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Fretless Jazz Bass Frequency Responce Project

K

kwelaman

Nov 25, 2008
5
0
Pittsburgh, PA
Disclosures
ME undergrad U PITT graduating at the end of the year- JOBS?
#1
Hey,

I've been lurking this site for quite a while now but never really had anything particularly interesting to say.

Until now...

I am a Mechanical Engineering undergrad a the university of pittsburgh. I got permission from one of my professors to do a little side project for some extra credit. It's a mechanical measurements class so it's pretty pertinent to course material.

This Saturday I'm going to go in and record the voltage coming from my MIM jazz in the Lab. It has a pretty bad dead spot around the C or D on the G string (might make myself a little amp from an op amp to get some more signal and filter to keep out aliasing). Once I have the data I'm going to do FFTs for each note over the period of time. I want to get an idea of what the response for various frequencies is at various times.

The dead spot is a pretty complex phenomenon... It seems to have to do with both the location and frequency of the note played. My hunch is that the bass is acting as some sort of high or low pass filter (with harmonic damping in the neck) but I want to look at the harmonics/ response to find out.

I'm also sort of assuming when I pluck the strings it is essentially the same as a unit impulse. The neck is basically a spring (of length that varies with note position) and the body/bassist is a damper.

I have a feeling that the shape of the bass lends itself to low frequencies- ie is a sort of low pass filter with a cutoff of like... 1000(?) because that is what the response sort of sounds like to my ears. Any way I think it's really cool. If any body is interested I can post my data/ results.

:D
 
#6
Hooray! I'm an ME also, and have always wondered about this. In my experiences, most basses have a dead spot in the same area. I also have noticed that changing the weight of the headstock moves it up or down! (try putting a small C-clamp on the headstock), so I suspect neck damping has a lot to do with it...
 
#7
somewhere on talkbass is a monster thread devoted to 3-d frequency charts of recorded bass notes, showing the relative strength of the various harmonics and how they change over the duration of the note.

very informative, even though it reveals the painful truth that even the deepest, fattest bass tone usually has very little of the true fundamental in it.
 
#9
yes!

you just beat me back to it, i had actually found it again by searching through your old posts! :)

"waterfall plots" is apparently the term for the charts you guys were doing. (and hat's off to you for those)
 
#10
Yup I have seen those graphs and that link... I think that's probably what got the idea in my head...

What I want to see is basically that... but looking at it for each note surrounding the dead spot on my fender :eyebrow:

I'm curious to see if the dead spot is filtering out all the frequencies or just the fundamentals... and what that looks like over time...

The vibrometer link is cool but I'm not convinced that captures the whole picture- I think the fact that your fretting the string and changing the length of the string has quite a bit to do with it.

I'm sort of mulling over in my mind how to model the system. I guess it's basically 2 springs (the string and the neck) plus some damping (the bass).

I want to look at the frequency response and the time responses to see if that is the case. I think I might also de-tune the g string and see what the response for a different note at the same place/ same note at a different place looks like.

I think if I can get a handle on what the responses really look like I can begin to get an idea of how to model the system... or the responses could be forms that are just totally beyond my understanding of dynamics (that would be a disappointment).

Can you guys believe I'm actually getting points in a class to do this...

I think this is the most interesting school has been since I started (and I'm a senior!)

Oh btw I'm going to check out what all this stuff looks like fretting the notes with my fingers and fretting the notes with a capo (took me a WHILE to dig up a capo... I really kind of hate those things...)
 
#11
It's been pretty slow going. The Lab equipment I used had way way to low of a sample rate and aliased badly. That data was useless. I then recorded my bass with a digital recorder at like 78K and got nice data. Unfortunately the only program I really know how to use for data analysis is matlab and it's having a really hard time with so much (relatively speaking) data. I made a few plots but to get them to actually run in a reasonable amount of time to see if everything was working I cut off the frequency at around 500HZ :scowl:

So... the higher harmonics that I really wanted to analyze around the dead spot on my bass still aren't there. Also I haven't gotten the scales on the axis of the graph quite right yet.

Anyway it's a start...

And waterfall graphs look freaking sweet :bassist:
 

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#12
Good first cut data on your D! Keep us posted as you can get some more data. Good catch on the aliasing too. You might have to find the biggest machine around to crank the data with matlab, or just let it run, but that's cool. There are Invalid Link Removed to matlab, but I doubt any of them are appreciably faster.
 
#13
OK Kwelaman....I graduated in the first couple of Engineering classes out of Benedum Hall (only with the EE's) in 1972. Didn't have much work upon graduating by a worse gap than you should be facing. Just retired today after 38 years in the Engineering business. You'll find something. Be flexible and remember you can always move on if it doesn't work out.

A dead spot has got to be some kind of "intonation" problem...the energy gets canceled instead of sustaining. You might try putting a strobe light on that while it's playing notes and sneak up to the dead spot from above and below it and watch what's going on.

BTW, what happened to the Steelers Sunday? One of my special pleasures here in the Boston area is pullin' for the Steel City boys vs. the Patriots. Not this time, tho!
 
#14
Yea... Though about the strobe light idea... Is a good one- I want to start with seein just how exactly the frequencies are being attenuated... My professor has a powerful computer I should have access to sometime soon >:)
 

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