Double Bass If it walks like a piezo, and quacks like a duck...

I'll add some science and some speculation. Yes, I think the "quack" resides in the low mids but 250 Hz is way too low for that. As discussed here many, many times, virtually all piezo pickups, if not loaded properly, produce a "honky" mid-range-heavy sound that, I believe, underlies the quack, at least in part. So, the first way to avoid these undesirable effects is to load the piezo properly (typically by an input impedance greater than or equal to 1 Mohm). I actually haven't experienced the "quack" with a high-quality piezo pickup loaded properly.

The fact that, when it does occur, it seems to act like a wolf-tone and seems to be excited more by some notes than others, is an apt description of a resonance in the response of the piezo/casing combination itself. Yes, I believe Chris is quite correct that it is the harmonics (overtones) of particular notes that excite the resonance.

The player cannot do much to control that resonance. It is best avoided by design. My informed take on this is that some manufacturers of piezo pickups spend very little time evaluating the physical properties and specs of their piezo elements and the encasing materials in order to produce a smooth response. Other manufacturers do. The latter produce higher-quality products that avoid the quack and a midrange response "hump."

Another factor that can contribute to the quack is over-driving the piezo or restricting its movement to the extent that it becomes highly non-linear. Many of us have experience this effect when a piezo pickup is too tight in the bridge wing.

So, in my experience anyway, the quack and other response anomalies of piezo pickups can be all but eliminated by choosing one of the better designed offerings and then installing and loading it properly. Still, from the standpoint of sound, even the best of the piezos aren't going to match a good microphone but they do have their practical advantages.

drurb said:

The player cannot do much to control that resonance. It is best avoided by design. My informed take on this is that some manufacturers of piezo pickups spend very little time evaluating the physical properties and specs of their piezo elements and the encasing materials in order to produce a smooth response. Other manufacturers do. The latter produce higher-quality products that avoid the quack and a midrange response "hump."

Another factor that can contribute to the quack is over-driving the piezo or restricting its movement to the extent that it becomes highly non-linear. Many of us have experience this effect when a piezo pickup is too tight in the bridge wing.

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I made an awful lot of piezo pickups, for various instruments in my time in an effort to avoid paying large amounts of dough for something I figured I could do myself.

I discovered that the mounting materials and position had more to do with "quack" than any other factor. The right input impedance and some creative EQ can help, but nothing I've found really eliminates it.

The casing and mounting have everything to do with it, IME. I tried rubber, plastic, PVC, wood, steel, aluminum, on the bridge, under the bridge, inside the bass, wherever. The amount of "pinch" the piezo suffers on the bridge wings, and the material the element is cased in seemed to have the biggest influence, rather than amplification or even preamps.

Eventually, I ended up using a long strip of piezo film under the bridge. The larger, flat element seems to have much less of a problem with "quack" than the smaller ones, and I'm now convinced that the softness of the top material eliminates the pinch (and thus, some of the honk) you get from the maple bridge material alone. I don't get a lot of piezo "clickiness" either.

Just my experience, but I'm always looking to improve, and would love to hear from other people about it.

WalterBush said:

...I discovered that the mounting materials and position had more to do with "quack" than any other factor... The casing and mounting have everything to do with it, IME... The amount of "pinch" the piezo suffers on the bridge wings, and the material the element is cased in seemed to have the biggest influence, rather than amplification or even preamps.

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I think this is what's called being in violent agreement! Thanks for adding your experience that reinforces every major point I made. I would like to note that one can, in fact, buy a piezo pickup where the values of the relevant electrical parameters and the interaction with and mass-loading of the casing materials has been carefully chosen and optimized so as to virtually eliminate the undesirable effects. Such devices have been discussed in these forums.

Just curious. How does the idea of 'pinching' apply to the Realist? It seems seriously 'pinched' by design.

I am an idiot when it comes to electrical stuff so please talk about this stuff in laypersons terms.

fingers said:

Just curious. How does the idea of 'pinching' apply to the Realist? It seems seriously 'pinched' by design.

