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How to get and interpret the results: the tap tones of Modes 2 & 5
First you will need to download and install the Audacity software from the ‘Sourceforge’ download page. This is a superb, free PC program to record and analyze sound picked up by your PC microphone. I always use mono recording as there’s only one mic! There’s also a version for the Apple Mac.
Now record the plate tap tones using this Audacity software by pressing the red button with the mouse pointer. This button is top left in the picture on the right - click on the picture to see it. I usually tap the plate in the right place about 10 times in 5 seconds.
Select the 2 silences at the either end of the waveform with the mouse left hand button pressed and delete them using the delete key. This leaves a good waveform to work with: you may need to adjust the mic sensitivity down so the mic is not overloaded.
Now select all the recorded waveform left in the window using “Control + A” keys pressed simultaneously and go to the ‘Analyze’ option on the top line of the window and select the ‘Plot Spectrum’ menu item that drops down. This calculates the Fast Fourier Transform (the FFT) of the selected waveform: that is it shows all of the resonant frequencies or tap tones present in the sounds picked up my the mic.
You will need to enlarge the ‘Frequency Analysis’ window, and bottom left select ‘Spectrum’, 16,384 (the sample window size), Hanning window, and finally ‘log’ (or logarithmic) display.
I’ve included 2 pictures of typical Audacity windows, one above showing the waveform, and one right showing the FFT screen. Click on them for the full size pictures. In this example of a front (belly) with a good bassbar fitted, Mode 2 peak is at 167 Hz, and, also shown selected with the cursor is the 333 Hz of Mode 5*. What’s so good about Audacity here is that the cursor pops automatically to the nearest peak and shows its frequency under the display, under the purple area bottom left. Be careful to read the ‘peak’ frequency and not the ‘cursor’ frequency! Now write it down - I keep a small transparent folder for each violin to record all results and calculations.
You can find instructions for using Audacity software here, or use a 10-part tutorial on YouTube here. Audacity also allows the export of data to MS Excel (or an OpenOffice spreadsheet) to calculate the energy at various frequencies.. It’s shown as an ‘Export’ button, bottom right of the Freq. Analysis window, and allows a graph of amplitude v. frequency to be drawn in the spreadsheet using the ‘X-Y plot’ option.
I’ve included here on the right a screen-dump showing the tap tones for a belly using some old software called ‘CoolEdit 2000’. It can be found on the web, but is only a 30-day trial. I’ve used it for years, as the FFT still works long after 30 days! It is my favourite waveform capture, with a very good FFT function. You can see here mains humm at 50 Hz (!), and then the Mode 2 peak, and the Mode 5 peak clearly. There are other small peaks too, and these are the other Mode resonance’s of the plate, and apart perhaps from Mode 1, are of little interest.
Note how broad each peak is: a good plate will have a very high, sharp (narrow) Mode 5 peak. The higher and stronger the better.
Another possible waveform/FFT software program is “Visual Analyser 2009 HR” which is also free to use, and has a pretty good real-time spectrum analyser as well as a standard FFT function you apply to a captured waveform. Note that the X-axis for the FFT does not plot to a log scale. The controls are a little quirky, see right.
You will need to use the ‘hold’ function to record a spectrum in real time - it works quite well, and is quick. Erase the spectrum by deselecting (un-ticking) ‘hold!’
Strobe Tuners
A company called Peterson make a range of strobe tuners, best described on this YouTube video. The latest is the ‘StroboSoft’ PC software for about $50. Strobe tuning techniques have been in use since Lloyd A. Loar’s outstanding work in the 1920’s on the Gibson F4 (and F5) mandolin plate tuning. They have a flat or rather carved front and back plate. His work has encouraged plate tuning on all kinds of instruments in the USA ever since, as the F4 and F5’s from that era are truly the ‘strads’ of mandolins! Roger Siminoff is the current US guru on plate tuning, and has published several books.
The advantage of ‘strobe tuners’ is they allow rapid visualisation of the tap tone’s pitch, and also its harmonics: i.e. a frequency and all of its octaves. This may require a compressor (as used with guitars) to stretch the tap tone out.
* Yes! An octave front plate, where Mode 5 is 2 x Mode 2’s frequency! Octave means twice the frequency, and it is what Carleen recommends.
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