Physics: Sound
Sound waves come from vibration of material objects. For example, vibrations of vocal cards in the larynx, and the vibrating strings of the sitar produce sounds. The frequency of the sound wave is same as the frequency of the vibrating source. A medium such as air, liquid or solid is required for transmission of sound. Solids and liquids are good conductors of sound whereas air is a poor conductor. Sound waves cannot travel in vacuum. Sound waves are longitudinal waves in gases and liquids, but they can be either longitudinal or transverse waves in solids.
Key Features of Sound
Pitch and Frequency
The sensation of a frequency is referred as pitch of a sound. A high pitch sound corresponds to a high frequency sound wave and a low pitch sound corresponds to a low frequency sound wave. The human ears are sensitive detectors to sounds with frequencies between 16 and 20,000 Hz. Any sound with a frequency below 16 Hz is known as infrasonic and above 20,000 Hz is known as ultrasonic. Cats and dogs are capable of hearing sounds of frequencies higher than 20,000 Hz. Dolphins can produce high pitched sounds of frequency as high as 1,00,000 Hz.
Loudness
The loudness of a sound wave depends on the amplitude of the wave. The bigger the amplitude, the louder the sound. The loudness of a sound is measured in decibels (db). Exposure to a noise level of 85 db or above can damage or impair hearing.
Whenever there is a need to increase loudness of a sound, it can be achieved by setting a greater mass of air into vibration. For example, instruments such as violin, guitar, sitar etc. have sound boxes attached to increase the loudness. In a loudspeaker, the vibrating cone has a large surface area and a large mass of air in contact with the cone is set into vibration to produce a loud sound.
The Speed of Sound
The speed of a sound does not depend on its pitch and loudness. At 0°C in dry air, the speed of sound is about 331 meters per sound and at room temperature in air it is 344 meters per second. Sound waves travel faster through warm air than cold air. It is calculated that for each degree rise in temperature, the speed of sound is increases by 0.61 metre per second. The speed in air slightly increases with presence of water vapour i.e. the speed of sound increases with humidity. The speed of sound also depends on the medium. It is high in solids, less in liquids, and the least in gases. For example, in steel the speed of sound is nearly 15 times as great as in air. The speed of sound is much less than the speed of light. This is the reason why thunder is heard much after the flash is seen. Similarly, the sound from an airplane does not appear to come from the plane at all, but from a point far behind it.
Reflection of Sound, Echo
Whenever waves meet an obstacle, they have the property of being reflected. When a sound wave reflects after hitting a distant object such as a wall, an echo is heard. But the minimum distance of the reflecting surface from the source of sound to hear an echo is 17 metres. If the distance is less than 17 metres, then the echo reaches us in less than 0.1 second and the echo cannot be distinguished as a separate sound. It gives the impression of the original sound being prolonged. This is called reverberation. Reverberation can also occur when a series of echoes are heard due to more than one reflecting surface.
The speed of sound can be measured by using an echo. For example, echoes of ultrasonic waves can be used to measure depth of sea-beds or finding the location of submerged objects. Ultrasonic waves are also used for finding faults in the interiors of solids and mapping of underground structures for oil and mineral deposits. Bats produce ultrasonic waves and use echoes to determine distance of the objects on their way. Ultrasonic waves are also used in medical diagnosis and treatment. Sound waves pass through various tissues, and from the pattern of echoes, tumours, lesions and other defects are detected.
Refraction of Sound
The character of sound waves to travel faster in warm air than in cold air causes bending of sound waves when they pass through successive layers of air that have different temperatures. This bending property is called refraction. On warmer days, the air near the ground is warmer than the air above and due to this the speed of sound waves near the ground is higher. It results in bending of the sound away from the ground. On colder days, the reverse will happen and the sound waves bend towards the earth. This is reason for hearing of sounds over longer distances on a cold day.
Resonance
Depending on the factors such as the elasticity and shape of the object, each vibrating object has a natural frequency. A resonance occurs when an object oscillates at its natural frequency, as a result of impulses received from some other system vibrating with the same frequency. Resonance can happen in different kinds of systems: acoustical, mechanical, electrical and optical. Resonance leads to increased amplitude of vibration. In some cases, the amplitudes that result from resonance can be disastrous. This is the reason for ordering soldiers to break up while crossing a suspension bridge. The resonant vibrations caused by the marching may severely damage the bridge. Oscillations also occur in an electrical circuit. A radio receiver is tuned to a particular frequency when the oscillating electrical circuit inside the radio is set into resonance with incoming signals.
Doppler Effect
The frequency of a wave changes depending on the motion of the source or observer. This is known as Doppler Effect. When the source approaches the listener, the frequency of a sound appears to be higher and vice versa.
Radar guns used by the police to check the speeding vehicles use Doppler Effect. The radar gun sends out a radio pulse and wait for the reflection. Then it calculates the Doppler shift in the signal to determine the speed of the vehicle. In astronomy, Doppler Effect is used to find out whether a star is approaching us or receding away from us. When a star is receding from us the light emitted from the star appears redder. Doppler Effect is also used to detect the rotation of a star or for tracking a moving object, such as a satellite, from a reference point on the earth.
Sonic Boom
A sonic boom is an impulsive noise similar to thunder caused by a supersonic (faster than sound) aircraft that produces a cone of sound called a shock wave.
Musical Scale
A musical scale is a group of pitches arranged in an ascending order. The diatonic scale includes the notes with frequencies: sa (256), re (288), ga (320), ma (341.3), pa (384), dha (426.7) and ni (480). The next note denoted by sa’ has a frequency 512, twice that of sa. The interval sa-sa’ is called an octave (8).
Noise Reduction in Recording Media
Music recording company Dolby Laboratories Inc. has developed techniques to reduce noise levels in recorded music. Dolby noise reduction works in tandem to improve the signal-to- noise ratio. Dolby A, Dolby B, Dolby C, Dolby SR and Dolby S are the noise reduction systems developed by the company.
