harmonic content of acoustic guitar.

[Racherik]

If this is the way you want to go, I suggest firing up a DAW generate a sin wave say 1khz. Play the signal through a studio monitor, record with your chosen mic. Adjust the newly recorded signals time line by 1ms for every foot from the monitor.

Then make printouts of the wave forms, anything that's not a perfect sin wave is distortion.

Rinse and repeat for different frequencies and mics.

The printouts give you something to include when you "write" your report making it look "scientific" enough to BS your way to a degree. Include a few screen captures of spectrum analyzer graphs and your good to go.


Racherik

 

[dintymoore]

... but what if this was something i really enjoyed, and designing speakers/microphones for electrovoice was exactly what i wanted to do in life?...

Then I'd say go for it! Really, when my gut tells me something's of value it doesn't matter what anybody else says. All I have is my views and opinions which could be polar opposites of what's actually right for you.

It's like shoes, what's right for me is almost guaranteed not to be right for you.

I don't mean to discourage you from what your instincts say might be good.

 

[miroslav]

thanks guys.

i'm not sure there'll be an opportunity for live demonstration, but there'll be audio and visual evidence of every recording included with the written work,,

but the problem is

if i say for example, an sm81 more accurately portrays the instrument, than an re20,they're going to say, ' how do you know'?

i mean, sure you use your ears and everyone will know which mic/placement sounds best, but i'm gonna have to prove how accurate the recording has been, by comparing to some kind of factual resource about, or measurement of guitar tone/harmonics.

what would that resource be?

A LIVE DEMONSTRATION!!!


Man...think outside the box...don't just do ANOTHER paper.

See...if you have the guitar and you can actually play it for them....THEN play the mic/recording....there you go.


A+

Guaranteed.

I mean...what you are trying to do is show which mic sounds closest to the *real thing*...right?
Well...you need to first demonstrate how the real thing sounds in order to do that...right?
There's no other way....is there?

 

[Steenamaroo]

sorry miroslav, i think i forgot to say to you, there's no piece of music as such.
it'll probably just be a collection of single notes or whatever.

plus,,,i'm a very very very low standard guitarist!!!

i have to admit, i love your idea, but in terms of ability and skill,it's not for me.

 

[Steenamaroo]

Then I'd say go for it! Really, when my gut tells me something's of value it doesn't matter what anybody else says. All I have is my views and opinions which could be polar opposites of what's actually right for you.

It's like shoes, what's right for me is almost guaranteed not to be right for you.

I don't mean to discourage you from what your instincts say might be good.

no sweat man.
i was sorta goin devil's advocate there, cos it's not something that really really really tickles me....but it could have been.

 

[mshilarious]

From a microphone perspective, this is a trivial problem. If you can afford one (or borrow one from a university lab), you use a NIST-traceable measurement microphone (or whatever the EU equivalent of that is). If you can't, you obtain a second-generation calibration, or failing that, any of the uncalibrated measurement microphones on the market. The frequency response of all such microphones will be essentially flat from 80Hz to about 10kHz, which is about all you can reasonably analyze anyway. Distortion at the sound pressure level generated by an acoustic guitar at 1 foot or so will be well under 1.0% THD, likely less than 0.2%, and will be mostly second-order. Given that guitars have a first harmonic that is many orders of magnitude larger than that, such distortion will be far less than your likely measurement resolution.

That was the easy part. From a guitar perspective, it's rather an interesting question; probably a good one for muttley as that is his specialty. The observed timbre (that is, the relative strength of various overtones) I imagine will vary by mic position. It also varies quite a lot according to how the instrument is played--primarily where it is plucked, and how, etc. So I don't think a objective response for "accuracy" can be stated. The variation in response can be easily measured, however. You can try to control for performance, or you can just create a large sample of notes and average them. Best of all would be to have a bunch of mics so you are analyzing the same performance, but that could get expensive.

A possible methodology would be to measure harmonic content at three positions close-miced (1 foot; lower bout, soundhole, 12th fret), and then at 3 feet aimed at neck/body joint. An anechoic chamber is not really required for this sort of analysis because the contribution of room reflections to measured amplitude of each harmonic is small. But use the largest room you can find anyway.

I would measure single notes across the full range of the guitar (say, play up fifths from bottom to top), allowing each note to decay to -60dB from peak, then create a waterfall graph of the resulting decay. Ideally, you would be able to restate each graph as a function of the overtone series rather than just frequency. Hint: use a pitch-shifting algorithm to normalize all pitches.

Next, you have to create a mean response for each mic position. Note that you will see the largest variation perhaps when you shift from wound to unwound strings. You can ignore that and average them anyway, or treat them separately. An easy way to generate the mean response is to mix all of the observations from each mic position, then you would end up with five waterfall graphs which you could then quantify the difference in overtone response.

I would finally point out that there are software programs such as Wavelab that have all of the tools required to perform this analysis without having to sort through a plethora of VSTs . . .

The great question is whether that will reveal anything different than a simple measurement of variation in frequency response; I don't know the answer to that offhand. I suspect it does; otherwise a recording engineer could simply use a single mic position and use EQ to compensate.

