Time for another technical blog. This month we’re taking a look at noise floors; starting with what a noise floor is, how it affects your recording and production, and how to make it better.
What is a noise floor?
First things first, we need to decide what we actually mean by a noise floor. Simply put the noise floor is the background noise on your recordings. To break that down further the noise floor is comprised of a number of different sounds.
1. Your equipment.
All electrical equipment makes a noise. I’m not talking about the sound from your hoover or radio, but the noise the electrical components make. Were you to ever try to record total silence, the resulting recording would consist of a hiss. This is the noise your microphone and interface (and anything else in the recording chain) makes and is referred to as self-noise.
2. Ambient noise.
Your microphone is there to capture acoustic energy. So it captures acoustic energy. This is the reason we try to make our studios/booths as soundproof as possible. It’s very difficult to achieve on limited resources in limited space, so we do face the prospect of sound ingress into the studio/booth which the mic will pick up. Plus if we have anything in the booth that makes a noise (other than our mouths) this will also add to the noise floor.
3. Electrical noise.
Hopefully, we’ll only have to deal with the first 2, but sometimes we can face electrical noise in our noise floor as well. This would usually be in the form of a ground loop (a loud low hum) interference on the USB lead (a higher pitched whine) or intermittent surges, pops and clicks when other electrical appliances are switched on or off.
How loud (quiet) should our noise floor be?
Now we’ve established that there will always be unwanted noise on our recordings, the next question is how much noise is acceptable? This is a slightly tricky question to answer properly, so the short answer is – if you notice the noise floor, it’s too loud.
The longer answer is that if you want to put a number on it, it depends on a couple of factors.
Firstly we need to bear in mind that sound is relative – there are few absolutes. What you need to ensure is that the levels of your voice recordings are sufficiently above the level of your noise floor that it covers it up (this is known as the signal-to-noise ratio). So if your noise floor is too high you can’t just turn your gain down as this will make your voice quieter as well, and the ratio between the two will be unchanged. Lowering of the noise floor is all about ensuring we can maintain recording with strong levels and minimal noise floor.
Secondly, we need to think about what the noise floor sounds like (more on this later). Hopefully, if you have a well-treated room, the noise floor will contain a small amount of hiss, but the majority of the sound will be a low-frequency rumble. But you may find that your noise floor is mainly hiss. Even if our level meters are telling us that a rumbly noise floor and a hissy noise floor are exactly the same volume, the hiss will be much more intrusive than the low frequency rumble. This is to do with the way our brains interpret the audio data our ears present it with rather than how sound actually works, but we’re not going to go into that here.
So bearing those 2 factors in mind, how much noise is acceptable?
When we’re recording with our peaks around -6dB, our noise floor should be a maximum of -60dB. This is for a low-frequency rumble noise floor. If we have a hissy noise floor -60dB is still very noticeable, so we’d need to take further measures to reduce it. Also, see the short answer above.
How to lower your noise floor
Even if your noise floor already meets the above criteria, you may be able to lower it further. This is not a bad thing to do! If your noise floor is louder than the above numbers you really should try and figure out why and deal with whatever issues you have in your studio.
The easiest way to figure out how loud your noise floor is is to record it. Set up your studio as normal and hit record. You may want to do a quick reference voice recording but then leave the room/booth. Record what your studio sounds like when it’s empty. This is your noise floor. Use your software to listen to it and take note of what the level meters are doing. If it needs attention you can boost the levels of the recording and have a listen to hear what kind of noises form your noise floor and then you’ll have a better idea of how to improve it.
Hissy noise floor
If the noise floor consists mainly of hiss my first port of call would be to have a look at your equipment. Better quality equipment will have lower self-noise. Did you buy equipment aimed at professional users, or hobby/project/home studios? Is your equipment in good condition? Is it all set up properly? Are you using the right drivers (that one’s particularly aimed at PC users)? If all that is right, I do have a theory about hiss that my albeit brief research doesn’t confirm, but it seems to work in practice. I think that if you have very low levels of noise ingress the incoming acoustic energy can sometimes move the diaphragm, but not with enough energy to make a discernible sound, and this comes over as hiss. In this case the smallest amount of additional treatment can help reduce the hiss. Materials for acoustic treatment shouldn’t usually be used in a soundproofing situation, but in a case like this, even an extra bit of acoustic foam can make enough of a difference.
