diy instruments apprehension engine part 2

DIY Instruments: The Apprehension Engine Part 2: Application

Following up on part one, this article walks through how to build and play elements of “The Apprehension Engine.” The original, as made by Mark Korven and Tony Duggan-Smith, is a work of art. It’s a single instrument with various ways of creating fear-inducing sounds. I’m not trying to duplicate their design, but instead illustrate how my favorite elements work. Building your own DIY instruments will bring a unique flavor to your film music. Using this guide will hopefully spark your creativity when working on your next film score that requires bone-chilling atmosphere.

For simplicity, I’m going to break each unique way of making a terrifying sound into its own Effect.

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Effect 1: Metallic Bones

This effect is achieved very simply, but produces a complex sound. Whether plucked or bowed, it gives a kind of dry, breathy sound. Hauntingly familiar, but definitely otherworldly. Check out the video below to see it in action:

note: the only post-processing done to these demonstration recordings is the addition of reverb.

Parts Needed

The Build

Pick up some metal rulers of varying sizes and affix them to the top of the resonating chamber. I used an old speaker cabinet as the base, and it worked really well. It had bolted-on feet that, with the addition of a wooden dowel, I could use to sandwich the rulers in place.

Varying the length and placement of the rulers will produce different sounds. The biggest difference in tone came from differences in thickness, material, and overall length of the individual rulers. Experimenting is half the fun with DIY projects.

entry-level horse hair violin bow
A basic horsehair bow is perfect for this

I also found that the bow strings needed to be heavily rosined in order to get a louder sound. So, I bought a very cheap bow – and with proper rosin, it works well for this application. I can’t imagine scraping a finely made instrument against sharp metal is the best thing for its longevity, so go with a cheap one here.

Playing and Recording

This effect produces a very quiet sound, so you’ll need a sensitive microphone. I used a small-diaphragm condenser mic, mounted on a simple stand. I plugged it right into my audio interface, and recorded into my DAW.

For playing with a bow, placing the microphone as close as possible to the ruler is essential here. The speaker cabinet helps produce low-end tones, but for the dry, breathy sound of the metal, you need to have the mic close to where the bow touches the metal.

If you are plucking them, the mic can be a bit further away, and focused more on where the ruler slaps against the resonating chamber.

For post-processing the sound, I added reverb, and that’s it. In fact, I used the same basic reverb preset on all the demonstration sounds in this article for consistency. The possibilities are endless, though. I’ve had great luck in pitch-shifting this sound, and adding effects like chorus to turn it into something unrecognizably horrifying. Experiment!

Effect 2: Glass Shards

If you’ve ever run your finger over the rim of a wine glass, you are familiar with what sounds a wine glass can make. If you instead use a violin bow, however, the results are much stronger, and much more piercing. Perfect for building tension or cutting through a low-end heavy mix.

Parts Needed

The Build

basic wine glasses
Basic wine glasses can be played with a bow

It doesn’t get any simpler than this – simply place the wine glass on a stable surface and go to town! I found that lightly holding the glass in place was necessary. While I used my hand, something like gaffer tape would work well too. Quick note: I placed it on the speaker cabinet as seen in the metal ruler video, but it could be any stable surface, it didn’t make a difference in the sounds produced.

Playing and Recording

Again, this is a simple one, though it does take some practice. Run the bow gently over the rim of the glass to produce a tone. Try to keep the bow rubbing against the same spot on the glass to get a continuous sound. If the bow wanders to other parts of the glass, I found it will dampen the vibrations.

Having a well-rosined bow is again crucial here, as the more friction results in louder notes, and different timbres. Pushing harder against the glass or bowing faster will create a bigger sound, and you may find different overtones. Listen to the video around :16 seconds in to hear the glass switch to a lower, harsher tone.

The microphone should be placed close to the glass. This effect can get loud, though, so you could back it off further than with the ruler effect. I used my small-diaphragm condenser microphone here, for the clarity of the high notes and its directional pickup pattern.

