Digital compressed-air speakers!

[CeramicMan]

Originally posted by bishopdante
...On another tack entirely I have heard that Sony have something very powerful cooking in their labs. Pure Digital Speakers. What you have is a bottle of compressed air and a bottle of vacuum connected via digitally controlled valves to the back of a horn. The valves recieve 1bit Bitstream signals at a very high sampling rate and can push the equivalent SPL of a 4kW PA out of a horn the size of a tube of toothpaste. Bass down to 20Hz, treble up to 50kHz. No amps, converters, wires, speakers or hungry PSU (since all the energy is stored as mechanical energy (vacuum/compression)). When the bottles run out you put them in a machine and pump them up again. Or buy new ones from the Sony Shop. I want some. The reason why they haven't been released yet are:

1 Sony are making enough money out of the boxed kind thankyou.
2 The valves they have at the moment are expensive and wear out pretty quickly.
3 A high power PA in a briefcase?! Noise pollution! Those things are basically weapons! With a van full of them you could probably knock down buildings!

Mmmm, the stuff of dreams, though I suggest that this thread forms a discussion of the feasability etc, and pros and cons of such an idea. I can see quite a few difficulties in making good sound out of valve controlled compressed air, so I think I'll focus on a few cons.

1) What would the valves be made from? They would have to have their own power amplifiers to supply the power required to open and close them. It's an interesting problem (as it always is) when trying to control a large signal with a small signal (transistors, relays, water taps, thermionic valves..... (in order of decreasing technology ))

2) It would give new meaning to the term "switching noise". It's like those old DOS programs that switched the PC speaker between 0 and 1 at a near-ultrasonic rate, to make it perform like a 1-bit delta sigma DAC. The squealing sound was most prominent in the quiet parts of the music due to the 50% duty cycle. In this case, I can imagine that vast amounts of white noise would be generated between a fully OFF state and a fully ON state. Either the valves would have to have an incredibly high slew-rate, or a linear variety should be considered instead.

3) For quality sound, look elsewhere. 1-bit delta sigma DACs can be regarded as an inferior technology to multi-bit delta sigma DACs. Add mechanical limitations to the mix and it gets worse.

OTOH, it could be ok for low frequencies. Vehicle exhaust emissions can create some seriously loud bass. Similarly, the trick would be to mechanically filter out all the high frequency distortion harmonics of the flowing air.

CM

[jjdche]

CM, you made some interesting points, but since I don't really know anything about audio/electronics theory what you said means absolutely nothing to me.

My concern is the way he described a bottle of compressed air and a bottle of 'vacuum'. The bottle of comperessed air makes sense, but a bottle of vaccuum really would do anything for you. Unless you are using EXTREMELY small quantities of air at relatively low pressures then it would make more sense just to vent the air to the atmosphere, the pressure difference would barely be affected.

The key to this design is how much air it actually consumes during use. If it uses alot then the tank for the air will be ridiculously heavy. Doesn't make much sense to make a speaker 'the size of a tube of toothpaste' when the power source weighs over 100lbs (oh, wait a minute!).

Another concern I have is how the change in tank pressure will affect the sound. As the amount of air in the source tank dwindles the pressure drop at the orifice will change, presumeably changing the sound of the speaker. That can't be an easy problem to deal with.

Of course you could hook up a little air compressor to the system that would keep it charged all the time. Compressors aren't typically very muscial though. Maybe Sony can come up with a subwoofer/air compressor.

[Svante]

The problem with this approach is that sound pressure from a piston is proportional to its acceleration, not its position. In some sense this is good, since it would be very hard to make the position of the piston oscillate between its end positions at ultrasonic rates. Now, most pistons are mass-controlled at the frequencies in question, ie they operate above their resonance frequency. Applying a force to that mass would give it an acceleration that is proportional to the signal, which would give a sound pressure that is proportional to the signal. Nice. But wait, that is exacly the case with the good old-fashioned dynamic loudspeaker...

The "problem" here is that the driver's motor has to be linear, just as any speaker has to. Actually, the system outlined above is pretty much an ordinary loudspeaker connected to a class D amplifier.

So, until we can build a large driver with a resonance frequency of 20 kHz+, the "best" we can do is class D with an ordinary speaker.

[johninCR]

Svante,
There's no piston to move. This is like a variable frequency air horn.

[Svante]

Ah, silly me, I should have read the first post better.

[CeramicMan]

Anyway, by calling it a type of horn implies that there is acoustic impedance conversion, and that sound waves resonate across a relatively narrow range of frequencies. High efficiency 20Hz to 20kHz reproduction would not be possible from a single driver, at least not without severe compromises.

20kHz reproduction would require accurate high-speed modulation of a relatively low volume of air, while low frequencies would mean that the same transducer also has to modulate an extremely large volume of air. There was a thread a while ago discussing the possibility of distortion due to frequency modulation caused by the Doppler Effect. Whatever subtle effects could be possible with ordinary speakers, it would be much worse here.

The question remains: how would a mechanical valve be constructed for such a device?

CM

[danielw]

I have actually seen such a thing. It was made by a group of mechatronics students. What they did was remove the motor of a big sw-driver, and attached a hose (don't know what type) to the back. Because of the construction of the hose, it would shorten when pressurised. With a compressor and fairly high-switching valves, they made a subwoofer. I don't remember correctly, but i don't think it reached above 100Hz. Did go pretty loud though

Daniel

[BassAwdyO]

the idea of a pressure/vaccum loudspeaker is pretty rediculis if you ask me. It is true there are only 3 ways to change pressure and changing pressure is sound. This is shown in the gas law PV=nRT if you want to change pressure you must change voume, temperature, or the amount of gas. basically this pressure/vaccum is changing the amount of gas by adding(from the high pressure tank) or subtracting(from the vaccum). the noises induced at higher frequencys would be impossible to avoid. just think about taking an air compressor and putting on the trigger nozzel. when you squeeze the trigger it makes the horrid psssss sound of air spray. thats what the sony horn would sound like.

[Havoc]

All been done before.

Check out the Auxetophone: http://www.dself.dsl.pipex.com/MUSEU...e/auxetoph.htm

And if you care to read old issues of the JAES you will find a reference to such a horn actually build for testing the sound propagation of a Saturn rocket.

"Vol. 13, Number 3 pp. 229 (1965)
Author: _ John K. Hilliard
Abstract: _This paper discusses the generation, propagation, and detection
of
acoustic energy over a wide frequency and power level range. In particular,
it describes an electro-pneumatic sonic generator which develops several
kilowatts of acoustic energy."