Design/mechanics question about condenser mics

[Tuneful]

On a cardioid condenser mic, there are some sort of vents behind the main capsule surface.

Do those vents pickup sound?

I'm wanting to put one inside an acoustic and only pickup the strings, but not the sound from the body.
But if the capsule picks up sound from the rear, then I probably can't do that.

 

[gecko zzed]

On a cardioid condenser mic, there are some sort of vents behind the main capsule surface.

Do those vents pickup sound?

I'm wanting to put one inside an acoustic and only pickup the strings, but not the sound from the body.
But if the capsule picks up sound from the rear, then I probably can't do that.

The vents are what determine (in part) the mike's pick-up pattern (cardiod, omni, etc.)

Try one and see what happens.

There is a current crop of acoustic guitars that have bridge transducers and internal mikes.

I'd rater use an external mike.

 

[Tuneful]

The vents are what determine (in part) the mike's pick-up pattern (cardiod, omni, etc.)

Try one and see what happens.

There is a current crop of acoustic guitars that have bridge transducers and internal mikes.

I'd rater use an external mike.

Well i've read that previously, but I'm not making sense out of it.
Cardioid's record a small range in front of the capsule. Is the information from the rear cancelled out somehow?
The issue here, is that i'm using a piezo saddle arrangement, and I only want to add the condenser at the soundhole to add some accuracy to the string sound. But I don't want to deal with the boominess from the sound cavity, in any way.

 

[darrin_h2000]

Thats why most of the time they are miced at the 12th fret.

 

[mshilarious]

Well i've read that previously, but I'm not making sense out of it.
Cardioid's record a small range in front of the capsule. Is the information from the rear cancelled out somehow?

Yes. There is a very detailed explanation in the sticky threads at the top of this forum.

 

[Tuneful]

Yes. There is a very detailed explanation in the sticky threads at the top of this forum.

That was the first place I looked. I do know that a cardioid is supposed to pick up the sound from a very tight area in front of the capsule. I've seen the polar patterns.
I know that outside of the 130 degree or so area in front of the mic, cancellation occurs.
But that still doesn't answer what would happen from the boom sound of an acoustic when it hits those vents, much less taking into consideration the wave form as it develops outwards from the guitar body, through the sound hole, and wraps around the front of the mic.
There's plenty that could be going on here, that I just haven't seen addressed anywhere.

 

[XLR]

That was the first place I looked. I do know that a cardioid is supposed to pick up the sound from a very tight area in front of the capsule. I've seen the polar patterns.
I know that outside of the 130 degree or so area in front of the mic, cancellation occurs.
But that still doesn't answer what would happen from the boom sound of an acoustic when it hits those vents, much less taking into consideration the wave form as it develops outwards from the guitar body, through the sound hole, and wraps around the front of the mic.
There's plenty that could be going on here, that I just haven't seen addressed anywhere.

Lower frequencies are less directional than higher freq. Check the pickup pattern on the spec sheet of your mic and see if it shows directionality by frequency range.

 

[Chili]

I'm wanting to put one inside an acoustic and only pickup the strings, but not the sound from the body.
But if the capsule picks up sound from the rear, then I probably can't do that.

Yeah, I don't think you're going to be able to do that. The body is what produces all the sound. Try micing the strings of an electric if you want to mic just the strings.

 

[Tuneful]

Lower frequencies are less directional than higher freq. Check the pickup pattern on the spec sheet of your mic and see if it shows directionality by frequency range.

What i'm trying to figure out, is the mechanics of how a cardioid mic actually cancels out boundary sounds.

Does it simply cancel, electrically, all of the pressure from the back of the capsule?
If that's the case, then it just might be able to eliminate the boom from the acoustic body.
But...
It seems to me that that the way it works could be a simple matter of it being less sensitive to sounds emanating from the rear of the mic. In a normal room situation, those sounds would be much lower in volume than the signal source that is directly in front of the mic.
In which case, all would be fine. Except if I put it inside an acoustic guitar soundhole, where the sound pressure level from the inside cavity seem to be louder than the strings.

