Re: Urgent!!! ISRA:
The number of "peaks" you define is the number of cycles. This varifies my postulation of mechancial variation due to clamping effects on the element.
Smaller dimensioned elements tend to be easier to clamp and thereby limit ring-time. Increased ring-time results in a higher Q-factor [Q=(fc/(f2-f1)] where f2 and f1 are the upper and lower 6dB drop frequencies from the fc or centre frequency.
For a polycrystalline piezo material you will need more damping to reduce the number of cycles over the referenced amplitude for a larger area element as compared to a smaller area element lapped to the same nominal resonant frequency.
As to specific assignments of bandwidth categories; i.e. what percentage bandwidth constitutes broadband or narrowband I do not know what the convention is. I personnally prefer a broadband probe with 1-1.5 cycles and this is usually about a 75-90% bandwidth. But I do not know if a 2 cycled pulse or something less than 70% bandwidth constitutes a narrow band probe.
Band widths greater than 100% are of course possible. But damping rigid crystalline materials to greater and greater amounts eventually kills all useful output. The best option for a useful high bandwidth signal can be had from PVDF and PVTrF materials. These have a very good receiver efficiency (about 7 times more than PZT).
Another potential source of variation is the electrical matching required if there is different capacitance between the electrodes of the large and small probes.
If using a negative spike pulser you can alter the frequency content of the ring by adjusting the pulse duration.
Why are you using the 'second' backwall signal instead of the first? Since you are testing these probes in contact mode you are coupling energy back into the probe's matching plate. Also, because you are testing the frequency content which is, as just discussed above, dependent on the damping of the element you have added another variable in the pressure put on the probe. Now you must consider effects of front loading as well as back loading.
Pages 65-100 of the NDT Handbook Second edition vol 7 by ASNT may be helpful to your design considerations.
: I sent this mail 10 days before but i don't know why you don't receive it.
: Dear expert
: Thanks for giving me of your time to reply my question
: by peak number?
: i mean the number of half peaks have an amplitude greater than 20% of the
: hiegst one (observed by viewing the second backwall echo received from the
: V1 block on the oscilloscope)
: the kind of probes are broad band normal contact probe (supposed to be
: typically constructed)
: a third question is
: is there any limit for the band width% to judge if it is narrow band or wide