What are the Characteristics of Sound Waves

Characteristics of Sound Waves

Sound travels in the form of a wave. Waves are characterized by three basic quantities. They are frequency, speed, and amplitude. Two of the main characteristics of sound are pitch and loudness, which in turn are determined by the frequency and amplitude of the wave, respectively. Therefore, to understand what pitch and loudness mean, we will first need to study the characteristics of waves. We will use the example of oscillations of a simple pendulum to explain the terms amplitude and frequency. Although a simple pendulum does not produce sound waves that we can hear, this example will help us observe oscillations and understand the terms involved. The diagram of a simple pendulum is given in figure. It consists of a small ball (called the bob) attached to a string, which is fixed at one end.

Amplitude: If we pull the bob sideways and leave it, it will oscillate back and forth for some time and then come to a stop. The position where it comes to a stop is called the mean position (position A in figure). The maximum displacement of the bob from the mean position during oscillation is called the amplitude of the oscillation. In figure, the maximum displacement occurs when the bob is at positions B and C.

Time period: When the bob moves from one position and comes back to the same position (moving in the same direction), it is said to complete one oscillation. For example, in figure, if the bob starts from A, goes to C, then to B, and then back to A, we say one oscillation is complete (look at the arrows in figure). The time taken to complete one oscillation is called the time period of the oscillation. It is measured in seconds.

Frequency: The number of oscillations per second is called the frequency of oscillation. For example, if the bob of the pendulum in figure moves five times through point B in a second, its frequency is 5 per second. The SI unit of frequency ‘per second’ is called hertz (Hz) in honour of the German physicist, Heinrich R. Hertz. When we say that a vibrating body has a time period t, we mean that it completes one oscillation in t seconds. Thus, in 1 second it will complete 1/t oscillations, which is its frequency.

Frequency and pitch: Sound waves are produced due to the to and fro oscillation of particles in a medium. If an object oscillates 80 times per second, it is said to have a frequency of 80 Hz. Frequency is considered an important characteristic of a sound wave because different frequencies sound different to us. You might have noticed the difference between the shrill voice of a child and the deep voice of a man. The shrillness of the sound produced is determined by the frequency of the vibrating body. Shrillness is also explained using a term called pitch. Higher the frequency of the vibrating body, the higher will be its pitch. A high-pitched sound appears shrill, and a low- pitched sound appears deep or gruff to our ears.

Amplitude and loudness: The amplitude of the vibrating body producing the sound determines the loudness of the sound. If the amplitude is higher, the sound produced is louder.

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LOUDNESS

Loudness of a sound depends on the amplitude of the vibration producing that sound. Greater is the amplitude of vibration, louder is the sound produced by it. The loudness of a sound also depends on the quantity of air that is made to vibrate. Loudness of sound is measured in decibel (dB) unit.

PITCH

• The shrillness of a sound is called its pitch. The pitch of a sound depends upon its frequency. Higher the frequency of a sound, higher is its pitch.
• The voice of a child or a woman has higher frequency than the voice of a man.
• The faster is the vibration of the source object, higher is the frequency and therefore higher is the pitch.
• Higher pitch of any sound corresponds to larger number of compressions and rarefactions passing a point per unit time.
  
  
• The stretched membrane of a tabla or mridangam produces sound of a higher frequency (or of higher pitch).

QUALITY

• Quality of a sound is also called timbre. The quality of sound is the characteristic which enable us to distinguish between the sounds produced by different sources.
• The more pleasant sound is said to be of rich quality.
• A sound of single frequency (called pure sound) is called a tone.
• A tuning fork produces the sound of a single frequency. The sound which is a mixture of several frequencies is called an impure sound (or note) is pleasant to listen.
• Different instruments, depending on their shape and size, produce different number of harmonics of different relative loudness. As a result, the sound produced by an instrument can be distinguished from that produced by other instruments.