ACOUSTICS AND ULTRASONICS

UNIT 3

QUESTION BANK

1 Question: Define wave distinguish longitudinal and transverse wave?

Solution:

In physics a wave can be thought of as a disturbance or oscillation that travels through space-time accompanied by a transfer of energy. Wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium —that is with little or no associated mass transport.

The emphasis of the last point highlights an important misconception of waves. Waves transfer energy not mass.

A wave is a vibratory disturbance in a medium which carries energy from one point to another point without any actual movement of the medium.

There are two types of waves:

• Longitudinal waves
• Transverse waves

Longitudinal Waves: A wave in which the particles of the medium vibrate back and forth in the ‘same direction’ in which the wave is moving.  Medium can be solid, liquid or gases. Therefore, sound waves are longitudinal waves. These waves travel in the form of compressions and rarefactions.

Longitudinal waves have the same direction of vibration as their direction of travel. This means that the movement of the medium is in the same direction as the motion of the wave. Some longitudinal waves are also called compressional waves or compression waves. An easy experiment for observing longitudinal waves involves taking a Slinky and holding both ends. After compressing and releasing one end of the Slinky (while still holding onto the end), a pulse of more concentrated coils will travel to the end of the Slinky. In the example of the Slinky, each coil will oscillate at a point but will not travel the length of the Slinky. It is important to remember that energy, in this case in the form of a pulse, is being transmitted and not the displaced mass.

Longitudinal waves can sometimes also be conceptualized as pressure waves. The most common pressure wave is the sound wave. Sound waves are created by the compression of a medium usually air. Longitudinal sound waves are waves of alternating pressure deviations from the equilibrium pressure, causing local regions of compression and rarefaction. Matter in the medium is periodically displaced by a sound wave, and thus oscillates. When people make a sound, whether it is through speaking or hitting something, they are compressing the air particles to some significant amount. By doing so, they create transverse waves. When people hear sounds, their ears are sensitive to the pressure differences and interpret the waves as different tones.

Transverse Waves: A wave in which the particles of the medium vibrate up and down ‘at right angles’ to the direction in which the wave is moving. These waves are produced only in a solids and liquids but not in gases. These waves travel in the form of crests and troughs. If a transverse wave is moving in the positive x-direction, its oscillations are in up and down directions that lie in the y–z plane. Light is an example of a transverse wave. For transverse waves in matter, the displacement of the medium is perpendicular to the direction of propagation of the wave. A ripple on a pond and a wave on a string are easily visualized transverse waves.

Transverse waves are waves that are oscillating perpendicularly to the direction of propagation

2 Question: Define sound? How are sound waves classified?

Solution:

Sound is a longitudinal wave which consists of compressions and rarefactions travelling through a medium.

Sound Waves

Sound waves of all the mechanical waves that occur in nature, the most important in our everyday lives are longitudinal waves in a medium usually air called sound waves.

Sound waves are of three types

(i) Infrasonic Waves The sound waves of frequency lies between 0 to 20 Hz are called infrasonic waves.

(ii) Audible Waves The sound waves of frequency lies between 20 Hz to 20000 Hz are called audible waves.

(iii) Ultrasonic Waves The sound waves of frequency greater than 20000 Hz are called ultrasonic waves.

Sound travels in the form of wave. A wave is a vibratory disturbance in a medium which carries energy from one point to another without there being a direct contact between the two points.

We can say that a wave is produced by the vibrations of the particles of the medium through which it passes.

The sensation felt by our ears is called sound. Sound is a form of energy which makes us hear. We hear several sounds around us in our everyday life.

3 Question: A cinema hall has a volume of 7500m3. It is required to have reverberation time of 1.5 sec? What should be the absorption in the hall?

Solution:

Given

Volume = 7500m3

Reverberation time T = 1.5 sec

We know that reverberation time is given by

T =   0.165 V/

T =   0.165 V/ A

Where V-Volume of hall in m3

A - Absorption coefficient

1.5 =  0.165 x 7500/A

A= 825 O.W.S

4 Question: A volume of room is 1200m3. The wall area of room is 200 m3. The floor wall area of room is 120m3 and ceiling area is 120m3 and. The average the sound absorption coefficient (i) for walls is 0.03 (ii) for ceiling is 0.80 (iii) for the floor is 0.06. Calculate the average absorption coefficient and reverberation time?

