Noise in Audio Engineering

Previous posts have talked about what happens when audio signals get too powerful, too loud. Distortion is what happens. That ain’t the same pork chop is what happens. For a refresher go here.

This week, let us look at kinda the opposite. If distortion is what we hear when things are too much, what is at the other end, the quiet, weak side of things?

Noise is at the other end.

NOISE and SIGNAL

Noise is anything that you’d rather not hear, basically. And Signal is the thing that you actually do want to hear.

• Watching sports on TV and hearing the announcer clearly = signal
• Spouse/Significant Other/Toddler w/Poopie Diaper/Pet Cat in Heat = noise

When we like how noise sounds, it isn’t noise anymore. It becomes signal.

Example: you’re recording drums. The snare is leaking into the tom misc. The snare leakage is noise. So, you put a bunch of gates on the tom mic and spend 45 minutes getting rid of all the snare leaking into the toms.

Cue the band, cue the drummer. Do the count in 1 2 3 4...

And it sounds like shit. The snare sounds like you mic’d up this monkey:

Because the leakage into the tom mics was actually HELPING the snare and the whole drum set. So, you pull off the gates, and now that leakage, previously noise, has become part of the signal.

At a live show, the audience is noise, the sound of the band is signal. In your car, the radio is signal and the sound of the engine, the wheels on the road, the wind rushing past the car is the noise. And suddenly, you hear a “pop” and then a flapping sound outside the car, and now the radio becomes the noise, so you turn it down to hear if you have a flat tire, because the road sounds are now the signal.

Please note that when Noise gets in the way of hearing the Signal there is a Problem.

SELF-NOISE and the NOISE FLOOR

Self-noise is the noise that a device makes when it’s turned on and power is running through it. If you aren’t running a signal through your console or your interface, and you turn up the speakers, you’ll hear hiss. Hopefully, the hiss will be very quiet, and you won’t hear hum along with it.

Hiss is the sound of the device working, the sound of electrons running around the circuit. This hiss is self-noise. All devices that have power flowing through them make noise. Your body generates self-noise, unless you’re dead.

At night, if it's really quiet, you might hear a whooshing in your ears and perhaps a very very quiet whining sound. If you put a cup or a shell to your ear you’ll easily hear the whooshing — remember as a kid when you put shell to your ear and could hear the ocean? It’s not the ocean. It’s blood flowing through your ear, reflected back into it by the shell. You’ll hear the same whooshing if you put a coffee cup up to your head, rather than a barista yelling or a tractor on a coffee plantation in Guatemala.

The whoosh is your blood flowing. The whine is your nervous system working. This is really quiet stuff, about the quietist things you can hear. We call this the Threshold of Hearing. This is like the sound of an ant picking its nose.

Now, you don’t normally hear this stuff in your day-to-day life because everything around you is noisier. Noise causes masking when the signal gets too quiet and falls below the noise. The limit to how quiet a signal you can have is how low the noise is. You can’t really go below the noise, so that bottom limit is called the Noise Floor. You can’t get lower than the floor, right?

The noise floor of a piece of audio equipment is typically really low. Guitar amps have more noise — how often have you heard a sustained guitar note decay away into the hiss of a guitar amp? It goes below the noise floor and then you can’t hear it anymore.

The noise floor is shifting thing. When you’re mixing live, is the hiss through a PA system really an issue? It might be during the sound check when the venue is empty. But once it fills up with people, the noise floor of the audience is considerably higher than the hiss of the PA and effectively masks it. And if your PA hiss is heard above the audience... jeez, you suck, you stumpy bastard.

SIGNAL to NOISE RATIO

You’re in the coffee shop talking to a friend. The friend who is talking is the SIGNAL — the thing you want to hear, and the background chatter, espresso machine sounds, etc., are the NOISE — the things you don’t want to hear. The louder the coffee shop gets, the louder your friend will have to be such that you can hear their signal over the noise.

Signal over Noise... let’s call this the Signal to Noise Ratio. S/N ratio. If this is a low number, the noise is loud and it's intruding on the signal. If this number is high, the noise is quiet compared to the signal. So, now you understand this bit more:

You still might not fully understand decibels, but we’ll get to that.

The S/N ratio is different for different types of equipment. It’s comparatively huge for microphones and really good preamps, and much less so for cheaper equipment, guitar amps, PA systems, etc.

Noise builds up. When recording, the ambient sound of the studio feeds into, oh, say a condenser mic, which adds some hiss, and then into a preamp, which adds a little more hiss, and then into various converters and devices, all of which add hiss. And all of this noise adds up, and that’s the noise floor. Then someone wacks a snare out in the studio, and that goes slamming through everything and it’s much louder than the noise. High S/N ratio. The snare rings out for a moment, then decays into the ambience of the room. And once it decays to a certain level, we’ll notice the noise again. S/N ratio is a fluid thing.

CAPTAIN OBVIOUS

This is frickin’ obvious but it must be said: you usually hear noise when things are quiet, when the signal is low and the S/N ratio is small.

Another frickin’ obvious thing that must be said: analog recording techniques were mostly developed to compensate for noise, especially tape hiss.

Tape hiss... the sound a piece of magnetic tape makes as it slithers over the heads of a tape recorder. The more tracks you have, the more tape hiss you’d get. Dolby, DBX noise reduction, noise gates, etc., were all developed to control tape hiss.

Digital recording was developed to totally get rid of tape hiss.

I can’t tell ya how much time I spent in my engineering career trying to get rid of noise. Automating mutes. Gates. Yada yada yada. I never really used DBX systems because I thought they sounded terrible, and if I was recording at a nice high level to really good tape, and was careful with muting, I could make a virtually hiss free record.

You can’t hear hiss when the band is cranking.

I cannot understand why anyone would make a plug-in that adds “authentic analog noise” to the signal chain. Restaurants are allowed to have a very low percentage of cockroach bits and rat crap in the food. Would you add cockroach bits and rat shit when you cook at home to get that “authentic restaurant taste?” Fuck no.

Next week we’ll put all of this together and figure out dynamic range and metering.

Be well. Stay safe.