Page 32 - Summer 2008

to sound pressure, microphones are designed to sense sound
pressure. To simplify matters greatly, the sound pressure in
the immediate vicinity of a vibrating body is proportional to
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acceleration. What if there were an accelerometer that had
enough bandwidth to be used as a contact microphone?
To explore this concept, a three axis accelerometer was mounted on an acoustic guitar to act as a pickup. While not ideal, we used an Analog Devices ADXL330, a three axis low- g accelerometer that has wider effective bandwidth than other traditional low-g accelerometers. It has a bandwidth up to 6 kHz on the x and y channels and around 1 kHz for the z axis. The expanded bandwidth allowed the MEMS accelerometer to gather useful information in the audio band. Since the output is analog it is easily instrumented and
can be used with standard audio recording equipment
The vibration of the instrument was measured and com- pared to the built in piezo pickup and to a MEMS micro- phone mounted near the guitar. The guitar used was a Fender Strat acoustic with a built in Fender pickup. An analog out- put MEMS accelerometer was mounted on a light-weight flex circuit and attached using beeswax on the guitar body at the bridge location, as seen in Fig. 7. The x axis of the accelerom- eter was oriented along the axis of the strings, the y axis per- pendicular to the strings and the z axis normal to the surface of the guitar. A MEMS microphone with a flat frequency response out to 15 kHz mounted 3 inches from the strings
was used as a reference.
A short sound segment was recorded using the
accelerometer, the built in piezo pickup and the MEMS microphone. The time domain waveforms for each transduc- er are shown in Fig. 8. No post processing was done on any of the audio clips.
Figure 9 shows an FFT-based spectrum of the piezo pick- up measured at one of the peaks in the time domain wave
Fig. 8. Time domain waveforms using different transducers (as indicated).
Fig. 9. Spectrum of piezo pickup.
form. This spectrum shows a response with a strong bass component. Indeed the actual audio file sounded excessively full with a lot of bass response. This sounds pleasing (depending on your taste) as the cavity resonance creates a fuller bass sound that is not the same as that heard when lis- tening to the instrument directly.
The MEMS microphone output is very flat and repro- duces the sound of the instrument very well. It sounds very natural, well balanced and true to life. The FFT-based spec- trum measured at the same point in time as the piezo pickup is shown in Fig 10(a). The frequency response of the MEMS microphone is shown in Fig 10(b) for reference.
Fig. 10 (a). Spectrum of MEMS microphone; (b). Frequency response of MEMS microphone.
Managing Acoustic Feedback 31