Mechanical FX

A DIY Guide To Unusual Audio Effects

Technique : Effects / Processing

Some of the best signature sounds have been developed through experimentation with mechanical devices. We offer some inspiration...

Paul White

When we think of creating effects in the studio, we invariably think first of electronic processing using hardware or plug-ins, but many of the great sound effects and sound processes heard both on film soundtracks and records were created by mechanical rather than electronic means.

Classic Mechanical FX

A prime example of this in the film world is the laser gun sound used in Star Wars, which started life as someone hitting the steel support wires for a radio mast or similar structure. The resulting twang has rather more attitude than most synthesized sounds.

It's the same in music, where one of the most established effects, the rotary speaker cabinet used with electric organs, works via a system of rotating baffles and sound guides rather than any form of electronic manipulation. Essentially the audio is fed through a crossover to split it into high and low frequencies, and each band is sent to its own amplifier. The low end feeds a conventional cone driver, facing upwards or downwards into a rotating drum that has an angled reflector inside and an aperture in the wall of the drum to allow the sound to escape. As the drum rotates, the sound is beamed out, rather like the light from a lighthouse. This results in an audibly complex modulation, involving level and frequency content, with a hint of Doppler shift for good measure.

One of the most famous and instantly recognisable mechanical effects: the Leslie rotating speaker. You can use similar principles to create DIY mechanical effects.

The high frequencies go into a fixed tweeter, which feeds into a rotating horn system, usually counterbalanced by a dummy horn. Again, the sound is sprayed out in all directions as the horn rotates and, as each rotor has its own motor, the speeds are never exactly the same, which adds to the complexity of the effect. The performer changes the rotation speed or applies brakes during the performance, and a novel characteristic of the system is the way in which, when the speed is changed, the rate of rotation ramps up and down due to mechanical inertia. Because of how the moving sound source interacts with surfaces in the room, the result is actually very complex and quite difficult to synthesize precisely — though there are some very plausible electronic emulations.

There's also the spring reverb, which uses the mechanical properties of a loosely tensioned helical spring (or, more often, two or three springs: see the picture at the top of the page) to transmit audio from a small transducer to a pickup at the other end of the spring. The complex way in which the sound travels along the spring, and is reflected back and forth along its length, produces the characteristic sound. By combining two or more slightly different springs, the sound can be made more complex.

The familiar plate reverb is also mechanical but, instead of a spring, it relies on a fairly large, thin, suspended metal plate. A transducer fed from the aux send of a mixer (and then amplified) drives the plate into vibration and the resulting reverberation is picked up using contact mics on the surface of the plate. Placing two different contact mics in different positions on the surface of the plate, then feeding these back as two channels panned left and right, creates a pseudo-stereo effect. Plates have a very dense reverb, with a slight metallic coloration, but they're still valued today. In fact, many electronic reverberation devices include plate emulations or, in the case of convolution reverbs, impulse responses taken from real plates.

A plate reverb (above) is a mechanical device that uses contact mics on a resonating metal plate. The plate reverb sound remains hugely popular, and has been painstakingly replicated in the software domain, Universal Audio's Plate140 (left) being one example.

A slightly less well-known early mechanical reverberation device was the Cooper Time Cube. I've never heard one of these working, but the principle involved feeding the sound from a loudspeaker down a long length of coiled hose (folded into a cube-shaped box, hence the name), then picking it up at the other end using a microphone. I'd imagine that some serious EQ would be necessary to make the sound useable, but as with so many mechanical devices, the unusual character was no doubt part of its charm. While apparently simple ideas, what occurs during all these mechanical reverberation processes is actually quite difficult to emulate electronically, though the modern generation of convolution processors can capture them pretty effectively.

Some of these reverb effects can be found in the most unlikely of places. A simple spring reverb system is used in the familiar 'toy' microphone which adds apparent reverb when you sing into it. Here, a plastic diaphragm is forced into vibration by the voice and this, in turn, feeds energy to the internal spring, producing an entirely acoustic reverb that uses the mic body and the plastic diaphragm as soundboards to amplify the sound. In fact, you can use these in the studio to add a quirky kind of reverb to your own mixes, by simply miking the result of singing or playing a sound into the mic. As the plastic body also acts as a soundboard, it is also well worth experimenting with contact mics (the sort of stick-on contact mics that are suitable for guitars should work just fine) as a means of collecting the reverberant sound without getting too much of the direct sound, and this should allow you to add reverb to sounds generated by loudspeakers, as well as the voice and other instruments played close to the microphone.

I mentioned earlier the laser gun sound from Star Wars. Another classic sound effect, which may generate useful ideas for the music studio, is another Star Wars favourite: the light sabre. The humming energy of this fictitious weapon was apparently created by feeding 60Hz US mains hum and buzz from a cheap portable cassette deck (discovered when it picked up interference from a nearby TV set) into a pair of loudspeakers. Then the sound designer simply moved a very directional shotgun mic between the two speakers, in time with the action on screen. What happens here is that the sound from the closest speaker reaches the mic slightly before the sound from the other speaker, so you get some frequencies cancelling and some adding, as you do with a phaser or flanger that uses electronic time delays. As the mic is moved between the speakers, the time differences and audio levels vary, which creates the apparent movement in the sound.

More DIY Tricks

You can obtain an interesting phasing effect by playing a track through a speaker and recording the sound while moving the mic around.