 

[miroslav]

sorry miroslav, i think i forgot to say to you, there's no piece of music as such.
it'll probably just be a collection of single notes or whatever.

plus,,,i'm a very very very low standard guitarist!!!

i have to admit, i love your idea, but in terms of ability and skill,it's not for me.

Don't sell yourself short, man.

You're not going to do a performance...you would be doing a demonstration.

Set up a small PA system (nothing elaborate).
You simply strum a couple of chords live...then play a prerecorded clip of those same couple of chords with Mic 1....then play live again...then clip of Mic 2...etc...etc.

Back that up with a technical paper with all kinds of 8x10 color glossy photographs and mathematical equations as needed.

Without the live demonstration...they will have to *take your word* which mic sounds closest to the real thing...and that doesn't prove anything.

 

[Steenamaroo]

ok,that's more than enough info to chew on!

thanks MSh for taking the time, there's a lot of useful info in there that i can draw on.

also,thanks equally to miroslav the motivational effort

much appreciated.

 

[timtimtim]

hey all.

not sure if this should go here or not, so please feel free to move it.

i'm thinking ahead to studying next year and trying to half plan what i want to do in my head. i'm doing a music production degree and will have to write a dissertation on an area of my choosing.
I've thought about investigating use of a few different microphone types and several microphone positions when recording acoustic guitar,
and how these factors relate to the harmonic content recorded when playing each pitch.

basically, i guess the idea is to 'prove' that a certain mic pointed a certain way, at a certain distance,is the most accurate representation of that particular instrument (within my own research).


the problem that i can't understand is, what would be the benchmark?
every instrument has different tonal qualities, and therefore different harmonic content right? so, what would be an acceptable scientific way of saying "this is how the guitar sounds, and as such, this is what we are aiming to get close to"?

hope that question makes sense!

thanks.

The only benchmark I can think of would be; does this type of mic and mic position near the guitar, give the same sound as the guitar does to my human ear at a "normal" listening distance, but that would be a subjective judgement and not measurable.

 

[grimtraveller]

I guess what I'm saying is that there's so many interesting things you could spend time on and this just isn't one of them.


and even if you did it, in the end... so what? There's no promise that I can see you'd net any real world gain.

I sometimes suspect that 99% of the things we do or have interest in, actually, in the grand scheme of things are utterly irrelevant and worthless. And so what ?!? Who judges their worth ? I took my kids to the pictures today. It won't contribute towards eliminating world starvation !

Steenamaroo, your idea means something to you. Even if every one of us poured cold soup on it, that shouldn't change something that you're thinking of doing.
Go for it. However you do it, do it. There's a neat logic behind the requirement of dissertations....

 

[mshilarious]

I sometimes suspect that 99% of the things we do or have interest in, actually, in the grand scheme of things are utterly irrelevant and worthless. And so what ?!? Who judges their worth ? I took my kids to the pictures today. It won't contribute towards eliminating world starvation !

Steenamaroo, your idea means something to you. Even if every one of us poured cold soup on it, that shouldn't change something that you're thinking of doing.
Go for it. However you do it, do it. There's a neat logic behind the requirement of dissertations....

My undergrad thesis became obsolete as I was writing it, and even if it hadn't it was still a piece of crap in terms of serious research. These things ain't about breaking new ground, they are about learning how to do quantitative research. Grad students do all of the exciting stuff, but they still aren't the ones doing the brainwork, that is the profs. Grad students would get the task of drudging through a couple hundred waterfall graphs quantifying the decay of each overtone

A dissertation includes a review of published research; I think I recall muttley or somebody posting something really sophisticated that mapped the distribution of overtones on an acoustic guitar's soundboard, or something similar. So I would dig up something like that, develop a hypothesis of what one would expect when recording with microphones at various locations based on that research and see if you can verify that hypothesis with your study method. That's about as good as it usually gets in undergrad stuff . . .




. . . I helped one of my classmates on whom I was sweet with her thesis--she was a bio major and was studying the efficacy of bacterial agent additives on waste sludge processing at a brewery's sewage treatment plant. I got to climb up the tanks with her to take pictures I think I would have rather set up some microphones instead . . .

 

[Steenamaroo]

These things ain't about breaking new ground, they are about learning how to do quantitative research.

i'm glad you said that, cos i had seriously started to question my outlook.
my last two years at college have been mostly comprised of doing stuff for the hell of it. it was a music production foundation degree, but we had modules in law, health, research (literally just to learn how to research stuff) etc etc.

i've sorta carried my old opinions with me to uni, where i'm thinking a lot of it is about carrying out the work, the way they want you to, rather than actually coming up with something amazing, or fulfilling your hearts desires.

in the ideal world you'd do a blend of both, but most things that you'd actually want to learn about are going to be completely subjective anyway?!no?

come to think of it, i pretty much spent two years throwing dinty's opinions at the tutors. why is there a compulsory research module at this level of education, when there's a kitted out studio sitting a few rooms down gathering dust?

A dissertation includes a review of published research; I think I recall muttley or somebody posting something really sophisticated that mapped the distribution of overtones on an acoustic guitar's soundboard, or something similar.

that's exactly the kind of thing i could draw on! much appreciated indeed!!!