Acoustics

Acoustics is the branch of physics dealing with the production, transmission and reception of sound. It matters enormously in the construction of theatres and auditoriums, where careless design leaves parts of the audience unable to hear clearly. A well-designed auditorium needs, at minimum: (1) sound heard SUFFICIENTLY LOUDLY at every point in the hall -- typically achieved by shaping the surface behind the speaker/performer as a PARABOLA with the speaker positioned at its focus, so reflected sound distributes uniformly across the whole audience (reflection, used carefully like this, actually HELPS maintain good loudness throughout); (2) echo and excessive reverberation ELIMINATED or minimised, mainly by making reflecting surfaces more absorptive (a full auditorium, with its many sound-absorbing bodies, also reduces echo compared to an empty one); (3) no unwanted FOCUSING of sound and no dead 'zones of silence' -- curved wall or ceiling surfaces, which can inadvertently focus sound onto a small region while leaving other areas quiet, should be avoided; (4) no echelon effect -- a distinctive, unwanted repeated-echo mixing caused by sound reflecting off a regular, evenly-stepped structure like a staircase, avoided simply by not using stair-type construction inside the hall itself; (5) the auditorium should be SOUND-PROOF when closed, so stray outside noise cannot leak in; and (6) internal fittings, seats and fixtures should themselves be quiet -- air conditioners rather than noisy fans, soft-action door closers, and so on.

Acoustics observed in nature. Several animals rely on sound, not light, for navigation and hunting. Bats fly confidently through total darkness by emitting short ultrasonic pulses (roughly 30 kHz to 150 kHz) and interpreting the returning echoes to locate obstacles and prey -- a natural SONAR. Dolphins use an analogous underwater system, but at much lower, sub-sonic frequencies (around 100 Hz), letting them detect objects roughly the size of their emitted wavelength (about 1.4 m) or larger.

Medical applications of acoustics. High-pressure, high-amplitude SHOCK WAVES, generated outside the body and focused by a reflector or acoustic lens onto a kidney stone, can break the stone into small, easily-passed pieces without invasive surgery, once the induced stress in the stone exceeds its tensile strength. Reflected ULTRASONIC waves from structures deep inside the body underlie ultrasonic (ultrasound) imaging, used for prenatal examination, detecting abnormal growths such as tumours, and studying heart-valve action. At very high power, ultrasound can even selectively destroy diseased (pathological) tissue, used in treating arthritis and certain cancers. …