Kinky Stardust (Acoustic Guitar Pickup)

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  Twins?

This page is about developing my very own acoustic guitar pickup design.

Lately I have been recording songs with my guitar and vocals. I used (above average quality) dynamic microphones and the results with that setup weren't very satisfying. There are several reason for that: There is very significant cross-talk between the two microphones when recording simultaneously, it is very susceptible to background noise and the handling is cumbersome (both mics need to be well aligned to both my guitar and my mouth). Usually those problems are mitigated (except for the background noise) by recording the instrumental first and then the vocals. But since i also would like to be able to do amplified live performances there is only one solution (i can see): using an acoustic guitar pickup. Well, and since they are rather expensive, I'm a hacker and lately have started to build my own audio equipment (e.g. a dual channel mic preamp), i decided to hack my own acoustic guitar pickup.

The Name: "Kinky Stardust"

Well, the name of the pickup is due to the resemblance of the mounted pickup to an eye patch. And especially on my white guitar it reminded me of "Ziggy Stardust" the fictional character by David Bowie which had his face painted white and wore an eye patch. The prefix "Kinky" is kind of an reiteration since Ziggy Stardust is an embodiment of kinkiness but I still felt I wanted to emphasize it, because the elastic strap used for mounting the pickup is a re-purposed sex toy made from re-purposed bike parts (i think it was made by the people behind http://bikesexual.blogsport.eu though i couldn't find that specific toy). I think it is worth noting that all the parts used are re-purposed parts i found in the whateverlab except for the audio jacks and the piezo.

My specifications for a good pickup

1. Good sound quality: Very low noise, a rather analogous and easy to correct frequency response, no artifacts (like the infamous piezo-quack), etc.
2. Easy to mount, without any modification to the guitar (even easily removable glue or tapes)
3. Doesn't inhibit, influence or hinder any guitar playing techniques (like e.g. soundhole based pickups prevent tapping the strings)
4. Doesn't (significantly) influence the acoustic characteristics of the guitar (e.g. doesn't try to pickup sound from the strings directly or is installed in the soundhole)
5. Very low susceptibility to background noise and feedback
6. Cheap and easy to build

Research on existing pickups

So, i did some research on usual pickup technologies and guess what i found? None of them really met my specifications. Also existing DIY solutions tended to fail my specifications utterly and were mostly very crude (gluing a piezo transducer directly to guitar and drilling holes in my guitar - seriously?). Well, after ruling out picking the sound up from the strings directly (which btw. only works with western guitars) i saw two options: Picking up vibrations of the corpus directly (e.g contact mic) or picking up "airborne" sonics (e.g. classic microphone). Both have their pros and cons: e.g a contact mic has a very low susceptibility to background noise whereas a classic microphone is more true to the sound of the guitar. That ment i needed a third option - a mix of both techniques. I figured acoustic emissions sensors (AES) would be a good starting point and I investigated different designs and decided to modify one of the approaches i found:

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  Acoustic Emission Transducer

The Approach

After some experiments i build a pickup to the above AES design and step by step came up with three major modifications (which actually make my pickup work very differently compared to an AES - but anyway it was my starting point):

1. There is no couplant and the wear plate doesn't directly touch the object. Instead i left an air cushion of about a 1.5 centimeters between them. Note that the chamber between object and case is airtight.
2. I also introduced a gap (about 3mm) between the backside of the piezo element and the dampening material and drilled a small hole (~ 1mm) in the dampening material, so that the backside pocket is not airtight.
3. I used hard rubber as case material (i tried metal first).

Theories™

The theories i have (IANASE® = I am not a sound engineer) behind those modifications are: An AES directly picks up the sound from a solid object (the guitar corpus) and therefor isn't susceptible to background noise. On the other hand i experimented with a classic AES setup and the sound was really bad (far from preserving the acoustic characteristics of the guitar) which in my opinion is due to the fact that sound behaves differently in different media (materials) and that the wear plate and the object are not perfectly connected (e.g. fused or glued). So i decided that while loosely adhering to the design i still wanted to measure changes in air pressure like a classic mic - hence the (air tight) air cushion. I Introduced the (not air tight) backside gap so vibration of the membrane wouldn't be inhibited - covering the hole actually results in less gain and a dampening of the higher frequencies. Well at that point i already was pretty satisfied with the results except for two issues: 1. Even the most careful handling (touching) of the guitar was picked up very loudly. 2. The FFT (on a Rigol oscilloscope) showed significant beat noise. After some theorizing I figured the beat noise was the result of the different speeds of sound conducted in the (solid) cover and the air cushion. So i replaced the metal cover with a hard rubber cover which actually fixed both issues. The beat noise was reduced to a very minimum and the handling noise became very acceptable. I'm aware that though I started with an AES the final pickup works very differently :)

The Result

1. Has very decent sound quality, with dampening on the lower frequencies and a bit stronger dampening on the higher frequencies - still resulting in an easy to correct curve.
2. Is rather easy to mount and doesn't require invasive modifications to the guitar.
3. Doesn't hinder guitar playing (at least as far as I'm aware of)
4. Hasn't been tested with different guitars but it seems to do a good job on preserving the characteristics of the guitar.
5. Has a very, very low susceptibility to background noise and feedback
6. is cheap and easy to build
7. Profit!

Comparisions, DSP Analysis, Graphs and Samples

TBD

Video and Pics

Usage video (Please note that i forged the audio, recording it a second time directly to my usb sound interface because i didn't have a decent microphone with me. gonna re-record tomorrow but believe me it sounds real good on the guitar amp. It also should be noted that i didn't do equalization, which would be easy to do, because i wanted to convey the unaltered sound)

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  Top View

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  Bottom View

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  Mounted

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  The "breathing hole"

Addendum (1): "The Toy"

I thought with a little imagination it is pretty obvious what kind of toy it was i re-purposed. Apparently i was wrong, since several people asked.

Anyway, I decided to draw a picture:

One Ring to rule them all, One Ring to find them, One Ring to bring them all and in the darkness bind them

Addendum (2): Alternative Use-Cases

Medical Applications

Security Research