Sound Online Test 9th Science Lesson 8 Questions in English

Human ear can hear only a particular range of frequency of sound that too with a certain range of energy. We are not able to hear sound clearly if it is below certain intensity. The quality of sound also differs from one another.

Sound needs a material medium like air, water, steel etc., for its propagation. It cannot travel through vacuum.

This process continues in the medium till the sound reaches our ears. It is to be noted that only the disturbance created by a source of sound travels through the medium not the particles of the medium. All the particles of the medium restrict themselves with only a small to and fro motion called vibration which enables the disturbance to be carried forward.

The sound disturbance which is carried forward in a medium is called Wave.

The waves that propagates with compressions and rarefactions are called longitudinal waves. In longitudinal waves the particles of the medium move to and fro along the direction of propagation of the wave.

Sound also is a longitudinal wave. Sound can travel only when there are particles which can be compressed and rarefied. Compressions are the regions where particles are crowded together. Rarefactions are the regions of low pressure where particles are spread apart. A sound wave is an example of a longitudinal mechanical wave.

A sound wave can be described completely by five characteristics namely amplitude, frequency, time period, wavelength and velocity or speed.

The SI unit of velocity of sound is m s-1.

The time required to produce one complete vibration (wave or cycle) is called time period of the wave. It is denoted as T. The SI unit of time period is second (s). Frequency and time period are reciprocal to each other (T = 1 / n).

The distance travelled by the sound wave in one second is called velocity of the sound.

The minimum distance in which a sound wave repeats itself is called its wavelength. In a sound wave, the distance between the centres of two consecutive compressions or two consecutive rarefactions is also called wavelength.

The wavelength is usually denoted as λ (Greek letter, lambda). The SI unit of wavelength is metre (m).

The number of vibrations (complete waves or cycles) produced in one second is called frequency of the wave. It is denoted as n.

The SI unit of frequency is s-1 (or) hertz (Hz).

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. If the vibration of a particle has large amplitude, the sound will be loud and if the vibration has small amplitude, the sound will be soft.

Amplitude is denoted as A. Its SI unit is meter (m).

Human ear can hear sound of frequency from 20 Hz to 20,000 Hz. Sound with frequency less than 20 Hz is called infrasonic sound. Sound with frequency greater than 20,000 Hz is called ultrasonic sound. Human beings cannot hear infrasonic and ultrasonic sounds.

Sounds can be distinguished from one another in terms of the following three different factors. 1. Loudness 2. Pitch 3. Timbre (or quality).

Pitch is one of the characteristics of sound by which we can distinguish whether a sound is shrill or base. High pitch sound is shrill and low pitch sound is flat. Two music sounds produced by the same instrument with same amplitude, will differ when their vibrations are of different frequencies.

Timbre is the characteristic which distinguishes two sounds of same loudness and pitch emitted by two different instruments. A sound of single frequency is called a tone and a collection of tones is called a note. Timbre is then a general term for the distinguishable characteristics of a tone.

The loudness of a sound depends on the intensity of sound wave. Intensity is defined as the amount of energy crossing per unit area per unit time perpendicular to the direction of propagation of the wave.

The speed of sound is defined as the distance travelled by a sound wave per unit time as it propagates through an elastic medium.
Speed (v) = Distance / Time.

If the distance traveled by one wave is taken as one wavelength (λ), and the time taken for this propagation is one time period (T), then
Speed (v) = One wavelength (λ) / One time period (T) (or) v = λ / T
As, T = 1 n, the speed (v) of sound is also written as, v = n λ.

Speed, v = n λ
Here, n = 2 kHz = 2000Hz
λ = 15 cm = 0.15 m
v = 0.15 × 2000 = 300 m s-1
Time (t) = Distance (d) / Velocity (v)
t = 1500 300 = 5 s

Sound propagates through a medium at a finite speed. The sound of thunder is heard a little later than the flash of light is seen. So, we can make out that sound travels with a speed which is much less than the speed of light. The speed of sound depends on the properties of the medium through which it travels.

The speed of sound is less in gaseous medium compared to solid medium. In any medium the speed of sound increases if we increase the temperature of the medium. For example, the speed of sound in air is 330 ms-1 at 0 °C and 340 ms-1 at 25 °C. The speed of sound at a particular temperature.

When the speed of any object exceeds the speed of sound in air (330 ms-1) it is said to be travelling at supersonic speed. Bullets, jet, aircrafts etc., can travel at supersonic speeds.

When an object travels at a speed higher than that of sound in air, it produces shock waves. These shock waves carry a large amount of energy. The air pressure variations associated with this type of shock waves produce a very sharp and loud sound called the ‘sonic boom’. The shock waves produced by an aircraft have energy to shatter glass and even damage buildings.

