Condenser mic question

Wailin

I have a paired set of stereo mics for recording guitar (t.bone SC140) which are giving decent enough results for a cheap mic. They didnt come with instructions and there are 2 switches on them that I'm not sure what they're for. First switch is close to the mic tip and has 3 positions labeled: _, then
-10db and -20db. What exactly is this for? (Yes I'm new to home recording)
The second switch also has 3 positions: -, followed by 75 and then 150. Again, what is this for?
Cheers guys.

JuneBug29

The first switches are pads. They reduce the signal by 10 decibels and 20 decibels, depending on which is selected.

The second switches are high pass (or low cut) switches. If you select 75, everything below 75 hz is cut out of the signal. Same with 150 for 150 hz.

The _ option on both leaves the signal untouched

TelePaul

Chances are your acoustic wont produce anything below 80hz anyways, or so I'm inclined to think...maybe someone can correct me if I'm mistaken.

TroutMask

Those HPFs (high-pass filters) don't 'block' everything below the cutoff frequency. The Fc (cutoff frequency) is the frequency below which (in the case of a high pass filter) the filter begins to attenuate. The rate at which it 'cuts off' is determined by the slope of the filter, and this is expressed in dB per Octave or dB/octave. A gentle slope is typically set at 6dB/octave, and you will hear substantial low frequency information below the Fc. A more 'steep' filter might have a response of 18dB/octave or 24dB/octave, and with this type of filter you will hear much less low frequency information below the Fc. The 'per octave' bit describes how much attenuation occurs after the cutoff frequency is crossed as you go down in frequency. In the case of a HPF set at 440 Hz with a slope of 6dB/octave, a frequency component at 220 Hz will be 6dB quieter. Still there, though not as apparent.
Only specialised filters such as 'anti-aliasing' filters used in DACs (digital to analogue converters) have very steep (almost rectangular) slopes. Even these are not perfect, hence the CD sample rate of 44.1 kHz which allows a little 'wriggle room' for the idiosyncracies of errant anti-aliasing filters.
So, it depends on the filtering circuit employed in your microphone, but you can take it that if you use HPFs there will often be frequency information below the cutoff point - and the amplitude of this information depends on the slope of the filter (and these are set by the manufacturer). :pac:

madtheory

Troutmask is right. Actually the most common HPF in a mic is a simple RC filter, which has a slope of only 6dB. This is almost always tailored to compensate for typical proximity effect on a close miked voice.

TelePaul wrote: »

Chances are your acoustic wont produce anything below 80hz anyways, or so I'm inclined to think...maybe someone can correct me if I'm mistaken.

Sort of. Room modes and the proximity effect of a directional mic will almost always conspire to emphasize frequencies at or below 80Hz. Your ears will tell you.

TroutMask

Until you get a DADGAD player - then your low D fundamental will be 73.4 Hz.
I prefer to do my rolling off on the channel, preferably with a sweepable HPF. Unless I'm live, and/or if the mic HPF is set at a more logical frequency than the available console EQ HPF

Wailin

Thanks guys, I haven't a clue what most of it means, not up to date with mic specs, Hz, etc! My ears can tell me what sounds best though and I'll just experiment until i find the best settings. I seem to get alot of sliding noises wheni move my fingers up and down the frets that comes out much louder than what it actually is, might have something to do with mic position I guess. Anyway, cheers for the advice.

TelePaul

TroutMask wrote: »

Those HPFs (high-pass filters) don't 'block' everything below the cutoff frequency. The Fc (cutoff frequency) is the frequency below which (in the case of a high pass filter) the filter begins to attenuate. The rate at which it 'cuts off' is determined by the slope of the filter, and this is expressed in dB per Octave or dB/octave. A gentle slope is typically set at 6dB/octave, and you will hear substantial low frequency information below the Fc. A more 'steep' filter might have a response of 18dB/octave or 24dB/octave, and with this type of filter you will hear much less low frequency information below the Fc. The 'per octave' bit describes how much attenuation occurs after the cutoff frequency is crossed as you go down in frequency. In the case of a HPF set at 440 Hz with a slope of 6dB/octave, a frequency component at 220 Hz will be 6dB quieter. Still there, though not as apparent.
Only specialised filters such as 'anti-aliasing' filters used in DACs (digital to analogue converters) have very steep (almost rectangular) slopes. Even these are not perfect, hence the CD sample rate of 44.1 kHz which allows a little 'wriggle room' for the idiosyncracies of errant anti-aliasing filters.
So, it depends on the filtering circuit employed in your microphone, but you can take it that if you use HPFs there will often be frequency information below the cutoff point - and the amplitude of this information depends on the slope of the filter (and these are set by the manufacturer). :pac:

Really interesting stuff, thanks!

TroutMask

Why thank you! :pac:

nickcave

TelePaul wrote: »

Chances are your acoustic wont produce anything below 80hz anyways, or so I'm inclined to think...maybe someone can correct me if I'm mistaken.

I've noticed that a high-pass filter can be useful when playing acoustic guitar (standard tuning), particularly when close-mic'ing. I'm not sure where those lower frequencies come form (subharmonics perhaps) but I'm just going on what my ears tell me.

PaulBrewer

It all depends on the track with acoustics - been said many times but it is true.

To add to Telepaul's comment your acoustic could well be producing sub 80hz - imagine the guitar with out the strings, as a wooden box it could well be producing low thuds from palm contact.

But as TP alludes to it may not be of musical value.

In fact, depending on the track for something dense musically one might well use a HP Filter to clean out sub 100Hz , 150 or even 200 in order to find it's place in a mix.

When done well the acoustic has the illusion of being 'full' but it's primarily other instruments that are 'filling in' that area below the HPF.

Similarly one might use a LPF to stop the acoustic taking up room in the hi hat, cymbals and top of vocals region.

tweeky

Good post Paul, foot thuds and the rumble of the number 6 bus are also good woofer fodder, especially for the home recorder.