Soundport and Weight Effects on Frequencies

[seeaxe]

This post relates to my first falcate braced guitar, which i have posted about elsewhere.

Having finally sorted out the tapping/VA stuff I decided to go back and reanalyse the guitar.

I had prepared 4 steel weights and bolted them all in to see what difference it made to the sound. Not a huge amount I have to say. However, I also have a soundport on this guitar (I like soundports) so decided to use the opportunity to check out what difference it might make to the response. I have seen a few articles about them but had not seen anything using any definitive measurement.

I made some weights out of 30 by 6 mild steel flat bar, cut them to fit in between the top and bottom linings and drilled them for an M6 bolt. They are about 110gms or so each, weights vary a little. I weighed the guitar for each added weight and subtracted the bare guitar weight to get the total added mass, including the bolt.

I made polystyrene plugs for the soundhole and for the sound port. I also made a half sized plug for the sound port as when I made my template for the soundport cut, I had no idea what difference it would make, I just chose a size I thought looked good. It would be good to check out what difference a smaller sound port would make.

I went through the tap test procedure for the bare guitar and then with each of the four different added masses. For each weight, I tapped with the soundport open, half open and closed. I only measured T1,1, T1,2 and T1,3 for the purposes of this check. I took three lots of taps, recorded peaks from the screen plot and then averaged them. Accuracy of reading the screen was better than 1Hz, but decimal Hz in the averages give a bit of a misleading impression of accuracy – around 1Hz was good enough for me.

Results

Soundhole weight response snip.JPG (81.01 KiB) Viewed 14197 times

Conclusions

No one will be surprised that the T1,1 and T1,3 didn’t vary much with added weight.

The added mass allows me to tune the T1,2 to 190Hz (my desired frequencies were 100, 190 and 240).

The T1,1 varies significantly with and without the soundport. Increasing soundport area increases the T1,1 and this in turn increases the T1,3. It should be possible to fine tune the T1,1 using the size of the sound port. A way of incrementally increasing it while you check frequencies would be a good idea.

If your T1,3 is too high, then adding a soundport will make that worse. I could do with dropping my T1,3 by 3Hz and will look at that another day. Just add some weight to begin with I guess.

Overall a fairly interesting exercise for me. Unfortunately, even though I am almost exactly on my desired frequencies, the guitar doesn’t yet have the brightness and sustain that its conventionally braced predecessor has so I’ll have to look elsewhere for that solution. Next step is new strings.

There’s a combination of added mass that gets me to 101/190 with the soundport half open size, that’s probably close enough for me, so that’s a good result. The soundport is on the righty side and so I need to block it off and make one on the lefty side anyway, so I might as well make it smaller. As a player, I'm not so keen on the added weight but I guess Ill get used to that.

Cheers
Richard

[johnparchem]

Interesting data, I had always planned on playing with the sound port size but never did. My falcate braced classical guitar all have very long sustain relative to my fan braced guitars. I do not use a lower transverse brace on my falcate guitars and I do on fan braced guitars. I am not sure how to think about brightness.

[seeaxe]

Good point about brightness John. Not sure if there's an objective way of measuring that. I had thought that was a function of some of the higher frequency resonances, havent looked in detail at those yet but there does seem to be a decent number of clear peaks further up the graph. The old guitar "sings", the new one doesnt.

Its probably more the lack of sustain thats disappointing (and contributing to the lack of brightness, might even be the same thing) . The newer guitar sounds for about half as long so something is draining that energy and thats what ill go looking for next.

This exercise (plus the monopole mobility check which was fine) has at least convinced me that pulling the top off and starting again wont help.

Onwards and upwards.

[kiwigeo]

Richard that classical is a southpaw?

[seeaxe]

Hi Martin

Its strung the right way (lefty) but i put the soundport in for when i lent it to my guitar teacher, so thats on the wrong ( righty) side. Ill block it off and tear it a new one on the other side.

I need to come up with a clever way of blocking off the soundport so that it can be opened up again if restrung for right handers. I have no lefty kids, they all take after their Mum, the lucky little beggars, and thats likely where it will end up.

[Dave M]

Richard this may or may not be relevant...

John I remember that you ended up using fairly tall falcate braces on your classicals. But I wonder what top thickness you ended up with?

I have a falcate classical sitting in a corner because it turned out so dull and I am planning on thinning the top seriously before possibly going for a retop

[seeaxe]

Thanks Dave, not good to hear but at least some common ground. Have you checked its monopole mobility? Might pay to do that before you pull it apart.

For my one, Ive since blocked off the old sound port, routed new almost elliptical (dont ask, i dont want to talk about it )sound port on the other side. Ive put 200gms of weight back in, redone the bridge and the saddle, making sure i have good bone on wood contact. Its a zero fret guitar so nut is not critical but i made a new one anyway. I havent rechecked frequencies but they will be close to perfect and everything sounds great.................except for the first string. Its not a frequency issue as it lacks sustain all the way up the fret board. Its got me proper hornswaggled. Have tried different strings, same result. When my teacher had it he didnt notice any issue with the 1st string, of course thats now where the 6th string is. Dunno.

Re soundboard weight, From reading other threads though i think the soundboard could be a lot lighter. At 196g its a bit porky if others can build a good sounding steel string with just over 200gms. Main Braces were 10 by 5 ( three plies or maybe 4, cant remember now). The next one i build will have a thinner top and narrower braces.
This has a good MM number but fails the bridge rotation test by a country mile. More dunno.

Im also going to check out what difference top radius makes to the I value. I may be over curving things.

If the next one is ok, then ill just chalk this up to experience, but its really annoying on a classical as a lot of the time the 1st string is carrying the melody and a deadish sound sticks out like the proverbial dogs bollix. If not ill revert to Torres fan bracing and get on with my life.

Cheers and thanks for those comments.