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Old October 27th, 2014, 10:13 AM   #1
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Microphone Hi-Pass filter use

Let me ask some of the more learned audio folks, is it common practice to enable the 'Hi-Pass filter' on Condenser microphones when feeding into a mixer? Or, for that matter, directly into a camera? Let me first say, this is primarily for VOX recording, not music.

In the past, I've enabled the 80Hz Hi-Pass as a 'general rule', to address HVAC rumble and to catch and mitigate any low-frequency noise that might occur.

If, I think I need more, I'll 'double-up', using the 80 Hz setting on my SD-302 Mixer. Although, I'm not sure it works that way.

J.
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Old October 27th, 2014, 10:55 AM   #2
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Re: Microphone Hi-Pass filter use

If you are recording voice, then as a general rule, I think it's fine to use the 80 Hz high pass setting. There is very little voice energy down this low, so the filter shouldn't have an adverse result. And yes, it will get rid of a lot of room rumble from HVAC, nearby vehicular traffic, etc.

Remember that the filter does not abruptly remove all energy below 80 Hz. It removes progressively more energy as the frequency goes further below 80 Hz.

One additional number that's missing from your post is the "slope" of the filter. It might be as little as 6dB per octave, or 18 dB/octave or more. For example, a 6dB/octave filter will remove 6dB more signal at 40 Hz, compared to 80 Hz. It will remove another 6dB (a total of 12dB) at 20 Hz, compared to 80 Hz. An 18 dB/octave filter will remove 18dB more signal at 40 Hz, compared to 80 Hz. It will remove another 18dB (a total of 36dB) at 20 Hz, compared to 80 Hz.

Filters with a shallower slope (e.g. 6dB/octave) may sound a bit more natural. Filters with a steeper slope (e.g. 18dB/octave) will be more effective at removing room rumble.
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Old October 27th, 2014, 11:08 AM   #3
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Re: Microphone Hi-Pass filter use

Quote:
Originally Posted by James Kuhn View Post
Let me ask some of the more learned audio folks, is it common practice to enable the 'Hi-Pass filter' on Condenser microphones when feeding into a mixer? Or, for that matter, directly into a camera? Let me first say, this is primarily for VOX recording, not music.
Yes. Very common.

Quote:
Originally Posted by James Kuhn View Post
In the past, I've enabled the 80Hz Hi-Pass as a 'general rule', to address HVAC rumble and to catch and mitigate any low-frequency noise that might occur.
That's what it's for.

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Originally Posted by James Kuhn View Post
If, I think I need more, I'll 'double-up', using the 80 Hz setting on my SD-302 Mixer. Although, I'm not sure it works that way.
It works exactly that way. The two HPFs will add. That is, if your mic has an 80Hz 6 dB/octave filter, and your mixer has an 80Hz 12 dB/octave filter, the two together results in an 80Hz 18dB/octave filter.

I did exactly this a few weeks ago. Had to use a hotel room for interviews, and the room they assigned shared a wall with the air handler for the entire floor. A low rumble that you could feel as well as hear, because it was also pretty loud. It was... less than ideal. So I walked the room listening for room modes and found a spot where the rumble was less bad. I put the interviewee chair in that null. Then I enabled the filter on my mic, and on my mixer both. The results are good enough that I didn't feel the need to add even more EQ in post. Basically a nicely sharp cutoff at 80Hz. Producer was shocked and pleased by the sound in the rough edit (before any post), always a good thing.
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Old October 27th, 2014, 11:29 AM   #4
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Re: Microphone Hi-Pass filter use

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It works exactly that way. The two HPFs will add. That is, if your mic has an 80Hz 6 dB/octave filter, and your mixer has an 80Hz 12 dB/octave filter, the two together results in an 80Hz 18dB/octave filter.
The only minor detail is what happens at 80 Hz. In theory, the specified frequency is the "knee" of the curve, where the filter is down by 3dB. So if you cascade two 80 Hz HPFs, the result (in theory) would be down 6dB at 80 Hz. So this moves the theoretical knee frequency, where the filter response is -3dB, upward a bit above 80 Hz. But yes, with the above example, the filter response at 40 Hz would be down 18 dB lower than at 80 Hz.
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Old October 27th, 2014, 01:21 PM   #5
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Re: Microphone Hi-Pass filter use

Only a few men with VERY deep voices reach down to 80 Hz.

But many places out in the Real World have ambient, environmental noise prevalent down in that range.
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Old October 27th, 2014, 02:40 PM   #6
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Re: Microphone Hi-Pass filter use

Mr. Greg Miller...the microphone in question is an A-T 4053b, the specs state, "Low Frequency Roll-Off 80Hz High-Pass Filter, Switchable".

