ServoDrive Subwoofers In Airport Noise Mitigation Research

Chris Hobbs performing jet noise research with ServoDrive and Sound Physics Labs subwoofers.

For decades, Wyle Laboratories has been conducting acoustical studies for government, military and aviation clients, with one specialty being the measurement and mitigation of noise in free space -particularly transportation noise. While acoustical walls have long been employed between highways and residential areas, only recently has the gulf between "acoustical theory" and reality been bridged as it applies to airports.

One recent example involves a Florida airport's plans to extend a runway. Prior to construction, sound in adjacent neighborhoods, and within the airfield, was measured and recorded. Wyle Labs Acoustician Chris Hobbs then developed a computer simulation demonstrating potential noise reduction.

After constructing a wall, but before completing the runway, Hobbs tested the barrier to ensure that it matched the simulation's predictions. "To verify our tests, we had to simulate the sound of a jet engine in both its spectrum and amplitude," he said.

To do this, Hobbs assembled an audio system consisting of six ServoDrive BassTech 7 subwoofers and eight Sound Physics Labs SPL-td1 full-range loudspeakers powered by QSC PowerLight amplifiers.

"It takes a great deal of low-frequency information to reproduce a jet takeoff, and the ServoDrive BassTech 7s are the only speakers we've found able to reproduce the needed amplitude and frequency spectrum in a reasonably-sized package. Weight and ease of transport are major considerations in these tests," Hobbs explains. "Previous speakers were much heavier, and more of them were required to achieve the same SPLs that just a few ServoDrive and Sound Physics Lab speakers can accomplish. A single ServoDrive BassTech 7 can exceed 114dB at 100 feet away."

Hobbs and Wyle Labs have also been evaluating active noise cancellation in free space, which is considerably difficult to achieve. Using digital signal processors (DSPs), active noise cancellation reproduces the noise signal both in its spectrum and amplitude, but with inverted phase. The actual noise (with normal phase) combines with the reproduced noise (with inverted phase) and the two cancel each other out, resulting in silence.

Hobbs typically places several microphones near the noise source to provide a first-order phase-reversal feed to the speakers. A second microphone is placed several hundred feet away to measure the system's "success" and provide additional information.

"DSP-based computer modeling helps us predict what sounds are going to do," says Hobbs. "The real difficulty is generating the acoustical power to create noise comparable to a jet aircraft. Our expertise and the ServoDrive BassTech 7's allow us to optimize the range of measurement and cancellation well beyond previous capabilities."

Wyle Labs plans to implement active noise reduction systems featuring ServoDrive BassTech 7s and SPL-td1s in various upcoming applications as it continues its commitment to quieting our modern lifestyle.

Chris Hobbs recently presented a paper at InterNoise 2000 in Nice, France, regarding his active noise control research findings and has presented papers and lectures in association with The Acoustical Society of America.