Acoustic Impedance Matching For Acoustic Sensing

VOLUME 4, APRIL 2011

Acoustic Impedance Matching For Acoustic Sensing

By: Mohd Faizal Abd Rahman (PhD Student)

In acoustic transducer design, impedance matching is a crucial part that needs to be considered in improving the performance of the device. This property relates on how much energy that is efficiently transferred when acoustic signal propagate between two different medium. The acoustic mismatch causes only a portion of energy to be transmitted, while the rest of it will be reflected. The more it differs, the more energy to be reflected which result the energy transfer become inefficient.

Acoustic impedance can be represented by[1]:

Z=pV Rayl


where Z=Acoustic Impedance
p=density
V=acoustic velocity

Based on this equation, we can anticipate the value of acoustic impedance, providing the value of sound velocity and density are known. So, when designing an acoustic transducer, this equation can be used as an initial step towards choosing the best material for our transducer. Due to this effect, different coupling environment need different design considerations. A good transducer for airborne application will not necessarily be good in water application. This why a good microphone will not perform as good as it is in water [2].

In some applications, the acoustic mismatch is overcame by introducing the matching layers or couplant between the medium where the propagation occurs. The introduction of these matching layers has the effect of reducing the obvious different in acoustic impedance, thus maximising the power transfer. For example, in ultrasonic medical diagnosis, gels which act as a couplant is used as a matching layer when operating the transducer[3].

References

[1] http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/ acousticimpedance.htm

[2] http://en.wikipedia.org/wiki/Hydrophone

[3] The technology of ultrasound scanning gels, A Sonotech Technical Paper, 2006