Official Luthiers Forum!

Does anyone know of a free, or relatively inexpensive program that will perform real time frequency analysis of an input audio signal?

I'm reading Trevor's books & this is a needed tool. I have Audacity, but it won't do real time, unless I haven't discovered that feature. With Audacity, you have to first record the sound, then select the region to analyze.


Thanks,
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 don't have Trevors book (yet), but why do need real time?

_________________
Jim Watts
http://jameswattsguitars.com

You need to measure the frequency of different vibrational modes of the plates - along the grain, across the grain and twisting. To do this, you need to hold the plate at a node & strike it at an anti-node which seems like it will take some trial & error.

With real time, you can hold, strike and see the results then change your technique to hone in on what you're trying to measure.

If you have to record it first, then select, then analyze, it would be very cumbersome.

Kevin Looker

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

If you want real time then you will need a spectrum analyzer function - I don't know if there is a free version out there. Being in the signal analysis business (for RF) I can say that recorded samples are a lot easier to analyze than real time (IMO).

I have been trying to work through Trevor's methods and I have been using the analysis function with Audacity. That part seems to work well. My problem is getting repeatable results so I can identify the frequencies of interest. Not having much luck there. Be interested to know how others are getting this data.

_________________
Steve Smith
"Music is what feelings sound like"

I just started playing with Spectrum View (it was mentioned in the appendix).

It appears to work, I just have to get used to the interface.

One feature I wish it had is being able to have the cursor stick to the spectrum plot & tell you what the value is. You have to manually position the cursor on the plot.

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 haven't played with that one but most spectrum analysis programs have a peak finder. It's a pretty easy thing to do in the software so I'd be surprised if they didn't put one in.

Edit: I downloaded the IPhone version and it is very limited. There is no option that I can find to identify peak frequencies/amplitude other than reading off the scale on the plot which is not accurate enough to be useful. Might have more functionality on the IPad or a Mac.

_________________
Steve Smith
"Music is what feelings sound like"

what is your opinion on strobosoft? you get both a precise tuner and spectrum analysis. I haven't used it but someone might be of more help explaining the good and bad side of using it

I use the IAnalyzerLite software on my IPhone. It doesn't necessarily do realtime (or it happens so fast you can't get information) but hold the board and strike it and it while recording. Now go back and the peaks will represent the different vibration modes and the software will give you the frequency and decible reading at the peaks. You might have to do this a couple of times holding the board in different spots so one of the vibration modes isn't muted.....but it's not difficult. Some of these frequencies can be low so difficlut to measure (or at least difficult with the built-in IPhone mic).

_________________
Formerly known as Adaboy.......

Mostly I use Audacity (non real time), but the two I've used with real time are Visual Analyser and WaveSurfer.
http://www.sillanumsoft.org/
http://sourceforge.net/projects/wavesurfer/
VA is basically a PC scope, standalone .exe, and has been solid. It has a very handy peak-hold feature. AFAIK, the spectrum analysis follows the Nyquist limit on frequency resolution, so typical resolution is ~2.5 Hz (whereas Audacity and WaveSurfer have some way of resolving peaks finer than the Nyquist limit... I'd like to know the theory on how they do that). Also the spectrum updates about every 2 Hz (not smoothly in time, but maybe there's a setting I haven't found to make it update faster).

The version of Wavesurfer I used a few years ago was a little buggy, but once I got it to work, I liked it. It draws smooth spectrums both in real time and frequency. There's a slight delay from real time (1/2 second?) as it gathers data and maybe averages it to display the finely resolved spectrum.

AFAIK, neither of these have the nifty auto-peak-finder like Audacity.

I haven't tried Spectrum View, so I can't compare these to it. I'd be interested if someone has tried either of the above and finds Spectrum View better.

_________________
David Malicky

Visual Analyser (free)
http://www.sillanumsoft.org/download.htm
Version 9 is, I believe what Trevore uses himself.
Advice/guidance on using it may be found here, or just ask:;
http://www.anzlf.com/viewforum.php?f=33&sid=4390d69d68e74f118cd69c1481f4c6b2

_________________
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.

I tried the IAnalyzer on the IPhone but pretty much couldn't see anything below 100 Hz. Figured maybe it was the IPhone mic. I also tried the mic on my Thinkpad with Audacity but was unable to get consistent results - the highest low-freq peaks for the lengthwise bending mode varied from the 20ish Hz to 60+ Hz. In all cases I've been trying to follow Trevor's instructions carfully.

So what are you guys using to input into the computer. A mic/preamp, the mic on the computer or ??

_________________
Steve Smith
"Music is what feelings sound like"

Personally I'm using a microphone (for drums to pick up lower frequencies better) into a laptop.

_________________
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.

I've got some external mics.

An old Sony ECM 909 that I used with my Walkman Pro.

I also have a Crown PZM with external power supply that I've adapted to use with my laptop.

