Acoustic Tap Test.
Tapping anything with a finger will reveal an universe of frequencies that can easily be captured on a spectrum analyzer. You can use an app on your smart phone and experiment with tapping. The spectrum analyzer from Keuwlsoft @ lhttp://www.keuwl.com let you choose different setting and save your graph as a picture or the data in a CSV file that you can then load in a spreadsheet and do further analyses.
Tapping may look like a mystic experience and at first I was skeptical about its usefulness. I now use a commercial program from Vitrins Technology @https://www.virtins.com/multi-instrument.shtml. which allows me to do tap testing efficiently. Tapping alone will not tell you the tonal quality of the instrument but when natural frequencies are correlated with the construction process it can be useful to the maker to assert different perspectives for further developments.
As I make my harps I can test parts of the harp with a tap and characteries its attributes and see their resultants when they are assembled without and with the strings on. This is what I will be sharing with you with the following graphs. The harp is a Wellspring 36 made with African Mahogany.
The top graph is the oscilloscope view of the sample. The bottom graph is the spectrogram. You can see the frequency scale at the bottom right. Frequency resolution on the bottom left. The 4 boxes on the right are from top to bottom, main frequency, Q factor, damping ratio, log ratio. The Y axis on the spectrogram is the microphone voltage which is related to the sound wave pressure (not Db, what you hear), the X axis is the frequency range selected for display. The settings are flat for weighting, hold for peaks and -3Db bandwidth for the calculations
Single tap test on the box frame
Before sound board glued
After sound board and back glued
After stringing sides of box test
Here we can see how the sides are responding differently as the harp is being assembled. We can see that there is a constant frequncy of 191 Hz on all the graphs which can be identified with the frame.
Single tap test on the Pillar and Neck
Test on Pillar/Neck before assembly to the sound box
Test on Pillar/neck after assembly to the sound box
Pillar Test after stringing
Neck test after stringing
The pillar/neck tap tests were the most surprising to me. This show that they are a significant factor in the ascoustic of the harp. They are rich in low frequencies that give a warm tone to the harp. The pillar and the neck have a dual response when the harp is stringed.
Single tap test on back of sound box
Back test before stringing
Back test after stringing
Here we can see the 3 main frequencies which can be associated with parts of the harp: 120 Hz with the back, 190Hz with the frame and 320Hs with neck/pillar.
Single tap Test on the sound board
Single tap test on sound baord after stringing
The sound board after stringing moved the resonance higher with more differentiations between the main 3 frequencies
Glissando from top to bottom graph
Spectrum unit in microphone voltage more useful for harp maker
Spectrum unit in dB V/Hz (voltage power per 100Hz bandwidth) more useful for musicians. It is more like what you hear.
Download and listen to the glissando HERE.
I kept it in the original format as a wav file so if you have an
acoustic software you may want to analyse it and compare it with my
graphs. See how the graphs tell you more than what you can hear. The
frequency range of the Wellspring 36 is from C2 to C7, that is from
65Hz to 2093Hz.
The first spectrum graph is upto 5KHz width with very sharp sound pressure peaks. It is more usefull to harp makers for identifying wood responses. I use this for my working reference as per my previous graphs.
The second spectrum graph is upt to 10KHz width using the dB V/Hz unit. It it more useful for musician as it is more related to what you hear. It shows very strong hamonics upto 4000Hz, weaker ones upto 9000Hz with a strong voice in the bass as well.
Does this mean it has an pleasant tone?, NO, IT DOES NOT. It takes more than audible capacity to make it a good sounding instrument. This method can not be used to validate a standard of excellence without significant bias.
The point of this is that the tap test can help the harp maker to identify and corrolate parts of the harp with specific freqnencies. Through experimentation and good record keeping the harp maker can see how the engineering of the harp can affect its acoustic. This is a self learning process. There is not text book about it. I encourage you to experiment with it with your smartphone first and then with more advance software if you find there is any value to it. Be curious, have fun with it. HERE is a shot video about it.
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