Sound Waves And Their Sources (2nd Ed, 1950)

[Music] three different sources of sound give voice to musical instruments vibr Columns of air as in the woodwinds vibrating surfaces such as drum heads and symbols and vibrating strings such as those of the string section but whether or not a sound is musical each of the countless sounds in our environment originates in some object that vibrates in this film we shall study how sounds are produced and the simple auditory differences of sounds to begin with what is involved in producing sounds to help answer this question we experiment first with the tuning fork when the fork is struck its prongs begin to vibrate these vibrations set up invisible waves in the surrounding air we can picture these waves as animated drawings striking the prongs distorts them like other solid objects these prongs resist Distortion they spring back and forth thus producing new distortions each movement or vibration sets up new disturbances or Soundwave impulses these continue as long as the prongs are in motion the speed with which sound waves move depends upon the medium through which they pass we will assume that these waves are being transmitted by air molecules every time the prongs move apart they push against the adjacent air in turn this movement affects the air further on in this segment of the wave we can see that some molecules are pushed relatively close together these represent condensations of air each time the prongs spring inward a partial vacuum forms in their vicinity air molecules nearby move into this vacuum leaving a rarified Zone behind thus there are alternate zones of condensation and rif faction of the air the number of times a given molecule moves back and forth in one second is the frequency of the wave in one complete vibration indicated here by the bracket sound travels a distance of one wavelength the velocity of any sound wave equals its wavelength times its frequency we can record pressure variations in sound waves with this kind kind of instrument an oscillograph sound waves are striking the diaphragm at the left the stylist records the sound waves as a graph later we shall use such graphs to represent sound waves condensations cause The Stylist to move upward ractions cause it to move downward so far we've seen how sound waves are produced Next we'll identify three auditory effects of musical sounds loudness pitch and quality we begin with loudness every sound sets up a disturbance in the transmitting medium the larger the disturbance or displacement of the individual particles the greater is the height of its graph above the neutral Line This displacement above the neutral line is called amplitude and it is amplitude that determines loudness as the F its loudness increases next let us consider pitch the auditory effect of frequency first we hear the sound of middle [Music] CA next we hear G below middle CA and now the sound of C below middle C what causes such differences in Pitch to explain such differences we picture two different sound [Music] waves here the lower Fork makes more complete vibrations per second therefore its frequency is greater [Music] it is evident that the one with the greater frequency has the higher pitch the frequency of a vibrating string depends upon the material it's made of and its density tension and length different instruments produce sounds of different tonal quality even when playing the same basic [Music] frequencies each vibrating may be made to produce different tones to explain some of the reasons for this let us observe this string here there is no motion at these end points or nodes the string vibrates along its full length and produces its longest possible wavelength this is the lowest or fundamental frequency of the string sometimes called the first partial if the string vibrates in two segments it produces a wavelength that's one half that of the fundamental this is the first overtone which may be called the second partial since wave length is inversely proportional to frequency the frequency of the first overtone is twice that of the fundamental when the string vibrates in three segments it produces its second overtone the third partial here the frequency is three times that of the fundamental but usually several different modes of vibration occur simultaneously the result is a compound waveform here we represent the fundamental tone supplemented by its first and second overtones differences in the audible components of a sound determine its Quality quality helps us recognize the characteristic sounds of various musical instruments and distinguish one voice from another with this oscilloscope we can represent as waves The Sounds being produced and see that their different modes of vibration as in the case of vibrating [Music] strings besides strings we've already considered vibrating [Music] surfaces the vibration from such diaphragms are often quite [Music] complex this diaphragm for example vibrates in a number of different [Music] segments the third of the principal sources of sound is the vibrating column of air the Organ Pipe like the woodwind produces Sound by means of such an air column often the exposed pipes of an organ are merely decorative and sounds are produced by the pipes in the organ LOF the wavelengths fitting the shorter pipes are shorter and their frequencies are greater than the longer pipes consequently the shorter pipes produce the sounds of higher pitch the different kinds of pipes produce different qualities of sound the organist uses stops and keys and pedals to admit compressed air into the pipes [Music] besides woodwind instruments and organ pipes another mechanism that depends upon a vibrating column of air for its sound is the vocal mechanism of man here sound waves result from vibrations of air that start in the vocal FS of the trachea these waves are reflected back and forth in the cavities of the mouth nose and head thus producing the complex sounds of speech and song so we've sampled a few out of the multitude of sounds in our environment and have studied the meanings of loudness pitch and quality from the vast number of vibrations that we interpret as sounds we may gather a wealth of meaning and enjoyment