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I recently finished reading the Gore / Gilet books (fantastic!) and on my next build I would like to try using their methodology for determining plate thicknesses and managing resonant frequencies. I don’t have a microphone and would like to buy one that is up to the task. Unfortunately, I’m really clueless about audio equipment. Is there such a thing as a microphone that can be plugged directly into a laptop via USB or will I need some other equipment or connections? Can anyone help me understand what I’ll need and perhaps point me to an example? Thanks in advance.

The frequency range evaluated for tap resonances are well within the range of any audio microphone you can buy. Depending on the age of the laptop its internal microphone may work. If all you want is to use it for evaluating tap tones and not higher end audio recording a $20 USB microphone would be fine.

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http://www.Harvestmoonguitars.com

I'm no expert either, but for the same reasons I just bought a $50 (probably overkill) drum microphone from Maplins in UK (similar to Radio Shack)
Drum mic because I wanted to be sure of a good response to low frequencies (down to 50 Hz IIRC) for estimating plate thickness.
Plugged it into the computer and it works really well, nothing else needed.
I also use it for guitar frequency response to taps using Visual Analyzer, again it works well.
Most laptops have a soundcard installed and audio input/output sockets these days, marked with a picture of a mic and headphones.
Check yours out.
An internal mic might work, but I get a clearer response from my external mic than I did with the internal one, and can position it anywhere I want.
If you have one, it doesn't hurt to try it.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Blue snowflake http://bluemic.com/snowflake/. Gets excellent reviews and is right price. Available lots of places including Amazon. I use mine for exactly that purpose. I'm using it with Peterson Strobosoft software -- couldn't get Visual Analyser to work right, but it seems many others use it.

One of my old students took a degree in acoustics from MIT. He said that any common electret mic, like the kind they use in laptops and almost everywhere else, is likely to have a very 'flat' response. Some of the expensive measurement mics are just tweaks of the common electret capsules, combined with a good pre-amp. I gather you're more likely to run into issues from the amp than the mic.

I don't think you need to worry about a flat frequency response. Since we're only diagnosing what the freqs are, and not their amplitude, it shouldn't matter, as long as the mic picks it up.

Seems like an IPhone mic might work with the right software/app. Since you would have to save the file and then review it later it's not real-time but might be fun to set-up too and certainly inexpensive to do since you don't have to buy a mic.

I've been using a Sampson USB mic for some years now with a free software named Audacity. Audacity is available I believe for Mac and PC (primitive computer... ).

If you are a Mac user at least on my Macs the Sampson is not automatically recognized when connected so you have to go to "system preferences", "sound", "input sources" and it appears there, select it and you are good to go.

Trevor is using a Shure PG57
viewtopic.php?f=10101&t=37642&p=493102&hilit=shure#p493102

Which can plug directly into the audio jack on a computer if you have the proper cable.

Kevin Looker

_________________
I'm not a luthier.
I'm just a guy who builds guitars in his basement.
It's better than playing golf.

I'm using a drum mic with a xlr to usb cable
For completed guitar resonances , most anything will do, but If you are using Trevor's method of tap testing for material properties, you need to be able to get down to 40Hz or so and many cheap mics intended for vocals roll off their response below 100Hz or so, often deliberately to avoid rumble.
A friend of mine was trying to do the material analysis with a cheap mic and just could not get it to register, plugged in mine and clear as a day

Anyone else using a condenser like a Rode NT2A? I bought it for recording vocals years ago, so I've used it for response curves.

Thanks to everyone for the information. Much appreciated.

Tim - thank you for the Blue Snowflake suggestion. I'm leaning in that direction because, as you said, it's reasonably priced, gets good reviews and is rated down to 35Hz, which seems lower than most.

You want a "reference microphone" if you want to accurately identify frequencies. These are special mics used usually to evaluate a room or instrument for its sonic proclivities. Microphones are generally designed for a purpose which can involve highlighting or attenuating certain frequencies, sound pressure levels, and pickup patterns. If you want to analyze frequencies I wouldn't use a mic that was designed for vocals or drums when reference mics are built for this very purpose.

