Eric Unger’s Acoustics from A to Z

MACHINERY of any kind 

Brings two main types of noise to mind.  

One’s from surface radiation 

Due to structural vibration. 

The other comes from pressured air 

As air is moved from here to there. 

Intake and exhaust noise usually predominates in engines or compressors, because intakes and exhausts are acoustic monopole sources that radiate sound efficiently. What is left when intakes and exhausts are quieted often is called “casing noise” – that is, noise radiated from the machine’s structural envelope as the result of its vibrations. In machinery whose internal components do not communicate directly with the ambient air, casing noise is all there is. 

Noise-radiating casing vibrations may result, for example, from internal pressure pulses, from hydraulic systems, from imbalance of rotating parts, from reciprocating elements, and from impacts and other interactions of mechanical components, such as those of bearings and gears. The latter have some particularly interesting aspects. 

At a small lunchtime conference some time ago, one of my colleagues pulled a ball taken from a ball bearing from his pocket, rolled it across the table and asked why this shiny, smooth ball should make so much broadband noise as it rolls across a polished wood surface. Try it; you’ll be surprised how noisy it is! When we later repeated the same experiment on a flat glass mirror, we again observed considerable noise. I don’t know whether any research has been done on this problem, but I conjecture that the tiny asperities on the surfaces interact, possibly producing local surface deformations and causing the surfaces to vibrate and thus to radiate sound. We didn’t try lubricated balls or balls with resilient covers, but I bet these would have produced a lot less noise. 

Combustion noise, which is responsible for the roar of furnaces and the “core noise” of jet engines, is due to nonuniform combustion, where there in effect occur local hot spots that behave as acoustic monopoles and thus radiate sound well. Temperature and density inhomogeneities behave as dipoles when accelerated in non-uniform flow.  Some flow-related acoustic phenomena involve feedback, such as those associated with edge tones and also some whistles. There also may occur thermal-acoustic feedback phenomena exemplified by the Rijke tube, which first was reported in 1859. As Lord Rayleigh describes it: “When a piece of fine metallic gauze, stretched across the lower part of a tube open at both ends and held vertically, is heated by a gas flame placed under it, a sound of considerable power and lasting for several seconds is observed almost immediately after removal of the flame.” As he goes on to explain, the air column in the tube is driven at resonance by periodic transfer of heat from the gauze to the air, with appropriate phasing resulting from the combination of convection with the acoustic pressure oscillations. This phenomenon differs from that of “singing flames,” in which acoustic pressures interact with the combustion process. 

And, why do transformers make noise, where these have no intakes or exhausts, nor internal moving parts? The answer is magnetostriction – slight changes in the dimensions of iron or steel components resulting from changes in the magnetic fields acting on these components. Practical and economic constraints make it difficult to reduce the noise produced by large power transformers at the source. But I’m glad that power companies have not let this get in the way of providing our homes with electricity; otherwise, we would have to watch TV in the dark. 

A noisy NOISE annoys an oyster  

And quiet noise annoys a cloister.  

Annoyance from a sound, it’s true  

Depends on what one wants to do.  

Shaped noise can be used to mask  

A sound that complicates a task. 

I must admit that I have no idea whether oysters can perceive any sound at all; I was just carried away by the cadence of the words. But I do remember hearing a paper concerned with sound perception by fleas that was presented at an Animal Bioacoustics session of the Acoustical Society of America a few years ago. At that time ultrasonic flea collars for dogs were being advertised aggressively and a study was carried out to determine their efficacy. This study, which was not sponsored by manufacturers of flea collars, found that: (1) fleas cannot perceive sound; (2) ultrasound emitted by the flea collar would be blocked and absorbed by the dog’s fur so that little sound would reach any fleas; and (3) in a comparison investigation, dogs wearing ultrasonic flea collars harbored a somewhat greater number of fleas than dogs without such collars. I don’t recall whether anyone concluded that dogs might be driven to distraction by the ultrasound. 

According to a paper presented by Douglas Barret of HMMH at the 1999 Summer Meeting of the Transportation Research Board, nuns objected to construction of a highway near their convent, insisting that quiet and serenity were essential to their work. They protested, even though the noise at the site was predicted to increase by a mere 10 dBA above the present 45 dBA. They may not have realized how valid their objections were. Highway noise levels typically are stated in terms of the energy-average levels observed during a day’s loudest one hour period – and changes in this noise level clearly do not account for the greater interruption of the evening quiet by short-duration loud noise intrusions from passing trucks. 

Quite a different situation exists in the “Land of the Rising Decibels,” as described in a recent newspaper article.  According to this article, the “Japanese are subjected to a variety of clatter that is perhaps unlike anywhere else in the world.” Not only do their vending and ATM machines talk to customers with electronic voices and escalators tell them to watch your step, but there also are demonstrators with bullhorns everywhere. Even in rural towns one can hardly escape from the ubiquitous public address systems which spew forth messages at all hours of the day and night. Some public address proclamations quoted in the article include, “Children, go home, it’s getting dark.” “Don’t use too much water, it hasn’t rained in recent days.” “Make sure the stove is off before you go to bed.” On trains, passengers are instructed to turn off their cell phones, with announcements that are much louder and more annoying than the telephones themselves. 

Although there is much quiet objection (pun intended) to this noise pollution, a citizens’ group organized about a decade ago to fight this pollution reportedly has had little success, largely because some of their cultural attitudes prevent the Japanese from expressing their discontent publicly. 

We’ve all heard that one person’s music is another person’s noise. But quiet may not be the optimum situation and what is noise to one person may be music to another. I’ve been in noisy offices of plant managers who were happy to hear the production machinery; they felt that they were making money as long as everything was running, and quiet was an indication of trouble. Some plant personnel could even identify problems from changes in the noise they heard in their offices and they objected strongly to any proposal to give them more quiet. 

In a recent survey of workers in cubicles in open-plan offices, about 70% reported that noise was the number one distraction, with conversational noise and the lack of acoustical privacy as the leading cause of acoustical dissatisfaction and stress. The most practical solution here consists of making more noise – adding a ‘masking’ noise to reduce the information content of the total noise perceived by a listener. The installation of sound masking systems has become more prevalent in recent years and studies have shown that use of such systems has resulted in significant productivity improvements.