Category: r2017 notes

Properties of Ultrasonic waves and Production of Ultrasonic waves

Piezo Electric Crystals – Principle, Construction, working

Principle and working of SONAR – Sound Navigation and Ranging

Determination of Ultrasonic Velocity in Liquid(Acoustical Grating Method): Principle, Construction and working

Ultrasonic Scanning Methods A, B and C Scan Displays

Sonogram Recording of movement of Heart: Principle and working

Characteristics of sound and Classification of Sound

PH8201 Notes r2017 notes

METALLIC GLASSES

The Metallic glasses are materials which have the properties of both metals and glasses.

Metallic glass = Amorphous metal

In general, metallic glasses are strong, ductile, malleable, opaque and brittle.

They also have good magnetic properties and high corrosion resistance.

METHODS OF PREPARATION and Principle

The principle used in making metallic glasses is extreme rapid cooling of the molten alloy. The technique is called as rapid quenching.

The cooled molten alloys are fed into highly conducting massive rollers at high speeds to give ribbons of metallic glasses.

PREPARATION OF METALLIC GLASSES

Principle

The principle used in making metallic glasses is extreme rapid cooling of the molten metal alloy.

This technique is called as rapid quenching.

Melt spinning system

A melt spinner consists of a copper roller over which a refractory tube with fine nozzle is placed.

The refractory tube is provided with induction heater as shown in fig.

The metal alloy is melted by induction heating under inert gas atmosphere (helium or argon).

The properly super heated molten alloy is ejected through the fine nozzle at the bottom of the refractory tube.

The molten alloy falls on the copper roller which is rotated at high speed.

Thus, the alloy is suddenly cooled to form metallic glass.

In this method a continuous ribbon of metallic glass can be obtained.

TYPES OF METALLIC GLASSES:

Metallic glasses are classified into two types:

(i)Metal –Metal metallic glasses

They are combination of metals

Metals Metals

Examples: Nickel (Ni) – Niobium (Nb)

Magnesium (Mg) – Zinc (Zn)

Copper (Cu) – Zirconium (Zr)

(ii)Metal –Metalloid metallic glasses

These are combinations of metals and metalloids.

Examples: Metals Metalloids

Fe, Co, Ni – B, Si, C, P

PROPERTIES OF METALLIC GLASSES

Structural properties

They do not have any crystal defects such as grain boundaries, dislocation etc.
Metallic glasses have tetrahedral close packing (TCP).

Mechanical properties

Metallic glasses have extremely high strength, due to the absence of point defects and dislocation.
They have high elasticity.
They are highly ductile.
Metallic glasses are not work-harden but they are work –soften. (work harnening is a process of hardening a material by compressing it).

Electrical properties

Electrical resistivity of metallic glasses is high and it does not vary much with temperature.
Due to high resistivity, the eddy current loss is very small.
The temperature coefficient is zero or negative.

Magnetic properties

Metallic glasses have both soft and hard magnetic properties.
They are magnetically soft due to their maximum permeabilities and thus they can be magnetised and demagnetized very easily.
It exhibit high saturation magnetisation.
They have less core losses.
Most magnetically soft metallic glasses have very narrow hysteresis loop with same crystal composition. This is shown in fig.

Fig. Hysteresis loop of iron based alloy in crystalline and metallic glassy phase.

Chemical properties

They are highly resistant to corrosion due to random ordering.
It highly reactive and stable.
They can act as a catalyst. The amorphous state is more active than the crystalline state from the catalytic point of view.

APPLICATIONS OF METALLIC GLASSES

Metallic – glasses also called as met glasses have found wide applications in different fields.

Structural application

They posses high physical and tensile strength. They are superior to common steels and thus they are very useful as reinforcing elements in concrete, plastic and rubber.
Strong ribbons of metallic glasses are used for simple filament winding to reinforce pressure vessels and to construct large fly wheels for energy storage.
Due to their good strength, high ductility, rollability and good corrosion resistance, they are used to make razor blades and different kinds of springs.

Electrical and Electronics application

Since metallic – glasses have soft magnetic properties, they are used in tape recorder heads, cores of high-power transformers and magnetic shields.
They use of metallic glasses in motors can reduce core loss very much when compared with conventional crystalline magnets.
Superconducting metallic glasses are used to produce high magnetic fields and magnetic levitation effect.
Since metallic glasses have high electrical resistance, they are used to make accurate standard resistance, computer memories and magneto resistance sensors.

