8.1 Origin of Sound Sound Sound is a variation in the pressure of the air of a type which has an effect on our ears and brain. These pressure variations transfer energy from a source of vibration that can be naturally-occurring, such as by the wind or produced by humans such as by speech. Sound in the air can be caused by a variety of vibrations, such as the following. Moving objects: examples include loudspeakers, guitar strings, vibrating walls and human vocal chords. Chapter 8: Sound Page 1
Moving air: examples include horns, organ pipes, wood and brass instruments, mechanical fans and jet engines. 8.2 Wave motion The mechanical vibrations of sound move forward, using wave motion. This means that, although the individual particles of material such as air molecules return to their original position, the sound energy obviously travels forward. The front of the wave spreads out equally in all directions unless it is affected by an object or by another material in its path. The waves are longitudinal in type because the particles of the medium carrying the wave vibrate in the same direction as the travel of the wave, as shown in following figures. The sound waves can travel through solids, liquids and gases, but not through a vacuum. Chapter 8: Sound Page 2
Sound Waves in Air A single-frequency sound wave traveling through air will cause a sinusoidal pressure variation in the air. The air motion which accompanies the passage of the sound wave will be back and forth in the direction of the propagation of the sound, a characteristic of longitudinal waves. Physics professor Clint Sprott of the University of Wisconsin shows one way to visualize these longitudinal pressure waves in his "Wonders of Physics" demonstration show. A loudspeaker is driven by a tone generator to produce single frequency sounds in a pipe which is filled with natural gas (methane). A series of holes is drilled in the pipe to release a small amount of gas. Igniting the gas produces flames for which the height increases with the pressure in the pipe. The pattern of the flames shows the pressure variation and can be used to roughly measure the wavelength of the pressure wave in the pipe. Low frequency High frequency Shown below is more detail on the attachment of the loudspeaker to the pipe. The loudspeaker is driven by the amplified output of a tunable oscillator. A series of small holes were drilled at regular intervals in the pipe. They appeared to be about 8 mm apart. This article was taken from: http://hyperphysics.phy-astr.gsu.edu/hbase/sound/tralon.html Chapter 8: Sound Page 3
8.3 Sound Calculations The velocity of sound is influenced by three factors: Wavelength () is the distance between any two repeating points on a wave. Unit: metre (m) Frequency (f ) is the number of cycles of vibration per second. Unit: hertz (Hz) Velocity (v) is the distance moved per second in a fixed direction. Unit: metres per second (m/s) For every vibration of the sound source the wave moves forward by one wavelength. The number of vibrations per second therefore indicates the total length moved in 1 second; which is the same as velocity. This relationship is true for all wave motions and can be written as the following formula. Where v = velocity in m/s f = frequency in Hz = wavelength in m Practical Example 1 A particular sound wave has a frequency of 440 Hz and a velocity of 340m/s. Calculate the wavelength of this sound. Answer v = 340 m/s f = 440 Hz =? m So So wavelength = 0.7727 m Chapter 8: Sound Page 4
8.4 Velocity of sound A sound wave travels away from its source with a speed of 344 m/s (770 miles per hour) when measure in dry air at 20 o C. This is a respectable speed within a room but slow enough over the ground for us to notice the delay between seeing a source of sound, such as a distant firework, and later hearing the explosion. The velocity of sound is independent of the rate at which the sound vibrations occur, which means that the frequency of a sound does not affect its speed. The velocity is also unaffected by variations in atmospheric pressure such as those caused by the weather. But the velocity of sound is affected by the properties of the material through which it is travelling, and table 8.1 gives an indication of the velocities of sound in different materials. The velocity of sound in gases decreases with increasing density since the molecules are heavier. Moist air contains a greater number of light molecules and therefore sound travels slightly faster in moist humid air. Sound travels faster in liquids and solids than it does in air because of the effect of density and elasticity of those materials. The particles of such materials respond to vibrations more quickly and so convey the pressure vibrations at a faster rate. For example, steel is very elastic and sound travels through steel about 14 times faster than it does through air. Table 8.1 Velocity of sound Material Typical velocity (m/s) Air (0 o C) 331 Air (20 o C) 344 Water (25 o C) 1498 Pine 3300 Glass 5000 Steel 5000 Granite 6000 Chapter 8: Sound Page 5
8.5 Frequency of sound If an object that produces sound waves vibrates 100 times a second, for example, then the frequency of that sound wave will be 100Hz. The human ear hears this as sound of a certain pitch. Pitch is the frequency of a sound as perceived by human hearing. Low-pitched notes are caused by high-frequency sound waves and highpitched notes are caused by high-frequency waves. The pitch of a note determines its position in the musical scale. The frequency range to which the human ear responds is approximately 20 to 20 000 Hz and frequencies of some typical sounds are shown in the following figures. Chapter 8: Sound Page 6
8.6 Cancellation of sound The nature of a sound wave means that the vibration of the wave has alternate changes in amplitude called phases. If a wave vibration in one direction meets an equal and opposite vibration, then they will cancel. The effect of this phase inversion in sound waves is to produce little or no sound and gives the possibility of cancelling noise. Chapter 8: Sound Page 7
8.7 Nature of hearing The ear is divided into two parts: the outer and the inner ear. The outer ear contains the cartilage that forms the external ears and a tube that directs sound into the inner ear. The eardrum is at the bottom of this tube which vibrates when sound reaches it. Connected to the eardrum is a series of bones called ossicles. A mechanism passes the sound as vibration via these bones into the inner ear. Here a liquid vibrates which contains tiny hairs which move with the sound vibration. When they move this is transmitted to the brain in the form of an electrical signal which is interpreted as sound. Having two ears gives humans the ability to detect from where sound is coming in stereo. Go to the following site for more information about sound. http://www.ndt-ed.org/educationresources/highschool/sound/introsound.htm Chapter 8: Sound Page 8
Exercise 8.1 1. Calculate the wavelength of sound in air at: i) 20 Hz ii) 50 Hz The velocity of sound in air is 340 m/s 2. The frequency of the note produced by a particular tuning fork is 320 Hz. Calculate the wavelength of the wave produced in the surrounding air if the speed of sound in air is 340 m/s. 3. If the sound from the tuning fork in (2) was detected under water, what frequency would be heard? 4. If the wavelength in (3) of water is 4.5m, at what speed does the wave travel through the water? Chapter 8: Sound Page 9