Solution:

Given

Volume = 1200m3

The average absorption coefficient is

A = a1s1 +a2s2 +a3s3 / s1 +s2 +s3

A = (0.03 x 200 + 0.80 x120 +0.06 x 120) / (200+120+120)

A = 0.2389 =0.24 (approx.)

Now the total absorption of room = As =0.24 x460 =110.40 O.W.S.

Reverberation Time T =   0.165 V/AS = 0.165 x 1200/ 109.2

= 1.80 seconds

5 Question: For an empty hall of size 20x15x10 m3 the reverberation time is 3.5 sec. Calculate the average absorption coefficient. What area of the wall should be covered by the curtain so as to reduce reverberation time 2.5 sec. Given the absorption coefficient of curtain clothe is 0.5

Solution:

A =as = (0.165) x (20x15x10)/ 3.5 = 138 m2

When wall are covered with the curtain clothe

2.5 = (0.165) x (20x15x10)/ 0.5x s

Therefore area of the wall covered by the curtain

s = 483- (2.5 x 138)  /2.5 x 0.5

110.4 m2

6 Question: Which method is used to produce high frequency ultrasonic waves? Discuss in details.

Solution:

Piezoelectric Generator or Oscillator is used to produce high frequency of range 500MHz. Let us discuss the production of ultrasonic waves by this method.

Principle:

This is based on the Inverse piezoelectric effect. When a quartz crystal is placed under the effect of an alternating potential difference, vibrations produce in the crystal. If the frequency of electric oscillations coincides with the natural frequency of vibrations of the crystal, the vibrations will be of large amplitude. When the frequency of the electric field matches the ultrasonic frequency range, then the crystal produces ultrasonic waves.

Electric circuit :

Construction:

      It is common base NPN oscillator circuit.

      Coils L1  and  L2 are primary coils of the transformer.

      Coil L2  is connected to the collector and L1 connected to the base.

      The coil L1 and variable capacitor C1 form the tank circuit of the oscillator.

      Quartz crystal is placed between the metal plates A and B so as to form a parallel plate capacitor.

      Quartz Crystal is connected to the secondary coil L3 of the transformer through which output or ultrasonic wave is obtained.

      The frequency of the oscillations can be changed by changing the value of capacitance.

Working:

As soon as the circuit is closed, the current starts flow through the circuit and charges the capacitor. After the charging is completed the capacitor starts discharging through the inductor L1, this energy is stored in the form of electric field in capacitor C1  and in the form of magnetic field in inductor L1.

 Due to this high frequency electric oscillations are produced in the tank circuit. Transistor is also produces electric oscillations. This energy or oscillations is provided to the secondary coil which is again fed to the quartz crystal. Thus oscillating electric field is converted to mechanical vibration of crystal.             

When the frequency of electric oscillations is equal to that of natural frequency of the crystal, resonance is achieved and the sound waves of maximum amplitude are produced. Thus by using inverse piezoelectric effect high frequency ultrasonic waves are produced.

Condition for Resonance:

Frequency of the oscillator circuit = Frequency of the vibrating crystal

Where, 

L1 is inductance of the circuit

C1 is capacitance of the circuit

t = Thickness of crystal slab

Y = Young's Modulus of material

ρ = Density of material

k = 1, 2, 3 ... (Integer Multiple)

Merits:

• High frequency Ultrasonic waves can be produced.
• This method is more effective than the Magnetostriction oscillator.
• The output power is very high
• We are able to get a stable and constant frequency of ultrasonic waves.
• It is not affected by temperature humidity

Demerits:

• Quartz crystal is very costly.
• Cutting and shaping the crystal is difficult.

7 Question: What is meant by ultrasonic?

Solution:

Ultrasonics are the sound waves of frequency above audible range (i.e) above 20,000 Hz (or) 20 KHZ. This sound wave cannot be heard by human ear, but it has many useful applications in engineering and medical fields.

8 Question: Are the ultrasonic waves electromagnetic waves? Give proper reasons.

Solution:

Ultrasonic waves are not electromagnetic waves because they are sound wave which does not consist of electric and magnetic vectors as in electromagnetic waves.

9 Question: Why are ultrasonic waves not audible to humans?

Solution:

The audible range of frequencies for human beings is between 20HZ to 20,000HZ. Since the frequency of ultrasonic wave is having above 20,000HZ, it is not audible to humans.

10 Question: Why not ultrasonic be produced by passing high frequency alternating current through a loud speaker?

Solution:

Ultrasonic cannot be produced by passing high frequency alternating current through loud speaker due to the following reasons.

