EL PROCESO DE FAENADO

• understand that sound waves are the vibrations or oscillations of the particles of a medium

• relate compressions and rarefactions of sound waves to the crests and troughs of a transverse wave representation from a cathode-ray oscilloscope (CRO)

• perform an investigation that enables you to gather information to determine the relationship between the frequency and wavelength of a sound wave in a medium of constant properties

• use a CRO to gather information about the

frequency, amplitude and velocity of sound waves and to observe and analyse the different sources of sound waves

• explain that pitch is related to the frequency of a sound wave, and volume is related to the amplitude of a sound wave

• explain that an echo is the reflection of a sound wave • identify the conditions necessary to hear an echo • describe superposition and present graphical

information showing the superposition of waves • use a CRO to observe the superposition of two sound

Sound is everywhere in our lives. Hearing is one of our dominant senses and people who lose the ability to hear usually take significant steps to restore it; for example, with hearing aids such as the bionic ear or cochlear implant. Sound is all around us, yet we often fail to notice it. Stop for a few seconds and listen to the sounds you can hear. Your sound-wave detector, the ear, is extremely sensitive. Most young people have the ability to hear sounds within the frequency range 20 Hz to around 20 000 Hz. Unfortu- nately, as you get older this range compresses as the ear loses sensitivity.

2.1

SOUND: VIBRATIONS IN

A MEDIUM

All sound waves are vibrations in a medium that result in pressure variations within that medium. In air, the presence of those vibrations you know as sound is relatively easy to detect. You can see the vibrations and their effect by placing a piece of paper in front of your mouth in contact with your lips and speaking loudly. You will feel the paper vibrate because of the pressure differences in the vibrating air or sound wave. Alternatively, place a piece of paper in front of a stereo speaker, turn up the volume and observe and feel the paper vibrating. Sound pressure waves can blow a person off their feet if the sound is loud enough (sufficiently high in amplitude).

The origin of a sound wave in any medium is always a vibration. The frequency of the original vibration determines the frequency of the sound produced by that vibrating object. If you touch any object producing sound you can probably feel those vibrations. The higher the pitch of the sound, the faster the rate of vibration of the object. The object’s vibration transfers some of its energy of movement to the medium that carries the sound wave or vibrational energy away from the source.

The drum is a good example of a device acting as a source of vibrational energy. As shown in figure 2.2, the back and forth vibrations of the drum skin produce air-pressure differences. This produces a vibration effect in the air particles that results in zones of high air pressure (compression) and zones of low air pressure (rarefaction).

Throughout this chapter much emphasis will be placed on the use of the cathode-ray oscilloscope (CRO) or alternative computer technology to see sound waves. Its use is a mandatory part of your study of sound.

A compression is a zone where the particles of the medium are pushed closer together. It is a zone of higher pressure.

A rarefaction is a zone where the particles of the medium are spread further apart. It is a zone of lower pressure.

Rarefaction Compression

Membrane of drum

Figure2.2 Production of a sound wave in air by the vibrating skin of a drum

Sound can travel within solids or liquids as well as in gases. You can feel or hear the vibrations you know as sound in a solid simply by touching the solid or placing your ear against it. Consider the following examples. • Victims of earthquakes who are trapped under rubble are often found because, rather than shout, they tap on a solid material and generate sound waves within it. The sound travels more easily through the overlying rubble within the solid material.

• The approach of a train can be detected in the track long before the sound of the train is heard through the air.

• The song of a whale can be heard hundreds of kilometres away with sensitive microphones designed to pick up those sounds underwater. Whales are thought to use this property of sound to communicate with each other over vast distances.

In all cases where sound waves are transmitted, the sound wave propa- gates through the material as a vibration of the particles in the form of a pressure wave. As mentioned in chapter 1, that pressure wave consists of compressions and rarefactions of the particles of the medium.

2.2

‘SEEING’ SOUND WAVES

The cathode-ray oscilloscope (CRO) is a device that allows us to view sound waves on a screen (see figure 2.3). The CRO plots or traces the amplitude of the input wave- form against time and displays the wave shape on the screen by means of a cathode-ray tube.

Figure 2.4 shows the trace of a sound wave from the screen of a CRO. The areas where the displacement of the wave is above the base line represent zones of com- pression. The areas where the displacement of the wave is below the base line represent zones of rarefaction. In reality, what happens is that the sound-wave energy is con- verted into an electrical signal at the microphone. The size of the electrical voltage induced at the microphone is a function of the pressure of the air striking the microphone diaphragm. That pressure differential changes the voltage to a higher or lower value as it passes into the CRO. That voltage input registers on the screen as a trace of a waveform — providing the trace of the sound signal.

You may have noticed how the shape of the trace is a sine wave. This is because this wave trace was produced by a constant-frequency source (tuning fork) that produces a pure tone. The wave’s traces are hence symmetrical about the base line. Often naturally occurring sounds are not symmetrical because they are not pure sounds of only one tone. The base line represents silence in this case. A trace showing only the base line would indicate no sound at all.