The light sabre effect was great in the film but there are also potential applications in the music studio. I wouldn't normally suggest doing this to a whole mix, but it can be effective on 'trippy' sounding vocals, especially if they already have a bright reverb added, and distorted guitar and complex-sounding keyboard pads also respond well. You can try to approximate the technique using any directional mic and a pair of studio monitors, the main proviso being that both speakers must be fed from the same audio source (ie. a single mono signal — the effect won't work with a stereo one). Record the result from the mic and you should hear a gentle, very organic-sounding phasing effect. It can be controlled fairly precisely by how quickly and how far you move the microphone relative to the two speakers. As the sound level picked up between the speakers will be less then when the mic is close to one of the speakers, you may want to experiment with mic distance to get the most consistent possible level, then use compression and/or mix automation to level up the parts afterwards.

Another trick you could try is to put a basic dynamic mic or pencil-style capacitor mic inside a length of plastic waste pipe so that the sound has to travel down the pipe before it reaches the microphone. This will make the microphone much more directional and also add a lot of tube-style resonance, which can be effective when used creatively. You can tune the tube resonance by moving the mic further up the tube to shorten the amount of tube in front of the mic.

Rotating Phasey Weirdness

Let's cast our minds back to the rotating speaker cabinet mentioned at the beginning of this article. If you can't afford the cost or space of a Leslie speaker, you can create DIY variations on this effect by using a stationary speaker and moving the mic, or by putting something like a slow-moving room fan between the mic and the speaker. If you have ever recorded a guitar combo, you'll already know that the tone of the recording changes as you move the mic across the front of the speaker cone. If you manually wave the mic to and fro in front of the speaker while someone is playing (or while you re-amp a DI part you've already recorded), variation is created in the tone as you record, which can result in a really nice sound that is halfway between a rotary speaker and phasing.

Swingers

Suspending a mic above the speaker and swinging it, rather like a pendulum, gives an effect that is somewhere between a rotary speaker and phasing. By setting up more than one mic in this way and varying the direction and degree of the swing, you can build more complex and interesting effects.

Another variation, this time creating a slow, regular sweep, is placing the amplifier face up, then fixing the mic cable to some point above it that allows the mic to swing, pendulum-style, over the speaker. Set the mic swinging gently and record the results. More complex results can be obtained by setting up two mics that swing at different rates and then summing their outputs to mono, as you'll get phasing characteristics caused by the path-length differences from the speaker to the two mics at any given time. Just make sure the mics don't swing into each other! Simple dynamic models are probably the best for this purpose, as they tend to be more rugged than capacitor mics, and using two identical mics will give the most pronounced effect. Two people can move the two mics independently in a more controlled way, to create a less cyclic modulation effect. If you independently compress the outputs from the two mics, to stabilise their levels as much as possible before summing them, the strength and consistency of the effect should be better.

Another option is to use a slow-moving, large-bladed fan placed between a fixed mic and speaker (or even somebody waving a reflective piece of board around between them!). If you try this, you should put a pop shield in front of the mic, otherwise the strong air stream might cause popping. While a fan will generate mechanical noise and air movement noise, the sound from a typical guitar combo will usually be loud enough to render this insignificant.

Talk Is Cheap

A couple of decades ago, another popular effect was the guitar 'talkbox', and it's still used even today. This simple device feeds the guitar sound to a small loudspeaker, up a length of hosepipe and into the performer's mouth. By changing the shape of the mouth, the instrument can be made to appear to 'talk' in a similar way to a vocoder. Clearly the technique isn't only limited to guitars, as synths or even complete mixes can be treated in this way.

Again, you can take the principles of the talkbox to create DIY alternatives. If you have a small speaker or practice amp, try taping the wide end of a plastic funnel over the speaker, so as to seal any air gaps, then fit the end of a piece of hose over the narrow end of the funnel. Put the end of the hose in your mouth (it's easiest if you tape it to your mic stand), set the audio running and just mouth the words! A mic set up as you would to record vocals will capture the processed sound, ready to record.

The guitar talkbox was famously used by Peter Frampton, Pink Floyd and others. As with other such mechanical effects, you can improvise some interesting DIY alternatives.

This effect is essentially a form of filtering, which is controlled by the resonant shape of the mouth, so it will be at its most effective if you start out with harmonically rich material such as a distorted guitar sound or a complex synth sound.

Summary

These are just a few examples of mechanical sound effects with DIY suggestions that you can try at home without any great financial outlay. There are lots more options to explore — it just takes a little lateral thinking and a willingness to experiment. I'll never forget the effect we discovered during a reader SOS visit, where the studio owner demonstrated a guitar playing technique that he'd developed using a lady's, er, personal massage device to excite the strings (as opposed to the lady). The motor also picked up on the guitar's single-coil pickups to he could use the speed control on the device to alter the pitch of the added drone. Apparently, he got some odd looks in the Ann Summers shop where he auditioned a number of such devices to find the most suitable, but it just goes to show that inspiration can come from any source!

The way interesting sounds can be created or modified never ceases to amaze me, and the best thing is that they usually cost little or nothing to try. The ideas can be very simple, or they can be more complex, such as the ill-fated Gizmo, financed by the band 10cc, which used a series of rotating, toothed, nylon wheels to physically bow the strings of an electric guitar. You may also find inspiration from ancient instruments, such as the Aeolean harp, which is played by the wind. You'll never know just how close I came to adapting my wind chimes to generate MIDI notes, so that I could let the wind choose the tunes while I chose the sounds! This would be easy enough to do using cheap piezo discs glued to each chime, feeding the signals to the pad inputs of a MIDI drum converter, such as an Alesis D5, and then sending the resulting MIDI notes out to the synth or plug-in of your choice. In the end, the results you get from experimenting in this way are usually very different to what everyone else is getting from plug-ins and they can help you to develop a truly innovative and unique sound. If you come up with anything particularly novel, please let us know! 

Published in SOS January 2007