 

[Chili]

Well, here's my best layman's explanation.

The vents on a single diaphram cardiod mic act like a labyrinth. It forces the sound wave to travel a longer distance which causes a slight delay to reach the backside of the diaphram. A sound wave is a cycle of compressed and expanded air molecules. When the compressed part hits the front of the diaphram it forces the diaphram to the move inwards. The same wave continues around the sides and through the vents and hits the backside of the diaphram a some point later in time, probably at the same time the expanded portion of the wave is reaching the front of the diaphram and moves it back outwards. So, the wave gets converted to an electrical signal.

When a wave comes from the sides or back, it hits the labyrinth first and then travels around to the front. So, with the time delay of the labyrinth and the time delay of the going all the way around the sides to the front, the wave is hitting the diaphram on the front and back side at the same time, cancelling itself out.

The time delay of the labyrinth is dependent on the wavelength of the frequency, so, the effect is more noticeable with higher freqs. This has something to do with the proximty effect of cardiod mics. I'm not sure if I fully understand it to explain it, but I believe it's because the compressed and expnaded portions of the wave are larger in low freqs vs. higher freqs, and somehow negates the time delay. So, when you get close to a cardiod mic it will improve low end response.

Somebody help me out if I've got it wrong!!!

 

[Tuneful]

Huh. So it is a physical cancellation as opposed to electrical.

I wonder if the physics of sitting in a soundhole with could screw that up somehow.

I might have to run some sort of waveform software so I can see what the mic is doing externally and just above the soundhole, and then inside the soundhole facing out.



For what it's worth, these are the two mic capsules i'm thinking of using at the present time.

Shure R184B

50 - 17,000hz
output noise - 27db
5-10vdc
output impedance 1,800ohms

OR:

Takstar HM-502 overhead microphone

50 - 16,000hz
9v - 52v phantom power
output impedance 2,000 ohm

 

[XLR]

My point was that if you position the mic in the guitar's near field - or /winces/ inside the guitar - the directionality of the cardioid pattern is going to give you less control of low frequencies than you may think it will. Plus, a cardioid's proximity effect is going to be a bigger problem with the lows than anything else you're asking about.

If I was going to try what you're suggesting, as an experiment, I'd use omni mic's. No proximity effect.

 

[mshilarious]

What i'm trying to figure out, is the mechanics of how a cardioid mic actually cancels out boundary sounds.

Interesting and useful to know, but what is most important about your application is that a cardioid mic crammed in a soundhole will sound boomy. Proximity effect of a mic placed in the port of an enclosure. What do you think is going to happen?

Does it simply cancel, electrically, all of the pressure from the back of the capsule?

Single-diaphragm cardioid mics cancel acoustically, not electrically, via a delay chamber. That doesn't work in the nearfield because of the amplitude difference of the signal incident on the front and rear (through the delay chamber) of the capsule.

If that's the case, then it just might be able to eliminate the boom from the acoustic body.

Nope.

 

[Tuneful]

My point was that if you position the mic in the guitar's near field - or /winces/ inside the guitar - the directionality of the cardioid pattern is going to give you less control of low frequencies than you may think it will. Plus, a cardioid's proximity effect is going to be a bigger problem with the lows than anything else you're asking about.

If I was going to try what you're suggesting, as an experiment, I'd use omni mic's. No proximity effect.

But with an omni mic, there's no doubt at all that the mic will take a big hit of the interior boominess.

 

[philbagg]

Not sure if this has already been answered (in a hurry, can't read ).

The vents are what cancel the sound from the sides and behind.
The sound from the rear hits the diaphragm at the front and the back (via the vents), so the diaphragm does not move in relation to the rear sound.
Sound from the front only hits the front of the diaphragm, not the rear, so the diaphragm moves freely in relation to that sound.

