Sound Waves Sound is a form of energy produced by a vibrating body. Sound waves are longitudinal.( and mechanical) 1.Production of sound Whenever a body is vibrating, it produces a disturbance in the surrounding air. This disturbance reaches our ear in the form of waves and hence produces sensation on the ear drum.
In the absence of air (medium), the sound cannot be heard, because sound cannot travel through vacuum. Experiment: Sound cannot travel through vacuum An electric bell is suspended from wires passing through the cork in a bell jar which is connected to an air pump. The bell is set ringing and then air is pumped out of the jar. As the air is removed, the sound becomes fainter until it may become inaudible although the bell can still be seen ringing. As the air is readmitted, the sound becomes louder once again.
Three things are necessary for the production, propagation, and observing the sound. 1- vibrating body ( a diaphragm of a drum , vocal cords, string in a guitar , speaker ) 2- medium ( air , water, solid body) 3- receiver ( ear , microphone ) 2. Characteristics of sound
Loudness of sound waves Pitch of sound waves Quality of sound waves
Loudness of sound waves – Related to amplitude. Larger the amplitude,, larger the loudness. It is measure using Decibels ( db ). Very loud sounds damage ears.
Pitch of sound waves- this means frequency of the sound waves. High pitch (treble, shrill or sharp voice) indicates high frequency & low pitch (bass, deep voice) means low frequency. High pitch sound waves can damage ear only if it is having high loudness Notes and harmonics in music also indicate a frequency.
Audible frequency range Infra Sounds
0 to 20 Hz
20 Hz to 20,000 Hz ( 20 kHz)
Above 20 kHz
Quality ( Timbre ) of sound waves Sound from different musical instruments are different when same note is played. A pure tone is the sound of only one frequency, such as that given by a tuning fork or electronic signal generator. The fundamental note has the greatest amplitude and is heard predominantly because it has a larger intensity. The other frequencies which are multiples of fundamental are called overtones or harmonics and they determine the quality of the sound.
3. Speed of sound in air Sound wave have a definite velocity of propagation in a given medium. A thunder of a cloud is usually heard a few seconds after lightning flash is seen. When a distant gun is fired, the flash of discharge is seen some time before the sound is heard. the time interval between the event occurring and it being heard by the observer is simply due to high velocity of light as it reaches the observer almost instantaneously. The simplest way of finding the velocity of sound is by arranging for a gun to be fired at a measured distance from an observer who measures the time between seeing the flash and hearing the sound. By using the following relation speed of sound can be measured Speed (v) = distance (s) / time(t)
The velocity of the sound in air at 0o C is 331 m / s. the velocity of sound increases with the increase in temperature and humidity.
4.Echo When sound is incident on a big surface, part of the sound energy is reflected. The reflected sound waves heard after a silence is called an echo. Human can distinguish an echo if the silent period is more than one tenth of a second. ( time threshold of hearing is 0.1 s)
Find the minimum distance needed to hear echo, if the speed of sound in air is 340 m/s. 𝑆𝑝𝑒𝑒𝑑 =
Experiment to show, reflection of sound waves
2 × 𝐷𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑖𝑚𝑒
Echo sounding is used to survey the depth and nature of the seabed. Knowing the speed of sound in a particular medium, one can calculate the distance between the sound source and the reflector by timing the echo Average speed of sound in sea water is 1200 m/s. It took 2 minutes to hear the echo. What is the distance to the sea bed?
5. Diffraction of sound
Questions 01. A teacher and two students are measuring the speed of sound. The teacher makes a loud sound by hitting two cymbals together.
Each student starts a stopwatch when they see the teacher hit the cymbals. They each stop their stopwatch when they hear the sound. (a) Describe how a sound wave moves through the air. (3)
(b) The students repeat the experiment and record their readings in a table. Student Time in s Andrew 0.44 0.46 0.44 0.48 0.43 Kefe 0.5 0.6 0.4 0.4 0.6 (i) State the precision of Andrew’s readings. (1) (ii) State the equation linking speed, distance travelled and time taken. (1) (iii) The teacher was standing 150 m from the students. Use the experimental data recorded by each student to complete the table below. Give your answers to an appropriate number of significant figures. (3) Student Mean (average) time in s Speed of sound in m/s Andrew Kefe (c) The students look in a data book and find that the speed of sound in air is given as 341 m/s. The students discuss their results.
Evaluate these conclusions.
02. CD players use digital signals to transfer information.
Earlier systems using vinyl discs produced analogue signals. (a) Describe the difference between digital signals and analogue signals.
(b) Give one advantage of using digital signals.
(c) A CD player is connected to this loudspeaker system.
The sound produced has a range of frequencies. Use ideas about diffraction to explain why different frequencies require different sizes of loudspeaker. (3) 03. a) Figure illustrates a sound wave travelling through the air.
(i) On Figure, mark clearly the direction in which the air particles are moving.
(ii) Use Figure to measure the wavelength of the sound wave.
(iii) The pitch of the sound wave is raised. State how the sound wave pattern would differ from that shown in Figure
04. A police car siren emits two different sounds P and Q. These are produced alternately. The diagram represents the sounds emitted.
Which sound is the louder and which has the lower pitch? 05 What is the approximate value of the highest frequency that can be heard by a young person? 06 A boy is stranded on an island 500 m from the shore.
He shouts for help, but all he can hear in reply is the echo of his shout from some cliffs. Sound travels at 340m/ s through the air. What is the time interval between the boy shouting and hearing the echo? 07
A pest control device emits sound at frequencies between 50 kHz and 70 kHz.The device is shown in the diagram.
HARMLESS TO CATS, DOGS & PEOPLE BUT RATS - MICE AND ANTS CAN'T STAND IT
GUARANTEED to rid any building of Rats, Mice, Ants and Spiders
What name is given to sound with a frequency greater than 20 kHz?
The frequency of a transmitted wave is 1800 Hz. Sound travels at 340 m/s in air. Calculate the wavelength of this sound wave in air.