Pink Noise

Adam Kumpf, a really smart guy at MIT, built a piezo pickup much like mine, although he installed his permanently in a guitar. On his page, he posted frequency response curves for his pickup versus a commercial piezo and a Fender Stratocaster. I liked this idea and given the varying results with my three pickups, decided to do something similar.

I'm guessing that the vibration of a guitar soundboard, especially when contacting human bodies of varying density, is complex. So, to replicate such an environment when testing each pickup, I conducted the following highly-controlled experiment. I taped each element to a thin wooden board using painters' tape and hung the board by string from a wooden ruler in front of my right monitor. The ruler was held down by the plastic lawn dwarf who resides on top of the monitor. I then played pink noise though the monitor and simultaneously recorded through each pickup.


The results show some of what I've heard so far with the three pickups. First, here is the frequency response curve of my pink noise sample (when adjusted for a 3 db slope, it's flat):

Here is the curve for the small piezo disc pickup:

For the large piezo disc pickup:

And, for the piezo film pickup:

All of the pickups have jagged responses, even when averaged. This isn't entirely bad, since you can use different microphones and pickups for various purposes, to emphasize or deemphasize different frequencies. But, these curves don't compare well with an average microphone curve.

Nonetheless, I think the small disc sounds pretty good. Its curve, though jagged, has some high peaks throughout the range. The big disc sounds loud and tinny. I'm not sure if that's evident in its curve, especially when compared to the small disc. The film pickup sounds the best but produces very little signal (evident in the amplitude of its curve; I had to blast the pink noise just to get that). Otherwise, I'm not sure what these curves show, except that frequency response in these pickups is uneven.

By the way, if you compare these curves with Adam's, you'll see that his are much smoother, even though his pickup is almost identical to my disc pickups. He told me that he didn't use pink noise but instead "recorded audio data over a series of chords and notes that spanned the entire range of the guitar and then did post-processing in audio software." This is, in a way, a more accurate way to test these pickups since an acoustic instrument would never produce pink noise but instead a smaller range of frequencies. I may try it, but all this has made me tired.

Size Matters

In the case of my piezo disc pickups, it's bad.

I made a new pickup using a larger disc (about 1 5/16" diameter). I constructed it in the same way as I had the first pickup -- by soldering it to a short piece of guitar cable and then dipping the disc and about an inch of the cable into Plasti-dip.

The large disc gives a strong signal, but distorts very easily, regardless of where I tape it on the soundboard. I don't know whether this is entirely a function of the disc's size or if there is something different about its composition compared to the smaller disc I first used. The sound quality is not great.

I recorded with the large pickup taped behind the bridge of my nylon string guitar. I plugged it directly into the Firebox. I then recorded the same bit with the smaller disc pickup, also taped behind the bridge and plugged directly into the Firebox. The difference in frequency response is significant, with the larger pickup sounding like it's playing through a portable AM radio. This isn't necessarily always bad though as I can used this pickup when I want a lot of midrange.

Large piezo disc pickup

Small piezo disc pickup

On Film

Today I built a pickup using piezo film. I purchased the film in two lengths from Edmund Scientific. I'm reluctant to recommend them as a source because, even the though their prices were low and the delivery was prompt, their website was not entirely honest about shipping costs or availability of items.

Of the two sizes of film they sent me, I used the larger which is 16mm X 73mm (almost 3 inches long). I thought this would fit nicely behind or in front of a bridge. Also, in his fantastic book on pickups, Getting a Bigger Sound, Bart Hopkin writes that the larger films give a stronger signal and a more representative sample of the sound board's movement. I may use the smaller films for a ukulele pickup.

To build this pickup, I followed the guidance from David Fittell's site. I followed most of his instructions, including using shielded cable and a female phone jack. I did not glue cork to the pickup or put heat shrink on the jack.


Using the new film pickup and later the chip pickup I'd already built, I recorded the theme from The Munsters, played on an Epiphone with very old strings. I recorded directly into my recording interface, a Presonus Firebox. This time I taped the pickups behind the bridge using painters' masking tape. The difference between the new film pickup and the disc pickup is significant. The disc gives a much stronger signal. But, the film pickup seems to have a more realistic frequency response.

You'll hear some hiss in the sample with the film pickup. I'm not sure if it's a hum resulting from not yet effectively contacting the ground of the jack to the foil or if it's because of a gain problem. The signal from the film pickup was so low I had the preamp on the Firebox turned up all the way.

The Munsters' Theme - piezo film pickup

The Munsters' Theme - piezo disc pickup

Hello

I've started making my own piezo pickups for guitars and ukuleles. I don't want to install them permanently in any instruments and so am experimenting with different means to attach them, scotch tape being my current favorite.

I've built only one so far: a 3/4 inch diameter piezo disc soldered to a short length of guitar cable. I bought the disc from Electronic Goldmine in a three-for-a-dollar package. I bought other discs as well, but these were the only ones with pre-soldered leads. Soldering wires to these discs is incredibly difficult. I pulled off the leads and used the solder already attached to the disc.

After I soldered together the the cable and the disc, I dipped the disc and an inch of the cable into black Plasti-Dip, a weird, and according to the State of California, carcinogenic substance. The idea for dipping the pickup in Plasti-Dip from Nicolas Collins' book Handmade Electronic Music. According to Collins, the Plasti-Dip strengthens the pickup connections, reduces hum, and reduces some of the high frequency resonance of the disc. All these things turned out to be true.
Because piezo pickups have high impedance outputs, they require preamplifiers for better sound. I've used some on hand and built one so far. Here are some sample recordings of the above plasti-dipped pickup and some preamplifiers. The samples are a Mel Bay scale exercise played on a nylon string guitar. I taped the piezo right near the bridge under the high "E."

The first sample was recorded with my recording interface, the Presonus Firebox. It contains its own preamplifiers so I could plug the pickup directly into it. Notably, all of the other samples must go through the Firebox. (Click on the pictures to play the clips.)




The second sample I recorded with the Digitech Genesis 1, an effects unit and amp emulator now discontinued. I bought it new about 6 years ago for $70. All of the settings were set half-way and all of the effects (compressors, noise gates, etc.) turned off. I set the amplifier to "Brit Combo," which supposedly emulates a Vox AC30 -- an odd setting for a nylon string guitar, but it sounded natural.


The third sample I recorded with the "Mint Box Piezo Buffer," a preamplifier I built. I was under the impression when I started building the first pickup that I would need some sort of special preamp, i.e. one that could handle very high impedances. This turned out not to be the case, but building this was lots of fun. The design is by an engineer named Scott Helmke. Scott is a very nice guy and answered many of my dumb questions while I was building this, such as why the tantalum capacitor started smoking the first time I plugged in the battery. The mint box sounds great through an amplifier, much better than a piezo alone. I am disappointed that the recording sounds not so good: too much midrange. But, I believe the Mint Box was designed for live settings.


Finally, I recorded the same exercise using an Audio-Technica Pro 37 microphone as a reference.


I'm happy with the sound of this pickup and think that, for accurate reproduction, the Firebox alone sounds best. I have some more discs and some piezo film I'll write about next time.