I am an idiot when it comes to electrical stuff so please talk about this stuff in laypersons terms.

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In my simple laymans terms the older piezos crystals like the ones in the Underwood were both thicker and harder than the Realist or Aptflex.
Those pickups are made out of a flexible film material that easier to get in a good position for sound to transfer. So the pressure on the Realist would be different than the pressure on the Underwood or Revolution Solo and what they are made out of is different stuff. All that said, I've always assumed that the position of the Realist puts it under greater stress. Ultimately, I don't think that piezos are the answer. Mics just seem to get better and better.

Ric

Chris Fitzgerald said:

Understood and agreed, especially about 250hz being too low. But there's somethig going on beyond "choosing one of the better designed offerings and then installing and loading it properly". I use one of the better designs which is installed properly and always loaded properly, and yet my results vary from amp to amp. The one amp which doesn't seem to quack at all that I've found is the Phil Jones flightcase. All of the EA and AI amps that I have used seem to have a bit of quack built in, but it can usually be defeated/mitigated with some EQ twiddling. The PJB amps I've played have a 4 MegOhm input impedance, which *might* be why they don't quack...OR it could be the way the preamp is voiced...OR is could be the type of speakers used...OR it could be the way the cabinet is voiced, etc.

Seems to me that a variable impedance knob would be a great place to start with any amp but I'm not sure what's involved in that or how hard it would be to make. If there was such a little black box similar to fdeck's HPFpre, I bet it would be incredibly useful to double bassists.

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The front end of my bass amp makes my DB sound awful. For some reason, the BBE Bmax that I was getting ready to sell doesn't, which means, now I have to keep it since I'm getting more DB work lately. I would have assumed the input impedance to be the same as my Genz.

The Summit Audio TD-100 I use for recording has a variable impedance knob, and it works well. One side=great for passive slabs, other side=great for piezos. No EQ, though, so I don't use it live, and it's not so little I could hang it off my bass.

Chris Fitzgerald said:

Understood and agreed, especially about 250hz being too low. But there's somethig going on beyond "choosing one of the better designed offerings and then installing and loading it properly". I use one of the better designs which is installed properly and always loaded properly, and yet my results vary from amp to amp. The one amp which doesn't seem to quack at all that I've found is the Phil Jones flightcase. All of the EA and AI amps that I have used seem to have a bit of quack built in, but it can usually be defeated/mitigated with some EQ twiddling. The PJB amps I've played have a 4 MegOhm input impedance, which *might* be why they don't quack...OR it could be the way the preamp is voiced...OR is could be the type of speakers used...OR it could be the way the cabinet is voiced, etc.

Seems to me that a variable impedance knob would be a great place to start with any amp but I'm not sure what's involved in that or how hard it would be to make. If there was such a little black box similar to fdeck's HPFpre, I bet it would be incredibly useful to double bassists.

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Hmmm... To the extent it is in the voicing or the response of the amp or speaker cab, then it should be apparent regardless of the input device. That is, you should get it with a piezo, a mic, or any other transducer. I wonder how much of the objectionable sound you hear is just a function of the amp. Hard to say from here.

I've never been a fan of the idea of variable input impedance. It's a parameter that alters frequency response in a way that's highly dependent on the device plugged into it. I much prefer optimization of input impedance followed by a really good tone shaping circuit (I prefer parametric ones, as on the EA amps). Optimization is what that switch does for different condenser mics that Chris mentioned. IMO, a variable impedance knob approaches (not "is," but "approaches") electronic snake-oil in many cases.

I use a Rev Solo 2, fdeck's pre-amp/HPF, an EA iamp200, and a Wizzy 12. No quacking, no honking. I can produce honking if I throw on a BassMax plugged into a less desirable rig. Just my experience.

fingers said:

Just curious. How does the idea of 'pinching' apply to the Realist? It seems seriously 'pinched' by design.

I am an idiot when it comes to electrical stuff so please talk about this stuff in laypersons terms.

Click to expand...

The Realist is encased in metal. The total design keeps the pressure on the piezo in the "proper" range. The encasing materials are a huge factor in all of this.

