Design/mechanics question about condenser mics

[Tuneful]

Tuneful, what microphone makes/models have you had hands-on experience with recording ac guitar?

I've mic'd with the usual suspects. Large capsule condeser, tiny radioshack electret, regular dynamic...

For me this isn't at all what's the "best" way to mic an acoustic guitar.
It's about what's a really effective way of capturing the sound with onboard mics, that will be mixed with an already onboard Piezo setup. Nothing more, nothing less.

Look, we all know how cheap it is to actually manufacture the parts that are used in the big$$ acoustic mic systems. The capsule that goes into that $75 solution probably costs a quarter to manufacture.
The exotic ones use multiple solutions, blended at output, just like i'm trying to do. And that equates to big$$ for solutions that may or may not be the best for my guitar.

And one thing I didn't mention that is fairly critical here, is that this is an acoustic electric, not unlike an ES-335 only a little bit thicker. So this thing never did have some sort of great dread sound in the first place. So however the sound/tone winds up being after it's mic'd is probably going to be good enough.

Now does this mean I don't care if it winds up sounding like a tin can? No, of course not.
But feedback is irrelevant to me. And so is using an external mic.

 

[Tuneful]

One mic somewhere on the surface of the lower bout as close as maybe the bridge. Adjust to taste. I don't think adding an upper bout mic will help; if the single mic is in the right place it should work as well as any other mic solution.

Here's a single dual-opamp phantom-powered three-mic circuit with a high-shelf filter on the omnis and a first-order low pass on the soundhole mic (M3), with gain or attenuation as required. If you need a more sophisticated filter on the soundhole mic, you can incorporate that into the feedback loop of U1B.

If you are using battery instead, dump the regulator circuit around Q1. In that case I'd probably reconfigure leaving U1B the same, but changing U1A to an inverting summing amp for all three capsules (U1B feeds U1A along with M1 and M2). Feed all of the capsule FET drains to the outputs rather than the sources to correct the inverted polarity of the summing amp. Actually if you like that better you can do it with phantom power as well, just do an impedance-balanced output instead. But I like the differential input at U1A because it helps cancel induced noise from M1 and M2, even though they are different capsules! For the same reason, you could also change U1B to a differential input amp instead of what I drew, just make it look like U1A but with + and - inputs coming from M3's source and drain.

If you add the piezo to the mix, you would probably want to go the summing amp approach though. Of course you'll need a separate buffer amp for the piezo. You can use another OPA277, but actually another PF5102 (or other FET, but I like that one) should do the trick with much less current.

Well the Piezo already has a pre-amp built into the guitar. But I was actually considering the use of either the Graphtech Ghost pre, which I think has circuitry built in for an extra mic solution.


Did you whip that schematic up on your own???

 

[XLR]

I've mic'd with the usual suspects. Large capsule condeser...

Which ones?

regular dynamic...

Which ones?

OK. Don't answer if you don't want to.

But this is a Sarah Palin answer.

 

[XLR]

It's about what's a really effective way of capturing the sound with onboard mics, that will be mixed with an already onboard Piezo setup.

Are you talking about recording or live sound? "Capturing" usually means recording.

 

[Tuneful]

Oh, and I ran into an interesting tidbit last night.
It seems that the Shure SM57/58 mics use the same capsule. Just different mic body.

So i'm guessing that a lot of the cardioid mics are the same capsule as the omni mics of the same brand/series.


Anyway, that brings up some basic questions about mics.

First, does anyone have a good recommendation for a mic capsule, most likely a small electret that I could use here? And I will be going with omni's, now that I understand the limitations of a cardioid in this instance.

And a change of direction sort of here, but what about using an actual small condenser capsule?
Not at the soundhole area, it would be too big.
But what if I put one say, right under the sound hole, in the middle of the air space. Like on a gooseneck maybe. Is building a small/short body for a capsule not a good idea?

 

[Tuneful]

Are you talking about recording or live sound? "Capturing" usually means recording.

This isn't for the purpose of live sound, but down the road I suppose it could wind up being used for that.

 

[Tuneful]

Which ones? Which ones?

OK. Don't answer if you don't want to.

But this is a Sarah Palin answer.

I've used Shure mics, EV mics, my current condenser is an MXL mic. And of course the radioshack electret mic which is what got me in this mess in the first place, since it actually sounds good.

