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Topic: Bass boost in microphone recording (Read 6292 times) previous topic - next topic
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Bass boost in microphone recording

Hi,

I'm inquiring about a recording issue for a friend of mine. She uses a condenser microphone attached to a usb audio interface (2 channels typical with instrument/guitar/line/mic inputs) via XLR, and the recordings have a big (upto ~30dB) bass boost at the low end starting from about 200-300Hz downwards. The quieter the low end the bigger the amplification it seems. We can't figure out why. Now I don't want to get into much details just yet, just ask for some general ideas where to look for the cause.

Could it be a audio filter circuit before or after the pre-amps? Or the pre-amps themselves? There's no hardware switch for a bass boost or any other filter on the audio interface. She uses a Mac though, and my hunch is that Mac OS X might have some option to boost microphone inputs just like Windows has. Although I find it odd that something like that would be there in the settings of an external audio interface. Yet the frequency analysis of quiet recordings show a boost in exactly the range where vocals typically are.

Or I guess (some) audio amplifiers tend to have a gain boost at the low and high ends, right? Also the microphone has a bass roll off, turning it on cuts the bass and the recordings sounds thin but the gain boost at the low end is still there. Amplification even seems to be bigger, it's just not rumbling anymore since the microphone didn't pass on the ambient noise in those frequencies to the audio interace. So ambient rumbling becomes a less noticable brown noise.

Now the issue is that the bass boost won't go away when she is recording louder noises or talking, it's a consistent constant rumble / low freq amplification of ambient noise that does not change when recording louder things.

The attachment shows recorded silence, but as I said the low freq ambient rumble is visible and audibly noticable when recording talking and singing, too.

I'm just fishing for general ideas, I mean in case it's a common thing.

Regards

Re: Bass boost in microphone recording

Reply #1 –
Quote from: Fandango on
Hi,

I'm inquiring about a recording issue for a friend of mine. She uses a condenser microphone attached to a usb audio interface (2 channels typical with instrument/guitar/line/mic inputs) via XLR, and the recordings have a big (upto ~30dB) bass boost at the low end starting from about 200-300Hz downwards. The quieter the low end the bigger the amplification it seems. We can't figure out why. Now I don't want to get into much details just yet, just ask for some general ideas where to look for the cause.

Could it be a audio filter circuit before or after the pre-amps? Or the pre-amps themselves? There's no hardware switch for a bass boost or any other filter on the audio interface. She uses a Mac though, and my hunch is that Mac OS X might have some option to boost microphone inputs just like Windows has. Although I find it odd that something like that would be there in the settings of an external audio interface. Yet the frequency analysis of quiet recordings show a boost in exactly the range where vocals typically are.

Or I guess (some) audio amplifiers tend to have a gain boost at the low and high ends, right? Also the microphone has a bass roll off, turning it on cuts the bass and the recordings sounds thin but the gain boost at the low end is still there. Amplification even seems to be bigger, it's just not rumbling anymore since the microphone didn't pass on the ambient noise in those frequencies to the audio interace. So ambient rumbling becomes a less noticable brown noise.

Now the issue is that the bass boost won't go away when she is recording louder noises or talking, it's a consistent constant rumble / low freq amplification of ambient noise that does not change when recording louder things.

The attachment shows recorded silence, but as I said the low freq ambient rumble is visible and audibly noticable when recording talking and singing, too.

I'm just fishing for general ideas, I mean in case it's a common thing.

Looks to me like the usual results of making a recording in a real world room with a microphone that has good bass response.

Most ambient noise occurs at low frequencies due to the nature of its source, and the fact that there is a lot more natural absorbency at higher frequencies.  Also, there may be relevant room resonances.

If you're recording or transmitting voice, add a high pass filter with at least 12 dB/octave slope, and raise the corner frequency as high as you can without ruining the natural timbre of the voice. Expect corner frequencies around 80 Hz or higher.

Re: Bass boost in microphone recording

Reply #2 –
What interface is it?

Re: Bass boost in microphone recording

Reply #3 –
Quote
Or I guess (some) audio amplifiers tend to have a gain boost at the low and high ends, right?
No.   Most preamps/amplifiers are designed to be "flat".

Directional microphones (most "good" mics are directional) exhibit bass-boost when close to the sound source (proximity effect).    Male announcers sometimes like to take advantage of it.    However, this will NOT affect ambient noise which comes from a distance.

Re: Bass boost in microphone recording

Reply #4 –
Ok, we found the source of the noise is actually some electronic device. It might be her laptop. The spectrum analysis had only a window size of 512, after increasing it a hum at 120 Hz was visible. I have a hunch that it's the LCD screen of the laptop maybe (120 Hz / 2 = 60 Hz).

The real ambient noise (sound waves) is definetely an issue, but the problem that is bothering her at the moment is the electromagnetic noise.

Re: Bass boost in microphone recording

Reply #5 –
Does the mic and the interface support using a balanced XLR audio cable?

Using that should take care of a common ground or interference hum and any noise from the PC or noise in the interface itself in the pre-ADC and possibly in the ADC stage too.

These days I always check mics, and if a dynamic mic is not balanced I re-solder it if possible.

And obviously make sure that your OS and drivers do not treat the mic-in as a Voice Chat input as those tend to use AGC or some other massaging on the audio that you really do not want.

Re: Bass boost in microphone recording

Reply #6 –
120 Hz could be a lot of things, starting from mechanical vibration from a 7200 rpm harddrive (maybe in combination with an unlucky room mode, I've seen that problem before). Are you in 50 or 60 Hz mains territory? If the latter, you may run into something power supply related or a ground loop issue, though balanced cabling should normally take care of that.