If we have to find the speed of sound in any material measuring sound vibrations at different points in the sample, what kind of piezoelectric sensors which have high sensitivity would be useful?
What sound frequency would you like to measure at, and is the material a solid, a liquid or a gas?
you can use piezoelectric materials like PZT-5 , acceloremeters, it depend on the cost
I have done dozens of sound speed measurements in solids, hundreds of measurements in water , dozens in other liquids such as ink and oil.
My preference for piezo material for a sensor head in liquid using low voltage transmit is APC 851.
My preference for solids is pvdf film with 100V transmit. Solids require parallel faces.
I typically use short pulses in the 1 to 4 MHz range.
Note that some materials are dispersive ie. The sound speed is frequency dependent.
I think PVDF is good candidate for measuring sound vibration. Since its surface impedance is close to air impedance.
I think it is time to use piezoelectromagnetics, more possibilities, etc.
For the measurement of sound propagation speed in different material is PVDF and Lithium Niobate piezoelectric materials. For the contact method generally a disc type transducers are used. The resonant frequency of interest (which results in resolution)is defined by it's thickness and diameter. Other methods are also available which are non-contact.
I would add that Electret microphones the size of a button are incredibly inexpensive and have a frequency response in some cases from 10 Hz to 30KHz. These are used to monitor earthquakes by the USGS in the U.S. and are very reliable.
Experimentalists state that to generate and detect acoustic waves in the piezoelectromagnetics (PEM), also known as the magneto-electro-elastic materials, is more preferable with such non-contact method as the electromagnetic acoustic transducer (EMATs). Also, such the two-phase materials possessing both the piezolelectric (PE),and piezomagnetic (PM) effects as well as the magnetoelectric effect can be used for smart materials. Anyway, PEMs can be used instead of PE and or PM. PEMs can give more possibilities for sensors, see my books for the non-dispersive waves' propagation in the PEMs and I have also just written one book on the thirty two new dispersive SH-waves' propagation in PEM plates. It is stated and well-known that SH-waves can possess a larger sensitivity.
You can use piezoelectric sistem with quartz resonator. Shifts of frequency and shift of q-factor of the system allow you to measure shear viscoelastic properties of material (mainly fluids).
If the medium is air, then condenser microphone is very good to detect sound vibrations.
I would concentrate on coupling considerations. How to get proper acoustic match on the contact of transducer vs. sample surface. What kind of materials are you measuring? The speed of sound is quite easy task if you need only longitudinal. 1 MHz transducer will probably be enough, and you get good penetration in usual solids. I only have experience of Olympus-IMS US (formerly Panametrics) transducers and Vallen-Systeme emission sensors. Both work fine.
Thank you very much all of you for your overwhelming responses.
To be more specific I have to measure speed of sound in steel where the velocity is very high and hence highly sensitive sensors are required.
Is your application under lab conditions or in the field subject to noise?
What accuracy level you are about to get? Speed of sound (londitudinal) should be around 5000--6000 m/s with most steels.
Since sound is propagated by deformation of the material it may be more accurate to use interferometry to detect the sound wave?
What kind of samples are involved? Piezoelectric crystals are usually narrowband so if the sample is bound to exhibit dispersion or anisotropy, you may have to 1) use multi-frequency techniques (e.g. pulses or several piezoelectrics tuned to different center frequencies), or 2) excite body resonances to infer sound speeds of various modes and polarizations. Unless you'd be satisfied with an average or ballpark figure for sound speed, the techniques differ whether your sample is crystalline, amorphous, porous, granular, with magnetic domains etc.
Since you are coupling to steel, none of the transducers which are based on flexing a membrane are useful (ie gas and liquid sensors designs are not good). You would probably do best with a PZT ceramic element in a rigid matrix (epoxy) within a faraday shield. The PZT and steel should have similar acoustic impedances.
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