knock sensors/piezos: what makes for a hot signal?

hello all --

tldr: what piezo will produce the hottest signal when struck? thicker ones, thinner ones, etc?

recently I repaired a roland octapad, an 8 pad midi drum controller that is played with sticks.
the piezos were from the mid eighties (estimation) and most of them had failed after many, many strikes

i replaced them with piezos and have been enjoying using it since, but i have had to use the utmost sensitivity setting
and even then, i do not have a good velocity range. from 0-127, i am only really able to get ~80-127.

the piezos i used were from ebay, and seemed a bit thicker and bulkier than the ones i took off the octapad. both are 35mm.

i see a lot of piezos on the market, including ones you peel out of speakers from awful radioshack -- can anyone shed some knowledge as to which are appropriate for the job?

thanks
nym

Don't get offended but have you ever heard of the concept of Amplification ?

thanks, yes! however, since it's already a product, presumably with its own internal amplification per sensor, i am hoping to simply drop a more sensitive piezo in place. there's very little room in the box for any additional circuitry.

in case this wasn't clear, this is not an arduino project, merely a question about piezos themselves, and what qualities make them more or less "hot"

oh, ok. So if you're not a Hacker, then the answer to your question is
the frequency response. Is that too vague or general ?
Well then you could read this:

http://scholar.lib.vt.edu/theses/available/etd-042299-150134/unrestricted/Chapter2.pdf
or this:

http://www.sem.org/PDF/Change_Piezoelectric%20Technology%20Review.pdf

You could look at this:
https://www.google.com/search?q=piezo+frequency+response&rlz=1C1GPCK_enUS504US504&es_sm=93&tbm=isch&imgil=GMMZgPHRcGmB-M%3A%3Bhttps%3A%2F%2Fencrypted-tbn1.gstatic.com%2Fimages%3Fq%3Dtbn%3AANd9GcQVwwxF602Vhe0idWrpqZjqEapcHQb18LdQWiJNAqtu-WXtwgWa%3B527%3B361%3BQDTmBQ3kHm20uM%3Bhttp%253A%252F%252Fcladlab.com%252Felectronics%252Fcomponents%252Fpiezos&source=iu&usg=__4OmTYXas6hQwbDOdEsW8Jlwl21I%3D&sa=X&ei=eZV2U526JIKEogSB9ILYBA&ved=0CC8Q9QEwAQ#facrc=&imgdii=&imgrc=GMMZgPHRcGmB-M%253A%3BQDTmBQ3kHm20uM%3Bhttp%253A%252F%252Fcladlab.com%252Fwp-content%252Fgallery%252Felectronics-misc%252Fpiezo-4khz-resonant-frequency-response.jpg%3Bhttp%253A%252F%252Fcladlab.com%252Felectronics%252Fcomponents%252Fpiezos%3B527%3B361

or look at this:

but in the end you have are back to "the frequency response"
Does that answer your question ?

yes, perfect. the sensors i used have too high a resonant frequency (2.8kHz) for what i need in my application. that pdf is full of excellent information.

it’s a shame that there’s only the “sensitivity” setting on the device itself.
it’d be much better if it gave more parameters, like note on threshold and upper velocity limit, then you’d be able to use practically whatever piezo you wanted (albeit with some loss of velocity resolution from the scaling). i think the newer spd30 has those parameters.