Electromagnetic Sound Probe

Listen

Recorded with a stereo pair of probes into a TASCAM DR-40X. No EQ or extra gain was added in post production. https://soundcloud.com/sonic-artefacts/sets/electromagnetic-field-sensor

Why I made it

I have always been interested in radio waves and the electromagnetic world around us. Ever since my teenage years I have been experimenting with winding my own little coils (or solder them out of broken devices) to amplify electronic machines and to reveal what is inside and buzzing away all day without us humans being able to hear it.  At home it finds the hidden voices of phone chargers, laptops, Wi-Fi routers, LED lamps, induction hobs and the compressor of your fridge. In the studio it reveals the inner workings of hard drives, synthesisers, guitar pedals and amplifiers, and in the workshop the rhythms of electric motors, power tools, soldering stations and 3D printers. Out in an urban environment, the field widens: trams and metro trains, shop security gates, card payment terminals, escalators, transformer stations and power lines all have their own electromagnetic signature. Each device turns out to have a voice of its own, from steady hums and drones to pulses, clicks and rhythmic chatter, making the probe as much a tool for listening as for composition. The probe is a mono, passive electromagnetic sensor, significantly more sensitive than common pickups such as telephone coils.

Ordering a pair

When you order two or more, I wind them slightly differently. That gives a more interesting stereo field, for the player and for the listener. If you want two identical probes (within 0.1 mH), add a note to your order at checkout.

Specifications

Type	Passive electromagnetic coil, mono
Powering	None needed
Inductance	7.5 mH
Frequency range	20 Hz to 95+ kHz
Resistance	42 Ω
Output	XLR, balanced (male)
Dimensions	Cork body Ø 24 mm, metal body Ø 20 mm, length 74 mm
Weight	32 g

Questions

How much gain does it need?

There’s no single right setting, because the probe’s output depends almost entirely on the source and the distance. It’s a passive coil (7.5 mH, 42 Ω), so its output scales with the strength of the field and how fast it changes. Pressed against a switch-mode charger it can deliver a hot signal, while a few centimetres away from a quiet device it’s down near the preamp’s noise floor.

Rough starting points on a typical recorder preamp (0 to about 70 dB of gain):

Weak or distant fields, subtle digital chatter: 50 dB and up

Strong sources, probe touching (chargers, transformers, motors, induction hobs): roughly 10 to 25 dB

Typical devices at close range (laptops, phones, LED drivers, routers, hard drives): roughly 30 to 45 dB

Set the gain for the loudest moment rather than the average, since sweeping the probe over a device can raise the level by 30 dB or more within millimetres. Find the hottest spot first, then explore from there. On 32-bit float recorders the gain setting mostly affects monitoring, although very strong fields can still overload the analog input. Keep headphone volume moderate and use a limiter if available. Thanks to its low 42 Ω impedance, the probe adds almost no noise of its own, so a quiet preamp pays off with weak sources. Near mains-powered equipment, a high-pass filter around 80 to 120 Hz keeps hum from eating your headroom.

Can I leave phantom power on?

Better not. Practical rule: a passive coil gains nothing from phantom power, so switch it off for that input. If you can’t, because it’s ganged with a channel that needs it, make sure the coil (and XLR cable you connect it to) is fully floating across pins 2 and 3, with nothing to pin 1 except the cable shield.

Does it fit the microphone mount?

Yes. The metal body is 20 mm, so it press-fits the Sonic Artefacts mount and other mounts for 19 to 20 mm bodies.