Page 31 - Summer 2008

show with studio sound quality is every musician’s dream, it has been virtually impossible. Even if sound reinforcement rigs sounded good, and if arenas had excellent acoustics, and if sound engineers knew everything there was to know about mixing sound and had the best gear available, there still would remain one obstacle on the road to sonic nirvana—feedback.
Acoustic pickups
There are several conventional ways of dealing with acoustic feedback. A primary method is to use different microphone technologies that are primarily focused on the directionality of microphone systems. This method does work up to a certain point, but requires constant manage- ment by sound engineers to adapt to the different and chang- ing characteristics of a stage venue.
A second method used is to employ contact pickups on the instrument itself. The technologies vary, but the basic idea is to sense the vibrations of the instrument’s body directly, rather than the sound it produces in the air. The advantage is obvious—practically no acoustic feedback—as these pickups are not sensitive to airborne sound. Unfortunately, the short- comings are many—finding a good-sounding location on an instrument body is notoriously difficult, piezo pickups’ sonic characteristics are far from perfect and they have high output impedance requiring special instrument inputs or direct boxes. They also tend to be large and can actually interfere with the natural acoustic behavior of the instrument itself.
This suggests the idea of a new form of a low mass “con- tact microphone.” Suppose we use a “surface transducer” that measures the acceleration of the instrument’s body, prefer- ably on more than one axis,6 with good linearity and so light- weight that it does not acoustically affect the instrument that it is measuring. Suppose further, that such a transducer has an interface identical to that of a traditional microphone— similar output level, output impedance, and power require- ments. In short, suppose that a musician can just plug this transducer into a microphone preamp or a mic input of the mixer like any other microphone.
Contact microphones
An attentive reader will notice the mention of “accelera- tion” in the preceding paragraph. Because our ears respond
Fig. 6. MEMS accelerometer 4 mm x 4 mm x 1.45 mm.
Low-g accelerometers can measure acceleration down to milli g’s but are typically bandwidth-limited to around 5 kHz. This limitation may be associated with the fact that few com- mercial applications of these sensors require significant bandwidth (the primary applications involve the detection of human motion or gravity driven acceleration events that are lower frequency dominated) and so there has been little motivation to develop sensors suited specifically for audio band measurement. The output of an accelerometer is typi- cally measured in millivolts per g and a three axis accelerom- eter has three separate output channels measuring the Cartesian x,y, and z axes separately. A typical low-g MEMS accelerometer is housed in a standard surface-mount pack- age, taking further advantage of the mature semiconductor manufacturing infrastructure. Measuring less than 4 mm x 4 mm x 1.5 mm, the product can fit into places unimaginable with traditional accelerometer technology and because of its very small size it does not cause mass loading or other changes in the response of the system that it is measuring. Figure 6 shows an example of surface-mount housing.
Acoustic feedback
Beginning with the introduction of the omnidirectional condenser and dynamic microphone in the mid 1920s,4 fol- lowed by Søren Larsen, the Danish scientist who first discov- ered the principles of audio feedback (also known as the Larsen effect), acoustic feedback has been a demon that few audio engineers are able to totally control. Acoustic feedback is an unavoidable fact of life in live sound production. The Beatles experimented with this audio artifact and decided to add it to their memorable introduction to “I Feel Fine” in 1964.5 Rock ‘n Roll then set out to tame the beast by embrac- ing it, thereby making acoustic feedback a striking character- istic of rock music. Electric guitar players such as Pete Townshend and Jimi Hendrix deliberately induced feedback by holding their guitars close to the amplifier sensors. As the fad waned, audio engineers continued to struggle with acoustic feedback’s often undesirable ear shattering effects particularly in live sound applications. In the perfect world of a well-appointed and acoustically treated recording studio, a high-end omnidirectional microphone will record an instru- ment with an astonishing degree of realism and fidelity. Artists who know and cherish this sound have long sought to be able to reproduce it on stage. Although recording a live
Fig. 7. Accelerometer mounted on Fender Strat Acoustic Guitar.
30 Acoustics Today, July 2008