How To Remove Your Bottom End – High Pass Filters

How To Remove Your Bottom End – High Pass Filters

Those of you who have had almost any kind of work-related conversation with me will have probably heard me mention high pass filters. I am something of a high pass filter evangelist, as I think these simple little processors can make all the difference to the quality of your recorded audio in a couple of quite important ways. So here we go VOs. My guide to what a high pass filter does and why you should be using one.


Supplying un-processed audio

Now I know that when you get your brief from your client they ask for un-processed audio, so you sent them exactly that – you record the script, you may edit it (depending on the client and the brief – you almost certainly take out the fluffs, burps and sneezes) and you send it off. You certainly don’t add compression or EQ. But in my opinion, although a high pass filter is technically an EQ process, it doesn’t count. By using one you will tighten up the audio you send and help it sound more professional. Your client won’t notice that you’ve applied it (if you apply it properly) but may well notice if you haven’t.


What is a high pass filter and what does it do?

We need to take a step back first to explain it properly. Sound is a form of energy – you may call it acoustic energy but physicists would probably call it kinetic energy. High frequency sounds have less acoustic energy than low frequency sounds – I’m sure at some point you’ve heard your neighbours TV or music coming through your walls; it’s the bass frequencies that come through and you hear the boom boom boom of the kick drum keeping you awake until 4 IN THE BLOODY MORNING! Higher frequencies simply don’t have the energy to pass through the walls. With this in mind, it’s always going to be the bottom end frequencies that take a greater effort to ‘tame’ in any recording environment.


Lowering your noise floor

A high pass filter attenuates bottom end frequencies by increasing amounts the lower they are, so by applying a high pass filter to your recordings you can hugely reduce the very low frequency sounds that may otherwise mar your recordings – the rumble of the main road 3 streets away or the occasional lorry passing your window, the washing machine going into a spin cycle 2 floors down, the person walking across the floor in the next room who’s managing to vibrate the floorboards enough to transmit the vibrations up your mic stand. It can help no end in lowering the noise floor in your studio, making it seem quieter than it is. It’s not uncommon for high pass filters to knock up to 10dB off the noise floor of a studio, and I have encountered studio where it’d even more than that.


Dealing with Plosives

High pass filter use #2 comes in the way of plosives. Plosives are those breath blasts you get from Ps and Bs when the burst of air you release from your mouth hits the diaphragm of the microphone. Yes you probably employ a pop screen to get rid of the breath blasts, but it’s not uncommon for the pop screen to not entirely solve the issue, and we end up with breath blasts on our recordings. And it’s not just the Ps and Bs to listen pit for, other sounds can be affected, and sometimes we can get what I call a ‘bottom end flappiness’ (no sniggering at the back) to the sound of the recordings. Again here high pass filters can help greatly.

The number of VOs who’ve come to me asking how best to clean up breath blasts as they’re manually editing each one and it’s taking ages, I simply apply a high pass filter over the entire audio file – it takes a few seconds to do – and the file sounds so much better. Not perfect, but much easier to deal with whatever pop may be left still.


Still worried that your client wanted unprocessed audio?

Don’t be. The frequencies that a properly set high pass filter works on are below the frequencies that naturally occur in the human voice so the program material that your client wants will be untouched by cleaning up the audio with a high pass filter.


Using high pass filters

Good in theory, but what about actually in practice?

Glad you asked. Here we go…

Here is me reading a bit of Gullivers Travels. It’s not a very good read because I was trying to accentuate the above issues. It’s unedited and unprocessed. You’ll hear some ‘silence’ (actually me stomping around a floor below my studio) and then the read. There is an undercurrent of bottom end flappiness which makes the read sound ‘scruffy’.


So now for exactly the same piece of badly read audio with the simple addition of a high pass filter.

Before you even play the audio you can see on Soundcloud’s crudely rendered wave form the ‘silence’ is quieter than on the non-high pass filter version. Can you hear how much cleaner the reading sounds?

Time for you to find out where your High Pass Filter is.


Want to check that your studio is professional quality? Rob’s Studio Consultation Service can help. Or, if you want to learn more about using high pass filters, get in touch to arrange some 1-2-1 training.


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