What lurks beneath

 No, it's not a horror movie, but it may well turn into a horror story if you're not careful.

I'm of course talking about audio interference, not the type you can hear, not immediately anyway.
We hear audio as longitudinal waves, i.e. the compression of air molecules,
but.....much more exists as electromagnetic (RF) waves.

So we can't hear them as they are electromagnetic, but they can, and do at times, wreak havoc
with our audio recordings.

So how do these electromagnetic waves get into our audio paths?

There are two main ways, both via capacitive or inductive coupling.
(Basically, as a result of poor shielding and isolation of circuits and cables).

1) Waves that are higher than the audio spectrum, but are picked up, rectified and the 
modulation of the main carrier, which is in the audio spectrum, is then heard.
We have all heard cellphone interference on audio equipment.
The actual carrier can be anywhere from 900 - 2200 MHz, definitely not audible,
however, the digital modulation will be heard after rectification.

Keep in mind that ANY semiconductor junction in electronic devices,
can act as a rectifier.
To make it worse, modern audio equipment has a combination of analog and digital
gain circuits, which makes equipment much more prone to interference if not
adequately shielded or fed with inferior cables.

2) The other type of potential interference, is from electromagnetic waves which are
in the audible range but are not heard as they are RF and not longitudinal waves.
Unheard of course, until our equipment picks it up, amplifies it and ruins our recording.
Rectification is not even required as they are already in the 20 Hz - 20 KHz range.

An example of this, is 50 Hz (mains) pickup.

To illustrate this, I built a RF adapter for my cellphone.
So it does not record using a microphone, but rather by picking up electromagnetic
waves that are in the range of 20 Hz and 20 KHz, actually up to 22 KHz.

First up, mains interference.
This was recorded 2 meters away from any wall plug and the antenna was a short piece
of wire.
Again, no microphone was used, this is purely an electromagnetic (RF) recording.

The fundamental at 50 Hz is clearly visible (and audible when recorded), plus
harmonics going all the way up to around 1 KHz.

So what else is lurking, just waiting to ruin your recordings?

A quick scan around the house, quickly revealed some candidates.

First off, the spark igniters on the gas stove.

The aerial is just a short screwdriver, this is what was picked up:

Those peaks are the sparks being created.
Zooming in, we see that they last for around 6 samples (@44.1 KHz sample freq.)
which puts them at 7.439 KHz.

The repetition rate is 3 sparks/second.

Since I'm sampling at 44.1 KHz, can "see" signals up to half of that (Nyquist rule),
so let's look at what harmonics it produces.
We can see the 2nd harmonic (at 14.698 KHz), the 3rd will be of no consequence as it's
pretty much on the edge of what someone with good ears will hear.

Nasty, and it manifests itself as short, sharp clicks.

Let's take another example, the garden.
Surely, other than the audible (with our ears), there is not much else.
Let's find out.
First, let's record with the built-in cellphone's mic.

PauloV · GardenAmbienceMic

Other than some birds and wind noise, not much else.

Aha, that is what you think, let's switch to RF mode:

PauloV · GardenNoMic - RF - Mode

What the ??
Let's look at what it is, it's frequency and spectrum plot.

So, it's clicking pulses with most of the energy concentrated at around 10 KHz
and just look at the level.

Yep, it's my electric fence.

This was recorded in the middle of my garden, with the "antenna" being a 50 cm 
piece of wire and about 4 meters away from the fence.

This then, simulates a crappy audio cable quite well.

I was lucky that it's not on my microphone recording as it was done all with the cellphone.
No cables, no external mixers, etc.
Had I being doing an outside broadcast or other recording with lots of external cables
and equipment, almost sure some of that fence energy would be on that audio recording.

I hope that by now, you are more aware of things that can catch you out,
and there are plenty more.

Electric motors (without suppression), geysers and other high current appliances switching
on and off.

Let's not forget about lights, light dimmers, ...........

The list is a long one, but by always using good quality shielded cables, as short as possible

and properly shielded equipment, you decrease your chances of being affected 
by what lurks beneath, unseen and unheard until it's too late.

This RF adapter is actually a rather handy thing, maybe I'll start making a few and selling them.

Expand your horizons