• Loud speaker cannot vibrate at such high frequency.
• Inductance of the speaker coil becomes so high and practically no current flows through it.

11 Question: Mention the properties of ultrasonic waves?

Solution:

 Properties

a)     The ultrasonic waves cannot travel through vacuum.

b)    These waves travel with speed same as sound wave travel in any given medium.

c)     In homogeneous medium the velocity of ultrasonic wave is constant.

d)    These waves can also weld some material like plastics and metals.

e)     They have high energy content.

f)       Ultrasonic waves get reflected, refracted and absorbed just like sound waves.

g)    They can be transmitted over large distances without any appreciable loss of energy.

h)     They produce intense heating effect when passed through a substance.

i)       The ultrasonic waves have high frequency.

j)       Because of their smaller wavelength Ultrasonic waves produce negligible diffraction effects.

k)     Ultrasonic waves can produce vibrations in low viscosity liquids.

l)       When the ultrasonic wave is absorbed by a medium, it produces heat because of high frequency and high energy and that energy is used to drill and cut thin metals.

12 Question: Can we use a copper rod in a Magnetostriction generator? Why?

Solution:

No, copper rod cannot be used to produce ultrasonics in magnetostriction generator because it is not a ferromagnetic material.

13 Question: Discuss Magnetostriction generator to produce ultrasonic wave? Also discuss its merits and demerits?

Solution:

Magneto-striction generator or oscillator

Principle: Magnetostriction effect: Magnetostriction is a property of magnetic materials like nickel or iron that causes them to change their shape or dimensions during the process of magnetization. i.e. when these material is placed in magnetic field parallel to its length it undergoes  changes in its dimensions.  This is called Magnetostriction effect.

Electronic circuit:

Construction

      In above figure we are using NPN Transistor

      In which battery is connected in such a way that emitter is forward biased and collector is reverse biased.

      Current can be produced by applying necessary biasing to the transistor with the help of the battery.

      The current produced in a circuit can be noted by the mill ammeter connected across the coil L.

      The ends of the ferromagnetic rod A and B is wound by the coils L1 and L.

      The coil L1 is connected to the base of NPN transistor The coil L is connected to the collector of the NPN transistor as shown in the figure.

      The frequency of the oscillatory circuit (LC) can be adjusted by the condenser C.
 

Working 

The rod is initially magnetized by the DC power supply. The transistor is properly biased. The battery is switched on and hence current is produced by the transistor. This current is passed through the coil L, this current causes a change in the magnetization of the rod. Now, the rod starts vibrating due to Magnetostriction effect.

When rod is vibrating and the coil is wounded over a vibrating rod, An emf is induced in coil L, this induces an emf to coil L1 & a part of it is feed as input to the base. Hence, this feedback system makes transistor operates continuously. The e.m.f. induced in the coil called as converse Magnetostriction effect. In this way the current is maintained in transistor so as the vibrations.

The frequency of the oscillatory circuit is adjusted by the condenser C and when this frequency is equal to the frequency of the vibrating rod, resonance occurs. At resonance, the rod vibrates longitudinally with larger amplitude producing ultrasonic waves of high frequency along both ends of the rod. 

Condition for resonance

Frequency of the oscillatory circuit = Frequency of the vibrating rod 

Where, 

L is inductance of the circuit

C is capacitance of the circuit

l is the length of the rod.
E is the young’s modulus of the material of the rod.
ρ is the density of material of the rod.

Merits:

• This Oscillatory circuit is simple to construct.
• Magnetostrictive materials are easily available at low cost
• Large output power can be generated by using this method. 

Demerits:
 

• It can produce frequencies up to 3 MHz only.
• As rod depends on temperature and the degree of magnetization so it becomes difficult to get a constant single frequency.
• As the frequency is inversely proportional to the length of the vibrating rod, so if you increase the frequency, the length of the rod gets decreased which is practically impossible.

14 Question:  What is the main difference in the quality of ultrasonic waves produced by piezo electric and magnetostriction method?

Solution:

15 Question  Discuss  Pitch, Loudness and Timber of sound wave ?

 Solution:

• LOUDNESS

The amplitude of a sound wave determines its loudness or volume. Thus the loudness of sound depends on its amplitude. The loudness of sound is proportional to the square of the amplitude. A roar of a lion is louder than a woman’s voice.

