Design/mechanics question about condenser mics

[Tuneful]

Shortening the scale length? Are you moving the bridge



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

I'm also replacing the bridge. That was why I thought to do that. The neck scale is 25.5, and i'm going to take it to 25 even.

 

[Chili]

If you are going to mount the mic in the position of the blue dot, whether you seal the sides or allow air/sound to flow to the vents, you are in essence, designing a new mic. Who knows what results you're going to get until you try it.

Maybe you should acquire a cheap guitar for experimentation before doing it on this guitar.

 

[Tuneful]

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).

That's what i'm planning on doing, and for exactly that reason. I'm going to mill a slot on the new bridge, and mount a set of stratocaster-style Graphtech Ghost saddles.
And I didn't want to throw too many ducks in the pond on what I thought was a quick question/answer, but i'm also going to put a pair of small electrets, one on each side of the rear corner of the fretboard, also facing up.
The one near the low E will pull in the lowest two strings most efficiently. The one near the high E will pull in the highest two strings most efficiently.
And the one in the middle, right smack dab under the two center strings will pull in those two most efficiently.
I intend to mix the three electrets as a single group, with some eq at the bottom end.
And the piezo saddles will recieve the eq at the top end. And of course, the electrets and piezos will have separate volume so as to be blendable.

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]

If you are going to mount the mic in the position of the blue dot, whether you seal the sides or allow air/sound to flow to the vents, you are in essence, designing a new mic. Who knows what results you're going to get until you try it.

Maybe you should acquire a cheap guitar for experimentation before doing it on this guitar.

Ok, I'll cross out that possibility, thanks.

 

[darrin_h2000]

Thank god theres a bunch of guys in labs with long white coats and pocket protectors who never got laid so we wouldnt have to understand the mechanics behind cardiod capsules.

 

[darrin_h2000]

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?

Why dont you make it easy and look at where Yamaha puts thier mic on thier compass series guitars...Im not sure if any other company uses a real mic inside thier guitars.

 

[mshilarious]

Look, I'm trying not to be a jerk, but what you are talking about designing is a potential phase disaster. Three cardioid mics with partially blocked vents a few inches apart, blending with a pickup? I mean, I don't really know, because not too many people are crazy enough to try that. But if you think about the acoustics of a guitar and how microphones work, it wouldn't be the first solution you'd want to experiment with. Seriously, you are missing most of the tone of the guitar, which is the soundboard.

You are also designing a rather significant circuit that I presume you'd want mounted on your guitar. It would take me probably a day to build that system, and let's just say that I solder more than the average person and I have a lot of very useful PCBs laying around What happens if after a week of circuit design and building, and drilling holes in your new fingerboard (I must say that swapping a fretboard and pulling a bridge is an awful lot of work for a 0.5" difference), it sounds horrible?

My guess is it will work some, but much worse than a single mic mounted somewhere neutral on the soundboard. You may or may not be happy with it, but I strongly recommend you establish a benchmark with a single mic mounted on the soundboard for comparison.

As for your technical questions, no, you won't get very much low-frequency rejection in the proximity zone. This is because rejection depends upon the *same signal* arriving at the front and rear of the diaphragm at the same time. Your problem is that you are conceiving of the bass output from the soundhole as being a point source. That is a reasonable assumption if the soundhole was 3 feet away. If the microphone is IN the soundhole, it's not a very good assumption.

Look, take one of these capsules, aim it away from your mouth, and sing. What do you hear? Here, I just did that experiment for you: this is an AT3060 with my face on the grille, first the front, then the back. Measure the difference in frequency response and sensitivity and you will have your answer.

Proximity Effect

 

[mshilarious]

Thank god theres a bunch of guys in labs with long white coats and pocket protectors who never got laid so we wouldnt have to understand the mechanics behind cardiod capsules.

Or how to spell, for that matter And Harvey Gerst has kids (one is an award-winning engineer) and I do too

 

[XLR]

Tuneful, experimentation is good! Kudos. Go for it. Let us know how it works out.