Metallic glasses as transformer core material

Metallic – glasses have excellent magnetic properties. When they are used as transformer core, they give maximum magnetic flux linkage between primary and secondary coils and thus reduce flux leakage losses.
In view of their features like small thickness, smaller area, light weight, high resistivity, soft magnetic property and negligible hysteresis and eddy current loss, metallic glasses are considered as suitable core materials in different frequency transformers.

Nuclear reactor engineering

The magnetic properties of metallic glasses are not affected by irradiation and so they are useful in preparing containers for nuclear waste disposal and magnets for fusion reactors.
Chromium and phosphorous based (iron chromium, phosphorous-carbon alloys) metallic glasses have high corrosion resistances and so they are used in iner surfaces of reactor vessels, etc.

Bio-medical Industries

Due to their high resistance to corrosion, metallic glasses are ideal materials for making surgical instruments.
They are used as prosthetic materials for implantation in human body.

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Properties of Ultrasonic waves and Production of Ultrasonic waves

Piezo Electric Crystals – Principle, Construction, working

Principle and working of SONAR – Sound Navigation and Ranging

Determination of Ultrasonic Velocity in Liquid(Acoustical Grating Method): Principle, Construction and working

Ultrasonic Scanning Methods A, B and C Scan Displays

Sonogram Recording of movement of Heart: Principle and working

Characteristics of sound and Classification of Sound

PH8201 Notes r2017 notes

Industrial Applications of Ultrasonic waves

Post author By Mr. Padeepz
Post date January 25, 2018
No Comments on Industrial Applications of Ultrasonic waves

Industrial Applications of Ultrasonic waves

The industrial Applications of Ultrasonic waves are explained in detail.

INDUSTRIAL APPLICATIONS

Ultrasonic waves find application in two major fields:

Engineering filed
Medical field

Application of ultrasonic waves in engineering and industry

Ultrasonic waves in the wide applications in engineering and industry as follows.

Non destructive testing (detection of flaws in metals)
Ultrasonic drilling
The ultrasonic welding
Ultrasonic drilling
A ultrasonic soldering
ultrasonic cutting and machinery
A ultrasonic cleaning
Sonar

Ultrasonic Non destructive Testing – Industrial Applications of Ultrasonic waves

Principle:

The basic principle behind the ultrasonic inspection is the transmission of the Ultrasound with the medium and the reflection or scattering at any surface or internal discontinuity in the medium due to the change in the acoustic impedance.

The Discontinuity means the existence of the flaw or detect or cracks or hole in the material.

The reflected or scattered sound waves are received and amplified and hence, the defects in the specimen are suitably characterized.

Block diagram of the Ultrasonic Flaw detector

Principle:

Whenever there is a change in the medium, then the Ultrasonic waves will be reflected.

This is the principle used in Ultrasonic flaw detector.

Thus, from the intensity of the reflected echoes, the flaws are detected without destroying the material and hence this method is known as a Non Destructive method.

Working:

The pulse generator generates high frequency waves and is applied to the Piezo-electric transducer and the same is recorded in the CRO.
The piezo electric crystals are resonated to produce Ultrasonic waves.
These Ultrasonic waves are transmitted through the given specimen.
These waves travel through the specimen and is reflected back by the other end.
The reflected Ultrasonic are received by the transducer and is converted into electric signals. These reflected signals are amplified and is recorded in the CRO.
If the reflected pulse is same as that o the transmitted pulse, then it indictes that there is no defect in the specimen.
On the other hand, if there is any defect on the specimen like a small hole or pores, then the Ultrasonic will be relected bby the holes(i.e.) defects due to change in the medium.
From the time delay between the transmitted and received pulses, the position of the hole can be found.
From the height o the pulse received the depth of the hole can also be determined.

ULTRASONIC SCANNING METHODS – A,B AND C SCAN DISPLAYS – Industrial Applications of Ultrasonic waves

In the Ultrasonic scanning methods, the principle, construction and working is the same as that of the Ultrasonic law detector.

Here, it is based on the position o the transducer and the output displayed in the CRO screen, we can classiy the scanning methods into three types

A-scan
B-scan
T-M scan or C-scan

All these three modes of scanning are obtained with respect to the pulses of Ultrasound transmitted into and received from the specimen.

The three modes are explained below.

A-Scan or Amplitude mode display

Amplitude mode display gives only one-dimensional information about the given specimen.

In this, a single transducer is used to transmit and receive the pulses from the specimen.