The loudness of sound is measured in Decibel (db)

 Figure: Showing Loudness of wave

Larger amplitude means a louder sound, and smaller amplitude means a softer sound. In Figure sound A is louder than sound B.  The vibration of a source sets the amplitude of a wave. It transmits energy into the medium through its vibration. More energetic vibration corresponds to larger amplitude. The molecules move back and forth more energetically.

The loudness of a sound is also determined by the sensitivity of the ear. The human ear is more sensitive to some frequencies than to others. The volume we receive thus depends on both the amplitude of a sound wave and whether its frequency lies in a region where the ear is more or less sensitive. If loudness exceeds 80 db, then the sound becomes physically painful.

• PITCH

The frequency of a sound wave is what your ear understands as pitch. A higher frequency sound has a higher pitch, and a lower frequency sound has a lower pitch. For instance, the chirp of a bird would have a high pitch, but the roar of a lion would have a low pitch.

The human ear can detect a wide range of frequencies. Frequencies from 20 to 20 000 Hz are audible to the human ear. Any sound with a frequency below 20 Hz is known as an infrasound and any sound with a frequency above 20 000Hz is known as an ultrasound.

Figure: Pitch of wave

• TIMBER OR QUALITY

Any sound consists of more than one frequencies and most of these additional frequencies are known as harmonics. The fundamental frequency is the lowest frequency waveform which is known as pitch of the note in music. For any sound to possess timbre, it must have one fundamental frequency and seven or more additional harmonics.

For any sound to be identifiable, the sound must have one fundamental frequency and seven additional harmonics and if any sound which doesn’t possess these, won’t be identified by the human ear.

Figure: Timber or Quality of Wave

If a particular note on a scale is played on two instruments say a piano and violin, it is easy to distinguish the tone of one instrument from that of the other without seeing it. We say that the quality (or timbre) of the note is different in each case.

Generally speaking, instruments do not give tones which are pure in the sense that they consist of a single frequency only.

The waveform of a note is never simple harmonic in practice. The nearest approach is that obtained by sounding a tuning-fork as shown in Figure (a). If the same note is played on a violin and a piano respectively, the waveforms produced might be represented by Figure (b), (c), which have the same frequency and amplitude as the waveform in Figure (a).

16 Question: Define any five Sound characteristics of sound wave ?

Solution:

• Wavelength
• Amplitude
• Time-Period
• Frequency
• Velocity or Speed

• WAVELENGTH

The distance between the centres of two consecutive compressions or two consecutive rarefactions is equal to its wavelength.

It is denoted by a Greek letter λ (lambda). The S.I unit for measuring wavelength is metre (m).

Figure: Wavelength of wave

The minimum distance in which a sound wave repeats itself is called its wavelength. That is it is the length of one complete wave.  We know that in a sound wave, the combined length of a compression and an adjacent rarefaction is called its wavelength.

The distance between the centres of a compression and an adjacent rarefaction is equal to half of its wavelength i.e. λ/2.

• AMPLITUDE

When a wave passes through a medium, the particles of the medium get displaced temporarily from their original undisturbed positions and the maximum displacement of the particles of the medium from their original undisturbed positions is called amplitude of the wave.

The S.I unit of measurement of amplitude is metre (m) though sometimes it is also measured in centimetres.

Amplitude is used to describe the size of the wave. The amplitude of a wave is the same as the amplitude of the vibrating body producing the wave.

• TIME-PERIOD

The time required to produce one complete wave or cycle is called time-period of the wave.

It is denoted by letter T. The unit of measurement of time-period is second (s).

Figure: Time period of wave

One complete wave is produced by one full vibration of the vibrating body. So, we can say that the time taken to complete one vibration is known as time-period

• FREQUENCY

The number of complete waves or cycles produced in one second is called frequency of the wave.

The frequency of a wave is denoted by the letter f.  The S.I unit of frequency is hertz written as Hz. Bigger unit of frequency is known as kilohertz (kHz). 1 kHz = 1000 Hz.

Figure: Frequency of sound wave

Since one complete wave is produced by one full vibration of the vibrating body so we can say that the number of vibrations per second is called frequency. A vibrating body emitting 1 wave per second is said to have a frequency of 1 hertz. That              is              1Hz is equal to 1 vibration per second.
Frequency of a wave is fixed and does not change even when it passes through different substances. The frequency of a wave is the same as the frequency of the vibrating body which produces the wave.

