3/4 DB Natural Frequency of Body

pi_r_squared · #1 03-01-2013, 07:22 PM

Admin, if this ought to go somewhere else, please move it accordingly.

Warning, geeky engineering/physics question ahead.

Started playing a 3/4 DB when a friend asked if I would try it, and have been enjoying it like crazy.

We were messing around with ways to amplify it the other day. The ways we amplified it aren't as interesting as something else we found.

It appears that the first fundamental natural frequency of the body is (nearly exactly) a 73Hz D. This was discovered as we were trying to mitigate feedback.

With any audio pickup we tried, from piezo to a lapel mic in the body, barely touching the D string would set up an almost instant feedback loop that sent the body resonating like crazy, even after the strings were damped. This was also apparent at a perfect fifth above the D (3:2 frequency ratio), not so much at a perfect fifth below the D (makes sense). Messed with this some more with the DB lying down near my amp, looking for frequencies that would "excite" the body.

After everything we tried, it seems that D is the first fundamental natural frequency of the body.

I'm not sure that any of this really matters, but my question is: Is this the result of some serious engineering effort? or more likely just the way things worked out? and if it's intentional, why the D? Maybe it really doesn't matter so much, just fascinated by the physics.

Thanks in Advance.

Hector Wolff · #2 03-02-2013, 10:42 AM

I am no acoustics engineer, but I think that the D string is the most resonant and best sounding string on my bass.

I also notice that when I tune up with with a digital tuner, I can definitely hear a marked increase in volume the moment that the D string snaps into perfect tune.

drurb · #3 03-02-2013, 11:04 AM

Quote:

Originally Posted by pi_r_squared

...This was also apparent at a perfect fifth above the D (3:2 frequency ratio), not so much at a perfect fifth below the D (makes sense).

Why does this make sense to you? A perfect fifth above will not ring the fifth below.

fdeck · #4 03-02-2013, 11:18 AM

I dunno, my bass seems to be most resonant around the A string. The mechanical engineers mess around with an acoustical analysis setup including a "force hammer" and accelerometers, that allows them to map out the resonances of large mechanical objects. I've thought about using similar gear to map out my bass. It would probably produce mountains of interesting data that I would have no hope of interpreting!

drurb · #5 03-02-2013, 11:28 AM

Yes, IIRC, the typical resonance is around the A.

ubassman · #6 03-06-2013, 01:13 AM

A few years back I walked into a violin shop only to see a guy holding a violin and singing into the F holes . Strange guy I thought . He was humming with a low rising 'wooo' tone ( a bit like the noise you make trying to make a scary ghost like sound). He had the strings damped with his fingers so that they wouldnt resonate.

You could hear the natural frequency of the violin body amplify his voice at a certain pitch. He was going through violins till he got one that had the same resonant frequency as one of the strings ( can't remember whether it was the G,D,A, or E -sorry!). The point that he was making was that if a violins natural frequencies where flatter or sharper than the open strings he put it back ...they were dull. The violin he ended up selecting absolutely sang when he started playing - I was impressed!. Seems to be the case that the whole violin was resonating sympathetically with the tuning rather than just being 'ok'.

Since then I make sure that any new basses that I try are firstly accurately tuned to an A at 440hz ('theres an App for that' !) and I have discovered that there can be quite some variety in natural frequency of instruments out there. The basses that I have all resonate around the A - and sound great . Try this at home and you can feel the vibration increase hopefully as you reach the A note with your hand on the back of the bass as you hum.

Can't guarantee you won't get strange looks from others though !

rgarcia26 · #7 03-06-2013, 07:54 AM

When I am playing my BG next to my DB sits the amp, when I play G the G string on DB start to vibrate.... what my bass resonance right?

iiipopes · #8 03-06-2013, 10:10 AM

My CCB has a resonance on the open D string as well. I thought it was due to the cheap construction. Now I know. Thanks.

uprightben · #9 03-06-2013, 11:53 AM

You know that you might be measuring the resonate frequency of the room your in, or the relationship between the bass and the room. I have different problem frequencies in different rooms that I gig in. This is why this kind of thing is normally measured with a hammer and some kind pickup plugged into a computer, you've got to take the room out of the equation.

I've worked on and played basses that ring out more on the A and others that ring out on the D. I wouldn't say that one or the other makes a better sounding bass, just different.

ubassman · #10 03-06-2013, 02:13 PM

Not sure that I agree Ben but I can completely understand your excellent point. Just thinking that the difference is that when you play on the strings the sound fills the room . The natural resonant frequency of the room would definitely try to resonate the bass.

The simple test that I have seen in action is different in that the strings are muted and the note is produced inside the instrument by humming into the f hole rather than filling the room with sound ( its a bit like mouth to mouth resuscitation throughout the F holes !). You can really feel the vibrations with your hand on the back when you hit the right note.

I agree that some ring out at different notes and I certainly am not saying that an A instrument is superior to a D just that an Ab doesn't sound as crisp and clear as a one that has a natural vibe tuned into an A .

Its a simple test that just seems to work! I discovered that I had an Ab instrument and when I retuned the instrument a half tone down from concert A it really sang but was a little muted and choked in concert tuning.

drurb · #11 03-06-2013, 05:07 PM

Quote:

Originally Posted by uprightben

...This is why this kind of thing is normally measured with a hammer and some kind pickup plugged into a computer...

Yep, it's called the impulse response and it would be the way to go about it. To find the resonant frequency, you examine the Fourier transform of the impulse response. Of course, not everyone is set up to do this.

Humming into the f-hole might be more practical.

fdeck · #12 03-06-2013, 08:43 PM

You could always use a spare pickup as an impulse hammer.

robobass · #13 03-07-2013, 02:08 AM

The first time I met David Gage he looked at my bass and in only a few seconds said that my D string was much louder than my A and this was an unhealthy situation. Personally I prefer basses in which all open strings have similar volume.

A simple way to evaluate resonance without a lot of gadgetry would be to take the bass outside away from any walls or structures and have a friend listen from a distance. Don't forget that what you hear standing over the bass or what your pickup might grab is different to what the listener or other players in the group hear.