Your comments remind me, though, of how everything in recording acoustic guitar is only truly learned by personal trial and error, and how terms, specs, etc. are of little value to a person unless those things are also understood by experience with their tactile effect on the sound of the recording.

How a mic's pickup pattern differs from one end of the freq spectrum to the other can be difficult to understand but it sounds like you need to get a handle on that. Not just in theory but in how it sounds with different mic placements, mic types.

It sounds like you need to get a handle on understanding proximity effect too - not just knowing the definition of the term, but how it sounds in different mic placements and types of cardioid mic's. Proximity effect is a big deal with ac guitar.

 

[Tuneful]

mshilarious, First let me say this - I love the hobbyist community. I tend to be a hard driver, with rarely ever a bad intent in my posts. Stubborn to a fault on an idea, until i'm convinced it's not a good one. I've got an extremely good track record of succesful one of a kind projects that i've done. In fact, I consider myself fairly cursed in that I seem to always go for the project that requires a ton of work.
So I can tell you whole heartedly, that I appreciate the time you and everyone else is putting into the posts.

As far as designing circuitry, my kit building days never took me that far.
I'll either copy something that already exists, or buy something.
So if neither exists already, then that will be my queue to modify my plan.

One thing I do know quite well is the physics of sound. That's why I know that if I put a mic on the upper and lower edge of the fretboard, that i'll get a very good balance of the two.
And I know that IF I could overcome the physics of the sound hole, that putting a mic in the middle would give a very good fill to the outside pickups.
Now electrically speaking, phase-wise and whatnot... that's where I need help from you guys. I'm definitely out of my league there, and I'm open to suggestion.

But let me point out that I have seen multiple mic solutions on acoustic with mics inside the body. Condensers, dynamics, piezos on sticky pads, in beams, you name it. There's no mic method that can't yield good results if implemented correctly. Again, this is where I need advice.

As far as the guitar goes, the fretboard/bridge replacement is under way for reasons unrelated to this project. But while i'm at it, I might as well go for the moon, you know?



Now, back to your comments. I have no problem switching over to omni mics to get this done.
In fact, I had actually thought this through, and was planning on using omni mics at the edges of the fretboard in order to take advantage of the boundary effect.
My only reasoning for the cardiod in the soundhole, is to avoid an imbalance between the amount of signal that mic takes in, vs the two on the edge.

By the way, I really like the idea of doing this with the 3 mics. But if it's undoable, I don't want to waste my time. I'm not THAT stubborn.

 

[Tuneful]

Thank god theres a bunch of guys in labs with long white coats and pocket protectors who never got laid so we wouldnt have to understand the mechanics behind cardiod capsules.

I can't help it. I need to know how everything works that I do. I'm that guy that will spend 20 hours or more, making sure that the way I connect the wood for a box, is as strong as is physically possible. Before I ever even make a single cut of wood.

One way or another, when this guitar is done I'll have an extremely good idea of what I was doing.

 

[Tuneful]

Why dont you make it easy and look at where Yamaha puts thier mic on thier compass series guitars...Im not sure if any other company uses a real mic inside thier guitars.

I might do that before this is done. This is that one project guitar, where the sky is the limit.
I'm not ruling out hand winding some small neodymium pickups, to make a small lipstick looking pickup that I could place in the bridge pickup position.
Maybe even a gooseneck pickup or two inside...

This part that i'm dealing with right now, is just my need to know. If it's sound, then I might run with it.
If not, maybe it will wind up being a pair of surface omni mics upper left/lower right. Anything for a possible good solution.

I like the idea of the mics being right off the strings for a hotter signal. I figure it might help me eliminate feedback, if I don't need to boost the signal as much.
And heck, if I have to buy a feedback eliminator, that's ok too.

 

[Tuneful]

Tuneful, experimentation is good! Kudos. Go for it. Let us know how it works out.

