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Double Bass DB Physics

Champagne

Champagne

Jan 22, 2012
472
31
West Chester, PA
www.woodcreststudio.com
#1
Bare with me here as I am a audio engineer and I play DB...

This morning, over a cup of coffee I was thinking about DB response curves, spectral curves, resonances, etc. I have recorded a dozen upright basses and even more electric basses and know certain notes on certain instruments have higher resonances than others. I can correct all this if it is too drastic with certain techniques to keep things sounding natural.

As I reached the 1/2 way point of my 16oz cup of coffee, it dawned on me:
If I set a DB in front of a subwoofer and mount a piezoelectric sensor on the sound board and sweep a sine wave frequency from 10hz up, I would be able to measure the resonance of the wood with the sweep and chart it. Along with the resonance curve, I should be able to see the complexity and harmonics in the wood. Knowing the sensitivity of the instrument could be a clue to what type of strings the instrument would like. For instance, with lighter more sensitive tops, more slinky/looser tension strings would get that board singing (speculating based on what I have read about string tensions and body types) Cool HUH? To the geek in me, YES!

Now I know that strings, neck projection, bridge height and player are variables, but wouldn't you think if you had the data from the sweep, then had the player play the bass as even as possible (meaning plucking and bowing) compare the results that similarities would be found? I think so, from experience as a player with time spent with my instruments. This brings me to another point: The sound post.

Sound post: If I was to take the sensor and mount it on the bridge, I could do the same sweep tone analysis. With comparing the data with the sweep data from the sound board, I would be able to see the transmission relationship of the sound post in the equation, then the only variable would then be the strings and player, further lessening variables and honing this physics experiment.

Now, lets move the sound post, do readings again and compare, or take the data and make a logical decision of which way to move the sound post for what we are after.

So if this is correct:
http://en.wikipedia.org/wiki/File:Soulpost2.gif

We could take our findings and place the "soul post" in an area what would achieve the balance of what we are after.

So to summarize, and if you followed me to here (kudos for baring with my insanity) this experiment could show us these things:

1) The DB resonances and the complexities of them
2) Wolf notes (maybe?)
3) Sound post transmission characteristics
4) That I am crazy

heh... is that all? Now I am thinking I just used my caffeine up. Well, just some food for thought from a physics/engineer mindset.
 
#3
You are not crazy--- but--- you may wish to consider the work that preceded your 16-oz coffee by centuries. :) For example, start right here.

Your resonance measurements at a specific location would, I am afraid, tell you about that specific location and not much useful about the instrument as a whole. One intuitive way to appreciate that is to consider what occurs when you reverse the process. That is, when you mount a sensor (microphone) at different places on the top plate and play the instrument. The results depend drastically on the location and positions even mm apart can produce radically different overall spectra. See here and here.

There's been quite a bit of work done regarding resonance patterns. It ain't so simple. :)
 
#5
Let empiricism rule. Do the sweeps with the sensor at various locations on the table. See what you get. :) Another important word/concept in all of this: "nonlinearities."
 
#6
drurb said:
You are not crazy--- but--- you may wish to consider the work that preceded your 16-oz coffee by centuries. :) For example, start right here.

Your resonance measurements at a specific location would, I am afraid, tell you about that specific location and not much useful about the instrument as a whole. One intuitive way to appreciate that is to consider what occurs when you reverse the process. That is, when you mount a sensor (microphone) at different places on the top plate and play the instrument. The results depend drastically on the location and positions even mm apart can produce radically different overall spectra. See here and here.

There's been quite a bit of work done regarding resonance patterns. It ain't so simple. :)
Click to expand...

That is fascinating. Thanks for the links. There was no doubt in my mind that this would wind up being much deeper than what I had wrote. I couldn't imagine instrument makers arbitrarily carving wood and thinking "hmm.. that should be good for this one. Let's glue the thing together and see what it sounds like. Hmmm sounds like crap, pull it apart and carve some more."

Those pictures of the resonance there of chladni patterns I came across a number of months ago. I had completely forgotten about them and that opens a whole other can of worms. I wonder if there is a way I would be able to do platter measurements, view and chart them to formulate an overall average. A strobe synced to a sound generator frequency would make this viewable to the naked eye and I suppose a 1000fps camera could see it too. I do see now static point measurements would be way too local due to the organic imperfect nature of wood. It makes perfect sense too since I have installed piezos on peoples instruments before and it is a hunting expedition and even down into micing an instrument where placement is critical to get the sound right in the first place so not to need any corrective shaping in the mix.

As far as nonlinearities go, that is one of the number 1 things most engineers seek out. That is the organic nature of sound that tickles the mind. Interesting... Very interesting.
 
#9
Champagne said:
As far as nonlinearities go, that is one of the number 1 things most engineers seek out. That is the organic nature of sound that tickles the mind. Interesting... Very interesting.
Click to expand...

Yup, nonlinearities are such a big part of my life. :)

Champagne said:
Every time I read that I laugh. :) If you are trying to pin that on me, I will keep wiggling my butt.
Click to expand...

I was being quite serious. The measurements tell the story. Empiricism, in that sense, does rule.

MostlyBass said:
I would do some research first. Check out back issues of ISB (International Society of Bassists) and do some googling. I'm not sure there's been exactly done what you're doing but there's been a lot of studies done.
Click to expand...

Check out the Invalid Link Removed and the Catgut Acoustical Society. You might be interested in this book as well.
 
#10
You are crazy for inviting this much work upon yourself, hours and hour, gallons of coffee, loss of hair and/or sleep... My guess (based solely on conjecture) is you'll find that there are soooo many variables and that each and every test subject will respond very differently and sometime contradict assumptions draw from past observations (example, once though we had cure for all wolf tones involving mode matching and tuning of tailpiece, worked beautifully on a problem specimen. Tried it again on the another bad wolf and got nowhere).

Over this past summer the VSA hosted some workshops at Oberlin college, the double bass Lutherie workshop and the Acoustic workshop coincided and I remember some cross collaboration and data collection was done on basses. I will try to dig up the Principle investigator(s) names. Might look at their methodology (the way I remember it was a Lavalier sized mic mounted in the "center" of the body while a striker "excited" the bridge, aka they hit it with another soundpost, hey data is useful relative to its data set and the PI was really set up for those small squeaky instruments).

Side experiment I would like to see or do someday would be to see responce curves, match with climate conditions, match with soundpost tention mesurments, along with players observations, all over a long amount of time (several seasonal cycles). Prehaps when I win the next powerball...
 
#11
I think the bigger challenge for me is certain test equipment that I may not have, so I will have to hone this daydream contemplation with what I do have. If I win the power ball or lottery I would have a lot more time too. I was given 2 of those scratch tickets for my birthday today that I have yet to scratch off so there is a chance! Realistically, I see messing around with this in mid january after I clear off the table from holiday and work deadlines.

I would appreciate the investigator's names if you do find them. The more I know about what has been done the better my understanding I will have which may just render the data I collect as a food for thought kind of thing.
 
#13
The Oberlin acoustic physics workshops are one of the best places in the world to gain insight into this type of information. It is a whole week locked up in the workshop with some of the headiest nerds in the world showing off their toys and research. You'd be surprised at the level of equipment in use- some of it very expensive, but a lot of it is within the range of everyone. I know I'll be at the next session as a repeat offender!

j.
 

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