Acoustics from A to Z:  I & J 

By Eric E. Ungar 

INSTRUMENTS for measuring sound

For many years have been around. 

Recent years have seen improvement 

In sensing sound, and also movement. 

Some systems are so fast and small 

They almost aren’t there at all. 

Ted Schultz, with whom I worked at BBN for many years, liked to tell the story of how he made measurements of airframe noise in one of the earlier Douglas transport aircraft. He used the only suitable filters available then, namely an analog system that permitted him to measure the noise level in one octave-band at a time. So, he had the pilot climb to a comfortable altitude and shut off the engines so that engine noise would not drown out the airframe noise, and then he measured the noise in one octave-band during a gliding descent. Then the pilot would restart the engines and repeat the process until Ted got data in all eleven octave-bands. 

With today’s instrumentation, one short glide would have sufficed to acquire all the data, analyze it in octave, one-third-octave, or narrow bands, display it and even print it out. And, today’s equipment would weigh at most only a kilogram or two, where Ted’s ‘portable’ system was portable by perhaps two people or a mule. 

Modern microelectronics and digital technology have made possible all sorts of compact and energy-efficient signal processors and recorders, with features too numerous to mention. Similar technology also has led to accelerometers with built-in processing chips that condition (e.g., amplify, filter, limit, integrate) the acceleration signal. It also has given rise to accelerometers that weigh only a few carats, where a carat (equal to 0.2 grams) is the unit in terms of which the weight of precious stones is usually stated. 

Modern technology also has led to laser systems that allow one to measure the vibrations of objects without attaching anything to them. These systems enable one to measure the motions of a given point on a vibrating object over a wide range of frequencies. Some such systems can even scan the surface of an object and generate a plot of its amplitude distribution. These systems have at least one drawback, in addition to their cost – they only work if the gross motion of the test object relative to the laser is small 

JETS make noise from mixing flow 

Of their exhausts with air that’s slow. 

Their turbines may act siren-like 

To generate a spectrum spike. 

Bypass fans give quiet thrust, 

For modern aircraft they’re a must 

Sir James Lighthill, who died in July 1998 at age 74, is credited with the development of the theory of jet noise. (You may have read that he succumbed while attempting a nine-mile swim around one of the islands in the English Channel – a swim that he had done earlier at least a dozen times.) One of his students, John E. Ffowcs Williams, tried to explain this theory to some of my colleagues and me while we worked together at Bolt Beranek and Newman some time ago. He showed us the basic equation, which covered an entire blackboard that wrapped around the room, and he discussed the meaning and implications of each term. Although he was unsuccessful in making me understand everything, he later went on to high academic positions at prestigious British establishments and was responsible for much work related to the control of noise of the Concorde supersonic transport. 

According to Lighthill’s eighth-power law, the sound power produced by a jet mixing with the ambient air varies as the eighth power of the jet velocity. So, a slower jet should be a lot quieter. This is precisely what is behind the relative quiet of fan-jet engines, which in essence produce a wider, slower air jet than do pure jet engines, yet provide the same thrust. In the newer large-diameter high bypass-ratio turbofan engines, jet mixing noise usually is not the dominant component; so-called core noise generated within the engines (due to combustion and density inhomogeneities) and the siren-like noise from fans, compressors and turbines take on more prominent roles. 

Powell and Preisser¹ reviewed the advances in aircraft noise reduction: “When normalized to total engine thrust, today’s new transports are about 20 dB quieter than those introduced in the 1950s. This reduction resulted from major engine cycle changes that improved fuel efficiency and incremental efforts that required careful optimization to preserve thrust and efficiency. Low-bypass-ratio turbofan engines introduced in the 1960s provided greater propulsive efficiency and lower noise.  But with jet exhaust no longer the primary noise source, further improvements in total engine noise required reductions in fan-generated noise. These resulted mainly from the elimination of inlet guide vanes, a decrease in the number and rotational speed of fan blades, and improved blade aerodynamic design. A major breakthrough was the fan blade passage frequency ‘cutoff’ design concept in which the BPF tone does not propagate outside the engine nacelle. In addition, advances allowed acoustic treatments to be designed or tuned for enhanced absorption of the fan tones. 

Active noise cancellation is also in the works, but hasn’t quite been reduced to practical installations, as far as I know. Eventually, only noise due to flow over the airframe itself will be left. This airframe noise should be relatively benign in general; in tests some years ago, researchers at Wright Field were unable to measure noise from aircraft in unpowered flight past a microphone array at times when crickets were active. 

¹ “Research for quieter skies,” C. A. Powell and J. S. Preisser, Aerospace America, August 1999.