Digging a little deeper on the A-T website the A-T 4053b states, "Low Frequency Roll-Off, 80Hz, 12dB/Octave".

Regards,

J.
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Old October 27th, 2014, 02:47 PM   #7
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Re: Microphone Hi-Pass filter use

Is it better to 'filter or Pad' the audio signal at the beginning of the 'audio chain', in this case the microphone, rather then to try and address it in the 'Mixer'? It would seem to make sense, but I may not understand the unforeseen consequences further up the Gain Staging chain.

Looking back, the 'video' portion of my education was a cinch compared the audio. : )

Regards,

J.
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Old October 27th, 2014, 04:23 PM   #8
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Re: Microphone Hi-Pass filter use

Yes it can be better to filter or pad at the mic end as most of the time it puts the circuit before the pre-amp inside the mic so can prevent overloading before it gets to your mixer.
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Old October 27th, 2014, 05:53 PM   #9
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Re: Microphone Hi-Pass filter use

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Originally Posted by Richard Crowley View Post
Only a few men with VERY deep voices reach down to 80 Hz.
Yes. For perspective, 80 Hz is about the second E from the bottom of an 88 key piano. Another way to look at that is the 20th key from the left. Find a piano and play it. It's pretty low.
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Old October 27th, 2014, 09:43 PM   #10
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Re: Microphone Hi-Pass filter use

Messrs. Miller, Watson, Crowley & Nattrass...thank you for your thoughtful and technically astute responses.

As I always say, DVInfo is a wonderful resource.

Best regards,

J.
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Old October 28th, 2014, 01:13 AM   #11
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Re: Microphone Hi-Pass filter use

IIRC, the SD302 mixer also has a 160Hz low-cut filter setting. Use much the same as the 80Hz, but for higher pitched voices... A handy filter for female voices.

It wouldn't be right for many male voices, unless in extreme circumstances.

But, yes, for dialog in most circumstances it's appropriate to start at the mic with the 80Hz rolloff.
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Old October 28th, 2014, 03:45 AM   #12
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Re: Microphone Hi-Pass filter use

For dialogue I tend to leave the 80hz high pass on all the time either at the mic end if it has one or the mixer, a 160hz can be useful if you are having to try and record voices outside in the wind or where there may be lots of LF rumble from air con, distant traffic or other machinery etc.

Voices recorded with the 160hz filter can always be warmed up again in post by adding some LF at 160-250 hz.

Of course it all depends on the mic and the mixer that is being used as they all have slight variations as to the effect of their high pass filters and a decent pair of headphones is essential but you should also be able to see the effect of the filters by looking at your meters whilst just listening to the noise floor that is present.
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Old October 28th, 2014, 05:32 AM   #13
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Re: Microphone Hi-Pass filter use

Bearing in mind all that has been said above about dB per 8ve, knee etc (and I know we are talking recording voices and not music here), but one common pitfall when recording female voice and guitar (or just guitar) is to overlook how low an acoustic guitar actually goes - as it is technically a transposing instrument it sounds an octave lower than most people think with the lowest E string around 82Hz and not 165Hz.

Fortunately it is perfectly possible to convey a musical note from a low instrument without its fundamental frequency (although that may seem counter intuitive) as the harmonics are what gives the note its character, but it is worth remembering all the same when considering HP filter use as it may well affect the quality of the lowest notes.
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Old October 28th, 2014, 09:25 AM   #14
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Re: Microphone Hi-Pass filter use

For dialogue I tend to leave the 80hz high pass on all the time either at the mic end
- Same here. Doubling up the 80Hz filters increases the LF rolloff slightly on the 4053/302 combo.
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Old October 28th, 2014, 12:34 PM   #15
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Re: Microphone Hi-Pass filter use

Mr. Colin McDonald & Mr. Seth Bloombaum...The information on female voices and acoustic guitars is interesting and makes sense.

If I understand you correctly, 'female voices and acoustic guitars', for example, are situations where you might consider using a the 160 Hz Hi-Pass filter?

The SD-302 Mixer has limited (no pun intended) EQ features, which could lend itself to being a little heavy handed with filtering. But, as Mr. Natrass points out, you can 'warm-up' the audio in post. Which is good information.

My intent, however, is to keep things as simple as possible and get the best possible results, maybe not perfect, with little or no audio manipulation in Post. Hence the reason I'm trying to do as much as I can 'up-front' to record a clean signal.

The reason for that is I don't currently have an audio S/W application, nor would I really know how to use it, if I did. My NLE (Vegas Pro) has a rudimentary audio section, 'mixing board', certainly not sophisticated by any stretch.

J.

Last edited by James Kuhn; October 28th, 2014 at 12:35 PM. Reason: Grammer
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