“Pipe organ” air-coupled broad bandwidth transducer

Background, Motivation and Objective
Air-coupled transducers are used to conduct fast non-contact inspections in NDT. Normally, the bandwidth of a conventional transducer can be enhanced, but with a cost to its sensitivity. However, low sensitivity is very disadvantageous in air-coupled NDT. This paper presents a methodology for improving the bandwidth of an air-coupled diaphragm transducer without sensitivity loss by connecting a number of resonating pipes of various length to a cavity in the backplate (see figure 1(a)). The design is inspired by the pipe organ musical instrument, where the resonant frequency (pitch) of each pipe is mainly determined by its length.
Statement of Contribution/Methods
The design, manufacture and experiment are divided into five steps: first, a fast 1D (in space) mathematical model is employed to ascertain the location of resonances and investigate the benefits of an increased number of pipes. Second, a slower but more accurate 3D finite element model provides the optimized parameters of the transducer. Third, a CAD model is built and a commercial stereolithography 3D printer is used to print the “pipe organ” backplate. Fourth, a passive diaphragm is attached onto the cavity/backplate. Finally, a 2D laser vibrometer is used to measure the average velocity of the diaphragm when applying an external sound source in order to estimate the bandwidth.
Results/Discussion
The average velocity of the passive diaphragm in the “pipe-organ” transducer is compared with the standard “cavity-only” transducer. The membrane velocity bandwidth increases with the addition of pipes emanating from the cavity. A common noise floor was defined for both devices as 6dB below the maximum average velocity of the pipe backed device (see figure 1(b)). The bandwidth of this new device was 2.3 times larger than the standard one. Further work is now underway to change the passive diaphragm to an active polyvinylidene fluoride (PVDF) diaphragm. This will allow the bandwidth of the transmission voltage response and the receiving voltage response to be calculated and compared with that of the standard device.