THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

2.1 Wave Nature of Sound and the Importance of Acoustics 15period T is simply the frequency f . The most common unit of frequency usedin acoustics (and electromagnetic theory) is the hertz (abbreviated Hz in the SIsystem), which is equal to one cycle per second. An acoustic signal may or maynot be audible to the human ear, depending on its frequency content and intensity.If the frequencies are sufficiently high (>20 kilohertz, which can be expressedmore briefly as 20 kHz), ultrasound will result, and the sound is inaudible to thehuman ear. This sound is said to be ultrasonic. Below 20 Hz, the sound becomestoo low (frequency-wise) to be heard by a human. It is then considered to beinfrasonic.Sound in the audio frequency range of approximately 20 Hz–20 kHz can be heardby humans. While a degree of subjectivity is certainly entailed here, noise conveysthe definition of unwanted sound. Excessive levels of sound can cause permanenthearing loss, and continued exposure can be deleterious, both physiologically andpsychologically, to one’s well-being.With the advent of modern technology, our aural senses are being increasinglyassailed and benumbed by noise from high-speed road traffic, passing ambulancesand fire engine sirens, industrial and agricultural machinery, excessively loud radioand television receivers, recreational vehicles such as snowmobiles and unmuffledmotorcycles, elevated and underground trains, jet aircraft flying at low altitudes,domestic quarrels heard through flimsy walls, and so on.Young men and women are prematurely losing their hearing acuity as the resultof sustained exposure to loud rock concerts, discotheques, use of personalcassette and compact disk players and mega-powered automobile stereo systems.In the early 1980s, during the waning days of the Cold War, the Swedish navyreported considerable difficulty in recruiting young people with hearing sufficientlykeen to qualify for operating surveillance sonar equipment for trackingSoviet submarines traveling beneath Sweden’s coastal waters. Oral communicationcan be rendered difficult or made impossible by background noise; andlife-threatening situations may arise when sound that conveys information becomesmasked by noise. Thus, the adverse effects of noise fall into one ormore of the following categories: (1) hearing loss, (2) annoyance, and (3) speechinterference.Modern acoustical technology also brings benefits: it is quite probable that theavailability (and judicious use) of audiophile equipment has enabled many of us,if we are so inclined, to hear more musical performances than Beethoven, Mozartor even the long-lived Haydn could have heard during their respective lifetimes.Ultrasonic devices are being used to: dislodge dental plaque; overcome the effectsof arteriosclerosis by freeing up clogged blood vessels; provide noninvasive medicaldiagnoses; aid in surgical procedures; supply a means of nondestructive testingof materials; and clean nearly everything from precious stones to silted conduits.The relatively new technique of active noise cancellation utilizes computerizedsensing to duplicate the histograms of offending sounds but at 180 degrees outof phase, which effectively counteracts the noise. This technique can be appliedto aircraft to lessen environmental impact and to automobiles to provide quieterinteriors.