The received or the reflected echo signals from the specimen is given to the Y-Plate and time base is connected to the X-Plate of the CRO, so that they are displayed as vertical spikes along horizontal base line as shown in the figure 1.10.1.

The height of the vertical spikes corresponds to the strength of the echo from the specimen.

The position of the vertical spike from left to right along the X-axis corresponds to the depth of penetration.

i.e, it gives the total time taken by the Ultrasonic sound to travel from transmitter to the specimen and from the specimen to the receiver.

Thus by passing Ultrasonic waves o known velocity and by noting the time delay, we can find the distance at which the detect or flaws are present, by using the formula.

Distance = Velocity x time

In ultrasonic flaw detector, A-scan method is used to detect the position and size of the flaws.

B-Scan or Brightness Mode Scan

B-scan or Brightness mode display gives a two dimensional image.

The principle of the B-Scan is same as that of A-Scan except with a small difference.

Here in the B-Scan the transducer can be moved rather than keeping in a fixed position.

As a result each echo’s are displayed as dots on the screen as shown in the figure 1.10.2.

T.M Scan or Time –Motion Mode or C-Scan display

This method is used to obtain the information about the moving object.

This combines the features of both A-Scan as well as B-Scan. In this the transducer is held stationary as in A-scan and echoes appear as dots in the B-scan.

Here, the X-axis indicates the dots at the relevant location and Y-axis indicates the movement of the object.

Therefore when the object moves, the dot also moves at a low speed.

Thus an object with the oscillatory movement will appear as a trace as shown in the figure 1.10.3.

Sonogram Recording of movement of Heart

Fetal Heart Movement

Principle:

It works under the principle of Doppler Effect

i.e., there is an apparent change in the frequency between the incident sound waves on the fetus and the reflected sound waves from the fetus.

Description:

It consists of a Radio Frequency Oscillator(RFO), for producing 2 MHz of frequency and RFA (Radio Frequency Amplifier) to amplify the receiver signal as shown in the figure.

Working:

The transducer is fixed over the mother’s absominal wall, with the help of a gel or oil.

RFO is switched on to drive the pulses and hence the transducer produces Ultrasonic waves of 2 MHz.

These Ultrasonic waves are made to be incident on the fetus.

The reflected Ultrasonic waves from the etch are received by the transducer and are amplified by RFA.

Both the incident and the received signals are mixed by the mixer and is filtered to distinguish the various types of sound and finally the Doppler shift or change in frequency is measured.

The movement o the heart can be viewed visually by CRO or can be heard by the Loud Speaker, after necessary amplification by AF.

Ultrasonic Imaging System – Industrial Applications of Ultrasonic waves

Principle of working

During the scanning of the body surface by Ultrasonic transducer, the Ultrasonic waves are transmitted into the patient’s body.

The echoes from the body are collected by the receiver circuit.

Since some echoes come from the depth, they are weak; therefore, proper depth gain compensation is given by DGC circuit.

Then these signals are converted into digital signals by an analog to digital converter and are stored in the memory of the Control Processing Unit (CPU) o a computer.

Meanwhile, the control unit in the CPU receives the signals of transducer position and TV synchronous pulses.

These signals generates X plate and Y plate address information’s or the T.V monitor and is also stored in the memory of the CPU.

The stored signals are processed and colour coded and is given to the digital to analog(D/A converter), which converts the digital into analog signal.

Finally the mixing circuit mixes the analog signals and TV synchronous signals properly.

The mixed signals are finally fed to the video section of the television monitor as shown in the igure 1.12.

The TV monitor produces the coloured Ultrasonic image of the internal part o the Body.

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Other links

Properties of Ultrasonic waves and Production of Ultrasonic waves

Piezo Electric Crystals – Principle, Construction, working

Principle and working of SONAR – Sound Navigation and Ranging

Determination of Ultrasonic Velocity in Liquid(Acoustical Grating Method): Principle, Construction and working

Ultrasonic Scanning Methods A, B and C Scan Displays

Sonogram Recording of movement of Heart: Principle and working

Characteristics of sound and Classification of Sound

PH8201 Notes r2017 notes

Determination of Ultrasonic Velocity in Liquid – Acoustical Grating Method

Post author By Mr. Padeepz
Post date January 25, 2018
No Comments on Determination of Ultrasonic Velocity in Liquid – Acoustical Grating Method

Determination of Ultrasonic Velocity in Liquid (Acoustical Grating Method)

Determination of Ultrasonic Velocity in Liquid – Acoustical Grating Method.

