INFRASONICON: Time-Compressing Infrasonic Recordings to Discover New Sounds, by Clark Huckaby
Recently, inexpensive portable digital audio recorders have made high-quality field recording easy, fueling interest in found sounds (Ref. 1). But such recorders, along with human hearing, are restricted to the audio band (20-20,000 Hz). I offer that it is chauvinistic to think that all interesting sounds must originate in this band. The infinitely wider range of 0 Hz (DC) to 20 Hz also offers possibilities (Note 0). Inaudibly low-frequency signals need simply be isolated and time-compressed to be heard as sound. The simplest form of time-compression is digital recording at a lower sampling frequency than the one used for playback. This is strictly analogous to time-lapse cinematography.
Since the low-frequency limit of human hearing is considered 20 Hz, lower frequencies are called infrasonic, literally "below sound." This term can apply to all periodic phenomena less than 20 Hz, not just atmospheric pressure waves, which are called infrasound. While an interesting topic, this project is not about infrasound per se, because I favor direct signal pick-up rather than acoustic pick-up. The reason for this is analogous to one of the reasons the electric guitar was born...
Efficient acoustical coupling requires more energy as frequency decreases (that's why subwhoofers are big and need powerful amplifiers). At the same time, signal losses over distance decrease as frequency decreases (that's why distant lightning goes "rumble" and nearby strikes go "crack"). Taken together, this means infrasound needs a lot of energy but travels long distances. Using microbarometers (infrasonic "microphones"), scientists can record infrasounds thousands of miles from their sources (Ref. 2). Most sources are energetic natural processes, but man can also contribute (e.g. rocket launches and large explosions). Local wind and global interference limit the signal-to-noise ratio of infrasound recordings.
What if you wanted to record a flickering candle flame? Let's say it flickers at 5 Hz, disturbing the atmosphere and theoretically radiating infrasound. Since the speed of sound is roughly 1000 feet per second, the wavelength is about 200 feet. But the source (flame) is at least a thousand times smaller, so acoustical coupling is highly inefficient, resulting in a very weak signal. Even if a sensitive enough acoustic transducer could be made, ambient noise from distant sources would likely prevent a clean recording. Acoustic pick-up of small-scale infrasonic phenomena is not practical.
In the audio band, Les Paul (1915-2009) had a similar problem in the 1930s: How do you isolate a guitar's sound and amplify it when there are louder drums and horns all around? The solution was to bypass acoustic coupling and use a "direct" transducer--a magnetic pickup for the strings themselves (and the rest is rock-and-roll history). By analogy, a direct infrasonic transducer for our candle flame might use light intensity (Figure 1). A direct pickup's signal probably isn't exactly like the corresponding infrasound (if any). But for discovering new sounds, this is not the main goal. The relevant goals are isolation, low noise, high resolution, and ease of collecting many diverse sounds.
|Figure 1: Direct infrasonic pick-up of a candle flame. The optical
probe (left) is clamped to a stand near the candle (right). See
Part 3 of Overview and
Example 1 in the
Sound Example Page.
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References (or Notes) Cited in Overview Part 1:
0. "Infinitely wider range of 0 to 20 Hz":
1. Found sounds:
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