For example: if 20 complete waves or vibrations are produced in one second then the frequency of the waves will be 20 hertz or 20 cycles per second.

• VELOCITY OF WAVE OR SPEED OF WAVE

The distance travelled by a wave in one second is called velocity of the wave or speed of the wave.

It is represented by the letter v. The S.I unit for measuring the velocity is metres per second (m/s or ms-1).

17 Question: What is the relationship between Velocity, Frequency and Wavelength of a Wave?

Solution:

Velocity = Distance travelled/ Time taken
Let v = λ / T
Where T = time taken by one wave.
v = f X λ
This formula is known as wave equation.
Where v = velocity of the wave
f = frequency
λ = wavelength
Velocity of a wave = Frequency X Wavelength

This applies to all the waves like transverse waves like water waves, longitudinal waves like sound waves and  the electromagnetic waves like light waves and radio              waves

18 Question: What is the relation between time-period and frequency of a wave?

Solution:

The time required to produce one complete wave is called time-period of the wave. Suppose the time-period of a wave is T seconds.
In T seconds number of waves produced = 1
So,  in 1 second, number of waves produced will be = 1/T
But the number of waves produced in 1 second is called its frequency.
Therefore, F = 1/Time-period
f = 1/T
where f = frequency of the wave
T = time-period of the wave

19 Question: List the requirement for good acoustics?

Solution:

Acoustics, the science concerned with the production, control, transmission, reception, and effects of sound. The term is derived from the Greek akoustos, meaning heard.

Acoustics in architecture means improving sound in environments. Although it is a complex science, understanding the basics - and making efficient and effective decisions - is much easier than you might think. The first step is to understand that there are two technical categories used in acoustics: soundproofing and acoustical treatment. Soundproofing means "less noise" and acoustical treatment, "better sound”.

According to classic acoustics theory there are five requirements which, when met, result in good acoustics:

• An appropriate reverberation time
• Uniform sound distribution
• An appropriate sound level
• An appropriately low background noise
• No echo or flutter echo

Acoustic requirement for good auditorium is as follows-

• The initial sound should be of adequate intensity.
• The sound should be evenly distributed throughout the hall.
• The successive nodes should be clear & distinct.
• Noise has to be taken care of.
• The size & the shape of the ball have also to be taken care.

These requirements can be achieved by following ways-

Site/location:-Before construction the first important factor to be considered is the location. For best acoustical quality of the hall, it should be far from railway tracks, industrial areas, airports, & highways, etc.

Size: - The size of the hall should be optimum, neither big nor small. It is small uneven distribution of sound will take place due to the formations of stationary waves. If size is too big reverberation time will be more that results in confusion & unpleasant sound.

Shape: - Instead of parallel walls spade walls are preferred, curved surfaces should be built with proper care.

Reverberation: - Reverberation time (T) should be neither too small nor too large. If it is small intensity will be weak. If large sound will be unpleasant. Thick carpets curtains, upholstered chairs, audience take care of reverberation. For lecture halls the reverberation time is approximately 0.5sec, for music concerts hall-1.5sec, for cinema theatres-2sec

Absorption: - Use of proper absorbent material enhances the quality of sound.

Echelon effect: - The regular intervals/space between staircase or railings give repeated echo, this makes the sound unpleasant, so thick carpets take care of this & wide gaps between staircases are generally preferred. No echo or flutter echoes must occur for the acoustics to be good. It is easy to prevent echo by installing a little sound-absorbing material on the wall. 
 

20 Question: Define absorption coefficient?

Solution:

The coefficient of absorption of material is defined as the ratio of the sound energy absorbed by the surface to that of the total incident sound energy on the surface.

Absorption Coefficient =

As all sound waves falling on an open window pass through, it can be assumed that an open window behaves as perfect absorber of sound and hence the standard of absorp­tion is taken as the unit area of an open window as a standard unit of absorption.

Thus, the absorption coefficient of a material is defined as the rate of the sound energy absorbed by a certain area of the surface to that of open window of same area.

The absorption coefficient of a surface is defined as the reciprocal of its area which absorbs the same sound energy absorbed at a unit area of an open window.

21 Question: What is Reverberation and Time of Reverberation?

Solution:

When a sound is produced in a building, it lasts too long after its production. It reaches to the listener a number of times. Once it reaches directly from the source and subsequently after reflection from the walls, windows, ceiling and flour of hall. The listener therefore receives series of sounds of diminishing intensity. By reverberation is meant the prolonged reflection of sound from the walls, floor and ceiling of a room.