 

[Harvey Gerst]

I'm wanting to put one inside an acoustic and only pickup the strings, but not the sound from the body.

May I suggest that you put some strings on a simple 1/2" thick piece of wood, put a mic near the strings, and see if that's a sound you really want?

The strings themselves make hardly any noise: they are thin and slip easily through the air without making much of disturbance - and a sound wave is a disturbance of the air. An acoustic guitar would be much quieter without the vibrations of its bridge and body. In an acoustic guitar, the vibration of the string is transferred via the bridge and saddle to the top plate body of the guitar.

The body
The body serves to transmit the vibration of the bridge into vibration of the air around it. For this it needs a relatively large surface area so that it can push a reasonable amount of air backwards and forwards. The top plate is made so that it can vibrate up and down relatively easily. It is usually made of spruce or another light, springy wood, about 2.5 mm thick.

On the inside of the plate is a series of braces. These strengthen the plate. An important function is to keep the plate flat, despite the action of the strings which tends to make the saddle rotate. The braces also affect the way in which the top plate vibrates. The back plate is much less important acoustically for most frequencies, partly because it is held against the player's body. The sides of the guitar do not vibrate much in the direction perpendicular to their surface, and so do not radiate much sound.

The air inside
The air inside the body is quite important, especially for the low range on the instrument. It can vibrate a little like the air in a bottle when you blow across the top. In fact if you sing a note somewhere between F#2 and A2 (it depends on the guitar) while holding your ear close to the sound hole, you will hear the air in the body resonating. This is called the Helmholtz resonance and is introduced below.

Another way to hear the effect of this resonance is to play the open A string and, while it is sounding, move a piece of cardboard or paper back and forth across the sound hole. This stops the resonance (or shifts it to a lower frequency) and you will notice the loss of bass response when you close up the hole. The air inside is also coupled effectively to the lowest resonance of the top plate. Together they give a strong resonance at about an octave above the main air resonance. The air also couples the motion of the top and back plates to some extent.

The Helmholtz resonance of a guitar is due to the air at the soundhole oscillating, driven by the springiness of the air inside the body. I expect that everyone has blown across the top of a bottle and enjoyed the surprisingly low pitched note that results. This lowest guitar resonance is similar. Air is springy: when you compress it, its pressure increases.

Consider a 'lump' of air at the sound hole. If this moves into the body a small distance, it compresses the internal air. That pressure now drives the 'lump' of air out but, when it gets to its original position, its momentum takes it on outside the body a small distance. This rarifies the air inside the body, which then sucks the 'lump' of air back in. It can thus vibrate like a mass on a spring. In practice, it is not just the compression of the air in the body, but also the distension of the body itself which generates the higher pressure.

To complicate things even further, the guitar top moves differently, depending on the note played. To see how the guitar top moves at different notes, there are visual tests called "Chladni Patterns". Here's an example:

http://www.phys.unsw.edu.au/~jw/guitar/patterns_engl.html

Finally, your statement of "It seems that the Shure SM57/58 mics use the same capsule. Just different mic body. So I'm guessing that a lot of the cardioid mics are the same capsule as the omni mics of the same brand/series." is wrong. No vents in an omni, but vents are needed in a cardioid mic. AFAIK, there is no Shure Omni mic that uses an SM57/58 capsule.

 

[XLR]

Which ones?

OK. Don't answer if you don't want to.

But this is a Sarah Palin answer.

I've used Shure mics, EV mics, my current condenser is an MXL mic.

That's still not an answer. That's OK.

 

[Tuneful]

That's still not an answer. That's OK.

I guess i'm just missing the point then. You could just skip the Palin part, and throw a guess out there...

 

[Chili]

May I suggest that you put some strings on a simple 1/2" thick piece of wood, put a mic near the strings, and see if that's a sound you really want?

Thank you Harvey. I suggested this to the OP 40 posts ago by suggesting he mic the strings on an electric guitar. I'm glad you chimed in to add support to the notion that he isn't going to be able to separate the guitar body of an acoustic from the strings.

 

[Tuneful]

May I suggest that you put some strings on a simple 1/2" thick piece of wood, put a mic near the strings, and see if that's a sound you really want?

The strings themselves make hardly any noise: they are thin and slip easily through the air without making much of disturbance - and a sound wave is a disturbance of the air. An acoustic guitar would be much quieter without the vibrations of its bridge and body. In an acoustic guitar, the vibration of the string is transferred via the bridge and saddle to the top plate body of the guitar.

