. The results are included at; the photobias requirement is mentioned in the Optical Probe's technical page. The optical tremolo experiments are an example of how the Infrasonicon can be used as an analytical tool, since it is linear.

Overview continues in Part 4==>

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References Cited in Overview Part 3:

5. TSL230 Light-to-Frequency Converter:

6. Photoplethysmograph:

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