High Pass (low cut) Filter On Microphone (on or off?)

[karma17]

Relatively new to field audio recording, and was just wondering if most use the high pass filter on the microphone. I've read in different forums and there never seems to be much of a consensus.

Some say absolutely turn it on because why would you want to record something that you don't want, for instance, low end rumble or wind noise.

I have read others to say they want the total soundscape and will deal with what they don't like later. But similar to raw in video, want everything.

What would you recommend or suggest?

 

[Joshua_G]

It depends on what and where you are recording.

For recording dialog outdoors, or indoors where there is, or there may be, loud low frequency noise - better turn the mic's LPF (Low Pass Filter) on. This can be done even just to be on the safe side. On human speaking voice, usually there is no useful information below 80Hz.

Though the low frequencies can be cut, or attenuated, in post - for one, not all people can, or want, to manipulate the audio track(s) in post. Second, loud noise may overload the recording preamp - which cannot be repaired in post.

For recording music - better turn the mic's LPF off. This way you'll capture the full tonal range, including the very low frequencies that some musical instruments may contain. Also, turning the mic's LPF off ensures linear frequency response on the low frequencies.

The same applies to recording voice, especially singing, in a studio, or in quiet and acoustically controlled environment.

 

[Jim Feeley]

Joshua's approach sounds good (<-- ha! get it? "sounds good" I crack me up).

Typical high-pass filters in mics designed for field use start rolling off low frequencies at 80Hz, and don't have a total drop off at 80Hz; rather the lower the frequency, the greater the cut. There's not a whole lot of activity in human dialog below 85Hz. So you're not going to lose much, if anything, dialog-wise. And avoiding all the lower-frequency handling rumbles, wind noise, etc, is a very good thing.

And anecdotally, it seems that cutting out that low-frequency noise/content helps cameras and cheap audio recorders create better audio tracks...Rather than waste bits and cycles encoding crap you don't want, they can spend more bits and cycles encoding stuff (e.g., dialog) you do want. But I haven't carefully tested this, and it doesn't seem to be an issue with good modern audio recorders such as those from Zaxcom and Sound Devices (sorry; I'm less familiar with Zoom recorders...I'd guess the F8 and F4 at least have the bandwidth to handle everything)....but what's the downside? Pretty much nothing.

Then you have cleaner tracks from post from the get-go.

But music, sound fx, and nature recordings, you probably don't want to use the mic's HPF...if, as Joshua says you have enough control over extraneous noise.

 

[karma17]

Thank you very much.

I really appreciate the thoughtful answers.

If I may ask one other question that's related.

On the recorder itself, you also have the option to cut lower end frequencies.

If you cut them with the microphone, do you still need to cut them on the recorder?

Or in other words, is there any logic in attempting to cut the low end on the microphone, then cutting it again in the recorder?

Just trying to have some method to my madness.

Thanks again.

 

[Rick R]

I flat out don't like the sound of HPFs on budget recorders at all, and they are usually in the digital stage, after the preamp. It's preferable to have them as far upstream (close to the mic capsule) as possible. I often use secondary HPF on my SD gear, though it's effect is negligible if around the same frequency as the mic's filter.

 

[ggrantly]

I only have a mic or two with low pass options. But I have always wondered what the best option is between:

Engage mic filter and SD mixer filter
Engage only SD mixer filter
Engage mic filter but not mixer

I've never tested this. Generally, I've just used the mixer filter and called it good. I'd be curious to here other opinions on this.

Grant

 

[Bruce Watson]

If you cut them with the microphone, do you still need to cut them on the recorder?

Typically, no. If I'm only going to use one, I typically use the one on the mic. But with some mixers / recorders like the Sound Devices gear has really good HPF too, so it may not matter which you use in that case.

Or in other words, is there any logic in attempting to cut the low end on the microphone, then cutting it again in the recorder?

Yes. Rarely. It makes some level of sense if both are rolling off at the same frequency -- in that case the two will "add" and you'll get a steeper roll off. For example, say the mic has an 80 Hz LPF that rolls off at -12 dB/octave. And your mixer has a similar HPF, 80 Hz at -12 dB/octave. Together this gives you 80 Hz at -24 dB/octave which is a nice steep roll off. Steep enough it may sound unnatural if there's not something there that really merits that steep a curve. So... not a default way of working. At least not for me.

Another consideration, if the two HPF have different "shoulder" frequencies, using both may make your total response curve get pretty funky where the two different shoulder frequencies are -- the curve may get "bumpy" and you may get interesting phasing artifacts as well. If in doubt, just use a single HPF.

Practical example: The one time I've done this, I got stuck with a bad room in a hotel to do some video interviews in. Right next to the room that held the air handler for the floor. Nasty boomy LF noise, most right at 60 Hz. I did the normal things trying to get a better room, which predictably amounted to nothing. Then I walked the floor of that room listening for room modes. I located a reasonable null where it didn't sound as loud as the rest of the room and set the "interview" chair there. Arranged some plants and furniture around to make a nice looking set for the video. Then mounted an AT 4053b on a boom pole, on a c-stand with a boom pole holder, so that it would be as close to the speaker as possible while still being just out of frame for the video. Turned on the HPF on the mic (80 Hz, -12 dB/octave). XLR cables into my tiny little SD MixPre-D, and turned on it's HPF (also 80 Hz, -12 dB/octave). End result was really pretty good. Just a trace of HVAC noise while all the dialog was perfectly clear. Happy and impressed client (who understood what they'd put me through), which is always a good thing.

 

[GaryNattrass]

Always best to use it on the mic as it is in circuit between the capsule and on board mic amp, this also goes for any -10db pad too as it can prevent overload at the front end.

ON for speech or outside use and off for music or full bandwidth recording and inside static use. I personally tend to leave the 80hz high pass on most of the time and some mics such as the AT875r already have it in circuit all the time.

High pass on mixers can be useful too annd tend to be more severe if an 80hz or 160hz option is available but once again this can help as it prevents unwanted LF from overloading the mic amps on the mixer or camera.

80hz tends to be good for general speech and 160hz for use in difficult situations such as high wind etc.

As a rule and when working as a dubbing mixer or sound supervisor at SKY etc I would tend to roll off at 80-100hz for any speech anyway and if I had bad location speech then use up to 160hz to get rid of any unwanted LF noise.

 

[karma17]

Thanks so much for explaining it so well. Really helps me.

 

[IronFilm]

And anecdotally, it seems that cutting out that low-frequency noise/content helps cameras and cheap audio recorders create better audio tracks...Rather than waste bits and cycles encoding crap you don't want, they can spend more bits and cycles encoding stuff (e.g., dialog) you do want. But I haven't carefully tested this, and it doesn't seem to be an issue with good modern audio recorders such as those from Zaxcom and Sound Devices (sorry; I'm less familiar with Zoom recorders...I'd guess the F8 and F4 at least have the bandwidth to handle everything)....but what's the downside? Pretty much nothing.

That seems very very unlikely to me. Not even with some very low end recorders such as say a Tascam DR60D (easily found for around a hundred bucks secondhand).

But just *MAYBE* way back in the past when the low end recorders couldn't do better than recording to low bit rate MP3 files, then maybe this line of logic made sense. However I can't see this having any relevance to today in nearly 2018.