Rumbly noise floor
If your noise floor is mainly low frequency rumble or sound ingress, then it becomes a bit more difficult as extra soundproofing measures will need to be taken. This can be tricky and/or expensive. If it’s all low frequency energy then the beloved high pass filter can do its thing and work its magic for you, but it won’t touch anything above the frequency it’s set to, so if you can hear the neighbours talking, the birds singing, or an approaching ice cream van, then the HPF won’t help you. Sorry. But at least you get ice cream.
If the noise is electrical noise then you may be in even worse trouble! Electrical noise can be difficult to find and cure. Leading culprits are using unbalanced cables (or cheap balanced cables), too many routes to earth, too few routes to earth, signal cables running parallel with power cables, neatly coiled signal cables or dodgy transformers on monitors, lights, nearly fridges, kettles, or one of your competitors hiring a Victorian street urchin to hide in your cupboard and hum.
How to lower your noise floor in post
So all else has failed, and you just can’t get your noise floor low enough. Or maybe your client has strict specs regarding the amount of compression and the noise floor levels (I’m looking at you ACX). How do you lower the noise floor to an acceptable level so you can not only pass muster, but also make your editing easier?
No prizes for guessing, but it’s our old favourite the high pass filter. Again, this is only going to work on very low frequencies, but it can make a significant difference because when we’re in post, we can custom make the HPF to suit the problem noise and our voice whereas when we’re using a HPF on a mic or interface it’s going to be a more gentle and fixed frequency and slope. I’ve very happily set high pass filters on jobs that have knocked 20dB off the noise floor on some recordings. And the good thing about HPFs is that they work not only in the gaps, but throughout our read as well. Unlike answer 2.
Answer 2 is using an expander. “What’s an expander?” did I hear? An expander is sort of like a compressor, but it works on the quiet bits rather than the loud bits. With a compressor, once the threshold is exceeded, the volume is turned down proportionally with the amount the threshold is exceeded by. With an expander the volume is turned down when the level gets below the threshold. It also works proportionally with the level below the threshold. They can be a little fiddly to learn how to use and to set, but once you’ve mastered it, it can be as transparent an effect as compression and HPFs can be. Some of you at the back are stroking your chins and thinking to yourselves “That sounds very much like a noise gate. Can’t we just use a noise gate?” No, you can’t! Noise gates make silence in the gaps whereas expanders let a bit of noise through (you control how much using the ratio control) Think of it a bit like noise gates totally close the gate, expanders leave it ajar. It’s easy to get a noise gate wrong, and it becomes obvious that you’ve used one. Plus suspicious minded producers see the silence in the gaps and start looking for what you’re trying to hide. Using an expander is more subtle and actually leaving some noise in can often make seamless edits easier.
Last resort time. Noise reduction software. I’m not going to go into the different types here because it’s taken bloody ages to write all this and it’s nearly tea time, but noise reduction should always be a last resort. Whatever type you use it will work by very different methods to those described above and you run the risk of damaging your audio. The closer the sound you want to remove is to the sound you want to keep the less you’ll be able to remove without risking introducing audio artefacts into your audio and thinning and distorting the voice.
It’s always best to tackle an audio problem at source, so lowering your noise floor at the earliest possible stage of your recording and processing chain is going to yield the best results, and once the problem is solved it will speed up your workflow, and you can concentrate on your performance without having to keep an ear out for problems. Everyone wins.
If you’re still with me, well done for making it through a marathon blog. There’s just time for the call-to-action selly bit then we can all go home and forget this ever happened.
If you need help diagnosing or lowering your noise floor, or just want reassurance that it’s ok, give me a shout – I can help you with a 1-2-1 voiceover studio consultation. Or, if you’re looking to improve your production skills I offer 1-2-1 technical training. In fact, you can give me a shout about all sorts of audio things – I was once described as ‘Mr know-it-all himself’ and someone else called me ‘A very nice man’.