The body
The body serves to transmit the vibration of the bridge into vibration of the air around it. For this it needs a relatively large surface area so that it can push a reasonable amount of air backwards and forwards. The top plate is made so that it can vibrate up and down relatively easily. It is usually made of spruce or another light, springy wood, about 2.5 mm thick.

On the inside of the plate is a series of braces. These strengthen the plate. An important function is to keep the plate flat, despite the action of the strings which tends to make the saddle rotate. The braces also affect the way in which the top plate vibrates. The back plate is much less important acoustically for most frequencies, partly because it is held against the player's body. The sides of the guitar do not vibrate much in the direction perpendicular to their surface, and so do not radiate much sound.

The air inside
The air inside the body is quite important, especially for the low range on the instrument. It can vibrate a little like the air in a bottle when you blow across the top. In fact if you sing a note somewhere between F#2 and A2 (it depends on the guitar) while holding your ear close to the sound hole, you will hear the air in the body resonating. This is called the Helmholtz resonance and is introduced below.

Another way to hear the effect of this resonance is to play the open A string and, while it is sounding, move a piece of cardboard or paper back and forth across the sound hole. This stops the resonance (or shifts it to a lower frequency) and you will notice the loss of bass response when you close up the hole. The air inside is also coupled effectively to the lowest resonance of the top plate. Together they give a strong resonance at about an octave above the main air resonance. The air also couples the motion of the top and back plates to some extent.

The Helmholtz resonance of a guitar is due to the air at the soundhole oscillating, driven by the springiness of the air inside the body. I expect that everyone has blown across the top of a bottle and enjoyed the surprisingly low pitched note that results. This lowest guitar resonance is similar. Air is springy: when you compress it, its pressure increases.

Consider a 'lump' of air at the sound hole. If this moves into the body a small distance, it compresses the internal air. That pressure now drives the 'lump' of air out but, when it gets to its original position, its momentum takes it on outside the body a small distance. This rarifies the air inside the body, which then sucks the 'lump' of air back in. It can thus vibrate like a mass on a spring. In practice, it is not just the compression of the air in the body, but also the distension of the body itself which generates the higher pressure.

To complicate things even further, the guitar top moves differently, depending on the note played. To see how the guitar top moves at different notes, there are visual tests called "Chladni Patterns". Here's an example:

http://www.phys.unsw.edu.au/~jw/guitar/patterns_engl.html

Finally, your statement of "It seems that the Shure SM57/58 mics use the same capsule. Just different mic body. So I'm guessing that a lot of the cardioid mics are the same capsule as the omni mics of the same brand/series." is wrong. No vents in an omni, but vents are needed in a cardioid mic. AFAIK, there is no Shure Omni mic that uses an SM57/58 capsule.

I could bore you to tears with my speaker building background, but suffice it to say that i've dealt with just about every finer point of sound that you can find in a book.
I've read every serious author under the sun, over the course of 25 years or so of speaker building.
D'Appolito to White, and everything in between.

But none of that effects the project here.

If I need to record a proper acoustic, I've got a '69 Mahogony/Spruce Fender acoustic that sounds just lovely on a mic.

Like I said, this thing is more of an acoustic electric.
Barely thicker than a Gibson ES-335.

So if I wind up having a slighty more balanced sound than some of the stuff I could buy, then cool.

If it winds up being slightly less balanced than the stuff I could buy, that's cool too.

But there's companies like L.R. Baggs, that pretty make a business out of sticking mics in every conceivable nook and cranny of acoustic guitars.
So I just don't see what all the fuss is about.

 

[XLR]

I guess i'm just missing the point then.

I believe that understanding a poster's baseline experience with mic's is necessary to communicate with them in any meaningful way about mic's. IME discussing mic's w/people, whatever their level of expertise, if a person has spent much time getting to know how to use a particular mic they usually know its make and model without a second thought. So usually it's no big deal to ask and get a quick simple accurate answer.

 

[Tuneful]

Thank you Harvey. I suggested this to the OP 40 posts ago by suggesting he mic the strings on an electric guitar. I'm glad you chimed in to add support to the notion that he isn't going to be able to separate the guitar body of an acoustic from the strings.

Ah, but see i'm not trying to do that at all.