The reverberation is defined as the persistence of audible sound after the source has stopped to emit sound. The duration for which the sound is stopped is called reverberation time. This time is measured from the instant the source stops emitting sound.

The time of reverberation is defined as the time taken by the sound to fall below the minimum audibility measured from the instant when the source stopped emitting sound.

According to Prof. W. C. Sabine, the standard reverberation time is defined as the time taken by sound to fall to one millionth of its intensity just before the sound is cut off.

22 Question: State Sabine’s formula for reverberation time ?

Solution:

According to W. C. Sabine, the time of reverberation depends up on

1. Size of hall,

2. Loudness of the sound,

3. Kind of music or sound for which hall is to be used.

Acoustics and Ultrasonic Reverberation Time

T =   0.165 V/

T =   0.165 V/ A

Where V-Volume of hall in m3

a - Absorption coefficient

S - Area of reflecting surface in a square meter

Absorption of hall.

23 Question: Discuss the factors affecting acoustics of buildings and their remedies?

Solution:

Acoustically good hall we mean that in which every syllable or musical note reaches an audible level of loudness  at every point of the hall and then quickly dies away to make room ready for the next syllable or group of notes. Following are the factors affecting architectural acoustics.

1. REVERBERATION

In a hall, if the reverberation is large there is of successive sounds which result in loss of clarity in hearing. However if the reverberation is very small, the loudness is inadequate Thus the time of reverberation for a hall should neither too large nor too small. The preferred value of the time of reverberation is called optimum the reverberation time. According to W. C. Sabine standard reverberation time is given by:

T =   0.165 V/

T =   0.165 V/ A

Where V-Volume of hall in m3

a - Absorption coefficient

S - Area of reflecting surface in a square meter

Absorption of hall

The reverberation can be controlled by the following factors:

• By providing windows and ventilators which can be opened and closed to make the optimum time of reverberation
• Decorating the walls by pictures and maps
• Using heavy curtains with folds
• The walls are lined with absorbent material such as felt, fibreboard, glass wool etc.
• Having full capacity of audience
• By covering floor with carpet
• By providing acoustics tiles

2.     ADEQUATE LOUDNESS

With large absorption the time of reverberation will be smaller which will minimize the chances of confusion and may go below the level of intelligibility of hearing. Hence suffi­cient loudness in every portion of the hall is an important factor for satisfactory hearing.

The loudness can he maintained at desired level by:

Using large sounding boards behind the speaker and facing the audience.

Large polished wooden reflecting surfaces immediately above the speakers.

Low ceiling are also useful in reflecting the sound energy towards the audience.

By providing additional sound energy using more number of speakers

3.     FOCUSING DUE TO WALLS AND CEILINGS

If there are focusing surfaces like concave, spherical, cylindrical or parabolic etc. on the walls or ceiling or the floor of the hall, they produce concentration of the sound in to particular region, while in some other parts no sound reaches at all. Thus there will be non-­ uniformity in the distribution of sound energy in the hall.

For uniform distribution of sound in the hall:

• There should be no curved surfaces. If such surfaces are present, they should be covered with absorbent material.
• Ceiling should be low. 
• Arrange speaker at the focus of parabolic reflecting surface. This will helpful to reflect beam of sound in the hall.

4.     ECHOES

An echo is heard, when direct and reflected sound waves coming from the same source reach the listener with time interval of about th second. It should be avoided as far as possible by absorption.

Echoes can he avoided by:

• Covering long distant walls with curtain or absorbent material
• Covering high ceiling with absorbent material

5.     ECHELON EFFECT

A set of railings, pillars or any regular spacing of reflected surfaces may produce a musical note due to regular succession of the echoes of the original sound to the listener. This makes the original sound confused.

This can be avoided by:

• Covering steps with carpet
• Covering flour with carpet
• Avoid pillars in the hall

6.     BALCONIES

There are chances of reflection of sound from the railing of balcony. This may lead to the problem like echelon effect or echoes.

This can be eliminated by:

• Adjust height to depth ratio as 2: 1
• Use grills and bars for railings instead of bricks

7.     SEATING ARRANGEMENT

This is one of the factors to be taken care at the time of arranging the seats.

It pre­ferred to arrange:

• Seats perpendicular to the direction of sound for better audibility
• Seats must be gradually elevated to take care of absorption of sound energy by human body.