The only effect added to the sound recording is reverb. To lose more of its recognizable “wine glass” quality, equalization and pitch-shifting can be a good bet here.

Safety note: while it hasn’t happened to me, playing a brittle wine glass too loudly could cause it to shatter. I recommend wearing work gloves and eye protection to be on the safe side.

Effect 3: Gates of Hell

This is a weird one. It doesn’t sound like anything I’ve heard before. It kind of has an organic quality to it, while being very recognizably metallic.

For those unfamiliar with a spring reverb unit, it is usually found in guitar amplifiers. An audio signal is sent through a transducer that causes springs to resonate. The waves are picked up at the other end of the spring, where their natural elastic quality has made the waves bounce around, thus producing a reverb-like effect. While this is probably a gross oversimplification of what is happening, it doesn’t matter because we are going to play it by itself, thusly:

Parts Needed

The Build

There’s not much of a build here. If you haven’t bought the spring reverb unit as its own part, you may need to cannibalize a cheap guitar amplifier. But, once you’ve got it out, you just need to plonk it down on a stable surface that will isolate it from extraneous vibrations.

We are using the transducers in the reverb as microphone pickups, so just connect the reverb unit to a mic input on your recording interface. In my case, my reverb unit had two RCA jacks, so I used an RCA-to-1/4″ adapter plugged into a mic-level input on my audio interface. If yours has a different jack, adjust the adapter.

Playing and Recording

This is a fun one. A spring reverb wired up like this can make all sorts of crazy sounds. Flick the springs to get crashing percussive sounds. Scrape a metal pick along the coils to get a tearing, wrenching metal sound. Play around with EQ to get rid of some of the high-end spring-y quality and pitchshift it down to get low, guttural growls.

ebow magnetic resonator
An ebow is a genuinely unique way to play a stringed instrument

My favorite way to play the spring reverb is with an Ebow. This device uses an electromagnet to cause the springs to vibrate at fundamental frequencies. And when the vibration is strong, slapping the device against the springs or twisting them under pressure yields harsh metallic howls and screams. To me it sounds like a dentist’s drill crossed with a chainsaw. Fun!

For post-processing, EQ an pitchshifting will go far. You don’t need much in the way of reverb (since it already has that quality), but I added it here to keep it consistent with the rest of the demonstration videos.

Effect 4: Heavy Metal

This effect is based on the original Apprehension Engine’s electric guitar-based strings and pickups. Maybe I’m just not one for subtlety, but I prefer to add some crunchy distortion. While any session guitar player can make their guitar scream with harmonics, the effect of the Ebow here achieves the same effect with infinite sustain, and without actually plucking the strings, which makes it more unsettling:

Parts Needed

The Build

There’s not much build here, unless you are building an all-in-one Apprehension Engine. In that case you’d probably be bolting the neck to a resonating chamber, and cannibalizing the guitar‘s wiring for its pickup. In my case, I just plug my guitar into my distortion pedal, plug the output into my audio interface, and go to town.

Playing and Recording

This is what the Ebow was designed to do – make guitar strings sing. Although, in my case, I like to make them scream. Experiment with different angles, placement, etc. I found that placing the Ebow over the heavier-gauge strings produces the best results. And, the closer I place it to the pickup, the more distortion I get.

One you have a string vibrating, it will keep going until you lift the Ebow away, so you could conceivably hold one note for hours. (That would make for a boring film score!) Play around with having the Ebow touch the string every so slightly as it vibrates – that makes an all-new distortion sound perfect for waking the dead.

In terms of effects, I used a basic distortion pedal between the guitar and the audio interface. In post, I just added the same reverb effect used on all of these demos.


There’s much more you can do with your own DIY Instruments, and we will explore that in future posts. Other parts of the original Apprehension Engine were not emulated here, but now that you’ve tried a few things yourself, why not take a stab at it? This is a great way to bring a fresh, unique sound to your film scores. Enjoy!

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