Acoustic Amplifiers

by Bart on December 28, 2010

 The term “acoustic amplifier” is sometimes used to describe sound amplification systems that don’t use electricity. I recently had some communication from Monte Thrasher on this subject (Monte is sound-adventurer/thinker/researcher, as well as visual artist, in the Los Angeles area). It got me thinking again about this rather fascinating topic. 

In the late19th and early 20th century heyday of pre-electronic inventiveness, several people came up with purely mechanical schemes for amplifying sound. Some of them worked in a manner analogous to electronic amplification systems that came later. In electronic tube amplifiers, a tiny electronic signal gives rise to a much stronger version of itself by means of a sort of gating system: through the use of vacuum tubes acting as gatekeepers, the relatively weak voltage fluctuations of the original signal are used to modulate the strength of a much larger voltage from an external source, thus recreating the pattern of the signal on a larger scale. A similar gating idea used in pre-electronic acoustic amplifying devices.

In the early devices, the external power source was compressed air. Imagine an air-gating system that can shut on and off the flow from a compressed air tank … or from a blower of some sort, or even human lungs.  If you can get the gate to open and shut at a frequency in the hearing range, then the resulting puffs of air will create an audible tone. If you can get it to open and shut in a pattern analogous to the wave form of a given sound, you’ll get a replication of that sound. In this way, the relatively small force required to operate the gate can be used to modulate the much greater power of a strong compressed air source, creating a stronger sound. Very clever!

Sound waves in the air typically have small amplitudes (the back-and-forth motion is small); and the force they exert is weak and diffuse. The same is often true (though a bit less so) for the vibrating bodies that generate those sound waves. So if you are to use these sources to operate an air gate, you’ll need to create a gating system that requires very little power to operate.

Let’s think about this: how can you create an air-gating system that requires the smallest amount of movement to let through a substantial puff of air? If you think in terms analogous to a garden gate or doorway, or even a faucet turning on and off, you’re not going to do very well – too much motion required to create an opening of any size. Somebody thinking about this in the early days came up with the idea of a dual-plate grid system. Imagine two flat, rectangular plates, perforated with identical grids of small rectangular holes. When the two plates are placed squarely one in front of the other, the holes line up and air is free to pass through.  When one plate is slid a bit to the side, the holes no longer line up, blocking the flow-through. If the holes are quite small, the amount of sideways motion required for blockage is small; yet if the number of holes is large, the amount of air allowed through when they line up can be substantial. With this approach a clever fabricator can create a gating system that allows a powerful air flow, yet requires relatively little force or amplitude of movement. And notice that this is not a simple on-off system; it’s capable of gradual closure and closure to varying degrees. This suggests that, in theory at least, it should be capable of reproducing the wave forms of the original signal with some subtlety.

Of course it helps to capture the strongest possible signal to begin with, since a stronger signal is more likely to have the power to operate the gate effectively.  This leads us to a question which perhaps should have been our starting point: what sound sources were these acoustic amplifiers being used to amplify … and, for that matter, what could they be used to for today?

For starters, acoustic amplifiers of this sort were workable with the phonograph disk sound recording systems of the day.  In these systems, most readers will know, the sound waves of the sound-to-be-recorded were captured by a diaphragm, whose motion in turn was transmitted to a stylus. The stylus was used to carve a wavering groove in the surface of a rotating disk or cylinder made of soft material, later hardened. (Notice that all of these pre-electronic systems faced the same problem: how to get the most, mechanically, out of sound waves that carry a minimum of mechanical energy.) For playback the process was reversed, with a stylus tracking in the groove and transmitting its motion to a diaphragm positioned at the base of a flared horn for projection into the air.  For a much stronger sound in playback, a similar stylus-based playback system could be used to operate the air-gate of an acoustic amplification system.

A system similar to first stage of the phonograph recording system could also be used for live amplification in real time. For this, the soundwaves of the sound-to-be-amplified were captured by a diaphragm, and the motion of the diaphragm used directly to operate the air-gate of the amplification system. 

Acoustic amplifiers were also used with musical instruments. I find this interesting to think about: long before anyone dreamed of making an electric guitar pickup, there were people anticipating the idea non-electrically. Many of these instrument amplification efforts, it appears from surviving information, centered on cellos and perhaps basses. These instruments are suitable candidates because their oscillating systems – the way the string drives the bridge and soundboard – are robust and forceful. The mechanism for transmitting motion to the air gate was attached directly to the cello or bass bridge.

So what did these acoustic amplifiers sound like?  Given a strong enough air source, according to contemporary reports, they could indeed be very loud; no one seems to dispute that. The sound, as reproduced, was accurate enough to be recognizable – e.g., speech amplified this way could be understood – but it appears that the frequency response was limited: great in the midrange, not so good toward the extremes.

You need links! Here’s a good article: http://www.aqpl43.dsl.pipex.com/MUSEUM/COMMS/auxetophone/auxetoph.htm

And to actually hear a phonograph with compressed-air amplification: http://www.youtube.com/watch?v=J7SV65DFNy8

 It’s tempting to think about the possibilities here for new musical instrument design.  I’ve long had an interest in instruments with forced oscillation. By that I mean instruments in which the vibration happens not because of the natural flexing back and forth of a string, membrane, bar or enclosed body of air, but because it is forced mechanically in some way. This could apply here: it’s not hard to imagine a gridded air-gate device with some sort of audio-frequency mechanical driver moving one of the grid plates back and forth.  This idea happens to be very similar to traditional sirens, which you can read about here:  http://windworld.com/features/gallery/musical-siren-built-by-bart-hopkin/. For such an instrument, in the tradition of wind instruments everywhere, human lungs might be a sufficient source of air flow.

Whatever the approach, there’s some fun to be had here with musical applications. Maybe some day I’ll try my hand at making something along these lines.  If you get to it before it do, be sure to let me know how it goes.

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