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SMART dampers may solve one of performing guitarists鈥� most excruciating problems: feedback. The screech of feedback has become more widespread as the tone of the acoustic guitar is increasingly sought by musicians.

鈥淔eedback has been a major problem ever since acoustic musicians decided they wanted to be able to hear themselves onstage,鈥� says Scott Nygaard, editor of Acoustic Guitar magazine and one of the best known guitarists in the US.

Now researchers at Duke University in Durham, North Carolina, have developed a way to banish feedback without affecting the sound of the guitar. An acoustic guitar鈥檚 soundboard 鈥� the holed, wooden panel beneath the strings 鈥� vibrates when the guitar is played. Feedback is induced when the sound from a speaker causes vibrations in the top plate. At certain resonant frequencies, these are picked up and amplified all over again, causing the characteristic howl.

Some guitarists stick weights, called passive dampers, on the soundboard, while others block the sound hole completely, stuff foam into it or use electronic filters. But all these measures affect the guitar鈥檚 tone. 鈥淵ou can always turn it down,鈥� says Duke researcher Steve Griffin. Strangely, this does not seem to be a popular option.

So Griffin and his colleagues in Duke鈥檚 mechanical engineering department have come up with a smart damper for the soundboard that prevents feedback without affecting the instrument鈥檚 tone. They tested their system on a Yamaha guitar, which on a laboratory test bench produced the worst feedback at a frequency of 239 hertz 鈥� somewhere between B and B flat below middle C.

The active dampers comprise two small slivers of a piezoelectric ceramic material, each half the width of a credit card and only one-quarter of a millimetre thick, tightly bonded to the soundboard. The sound signal from a controller is fed to an amplifier. At the feedback frequency, the controller applies a voltage to the 鈥減iezoceramic鈥�, causing it to deform. This damps the soundboard鈥檚 surface, limiting its ability to vibrate. As the volume increases at the resonant feedback frequency, the deformation increases and the damping effect is boosted. In tests with the active dampers, the researchers found they could play the modified guitar more than twice as loud as the undamped guitar before feedback started 鈥� a diference in loudness of up to 7 decibels.

The team now hopes to develop a commercial version of their invention.