Official Luthiers Forum!

Hi all, I've just finished a 4 string acoustic bass. I'm getting a warble/whistle when I play particular notes, which at times is very clear, then at other times disappears almost completely. I'm hoping you can help me establish what the cause is, and whether I can do anything about it. Here's a link to the recording (for some reason OLF wouldn't let m upload either m4a or mp3 files....)

https://drive.google.com/file/d/1BO2EIT ... drive_link

The bass is very responsive. The Monopole Mobility is 21 which seems very high, though interesting the bridge rotation is only 1.4 degrees even with D'Addario Chrome flatwounds which are pretty high tension.

The Air, Top and Back resonant frequencies are as follows:

82, 164, 226

I note a couple of things. The Air resonance is exactly half the top resonance. The Back resonance is somewhere between A and Bb, which is where the warble tends to occur.

Strangely, the warble seems to come and go. Sometimes it will have disappeared almost completely, other times it's very clear, like in the recording above.

I have tried blocking off the sound hole, to see if the 82/164 is the problem. The resonant frequencies were as follows:
- , 155, 214
But the warble was still there.

I've also tried adding 19g of putty to the bridge (soundhole uncovered). Again the resonances moved, as follows:
82, 155, 226
But the warble was still there. Or was sometimes still there, then would disappear.

The guitar is newly built, so obviously I'm hoping it's not a loose brace, or badly glued bridge. There's nothing obviously rattling when I shake it!

Does anyone have any thought about how to establish where the problem lies, and what to do about it? If anything. Acoustic basses are obviously super sensitive to wolf notes and mis-matched resonant frequencies. That's how I build them, so I've only got myself to blame

Edit: I should say I've already checked that the saddle and nut are flat-bottomed, and changed the strings.

I made a little "drum stick", a ball of cheese cloth wrapped around the end of a piece of dowel to tap tops and backs for voicing and for looking for something like a loose brace. I'd make one up and tap all over the top and back and see if you hear anything rattle. I also use a set of tuning forks similarly, energize them and press the handle end around on the top/back and see if something sets off your offensive note. It might help find the culprit and give you something to deal with.

I couldn't get access to the GDrive file.

Good luck!

ps -- Make sure there is a little tension on the truss rod and nothing else is loose like tuner parts, etc.

_________________
Brian R, Wood Mechanic
N8ZED

Which 'particular notes'? Is it simply pitch related, or does it happen only on certain strings/frets? Can you think of anything that correlates with the 'coming and going', such as temperature, humidity, time of day, phase of the moon, wholesale price index, or the height of skirts? All the usual variables....

I had a guitar that had a sort of 'warble' on the open G string due to a near-match with the 'main top' resonance. It was subtle: I didn't notice it until I saw the variation in a spectrogram; after that I could hear it. I took the guitar to a convention where I gave a talk on 'wolf' notes, but in the lower humidity the resonant pitch changed, and the warble went away.

For sure check for "back buzz", where the portion of a fretted string between the nut and finger can rattle against other frets if the nut slot and neck relief are too low. Sometimes it will activate by sympathetic resonance when other strings are played. The rattle seems to only activate on higher amplitude plucks in the recording, which fits with the string needing to swing far enough to make contact with the frets.

If you have any side braces butt jointed to top braces, I've had those rattle before. The recording does have a "woody" sound to it, so bracing is a definite possibility. Did you try thumping the box all over while you had the strings off?

Could be truss rod rattle, but I haven't personally encountered that one so I'm not sure how to test for it.

The thick windings on heavy gauge strings are more prone to rattling against the nut and saddle than guitar gauges, but if it's on the same note after changing strings then that's unlikely.

It sounds like string harmonics. It is in tune, and doesn’t sound bad, it just isn’t supposed to be there! Does it on different strings? If you change the string does it still do it?

Ah. You did change strings. It doesn't sound like rattling. I had a cello that made a crunchy rattle with a fast decline, that I could hear when plucking string very hard. That was the nut.

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Why be normal?

I listend to your clip of the buzz note. It sounds exactly what I ran into a few years ago, I had a repair in that involved new frets and repair a small crack. It needed new tuners but that was not part of the repair.
I had almost exactly the same buzzing tone on specific strings. I had the guitar an extra week trying to find what it was.
It turned out to be one of the ferruels on the tuner. I didn't notice it at the time but it didn't fit as tightly as the others and was the cause of the buzz.
I am not saying that is what it is but you should check small things like a strap button, pickguard , screws and such.

Hope you find it, I know first hand how frustrating it is.

Bob