Kevin Looker

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

You can do it all with VA, but it's not the most convenient for "real-time", for which I prefer G-tune (a guitar tuner with a spectrum analyser), which will give frequency and a picture of the peak it is triggering on. There are probably many other ways of doing the same thing.

For help in setting up VA, see this or this (or both). The frequency resolution on spectrum analysers depends on how you set them up. These instructions are for a set-up with 0.67Hz "buckets".


I haven't found the peak hold. Maybe in a later version than v. 9 that I use. (I tried some of the later versions, but they took me more clicks to get to where I wanted, so I reverted to v9.)

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

http://www.goreguitars.com.au

My old (35 yrs!) Sennheiser mic featured in the book died (probably dropped one too many times...) and I now use a Shure PG57 straight into my Toshiba Tecra laptop with mic boost on. I get cr@p if I try to use its in-built mic, same if I try to use a cheapo electret mic. I think any dynamic mic will probably work fine, but it may be dependant on you computer/sound card.

The main trick to getting clean plots is to do one (and only one) tap per buffer update. Be sure that you don't get a "micro-double-tap" by allowing the hammer to bounce more than just the once off the target.

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

http://www.goreguitars.com.au

Thanks for the advice, I'm determined to get this. Maybe the lousy results I got were due to the laptop mic. I've got a Behringer dynamic mike I'll try as soon as I can find the 1/4" to 1/8" adapter to plug it in. I'll also give VA a try.

_________________
Steve Smith
"Music is what feelings sound like"

I've also found the built-in laptop mics to not be accurate enough. I use a Blue Snowflake or MXL USB.007 (USB condensers) and like them both. The Snowflake is nicely portable. The Samson Go Mic came out more recently and has better specs than the Snowflake, for less $.

Trevor, thanks for the info. I've been using VA 2011, which behaves oddly when I try lowering the sampling frequency below 44kHz (with 16k FFT) -- the main spectrum screen shows "499" for most of the X-scale tick marks (500 Hz max) and the program seems sluggish. Trying it out, though, it does process data normally. I just tried VA 9 like you use and it works just fine with those settings. But that version doesn't like my USB.007 mic. The peak hold feature is on the main window -- "Main" tab on lower right > Y-axis > Hold checkbox. I see it in VA 9.0.6 but I don't know if it works.

So, it sounds like VA 9 and a regular mic is a good combo for real time FFT.

_________________
David Malicky

David, thanks for the heads-up on the "Hold" function.

Kevin, the Frequency meter in VA also has a hold function, which should get you the tap frequencies in real time. The frequency it shows is the one with the highest amplitude in the spectrum, which isn't always the one you want, which is why it's useful to see the spectrum, too, and why G-tune is convenient. G-tune also lets you "bracket" the frequencies of interest, so you can force it to trigger on the peak you want. You can't do that in VA (at least, AFAIK).

If you use the "Hold" feature in VA's spectrum analyser, once you have the screen held, if you click and hold on any peak it will give you the frequency and amplitude of the 11 data points around the peak, with the spectral resolution that you set (usually 0.67Hz).

There seems to be no end to the functionality of these things!

I'll also point out that I'm still using Windows XP which works very reliably, and I guess is why people see quite a lot of variation in performance of these packages as they use different operating systems. And my laptop, though quite old now, is fully optioned and is PDQ.

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

http://www.goreguitars.com.au

When I am measuring the Radiation Ratio I just find the main resonance by moving my pinching hand to the node and tapping at the middle. Then I record several bonks and do an average to find the frequency.

I don't have Trevor's book (although I should), but when measuring the spectrum of a guitar body I always use the record function in Audacity. If you make a bad bonk, you can subtract it out by unselecting that portion of the recording.

_________________
Ken

Trevor , looks like g tune is a plug in and requires a host?? what are you using???

_________________
MK5acoustics.com

I just downloaded VA.

I like it!

Should be more than sufficient.

Thanks.

Kevin Looker

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

Seems there's lots of things called G-Tune these days!

This is the one you want. Full spec. here (half way down the page).

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

http://www.goreguitars.com.au

sure seems to be a few...one vst plug in I downloaded into audacity , but couldn't get it to work

thanks for the link..will give it a try...

I have Peterson's strobosoft downloaded on my old Iphone, well worth the 10 buck I payed for it!!! ...I have since switched to a windows based phone and keep my old Iphone around and charged just to use it as a tuner

_________________
MK5acoustics.com

strangely, the download gives you a executable file that installs a VST instrument (albeit an excellent one), but not GTune. Go figure...

Maybe they´ll fix it in the meantime.

_________________
member of the guild of professional dilettantes

Well, I just followed the instructions on the web page linked above and got a perfect installation of G-tune 2.5. So it's either fixed or there was some "operator error" above!

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

http://www.goreguitars.com.au