Plasma Sonic Speaker

What is a Plasma Sonic Speaker?

A Plasma Sonic Speaker is basically an arc between two point (or in free space aka Tesla Coil) that emits sound. This I believe was discovered when experimenters started trying to transmit audio via Amplitude Modulated Radio Signals using high voltage coils and spark gaps.

How we produce sound with an arc:

Sound is caused by pressure waves in the surrounding medium (air). This can either be achieved by pushing the air (normal speaker) or expanding the air.

How do we go about expanding and contracting air molecules quickly enough to hear sound?

We expand the air in a confined environment by varying the air molecules temperature surrounding an arc. By modulating the duty cycle or intensity of the drive of a high voltage coil from an audio source, we change how "hot" the output arc is. The heat from the arc directly corresponds to the amplitude of the input audio signal. By expanding and contracting the air molecules we change an AM signal into a Longitudinal wave (aka sound).

There drive frequency of the transformer needs to be higher than the maximum audio frequency signal to retain the clarity of the sound. Around 45Khz or more is adequate.

What kind of sound quality can I expect?

The sound produced by the arc is very audible and very clear. Although there is one thing to note about a plasma sonic speaker. Since we are only moving a small amount of air, it only produces high frequency sounds, eg a Tweeter. Bass sounds are not reproduced very well by this. I have been running a small bass speaker alongside the arc using a simple frequency crossover, just to add a little more of a dramatic response from the people I demonstrate this to :-)

Having said that, it's very good on it's own.

Heat and ozone production are another side effect from this system. Ozone production is fairly small because the arc is quite intense and is AC, so this isn't a big problem, but Heat is! upon running this system for a few minutes, you will find that the electrode get to nearly red hot temperatures! So the best thing to do, is use large bolts and keep the arc away from any surfaces, and limit max running times to 5 minutes.

Requirements:

- High Voltage AC transformer (see the one I made earlier ;-) here)

- Power Supply capable of outputting 6-10 amps at 24v (2x 12v batteries are best)

- Audio source (headphones out from anything is fine, but make sure you don't mind if it gets destroyed from spikes!)

- Oscilloscope, Frequency meter etc.

- Time and patience!

+ everything in the circuit diagram.

Circuit:

Build this on a veroboard or pcb. Due to the high frequencies involved, this is not something to do on a breadboard.

Transistors can be substituted for others but must be N channel and have a high drain current and high frequency dv/dt. They must also be mounted on heatsinks and fan cooled for extended runs! You have been warned...

Finished Driver Board:

Testing:

My setup can be seen in the above picture	The HV transformer, spark gap and fluro tube setup

Place a flurotube in the vicinity of the HV coil to monitor what the output is doing. (Fluro tubes are great tuning aids for all HV projects, the brighter the tube gets, the more voltage there is in the area).

Connect it all up (use a current meter in the circuit for the first test) and make sure that there is air flowing over the transistors. Make sure the current isn't above 12A (remember this is 24x12=288 Watt). If it is, turn off check everything and alter the pwm potentiometer and try again.

If the arc doesn't start between the 2 electrodes, either turn the system off and decrease the gap, or try bridging the gap with something non-conductive (eg a ruler) with a croc-clip on the end to see if will start. You may also need to alter the input volume. Try altering the pwm potentiometer first. Whilst looking at the current meter you will notice you can control how much the coil is drawing. This is altering the PWM width to the coil and also changes the characteristics of the output power, arc intensity and sound modulation. You need to find an equilibrium between them. I have found around 50% to 70% is good.

Optional Crossover Bass circuit:

Use a 3.5mm audio splitter with the above circuit in-between the audio source and driver board. The output from the crossover can go to a speaker with an amplifier, eg. a pc speaker set.

Pictures:

Bright orange arc produced by the sodium vapour.	Notice the vapour from the right electrode.	All these arcs produced crystal clear high frequency audio.

 

Video:

A vid of the Plasma Sonic Speaker in action. WMV 2.53Min 40Mb (Right click and choose Save As)

Thoughts:

This is really impressive to show someone, as they won't understand it and think that it's magic! Only we shall know what causes this effect.

Another thing to try on the electrode terminals are crystals of salt. It will change the colour of the arc to an orange rather than the usual blue/purple. This is due to sodium ions that are released from the salt when heated. The same effect used in Sodium Lamps you see as street lights.

Read this:

The output from my HV AC coil is fairly dangerous as it is a high current output (around 50+mA) designed for powering my Tesla Coils. If you build the one I have described, which you probably will! , just bear this in mind and don't get stupid with it.