The Apprehension Engine: why this horror instrument sounds so wrong

Right, the Apprehension Engine. If you’ve spent any time in horror music corners of the internet, you’ve seen it: a wooden box covered in metal rulers, rods, a spring tank and a single string, with someone bowing it and making a noise that sounds like the house is about to fall on top of you.

I absolutely love this thing. Not because it’s a fancy gadget, but because it’s the purest example of something I bang on about all the time: horror sounds best when it’s built from real, physical, slightly broken sounds.

So let’s look at what it actually is, why it sounds so wrong, and how you can get that kind of sound in your own tracks without one.

What is the Apprehension Engine?

It’s a handmade acoustic instrument made for one job: producing disturbing sounds.

The story, as Colossal and Synthtopia tell it, is that the film composer Mark Korven was tired of using the same digital samples and wanted original sounds for his scores. So he worked with guitar maker Tony Duggan-Smith to build an instrument that could make them. Orchestral Tools puts it the same way on its Korven page: Korven “together with luthier Tony Duggan-Smith” developed it.

Korven is the composer behind The VVitch, The Lighthouse and The Black Phone. Guitar.com, in its 2022 feature on the instrument, says the Apprehension Engine has been used in his work on The Lighthouse and The Witch, and that it made its debut at the Toronto International Film Festival.

Here’s the thing I find brilliant. Korven himself told Guitar.com that his original was one of the “slapped-together” ones, “filled with quirks and problems”, and that “it took me a while to figure out that I just had to use the problems as advantages in order to find new sounds.”

That’s the whole of horror music in one sentence. Use the problems.

These days the instruments are made to order in Ontario. The official site, apprehensionengine.com, sells them on a 25% pre-order deposit for a base model, with an estimated six months to build. Guitar.com reported that around 25 more had been built and sold since the original, many to well-known composers, and that one of the first went to Trent Reznor.

What’s on it?

Pulling together what the maker’s site and Guitar.com describe, the main sound sources are:

• A slack string with a pickup, played with an E-bow and, in Korven’s setup, a lot of distortion.
• Steel rulers and rods that can be bowed, hit and plucked. The base model comes with a bow for the rulers and other bowing surfaces.
• A spring reverb tank, which Guitar.com says came from a Fender amp and can be played on its own.
• A primitive hurdy-gurdy.
• Friction mallets dragged across the body of the instrument.
• Hidden piezo pickups inside the box, plus a magnetic pickup.

Guitar.com also notes that when Korven plays it, he usually runs it through a harmoniser, loop pedals and a distortion pedal. Worth knowing. A lot of what you hear in those videos is the instrument plus processing.

Why it sounds so wrong

When I talk about what makes horror music scary, I always come back to the same idea: combine ingredients that don’t get on with each other. Two notes that clash. Two instruments from different worlds. Something slightly out of tune. The Apprehension Engine is that idea built out of wood and metal.

The slack string never sits still

A tight string holds its pitch. A slack string wobbles, bends and drifts with every tiny change in pressure. Korven put it perfectly to Guitar.com: “Every little 16th of an inch changes the sound totally when you have a slack string and a ton of distortion.”

That’s the microtonal, slightly out-of-tune feeling that makes your ear uneasy, and it happens on its own.

The E-bow has no attack

An E-bow keeps a string vibrating without you plucking it. So the note just appears, and it can hang there for as long as you want. No attack, no decay. It’s the same reason a sine wave with a slow attack feels like a ghost: you can’t hear where the sound came from.

Bowed metal doesn’t behave like music

Bow a violin string and the overtones line up in a neat, musical series. Bow a steel ruler or rod and they don’t. You get groans, shrieks and pitches that sit between the notes. Your brain hears something that’s almost a note, can’t settle on it, and that’s the weird face in the mirror.

Everything resonates

It’s all in one wooden box. The string sets off the springs, the rulers ring into the body, and the piezo pickups catch the lot. The spring reverb gives it that metallic, rattling space. Nothing is ever clean or isolated, and that’s exactly what you want.

Put those together and you get sounds that feel like found sounds, which, as I say in the course, are essentially the sounds of the unseen. Something is making that noise. You just can’t tell what.

How to get these sounds without one

Most of us aren’t going to have one in the studio. Good. Limitations are where your own sound comes from. Here’s what I’d do.

1. Start with Korven

The quickest route is Orchestral Tools’ Korven library, made with Mark Korven himself. Its listing includes sounds from the Apprehension Engine alongside hurdy-gurdy, oktav harpa and symphonae, and Orchestral Tools’ page has an instrument simply called “Apprehension engine”. If you want those exact textures in your DAW, that’s where they live.

2. Bow something metal

You don’t need a special bow, but a cello or bass bow with plenty of rosin helps.

1. Clamp a steel ruler to a desk so most of it hangs over the edge.
2. Bow the free end slowly. Change how much is hanging over and the pitch changes.
3. Try the edge of a cymbal too. Bow it slowly and it’ll groan, then scream.
4. Record it with whatever you’ve got. Your phone is genuinely good enough. We’re capturing texture, not building a sample library.

3. Put a contact mic on a spring or a sheet of metal

A cheap piezo contact mic picks up vibration through the object rather than the air, which is exactly how those hidden pickups in the Apprehension Engine work. Stick one on a spring, a baking tray, a radiator or a metal bin. Scrape it, flick it, drag a mallet or a superball across it. The sounds you’ll get are bonkers. If you want to take contact mics further, Field Kit is a boxed workstation built around contact microphones, motors and sensors.

4. Process it the way I process everything

Once you’ve got a recording, run it through my usual handful of tools:

• Reverse it. Often that alone is enough.
• Pitch it down an octave, or two. Watch it turn from a squeak into a monster.
• Add saturation or distortion for grit. Remember Korven’s “ton of distortion”.
• Chuck a big reverb on it. A spring reverb if you have one, to nod to that metallic tank.

Ask yourself “what happens if…?” at every step. That’s how my signature sounds start.

5. Or grab my free pack

If you’d rather start with something already recorded, my free Flesh & Bow pack is over 60 samples I recorded on cello, violin and voice, dry and in mono. The drones, string impacts and vocal effects are a brilliant starting point for this kind of scraped, unstable sound. Layer one under a Korven patch and see what happens.

Wrapping up

The Apprehension Engine sounds wrong because it’s built from things that don’t behave: a slack string that won’t hold pitch, an E-bow with no attack, bowed metal that sits between the notes, and a box where everything rattles into everything else. You can buy one, you can load up Korven, or you can bow a ruler, mic up a spring and mess about until something scares you. Out of tune, unstable and a bit broken. That’s good.

Happy composing! Rich