The intensity of sound heard at a place depends on the following five factors. I) Amplitude of the source. ii) Distance of the observer from the source. iii) Surface area of the source. iv) Density of the medium and v) Frequency of the source.

The laws of reflection are: • The angle in which the sound is incident is equal to the angle in which it is reflected. • Direction of incident sound, the reflected sound and the normal are in the same plane.

When we shout or clap near a suitable reflecting surface such as a tall building or a mountain, we will hear the same sound again a little later. This sound which we hear is called an echo. The sensation of sound persists in our brain for about 0.1 s. Hence, to hear a distinct echo the time interval between the original sound and the reflected sound must be at least 0.1s.

The total distance covered by the sound from the point of generation to the reflecting surface and back should be at least 340 ms-1 × 0.1 s = 34 m.

A sound created in a big hall will persist by repeated reflection from the walls until it is reduced to a value where it is no longer audible. The repeated reflection that results in this persistence of sound is called reverberation.

In an auditorium or big hall excessive reverberation is highly undesirable. To reduce reverberation, the roof and walls of the auditorium are generally covered with sound absorbing materials like compressed fibreboard, flannel cloths, rough plaster and draperies. The seat materials are also selected on the basis of their sound absorbing properties.

There is a separate branch in physics called acoustics which takes the aspects of sound in to account while designing auditoria, opera halls, theatres etc.

Animals, such as bats, dolphins, rats, whales and oil birds, use echolocation, an ultrasound technique that uses echoes to identify and locate objects. Echolocation allows bats to navigate through dark caves and find insects for food. Dolphins and whales emit a rapid series of underwater clicks in ultrasonic frequencies to locate their prey and navigate through water.

Distance (d) = velocity (v) × time (t)
Distance travelled by sound when gun fires first time, 2d = v × 5 —— (1)
Distance travelled by sound when gun fires second time, 2d – 620 = v × 3 ——– (2)
Rewriting equation (2) as,
2d = (v × 3) + 620 ——- (3)
Equating (1) and (3), 5v = 3v + 620
2v = 620
Velocity of sound, v = 310 ms-1

Ultrasonic sound is the term used for sound waves with frequencies greater than 20,000Hz. These waves cannot be heard by the human ear, but the audible frequency range for other animals includes ultrasound frequencies. For example, dogs can hear ultrasonic sound. Ultrasonic whistles are used in cars to alert deer to oncoming traffic so that they will not leap across the road in front of cars.

The waves are detected, analysed and stored in a computer. An echogram is an image obtained by the use of reflected ultrasonic waves. It is used as a medical diagnostic tool. Ultrasonic sound is having application in marine surveying also.

Ultrasonic waves are made to reflect from various parts of the heart and form the image of the heart. This technique is called ‘echo cardiography’. Ultrasound may be employed to break small ‘stones’ formed in the kidney into fine grains. These grains later get flushed out with urine.

SONAR stands for Sound Navigation and Ranging. Sonar is a device that uses ultrasonic waves to measure the distance, direction and speed of underwater objects.

Let the time interval between transmission and reception of ultrasound signal be ‘t’. Then, the speed of sound through sea water is 2d /t = v. This method is called echo-ranging. Sonar technique is used to determine the depth of the sea and to locate underwater hills, valleys, submarine, icebergs etc.

We know, distance = speed × time
2d = speed of ultrasound × time
2d = 1531 × 3.42
∴ d = 5236 / 2 = 2618 m
Thus, the distance of the seabed from the ship is 2618 m or 2.618 km.

The electrocardiogram (ECG) is one of the simplest and oldest cardiac investigations available. It can provide a wealth of useful information and remains an essential part of the assessment of cardiac patients. Thus, an ECG is simply a representation of the electrical activity of the heart muscle as it changes with time. Usually it is printed on paper for easy analysis. The sum of this electrical activity, when amplified and recorded for just a few seconds is known as an ECG.

The outer ear is called ‘pinna’. It collects the sound from the surroundings. The collected sound passes through the auditory canal.

We are able to hear with the help of an extremely sensitive device called the ear. It allows us to convert pressure variations in air with audible frequencies into electric signals that travel to the brain via the auditory nerve.

At the end of the ear is eardrum or tympanic membrane. When a compression of the medium reaches the eardrum the pressure on the outside of the membrane increases and forces the eardrum inward. Similarly, the eardrum moves outward when a rarefaction reaches it. In this way the eardrum vibrates.

The vibrations are amplified several times by three bones (the hammer, anvil and stirrup) in the middle ear. The middle ear transmits the amplified pressure variations received from the sound wave to the inner ear.

In the inner ear, the pressure variations are turned into electrical signals by the cochlea. These electrical signals are sent to the brain via the auditory nerve and the brain interrupts them as sound.