A good reference mic is going to have a flat frequency response from 20 to 20k hertz. The better the quality the flatter the response at high frequency. It doesn't have to cost an arm and a leg. If you get one make sure it has a foam padded case and make sure you store it in the case.

http://www.sweetwater.com/insync/reference-microphone/

It gets a little complicated. Reference mics are usually designed to work at a certain sound pressure levels (SPL). A mic is designed to analyze drums might not be the best choice for guitars. If you look at these notice that some of them are labeled as "drum" or "vocal" reference mics. This really means that the mic was more designed for the SPL delivered by those particular sources. Look for one labeled "instrument" or "room". Those will probably be designed for lower SPL.

After researching this a while... http://realtraps.com/art_microphones.htm ...it looks like the best value would be the Nady CM100 at about $40 ... http://www.nady.com/cm100.html

Don't be misled by those charts at Realtraps. They don't represent the frequency response of the mic per se. They represent the source in an open room (I'm pretty sure). The real frequency response of a good reference mic will be flat flat flat .... this is the chart for the Nady mic ... http://www.nady.com/manuals/microphones/cm100.pdf

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I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

Again, I don't believe you need anything like that. Reference mics are used for doing room sweeps to find the amplitude differences between frequencies and stuff like that. For example finding out what freqs are higher or lower (or doubled or canceled etc.) from the mix position to make better mixing decisions. We're not trying to do that. We are not measuring response curves to find relative amplitudes, just frequency peaks.

Therefore, any mic that can pick up the freqs will do. Even the ones with low end roll off, cause a roll off is not a cut off.

That being said, 40$ seems a decent price to pay. Myself, I use my iPhone or iPad, but as of yet have never needed to measure down to 40hz...

Trevor has a PHD, wrote the book, and uses the Shure PG57.

It's relatively cheap, rugged & plugs into my laptop using this cable.

http://www.sweetwater.com/store/detail/XVM115F/

Kevin Looker

_________________
I'm not a luthier.
I'm just a guy who builds guitars in his basement.
It's better than playing golf.

Hey Charlie, So this is your first go with a "scientific" approach? Being a user of the time tested "seat of the pants" approach I would be curious to know how the treatment of the top will differ from how you would have treated it in the past when you get to that point.

Thanks,
Danny

This says it all really! It'll be the next mic I buy.

For better or worse I ordered the Blue Snowflake this morning because it was a bit less expensive than the Shur but has a wider frequency response range (35 - 20,000 Hz for the Blue versus 50 - 15,000 Hz for the Shur). I will report back after I receive it and have had a chance to use it a bit in case anyone is interested.

Thanks again for all the input guys!

Hey Danny, I've been a "seat of the pants" kinda guy too but want to give the Gore /Gilet methods a try on a short scale dreadnought I'll be starting soon. I'll start a build thread in the doc-u-build section when I get started. Wish me luck.

Charlie: If it helps put your mind at ease, I've also got a PhD, have read the books, and have programmed Trevor's equations into an Excel table (similar to Table 4.5-3) where I store and compare all my tops and backs. Been doing this for almost a year.

I purchased Peterson's StroboSoft and the Snowflake some years ago when I was looking at Siminoff's Tap Tuning methods. I looked at Audacity, G-tune and Visual Analyser (all free, I think), but found the tap response averaging in StroboSoft Deluxe to be ideally suited for vibration mode measurements. I can get comparable results with the mic in my laptop, but the Snowflake makes it much easier to position consistently where I'm holding and tapping plates. Your tapping tool is probably more critical -- mine is very hi-tech: a drafting eraser stuck on the end of a long shafted electrician's screwdriver.

It is important to understand that Trevor doesn't claim his parameters lead to an "ideal" thickness, rather they allow you to generate equivalent vibrational stiffness across a variety of individual plates with different physical properties. His vibrational stiffness constants are those he's found work well for his instruments and bracing techniques. I found them a little thick compared to my experience, so adjusted the constants downward a tad.

Tim – thank you, it does give me confidence knowing you are having success with the snowflake and have been using it for years. It looks like a nice design and I like the idea of being able to hang it on my laptop monitor while tapping.

You’ve piqued my interest in StroboSoft. Does it produce the same style of graphical output as is shown in Trevor’s books?