Ok. So another silly question. I've always found the Realist to be the least 'piezo' sounding piezo. Honk included. I've always attributed that to the Realist having a more thumpy, less midrange-y tone partially due to the location between the bridge and the body. Does that (both the tone type being not heavy on the midrange and the location) really have anything to do with it or is it all about loading and construction?

fingers said:

Ok. So another silly question. I've always found the Realist to be the least 'piezo' sounding piezo. Honk included. I've always attributed that to the Realist having a more thumpy, less midrange-y tone partially due to the location between the bridge and the body. Does that (both the tone type being not heavy on the midrange and the location) really have anything to do with it or is it all about loading and construction?

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IMO, it's all of the above. Location is another huge factor.

I find the problem usually in the 500-800hz range. I use a Full Circle as well, and while it is better than a good deal of pickups on the market it still does have this "piezo quack" when you dig in. Agreed about the amp/loading. Although, I find that I can get rid of the quack with my GK MB150S while the AI takes a bit more work...

As a former manufacturer of piezo pickups, and the first unfortunate soul who crossed Gibson's sights regarding the application of piezo-Kynar films, the "quack" has very little to do with the input impedance of the amp but has a whole lot to do with the mechanical properties of the piezo crystal and anything that it is mounted to.

If you were to drive the crystal with a swept tone (driven by a suitable amplifier), you will find a resonant frequency that the piezo's output will peak at, this will be the "quack" frequency. It varies with crystal size, mounting surfaces, bonding agent, etc. This is exactly why piezo crystals are used as buzzer elements, tune the oscillator to the resonant frequency of the crystal assy. and things get VERY efficient.

Great thread, Chris!

I'm learning a lot from TB's expert residents, and I am most appreciative of their professional comportment; these gentlemen suggest a level of behavior that repesents the very best of TalkBass; professionalism worthy of emulation.

Thanks to everyone who has so generously contributed.

agedhorse said:

If you were to drive the crystal with a swept tone (driven by a suitable amplifier), you will find a resonant frequency that the piezo's output will peak at, this will be the "quack" frequency. It varies with crystal size, mounting surfaces, bonding agent, etc. This is exactly why piezo crystals are used as buzzer elements, tune the oscillator to the resonant frequency of the crystal assy. and things get VERY efficient.

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Okay, I'm quoting myself:

drurb said:

The fact that, when it does occur, it seems to act like a wolf-tone and seems to be excited more by some notes than others, is an apt description of a resonance in the response of the piezo/casing combination itself.

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Notice that I mentioned a resonance in the piezo/casing combination. You are quite correct, agedhorse, that the undamped piezo transducer has a strong resonant frequency. This is not unlike the fact that raw loudspeakers also have a resonant frequency. As I'm sure you know, one of the tricks behind cabinet design (Thiele-Small alignments) and, likewise, a trick behind piezo casing design for a pickup is to load the transducer in such a way that the total system has a reasonably smooth response. With piezos, the problem is that the resonance can be smack dab in the passband (in English, the band of frequencies you want to use). Still, by choosing the particular piezo element carefully and optimizing the mechanical loading provided by the case and damping materials, one can quite successfully create a piezo/casing mechanical "system" with minimal resonance in the passband. This is precisely what the most successful manufacturers of piezo instrument pickups have done.

Chris Fitzgerald said:

By this definition, then the difference in sound with the same bass/player/pickup through different amps/speakers could be attributed to the frequency curve/response of the rest of the signal chain (preamp, EQ voicing, speaker and cabinet resonance) in relation to the resonant frequencies of the crystals and mounting material in the pickup, right?*

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Precisely correct. The non-flat response of the amp/cabinet can offset the non-flat response (peaked at the resonant frequency) of the piezo pickup. This is why, as I mentioned, a good tone-shaping circuit is of great value. With regard to input impedance you are correct again. The input impedance acts to load the pizeo electrically (as opposed to mechanically) and mostly affects the low-end response as opposed to the resonant frequency, although it will can affect that as well. Now we're having fun!