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Knuckle Tapping


Michael_Molnar

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Fred Nichy asked that I post this for him. So please direct your questions to him. - Mike

I’ve never learned much from knuckle tapping an instrument, but would like to describe a recent incident tapping the back of a viola I made that resulted in a wolf tone-like response, similar to a bowed wolf note. This is a viola I made in june’03. Out of curiosity I tapped two other selected violas I made in Mar’01 and Dec’01 and they seemed to have different responses when tapped, so I recorded the tap sounds using Soundsnip, a sound analyzing software program. Results are shown on the attachment. Each instrument was tuned, the taps were made in the lower bout on the bass side roughly in the middle of the cheek where the wolf note was first detected. The instrument was held by the neck and strings were damped with fingers. The pick-up mic used was small enough to fit in an S-hole which was suspended outside near the tap area. Typically 2 or 3 taps were made in a run to get a representative one to examine. For example, on March’03 tap printout, 3 taps are shown in the horizontal timeline section and the tap with the patchwork lines over it is the one selected to show the frequency composition of the tap by analysis with Soundsnip. Looking at just the horizontal sound input in the graph of Jun’03, it shows the sound from a tap is broken up into segments of a pulse of sound, no sound, sound, etc, until it fades. It sounds very similar to a bowed wolf note. Removing the strings also removed the wolf note.

The data recorded from tapping Dec01 differs from taps on the others by being a very long continuous sound as shown by the long black input. When this instrument is bowed the after-ring on the lower strings is very long, lasting around 10 seconds. In contrast, sound from a tap on the March’01 viola has virtually no after ring, appearing as a brief burst from a tap. Also, it was necessary to tap harder to get a loudness response similar to the other two instruments.

The following are some comments about these instruments:-

A professional violist who plays a Guarneri and in the past played many of my instruments thought the Mar’01 viola was my best. Interesting that this is the one with barely any after ring. Recently a young player who is trying to get a career in an orchestra played the three instruments and selected Dec’01 as the one she liked, saying the sound was “chocolatey “ and had greater projection. Measured by a db meter it does have a slightly greater projection. The top also had about 400 holes drilled inside the top plate, the top thickness where drilled was ca 1.5mm. The holes drilled in the area of the lower bout were 5mm diameter, the rest 3mm, and a strip about 1½ inches along the midline almost from block to block was not drilled. I had forgot I did this until I removed it to try another top. Also, I had experimented with trying to tune the back by tapping and removing wood. This was not a good instrument when made, so I suspect the long after-ring and playing quality are the result of these later modifications. Just a guess. The remaining viola, June’03 with the wolf note-like tap sound has a quick response, bright sounding and no evidence of a wolf note when played. A violinist of our quartet liked this viola very much, probably because it plays somewhat like a violin.

Outside of finding a wolf note in a knuckle tap I wonder what is gained if someone collects such data on instruments they make. I know little about acoustics so hopefully somebody might have some comments. Thanks.

- Fred Nichy

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Fred Nichy asked that I post this for him. So please direct your questions to him. - Mike

The top also had about 400 holes drilled inside the top plate, the top thickness where drilled was ca 1.5mm. The holes drilled in the area of the lower bout were 5mm diameter, the rest 3mm, and a strip about 1½ inches along the midline almost from block to block was not drilled. I had forgot I did this until I removed it to try another top.

- Fred Nichy

I have read in a recent VSA journal about a technique called 'ribbing' ( I think ) where a bit of soft wood is removed on the inside of the top plate between the hard grain lines with a fine pointed scrapper. This technique still maintains the stiffness while removing a bit of mass. I never heard about the idea of removing wood by drilling dimples into the top. Interesting idea.

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I might tap for open seams, cracks, a quick check for wood thicknesses in haste or desperation or general boredom. Tap Tap.

If your plate is covered in dimples then it's likely you set the depth stop on your drill press to the wrong depth before thicknessing and you're looking for an excuse to 'hide the dimples'.

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I might tap for open seams, cracks, a quick check for wood thicknesses in haste or desperation or general boredom. Tap Tap.

If your plate is covered in dimples then it's likely you set the depth stop on your drill press to the wrong depth before thicknessing and you're looking for an excuse to 'hide the dimples'.

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Hi Ben- I did what you're mentioning while final depth-drilling on another inst and then had a mind change to leave the lower bout thicker so wound up with the lower inside like it got hit with number 4 bird shot. This plate with ca 400 holes was supposed have a drilled hole in every centimeter so that you would have a layer of plate 1.5mm thick and a structural network, the thicker portion. I know, wierd, but i couldn't resist doing it. I'm pretty sure i glued it on to test to see it would collapse, but then forgot. I'm pretty sure i can double the number of drilled holes and will do that shortly. I'll record some hammer tap tones before and after. fred

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+++++++++

Knuckle tapping for me is to tell how badly a crack is, not to know how good a violin is .

How good a violin is I have to play it to know. Why trust anything else.

I'm sorry i didn't stress that it was a way to collect data from the completed inst before varnishing. The Dec'01 inst was a poor inst when made, and if i had kept track of what i did afterwards it would have been valuable because the inst improved significantly. fred

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