Your comments remind me, though, of how everything in recording acoustic guitar is only truly learned by personal trial and error, and how terms, specs, etc. are of little value to a person unless those things are also understood by experience with their tactile effect on the sound of the recording.

How a mic's pickup pattern differs from one end of the freq spectrum to the other can be difficult to understand but it sounds like you need to get a handle on that. Not just in theory but in how it sounds with different mic placements, mic types.
Well, at the very worst I wind up using some prebuilt, very small, one-per-mic pre-amps. That way, I can simply make sure each signal is eq'd to my satisfaction before it gets mixed together.


It sounds like you need to get a handle on understanding proximity effect too - not just knowing the definition of the term, but how it sounds in different mic placements and types of cardioid mic's. Proximity effect is a big deal with ac guitar.

..........

 

[mshilarious]

As far as designing circuitry, my kit building days never took me that far.
I'll either copy something that already exists, or buy something.
So if neither exists already, then that will be my queue to modify my plan.

Blender preamps are common, but I don't know about the three-mic input requirement. That would at least require some consideration of blending the three mics before the commercial blender pre, and you might need to consider filter circuits as part of that circuit.

One thing I do know quite well is the physics of sound. That's why I know that if I put a mic on the upper and lower edge of the fretboard, that i'll get a very good balance of the two.
And I know that IF I could overcome the physics of the sound hole, that putting a mic in the middle would give a very good fill to the outside pickups.

See, I think your problems are acoustic, not electric. The physics of sound say that multiple mics can create phase cancellations. That is ameliorated somewhat with surface-mounted mics, but at the distance you are talking from the strings I wouldn't count on that.

The physics of sound also say that every time you double the number of open mics gain before feedback drops 3dB. Sound engineers generally strive to have as few open mics as possible on a stage. Having multiple mics on a guitar doesn't help; you can compensate by turning all of them down, but the nature of the multiple mic system could mean the system has resonant frequencies that will be more complicated to ring out than a single mic.

The physics of the soundhole are quite simple: it's bassy. You can compensate for that with a filter circuit, but the required correction will be greater with a cardioid mic than an omni. I don't think all of your troubles go away with the omni; the filter circuit is still required, but it's probably 6dB/octave rather than 12dB/octave. And still you have the problem that you aren't capturing the natural sound of the guitar. Sure, you could stick a tiny omni underneath every string and blend them, but that's more akin to an electric guitar sound than an acoustic. If that's what you want use a magnetic pickup.

But let me point out that I have seen multiple mic solutions on acoustic with mics inside the body. Condensers, dynamics, piezos on sticky pads, in beams, you name it. There's no mic method that can't yield good results if implemented correctly. Again, this is where I need advice.

But I can't give you advice unless I build the system and determine how to compensate for its idiosyncrasies electronically. This is not an electronic problem, it's an acoustic problem that requires an electronic solution. Eliminate the acoustic problem and the circuit becomes very very simple.

So sure, anything can be implemented, but does it actually sound better than a different solution? That's why I suggested a control technique for comparison.

Now, back to your comments. I have no problem switching over to omni mics to get this done.
In fact, I had actually thought this through, and was planning on using omni mics at the edges of the fretboard in order to take advantage of the boundary effect.

The boundary effect is not a function of polar pattern, it's an acoustic phenomenon at a surface. There are cardioid boundary mics although most are omni. Given the boundary principle, an omni is perhaps a more obvious choice although anything works.

My only reasoning for the cardiod in the soundhole, is to avoid an imbalance between the amount of signal that mic takes in, vs the two on the edge.

Has more to do with distance from source than rejection. If the mic is flush with the soundhole, the difference in distances from middle strings to side strings would have a larger effect than the cancellation at 30 degrees off-axis, which is practically none. Of course those differences in distance are also potential sources of phase cancellation, but we've discussed that.

Look, your system is quite easy to audition without drilling: buy the three mics you want to use and patch them into a regular ol' mixer. Aim them at your guitar at the same distance they would be installed. We have already learned that the cardioid pattern doesn't matter in terms of the soundhole, so it should sound roughly the same whether aimed at or out from the guitar.