 

[Tuneful]

Interesting and useful to know, but what is most important about your application is that a cardioid mic crammed in a soundhole will sound boomy. Proximity effect of a mic placed in the port of an enclosure. What do you think is going to happen?

I'm not sure if I explained this properly, but the mic will be facing out. Literally a quarter inch from the center two guitar strings.




Single-diaphragm cardioid mics cancel acoustically, not electrically, via a delay chamber. That doesn't work in the nearfield because of the amplitude difference of the signal incident on the front and rear (through the delay chamber) of the capsule.

If i'm following Chili's explanation correctly, wouldn't sound coming from the rear of the mic be more easily canceled out?

And I'm actually counting on some of the wave form from the body to exit the soundhole and wrap around the front of the capsule.
My only concern is that I wouldn't want a ton of it.


Nope.

...........

 

[mshilarious]

But with an omni mic, there's no doubt at all that the mic will take a big hit of the interior boominess.

There is also no doubt that a cardioid will have more. Cardioid mics:

- aren't as directional at low frequencies;
- have a *rising* bass response in the nearfield.

The soundhole is a port, it's not where "all the sound comes out", just the low frequency sounds. Try this experiment: put your ear next to the soundhole. Play a low E, then a high E. Repeat with your ear near the lower bout. There is little difference in the high E, yes? But the low E is quite different. The natural sound of the guitar is a combination of the two, but more soundboard than soundhole.

The soundboard also acts as a boundary surface for a soundboard-mounted mic, which increases the sensitivity of the mic across the entire spectrum (dependent upon the size of the boundary, but practically speaking for a guitar).

The soundhole-mount mic systems you see have more to do with convenience and appearance than physics. That said, I imagine there is quite a lot of compensation designed into either their capsule or electronics. Also those are typically systems designed to blend with a piezo. So you could rely on the piezo for lows and roll off lows massively in the mic, while still avoiding the piezo quack on the highs (again through careful circuit design & filtration).

I would prefer a piezo combined with a soundboard-mounted mic, using the piezo only to the extent it is necessary to avoid feedback.

In a studio I wouldn't put a mic closer than 1' to any part of the guitar.

 

[Tuneful]

Let me fill in some details on this guitar, and maybe you guys can fill in some blanks for me.

If you look at the pic, the red dot is where the mic will go, facing up.

Now, I happen to be replacing the fretboard on this guitar, and i'm going to a shorter scale.
The effect of that, is that if I want, I can either shorten the back of the fretboard, or I could possibly keep it the same length, and have a good inch of blank fretboard that I could drill into and mount the mic inside. (The blue dot)

This brings up a question.
What effect if any, would there be to blocking the side chamber on such a mic?

 

[mshilarious]

Let me fill in some details on this guitar, and maybe you guys can fill in some blanks for me.

If you look at the pic, the red dot is where the mic will go, facing up.

Now, I happen to be replacing the fretboard on this guitar, and i'm going to a shorter scale.
The effect of that, is that if I want, I can either shorten the back of the fretboard, or I could possibly keep it the same length, and have a good inch of blank fretboard that I could drill into and mount the mic inside. (The blue dot)

Shortening the scale length? Are you moving the bridge

This brings up a question.
What effect if any, would there be to blocking the side chamber on such a mic?

You will turn it into an omnidirectional mic with a large resonant peak, likely somewhere in the midrange.

 

[Tuneful]

Not sure if this has already been answered (in a hurry, can't read ).

The vents are what cancel the sound from the sides and behind.
The sound from the rear hits the diaphragm at the front and the back (via the vents), so the diaphragm does not move in relation to the rear sound.
Sound from the front only hits the front of the diaphragm, not the rear, so the diaphragm moves freely in relation to that sound.

So with the mic facing out of the soundhole, wouldn't that make it less likely that the bass from the cavity would get added to the mic?