8.     EXTRANEOUS NOISE AND SOUND INSULATION

In a good hall no noise should reach from outside. Noise may be defined as unwanted sound such as:

Outside Noise: street traffic, hammering, drilling, operating machinery, moving of furni­ture, electrical generator etc.

Inside Noise: machinery, typewriters, telephone, mobiles, projector etc.

This extraneous noise can be avoided by:

• Avoiding openings for pipes and ventilators
• Allotting suitable locations for doors and windows
• Using heavy glasses to doors and windows
• By providing double wall construction with air space between them
• By interposing layers of some acoustical insulators
• Use of soft floor finish e.g. Carpet, rubber etc.
• Insulating machines like refrigerators, lifts, typewriters, projector etc.
• Constructing small sound proof cabin for machine and office staff
• Making hall sound proof

9.     FREEDOM FROM RESONANCE

If the frequency of the created sound is equal to original sound, then the original music will be reinforced. Due to the interference between original sounds is distorted. Enclosed air in the hall also causes resonance.

This can be avoided by:

• Using absorbing material on reflecting surfaces
• Providing decoration which include holes in the design on interior wall
• Using ventilators whenever necessary

24 Question: Discuss various method used to detection ultrasonic waves? 

Solution:

Ultrasonic waves propagated through a medium can be detected in a number of ways. Some of the methods employed are as follows:

(1) KUNDT’S TUBE METHOD

Kundt devised an experimental technique in 1889 to study the transmission of sound in different materials.

Kundt’s tube consists of a horizontal glass tube about 1 m long and 5 cm in diameter. One end of the tube has an adjustable piston and the other end has a loosely fitted cardboard cap that is firmly fixed to a metal rod. The metal rod is clamped in the middle at B on a horizontal table to ensure minimum disturbance during the use of Kundt’s tube. A small amount of lycopodium powder is scattered in the tube. When ultrasonic waves are incident on the tube and pass through it, the lycopodium powder collects in the form of heaps at the nodal points and is blown off at the antinodal points . The distance between subsequent nodes is then equal to half the magnitude of the wavelength of ultrasonic waves. This information can then be used to determine the frequency of the waves.

Figure: Kundt’s tube

This method cannot be used if the wavelength of ultrasonic waves is very small i.e., less than few mm. In the case of a liquid medium, instead of lycopodium powder, powdered coke is used to detect the position of nodes.
 

(2)SENSITIVEFLAM METHOD
 

The formation of nodes and antinodes in the presence of ultrasonic waves can be exploited in another interesting way to detect and determine the frequency of the waves A narrow sensitive flame is moved along the medium. At the positions of antinodes, the flame is steady. At the positions of nodes, the flame flickers because there is a change in pressure. In this way, positions of nodes and antinodes can be found out in the medium. The average distance between the two adjacent nodes is equal to half the wavelength. If the value of the frequency of ultrasonic wave is known, the velocity of ultrasonic wave propagated through the medium can be calculated. 

(3) THERMAL DETECTORS
 

This is the most commonly used method of detection of ultrasonic waves. Whenever an ultrasonic wave propagates through a medium, it causes alternate compressions and rarefactions in the medium. Due to these compressions and rarefactions the temperature of the medium change at the nodes while remaining almost constant at antinodes. A thermal detector comprises of a fine platinum wire whose resistance changes at the nodes due to these temperature variations. The complete thermal detector uses the fine platinum wire as one of the arms of a sensitive bridge arrangement. Using this bridge arrangement, changes in the resistance of the platinum wire at the nodes can be measured as a function of time. These measurements can then be used to determine the frequency of ultrasonic waves. As the detector element is moved through the medium, the bridge remains balanced at antinodes but gets off-balance at nodes.

 

(4) QUARTZ CRYSTAL METHOD OR PIEZOELECTRIC DETECTOR
 

This method is based on the principle of Piezo-electric effect. Piezoelectric crystals have the ability to develop an electric potential when a stress is applied across certain faces of the crystal. This phenomenon can be used to detect ultrasonic waves. One pair of faces of a quartz crystal (piezoelectric material) is subjected to ultrasonic waves. An alternating potential then develops across the perpendicular faces. This potential can be amplified and measured to detect the presence of ultrasonic waves.

When one pair of the opposite faces of a quartz crystal is exposed to the ultrasonic waves, the other pairs of opposite faces developed opposite charges. These charges are amplified and detected using an electronic circuit.