If you look back to the original question, it is asking whether the ports on a cardioid can be overwhelmed by being inside an acoustic cavity.
This was relevant because it was going to be used with, blended with, a pair of omni mics.
And since the clear answer to the original question is yes, then I won't be using a cardioid.

And that being the case, I don't have the slightest care in the world if the 3x omnis pickup all the tones off of the guitar.

This was never about eliminating the sound of the guitar, it was about the boominess overwhelming one mic out of 3, and making it impossible to blend with the other 2.

 

[Chili]

This was never about eliminating the sound of the guitar, it was about the boominess overwhelming one mic out of 3, and making it impossible to blend with the other 2.

I'm wanting to put one inside an acoustic and only pickup the strings, but not the sound from the body..

I guess I didn't understand the way you worded it.

 

[Tuneful]

I believe that understanding a poster's baseline experience with mic's is necessary to communicate with them in any meaningful way about mic's. IME discussing mic's w/people, whatever their level of expertise, if a person has spent much time getting to know how to use a particular mic they usually know its make and model without a second thought. So usually it's no big deal to ask and get a quick simple accurate answer.

I'm not going to be putting an external mic on this guitar. No ifs ands or buts about it.
So now that i've pointed out how irrelevant that is to this situation, maybe you could proceed?

 

[XLR]

I'm outa here dudes.

/thread unsubscribed/

 

[Tuneful]

I guess I didn't understand the way you worded it.

That comes in later when I point out that two other mics are being used.

The last thing in the world a thread starter should ever do is load up a 1,000 word question. I know speaking for myself, I won't read that long of a question. First of all, in my experience you get a bazillion people who won't read the entire question, and then will load the thread up with answers that don't pertain to the actual issue.

In this case, asking how the cancellation on a cardioid works, was all I really needed to know.
Now I know that no matter what configuration I go with, I can use omnis.

Unfortunately, by trying to stick to cliff notes, it erupted anyway. But that's life in the forums.
I can live with that.




edit*, I just noticed the second part of the quote. I can see where that got the responses it did.
I didn't mean the outside of the body, only the inside. I actually said that a few times elsewhere.

 

[Harvey Gerst]

I could bore you to tears with my speaker building background, but suffice it to say that I've dealt with just about every finer point of sound that you can find in a book. I've read every serious author under the sun, over the course of 25 years or so of speaker building, D'Appolito to White, and everything in between.

But none of that affects the project here.

Having dabbled a bit myself in speaker design and building, I agree that the subject of speaker building is not related to your questions, but the concepts I quoted are. The sound of a guitar comes from two places:

1. A specific area of the guitar top, depending on the note frequency.
2. The body volume and port, which acts like an amplifier for low frequencies.

Speaker cabinets are generally designed for stiffness and freedom from spurious resonances, caused by vibrating cabinet walls. Only the speaker should move, and the air inside the cabinet, tuned by the port.

The strings are most similar to a speaker voice coil, while the guitar top would be more accurately compared to the function of a speaker cone. Removing the speaker cone and just miking the output of a voice coil wouldn't be too useful.

 

[mshilarious]

I could bore you to tears with my speaker building background, but suffice it to say that i've dealt with just about every finer point of sound that you can find in a book.
I've read every serious author under the sun, over the course of 25 years or so of speaker building.
D'Appolito to White, and everything in between.

Yeah? You ever design a speaker that Hendrix used?

Seriously, I don't get you. Sure, an electret capsule costs $0.25, but not at Radio Shack, they charge $2.99. The better capsules cost $1-$2 in quantities of 500, but you probably don't want 500, so figure $2-$3. For your purpose, I would try Panasonic WM64PNT, which would be relatively easy to flush-mount in a fingerboard. I would further load it with the high-shelf cut circuit I drew, because otherwise it will be way too bright.

So if you want to save money on the capsule, why buy a commercial preamp? Markup is the same there. The difference is the engineering of the circuit which was probably done with the limitations of the capsule and the placement in mind.

Anyway, enjoy your project, however it turns out you will probably declare it a success irrespective of how it actually sounds. You have to understand that Baggs' market is based more upon appearance and convenience than the best possible quality sound. And I think your goal of avoiding EQ is very naive. The simple filter circuits I dropped in that schemo are likely not even close to sufficient, just enough to not totally sound like poo.

Again, take your RS capsule and tape it to your soundboard. Try to beat the quality of that sound with whatever other arrangement you can think of. My money's on RS.