Fundamentals Of Acoustics (1950)

acoustics is the science that deals with sound in this film we will interpret some of the basic principles of acoustic invisible waves of sound behave much like visible waves of water when water is disturbed waves form that move away from the center of disturbance the waves move until they strike an obstacle and the reflected back or until their energy is dissipated sound waves travel in a similar manner sound in air travels about 1,100 feet a second since light waves move much faster than sound waves the cause of a sound may be seen before the sound is hurt reflected sounds are called echoes echoes particularly in doors are not distinct because the successive reflected waves interfere with the original weight upon striking and obstacles such as a wall part of the wave energy changes into heat our passes through the obstacle and part is reflected the dashed lines represent reflected waves that are nearly dissipated you have been hearing this explanation under nearly ideal conditions if the walls of a room are lined with a hard not absorbing materials they reflect most of the sound wave the sound of this kind in this story they call the reflected waves have fired for overall any time before they are this pace if the walls are covered with some thick porous material they absorb much of the energy of the sound waves sound produced under this condition lacks the live quality that usually characterizes indoor speech the live quality is lost because there is so little reflection a curved surface in an auditorium may be detrimental to good room acoustics because it reflects and focuses the sound waves much as a curved mirror reflects and focuses light waves hear echoes follow the original sound by an appreciable time interval and interfere with clear reception such an obvious structural defect can of course be avoided in the construction of a building often there are other conditions that affect listening enjoyment to study telephone acoustical engineers may use the oscilloscope this instrument pictures invisible sound waves by translating them into visual patterns then the visual patterns can be studied here for example is the pattern of a group of wood with musical sounds produce regular pattern noises produce irregular patterns even at low intensity noises tend to irritate the listener musical sounds tend to be enjoyable if a study of Soundwave patterns reveals undesirable components steps may be taken to control audible sounds are transmitted to the brain by the ear sound waves first reach the outer here this is the part of the year that collects sound waves and directs them to the eardrum changes in pressure caused by sound waves affect the eardrum as they would affect any other diaphragm so the pressure changes caused by longitudinal waves of sound in air or influenced cause to hear drums of vibrate just beyond to hear drum is a cavity called the middle here in this part of the here are the three small bones the transmitted vibrations of the eardrum to the inner ear the inner ear which is spiral in shape is filled with fluid when the bones of the middle here vibrate the vibrations are transmitted to this flew within the spiral-shaped inner here is a membrane in which nerve endings of the auditory nerve are located vibrations in the fluid excites these nerve endings and transmit nerve impulses along the auditory nerve to the brain when the sound wave impulses arrive at the brain the person experiences the sensation of hearing the healthy normal human ear is sensitive to a wide variety of sound we can represent the range of human hearing on a graph normal hearing ranges inaudible intensity or loudness from the faint rustle of leaves represented by the lowest part of the curve to the boom of distant artillery represented at the top in terms of audible frequency the average person hears from about 20 cycles to about 16,000 side sector that's considerably more than the rid of the piano scape the range or area of hearing varies between individuals as shown by the zone between the two outer sets of line modern electrical reproduction approaches the audible frequency limits of the original sound employed when normal amplification is reduced in volume hearing is affected notice a change of quality and speech caused by this unnatural baggage eliminating the lower frequencies makes another change in South this causes and unnaturalness particularly in the vowels eliminating all frequencies above three thousand cycles causes another important change this change in audible frequencies affect the consonant and finally here is sound with both high and low frequencies eliminated this kind of distortion produces a very noticeable effect on musical sounds as well as on speech in singing p overtones of the highest note may reach about 7500 vibrations second these sound waves of course are audible the oscilloscope can also reveal sounds that are not audible to our ears one way to produce inaudible sounds is to use an audio oscillator first the dials are turned to the range of audible sound now the oscillator is producing a 30,000 cycles sound we can't hear but this is how it looks on the oscilloscope screen and here is a 50,000 cycle sound such frequencies above the range of hearing are super sonic or ultrasonic sounds super sonic devices all sonar are used to detect objects underwater wartime detection may lead to the destruction of enemy submarines in the air the supersonic sound barrier has been overcome so that speeds greater than the speed of sound are now attained acoustical research progresses on many fronts for example this room called an anechoic chamber is constructed to absorb nearly all sound wave the absorbent materials are packed into cones in this room engineers can test the acoustical properties of microphones loudspeakers and other equipment with almost no interference from echoes the results of acoustical research can be applied to all kinds of situations involving sound principles of acoustics are of particular significance or it is through the application of these principles that sounds are transmitted from their sources to the listen