Thanks for the clarification on the vibrational stiffness constants. It will be interesting to see how using Trevor’s works out for my first application of them, which will be a short scale dreadnought. I’ll take good notes so I can make adjustments on future builds if need be.

Thanks again.

The short answer is "yes". Trevor uses an old version of G-tune, at least for his target plate thicknesses. That software has been discontinued and the company now recommends Peterson's StroboSoft: http://www.petersontuners.com/index.cfm?category=143. You'll need the "deluxe" version 2 in order to access the tap-averaging capability, which automatically rejects "outliers" from bad taps. It currently sells for $99.99, but you can trial a full version for 2 weeks for free. Peterson gives registered users free upgrades, but I've only ever seen one upgrade release for this product. The website claims that it is NOT 64-bit compatible -- which simply means that 64-bit systems need to install in a compatible mode -- and it runs fine. (Like many others here, I also use their iPhone app for tuning and for intonation.)

A response I got from Trevor early on when I was questioning some of the data I was generating might be of interest:

G-Tune is still available from here, for those interested. JHC call it a semi-functional free version, but it seems that it can be registered and updated to full functionality.

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

That's the part I couldn't figure out. Presumably if you were registered before the relationship was formed with Peterson, you have a functional version and can get access to updates, but I never figured out how to register G-Tune as a new user. I already had a StroboSoft license anyway, so didn't pursue it any further.

Tim – thank you for the info on StroboSoft. I think I’m going to give the free Visual Analyzer a try but will keep the StroboSoft in mind in case VA doesn’t work out. Thanks, too, for sharing the quote from Trevor on wood properties. Very insightful!

Trevor – thank you for the G-Tune link, and the books!!

Off topic, I know, but Tim quoted Trevor as saying:
"The Red (especially) and the Sitka are much denser (>30%) than the Engelmann/Lutz, so to hit what is essentially a modal frequency target (which is what my formulae do) they have to be made stiffer, therefore thicker, therefore heavier, therefore thicker...."

I thickness to a target stiffness, more or less, and let the frequency float (more or less). Denser softwood does have higher Young's modulus in general, so I usually make those tops thinner. They do end up heavier, of course, but not all that much heavier unless they're outliers, with lower than expected E values along the grain relative to the density. I don't see enormous deviations in frequencies.

I do my stiffness and damping tests the old way. Plates are set on foam pads and a signal generator is used to find the fundamental bending modes along and across the grain. The motion of a small piece of iron stuck to the top, usually in the center of the end, is read with a magnet and coil driving a DVM (MV-AC) to find the peak frequencies and 3dB down points. These are then used to calculate the E and Q values in the usual way.

This takes a bit more time than simply tapping and running an FFT on the computer. One thing that repays it is that you can see when the bending mode shapes are distorted, due to another mode close in pitch, or changing runout in the piece, for example. These things don't always show up in a tap spectrum. Basically, when the node lines are curved you're looking at a combination of properties of two stiffness along and across the grain, so the numbers are not exactly what you think. Looking at the modes won't disentangle them, of course, but at least you know when things are not what they might seem.

Also, it's wise to keep in mind that the equations we use to calculate the E values are limited in accuracy. For one thing, of course, you can't get results more accurate than your least accurate measurement. I use a metric dial caliper for thickness, and can interpolate to maybe .5mm, or 10% of a 5mm thick piece. On top of that, the equations themselves are simplified to make them solvable. According to one researcher they actually only give you a result that it within about 10% of the 'real' value, even if your measurements are accurate. There are other issues as well.

The point is that those numbers you get are better than nothing, by a long shot, but not etched in stone. The more care you put in, the better and more useful the numbers are likely to be, but they will never be definitive.

In light of that one thing I was told when I started out in the measurement game takes on more meaning. What's going to hurt you in taking these measurements will be really egregious narrow-band deviations in the apparatus. That is; if the mic has a very strong peak or dip at a particular frequency, and the resonance you're looking at is in that range, you're going to get bad data. However, any mic that does that is probably going to sound terrible when you use it to record normal speech or music. So; just record something and play it back. If it sounds OK, then the mic is probably good enough.