UNIT -3

SOUND ENGINEERING

Q1) A cinema hall has a volume of 7500m3. It is required to have a reverberation time of 1.5 sec? What should be the absorption in the hall?

A 1)

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

Q 2) A volume of the room is 1200m3. The wall area of the room is 200 m3. The floor wall area of the room is 120m3 and the ceiling area is 120m3 and. The average sound absorption coefficient (i) for walls is 0.03 (ii) for the ceiling is 0.80 (iii) for the floor is 0.06. Calculate the average absorption coefficient and reverberation time?

A 2)

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

Q 3) 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 by 2.5 sec. Given the absorption coefficient of curtain clothe is 0.5

A 3)

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

When wall is 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

Q 4) Which method is used to produce high-frequency ultrasonic waves? Discuss in detail.

A 4)

Piezoelectric Generator or Oscillator is used to produce a 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 are produced 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 a common base NPN oscillator circuit.
• Coils L1 and L2 are the 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 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 to 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 the electric field in capacitor C1 and the form of the magnetic field in inductor L1.

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

When the frequency of electric oscillations is equal to that of the natural frequency of the crystal, resonance is achieved and the sound waves of maximum amplitude are produced. Thus by using the 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 the inductance of the circuit

C1 is the 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)

Advantages:

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

Disadvantages:

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

Q 5) What is meant by ultrasonic?

A 5)

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

Q 6) Are the ultrasonic waves of electromagnetic waves? Give proper reasons.

A 6)

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

Q 7) Why are ultrasonic waves not audible to humans?

A 7)

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

Q 8) Why not ultrasonic be produced by passing a high-frequency alternating current through a loudspeaker?

A 8)

Ultrasonic cannot be produced by passing a high-frequency alternating current through a loudspeaker due to the following reasons.

• The loudspeaker cannot vibrate at such a high frequency.
• The inductance of the speaker coil becomes so high and practically no current flows through it.

Q 9) Mention the properties of ultrasonic waves?

A 9)

Properties

a)     The ultrasonic waves cannot travel through a vacuum.

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

c)     In a homogeneous medium, the velocity of the 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 an intense heating effect when passed through a substance.

i)       The ultrasonic waves have a 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.

Q 10) Can we use a copper rod in a Magnetostriction generator? Why?

A 10)

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

Q 11) Discuss Magnetostriction generator to produce ultrasonic wave? Also, discuss its merits and demerits?

A 11)

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 this material is placed in the magnetic field parallel to its length it changes its dimensions.  This is called the Magnetostriction effect.

Electronic circuit:

Construction

      In the 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 are wound by the coils L1 and L.

      The coil L1 is connected to the base of the 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 coil L, this current causes a change in the magnetization of the rod. Now, the rod starts vibrating due to the Magnetostriction effect.

When the 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 the transistor operates continuously. The e.m.f. Induced in the coil called a converse Magnetostriction effect. In this way, the current is maintained in the 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 the inductance of the circuit

C is the 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 the material of the rod.

 

Advantages

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

Disadvantages
 

• 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.

Q 12) What is the main difference in the quality of ultrasonic waves produced by the piezoelectric and magnetostriction method?

A 12)

Q 13) List the requirement for good acoustics?

A 13)

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

The acoustic requirement for a 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 of.

These requirements can be achieved in the following ways-

Site/location:-Before construction the first important factor to be considered is the location. For the 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. Is the small uneven distribution of sound will take place due to the formations of stationary waves. If the size is too big reverberation time will be more that results in confusion & an 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. 

Q 14) Define absorption coefficient?

A 14)

The coefficient of absorption of the 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 a 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 the open window of the 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.

Q 15) What is Reverberation and Time of Reverberation?

A 15)

When a sound is produced in a building, it lasts too long after its production. It reaches the listener several 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 a series of sounds of diminishing intensity. 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 emitting sound. The duration for which the sound is stopped is called reverberation time. This time is measured from the instant the source stops the 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.

Q 16)  State Sabine’s formula for reverberation time?

A 16)

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

1. Size of the 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 the hall in m3

a - Absorption coefficient

S - Area of reflecting surface in a square meter

Absorption of the hall.

Q 17) Discuss the factors affecting the acoustics of buildings and their remedies?

A 17)

The 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 the 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 are successive sounds that 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 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 the 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 with pictures and maps
• Using heavy curtains with folds
• The walls are lined with absorbent material such as felt, fibreboard, glass wool, etc.
• Having a full capacity of audience
• By covering the 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 be maintained at the desired level by:

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

Large polished wooden reflecting surfaces immediately above the speakers.

Low ceilings 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 a concentration of the sound into a 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.
• The ceiling should be low.
• Arrange speaker at the focus of parabolic reflecting surface. This will help to reflect a 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 a time interval of about th second. It should be avoided as far as possible by absorption.

Echoes can be avoided by:

• Covering long distant walls with a 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 the 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 the balcony. This may lead to the problem like echelon effect or echoes.

This can be eliminated by:

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

7.     SEATING ARRANGEMENT

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

It pre­ferred to arrange:

• Seat perpendicular to the direction of sound for better audibility
• Seats must be gradually elevated to take care of absorption of sound energy by the 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 finishes e.g. Carpet, rubber, etc.
• Insulating machines like refrigerators, lifts, typewriters, projector, etc.
• Constructing a small soundproof cabin for machine and office staff
• Making hall soundproof

9.     FREEDOM FROM RESONANCE

If the frequency of the created sound is equal to the original sound, then the original music will be reinforced. Due to the interference between the 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 includes holes in the design on an interior wall
• Using ventilators whenever necessary