SOUND

Production of sound When you place your fingertips against the front of your throat, you can feel the vibration of your vocal cord when you make a noise. If we bang a tuning folk on a cork, the prongs vibrate and we hear a sound. If the vibrating prongs quickly dipped into water, we see that water splashes.

Propagation of sound wave These examples shows that sound is caused by vibration. Any vibrating object can be a source of sound waves. Sound waves are longitudinal in nature. Propagation of sound wave When a loudspeaker cone vibrates, it moves forward and backward very fast. This squashes and stretches air infront. As a result a series of compressions (‘squashes’) and rarefactions (‘stretches’) travel out through the air. These are sound waves. When they reach your ears, they make your ear-drum vibrate and you hear a sound.

Compression Compression is the region where the particles are close together and has high pressure. Rarefaction Rarefaction is the region where the particles are further apart and has low pressure. Audible frequencies Human ear can only hear sounds with a frequency of between 20Hz to 20 000Hz (20Hz to 20kHz). This frequency range is called range of audible frequencies.

Sound waves need a material to travel through This material is called a medium. Without it, there is nothing to pass on any oscillations. Sound cannot travel through a vacuum (completely empty space). The experiment below is to prove that sound waves need a medium to travel through: Put the electric bell into a jar and remove all the air inside the jar with the help of vacuum pump. Then connect the bell into the battery and switch on, since the bell ring no sounds hear the outside of the jar. This means sound required medium to travel through.

Speed of sound wave Sound can travel through solids, liquids, and gases. Speed of sound varies with the medium through which it travels. Sound travels faster in solids (concrete 5000m/s), then in liquids (1400m/s in pure water) and then in gases (330m/s in air). High speeds in solids are due to the strong force of attraction between molecules. Speed of sounds depends on temperature.

Measuring of speed of sound in air Apparatus needed: stopwatch, starting pistol and measuring tape (trundle wheel). Procedure: Observers A and B are positioned at a known distance d apart in an open field. Record the distance d with the measuring tape. (d must be about 1km) Observer A fires the pistol Observer B, on seeing the flash of starting pistol, starts the stopwatch and then stops it when he hears the sound. The time interval t is recorded. Repeat the time taken t and find the average value of time. Then calculate the speed of sound s in the air by using the following formula. s= d t

Echo sound Sound waves can be reflected by large, flat and hard surfaces like walls and cliffs. If the reflected sound is heard after an interval of time, it is called an ECHO. Echo sound can be calculated using the formula: s= 2d t Worked example: To estimate the width of a valley, a climber starts a stopwatch as he shouts. He hears an echo from the opposite side of the valley after 4.0 s. s= 2d t 2d = s x t 2d = 340 x 4 d= 1360 2 d = 680 m

Characteristics of sound The characteristics of sound can be shown by connecting sound producing source to C.R.O.

Loudness The loudness of a sound wave depends upon the amplitude of the wave. A sound wave with a larger amplitude contains more energy and is therefore louder.

Pitch The pitch of a sound wave depends on the frequency of the wave. Sound of higher frequency (shorter wavelength) has a higher pitch.

Sound quality (timber) The quality of the sound depends on the shape of the waveform.

Ultrasound Uses of ultrasound The sounds above the range of audible frequency (20Hz to 20 kHz) are called ultrasounds. Uses of ultrasound Ultrasounds are used to pre-natal scanning (womb scanning) and also to examine the inside of human body.

It is also used cleaning delicate (sensitive) machinery It is also used cleaning delicate (sensitive) machinery. The machinery is immersed in tank of liquid, then the vibration of high power ultrasound are used to dislodge (remove) the bits of dirt’s and grease.

Ultrasound is also used to quality control (detect flaws in metals) Ultrasound is also used to quality control (detect flaws in metals). The echo sounding principle can be used detect flaws in metals. A pulse of ultrasound is sent through the metal, the there is a flaw (tiny gap) in the metal, the reflected pulse are picked up by the detector and shows on C.R.O screen.

Pulses of ultrasound are sent into the metal from a transmitter Pulses of ultrasound are sent into the metal from a transmitter. A detector is placed next to the transmitter on the front surface of the metal. Diagram below shows the oscilloscope trace of the ultrasound pulses produced if the metal contains no cracks.

Ships use echo – sounders to measure the depth of water beneath them Ships use echo – sounders to measure the depth of water beneath them. An echo – sounder sends pulses of ultra sound downwards towards the seabed, then measures the time taken for each echo (reflected sound) to return. The longer the time, the deeper the sea.

The diagram below shows cathode-ray oscilloscope (c. r. o The diagram below shows cathode-ray oscilloscope (c.r.o.) trace of the pulses of ultrasound sent from the ship and the reflected pulses.

Bat uses ultrasound to find insects and other things in front of it Bat uses ultrasound to find insects and other things in front of it. It send out ultra sound pulses and use its specially shaped ears to pick up the reflections.