Characteristics-of-a-sound-wave

CHARACTERISTICS OF A SOUND WAVE.

Sound is a longitudinal wave which consists of compressions and rarefactions travelling through a medium A sound wave can be described completely by five characteristics : Wavelength, Amplitude, Time-period. Frequency and Velocity (or Speed). All these characteristics of a sound wave are described below.

consider the longitudinal sound waves ABCDE and EFGHI formed by the vibrations of a tuning fork The first sound wave starts from the normal density position A and after going through compression along ABC and rarefaction along CDE, it returns to the normal density position E. Thus, the portion ABCDE represents one complete sound wave. The next sound wave starts from point E and goes up to point I (and so on). Thus, the above Figure shows two complete sound waves : from A to E, and from E to I. We will now describe the wavelength, amplitude, time-period, frequency and velocity (or speed) of the sound wave.

1. Wavelength.

The minimum distance in which a sound wave repeats itself is called its wavelength. In most simple words, it is the length of one complete wave. The wavelength is denoted by the Greek letter lambda, 2. In a sound wave, the combined length of a compression and an adjacent rarefaction is called its wavelength.For example, in Figure 13, the length of compression is AC and that of the adjacent rarefaction is CE, so the combined length AE is equal to wavelength. In a sound wave, the distance between the centres of two consecutive compressions for two consecutive rarefactions) is also equal to its wavelength. In Figure 13, the distance between the centres of two consecutive compressions is PR, so the distance PR represents wavelength of the sound wave. Similarly, the distance between the centres of two consecutive rarefactions is QS, so the distance QS also represents Wavelength. Please note that the distance between the centres of a compression and an adjacent rarefaction is equal lo half the wavelength 6). For example, the distance PQ in Figure 13 is half wavelength 6). The SI unit for measuring wavelength is metre (m). Sometimes, however, centimetre unit is also used for expressing wavelength.

2. Amplitude

When a wave passes through a medium, the particles of the medium get displaced temporarily from their original undisturbed positions. The maximum displacement of the particles of the medium from their original undisturbed positions, when a wave passes through the medium, is called amplitude of the wave. The term amplitude is, in fact, used to describe the size of the wave. In Figure 13, PB is the amplitude of the wave. The amplitude of a wave is usually denoted by the letter A. The SI unit of measurement of amplitude is metre (m) though sometimes it is also measured in centimetres. It should be noted that the amplitude of a wave is the same as the amplitude of the vibrating body producing the wave.

3. Time-Period

The time required to produce one complete wave (or cycle) is called time-period of the wave. Now one complete wave is produced by one full vibration of the vibrating body. So, we can write another definition of time period as follows: The time taken to complete one vibration is called time period. Figure 13 shows two complete waves (one wave from A to E and another wave from Eto 1). Suppose these two waves are produced in 1 second. Then the time required to produce one wave will be 5 second or 05 second. In other words, the time period of this wave will be 0.5 second. The time-period of a wave is 5 denoted by the letter T. The unit of measurement of time-period is second (s).

4. Frequency

The term frequency tells us the rate at which the waves are produced by their source. The number of complete waves (or cycles) produced in one second is called frequency of the wave. Since one complete wave is produced by one full vibration of the vibrating body, so we can also say that: The number of vibrations per second is called frequency. If 10 complete waves (or vibrations) are produced in one second, then the frequency of the waves will be 10 hertz (or 10 cycles per second). The frequency of a wave is fixed and does not change even when it passes through different substances. The SI unit of frequency is hertz (which is written as Hz). A vibrating body emitting 1 wave per second is said to have a frequency of 1 hertz. In other words, 1 hertz is equal to 1 vibration per second. Sometimes, however, a bigger unit of frequency called kilohertz (kHz) is also used (1 kHz = 1000 Hz). The frequency of a wave is denoted by the letter f, though in some books, they use v (nu) to denote frequency. The tuning forks are often marked with

numbers like 256,384 or 512, etc. These numbers signify the frequency of vibration of the tuning forks. For example, a tuning fork of frequency 256 means that its prongs will make 256 vibrations per second and emit 256 complete sound waves per second when hit on a hard surface. It should be clear by now that the frequency of a wave is the same as the frequency of the vibrating body which produces the wave.