Record that and blend to taste. Note any EQ corrections you have to make and decide how to handle that when you build the preamp.

Compare with two other techniques: one of the mics attached to the lower bout, and a pair of the mics aimed at the 12th fret and lower bout from 18" away. If you can get your system to sound better than (or at least as good as) either of those approaches using a reasonable circuit, then install it in the guitar.

One last thing to consider with these little capsules is that attention must be paid to circuit design to prevent overloading their FET. An acoustic guitar string at 1/2" could be rather loud. This varies a lot by the capsule in question, so again it's hard to make specific recommendations.

 

[Tuneful]

Ok mshilarious, I'm pretty much up to speed with what you're saying here.
I suspect the issue of the signal level loss with multiple mics will prompt me to go with less mics than the three I was thinking about.

So let me try and throw some more stuff/questions your way.

First, my plan is to definitely test out locations, via a piece of butyl underneath each pickup. That way I can easily move them around in whatever increments I need to find the best sweet spots.

As fas as putting a mic on the lower bout, are you speaking about the surface of the bout, or inside the bout?

And what about one on the upper and lower bout corner(surface)?
It seems that there would be a good chance of getting a full and balanced sound that way.

What about internal mics, say on maybe short clip-on goosenecks?

Oh, and the saddle piezos are pretty much a done deal, since the guitar already has a under saddle piezo pre built in.

 

[Tuneful]

edit*
"Has more to do with distance from source than rejection. If the mic is flush with the soundhole, the difference in distances from middle strings to side strings would have a larger effect than the cancellation at 30 degrees off-axis, which is practically none. Of course those differences in distance are also potential sources of phase cancellation, but we've discussed that."

I forgot to point out that the point of the three pickups was to have pairs of two strings that are equally distant from a pickup.
The two side pickups would be one string further from the second and fifth strings.
But so would the middle pickup.
This would have the effect of having two pickups the same distance from the same string, which would provide extra gain for that string.
I'm guessing that it would be nearly a perfect blend of all six strings.

 

[Shadow_7]

This all sounds very problem creating, than solution making IMO. Is there a particular sound that you're going after? Is there some other way to achieve it(steele / nylon / gut strings?)? As someone else said to me, once you make a mic mobile (part of the instrument), invariably that mic is going to wander in front of a speaker and create feedback. Manifest destiny or whatever. At any rate I wouldn't want a mic that close to a guitar. Each string is a different and noticeable distance away from the mic element when you're that close. And other rattlings / rumblings and the likes when it's in physical contact with an instrument being stroked, beaten and strummed. At which point it doesn't really matter how good the mic is and how well placed it is. How do you drill a hole in a guitar and not alter the sound that you were trying to mic in the first place? Somedays I'm just glad I play trombone. Only two viable options, on bell, or on stand.

 

[XLR]

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

 

[mshilarious]

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.

 

[Tuneful]

This all sounds very problem creating, than solution making IMO. Is there a particular sound that you're going after? Is there some other way to achieve it(steele / nylon / gut strings?)? As someone else said to me, once you make a mic mobile (part of the instrument), invariably that mic is going to wander in front of a speaker and create feedback. Manifest destiny or whatever. At any rate I wouldn't want a mic that close to a guitar. Each string is a different and noticeable distance away from the mic element when you're that close. And other rattlings / rumblings and the likes when it's in physical contact with an instrument being stroked, beaten and strummed. At which point it doesn't really matter how good the mic is and how well placed it is. How do you drill a hole in a guitar and not alter the sound that you were trying to mic in the first place? Somedays I'm just glad I play trombone. Only two viable options, on bell, or on stand.

The varying distance from different strings to mic is precisely what i've been wanting to avoid here.
That's why I thought to do the 3x small mics evenly spaced near the edge of the soundhole. I want to avoid heavy eq'ing as much as possible, if not entirely.