Acoustical Grating Method Principle:

When ultrasonic waves travel through a transparent liquid, due to alternate compression and rare action, longitudinal stationery waves are produced.

If monochromatic light is passed through the liquid perpendicular to these waves, the liquid behaves as diffraction grating.

Such a grating is known as Acoustic Grating.

Here the lines of compression and rareaction act as transparent light waves.

It is used to find wavelength and velocity(v) of ultrasonic waves in the liquid.

Acoustical Grating Method – Construction;

It is consists of a glass tank, filled with the liquid.

A piezo-electric (Quartz) is fixed at the bottom of the glass tank and is connected with piezo-electric oscillatory circuit as shown in the figure 1.7.

An incandescent lamp is used as a monochromatic source (S) and a telescope arrangement is used to view the diffraction pattern.

A collimator consisting of two lenses L1 and L2 is used to ocus the light effectively in the glass tank.

Acoustical Grating Method – Working:

(i) When the piezo-electric crystal is kept at rest:

Initially the piezo-electric crystal is kept at rest and the monochromatic at light is switched ON.

When the light is focused in the glass tank filled with the liquid, a single image, a vertical peak is observed in telescope.

i.e., there is no diffraction.

(ii) When the piezo-electric crystal is set into vibrations:

Now the crystal is put into vibrations using piezo-electric oscillatory circuit.

At Resonance, Ultrasonic waves are produced and are passed through the liquid.

These Ultrasonic waves are reflected by the walls of the glass tank and form a stationery wave pattern with nodes and antinodes in the liquid.

At nodes the density of the liquid becomes more and at antinodes the density o the liquid becomes less.

Thus, the liquid behaves as a directing element called acoustical grating element.

Now when the monochromatic light is passed the light gets directed and a diffraction pattern consisting of central maxima and principle maxima on either side is viewed through the telescope as shown in figure 1.7.2 as well as in 1.7.3.

Calculation of Ultrasonic Velocity

The velocity of Ultrasonic waves can be determined using the condition.

Thus, this method is useul in measuring the wavelength and velocity of ultrasonic waves in liquids and gases at various temperatures.

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Properties of Ultrasonic waves and Production of Ultrasonic waves

Piezo Electric Crystals – Principle, Construction, working

Principle and working of SONAR – Sound Navigation and Ranging

Ultrasonic Scanning Methods A, B and C Scan Displays

Sonogram Recording of movement of Heart: Principle and working

Characteristics of sound and Classification of Sound

PH8201 Notes r2017 notes

Sound Navigation and Ranging (SONAR)

Post author By Mr. Padeepz
Post date January 25, 2018
No Comments on Sound Navigation and Ranging (SONAR)

Sound Navigation and Ranging (SONAR):

Sound Navigation and Ranging (SONAR) are explained in this page.

SONAR – Sound Navigation and Ranging

Principle:

It is based on the principle of Echo – Sounding.

When the Ultrasonic waves are transmitted through water, it is reflected by the objects in the water and will produce an echo signal.

The change in frequency of the echo signal, due to Doppler Effect helps us in determining the velocity and direction of the object.

Description:

It consists of timimg section which triggers the electric pulse from the pulse generator.

This pulse generator is connected to the transducer so that ultra sonic can be produced.

The transducer is further connected with the CRO for display.

The timing section is also connected to the CRO display or reference of the timing at which the pulse is transmitted as shown in the block diagram.

Working:

The transducer is mounted on the ship’s hull without any air gap between them as shown.

The timing at which the pulse generated is recorded at the CRO or reference and this electrical pulse triggers the transducer which is kept in hull of the ship to produce ultrasonic waves due to the principle of inverse piezo electric effect.

These ultrasonic waves are transmitted through the water in sea.

On stricking the object the ultrasonic waves (echo pulses) are reflected in all directions as shown

Cavitation:

Definition:

The Ultrasonic sound waves that propagate into the liquid media result in alternating high pressure (compression) and low-pressure (rarefaction) cycles, with rates depending on the frequency.

During the low pressure cycle, high intensity ultrasonic waves create small vcuum bubbles or voids in the liquid.

When the bubbles attain a volume at which they can no longer absorb energy, they collapse violently during a high pressure cycle.

This phenomenon is termed cavitation.

During the low pressure cycle, high intensity ultrasonic waves create small vacuum bubbles or voids in the liquid.

When the bubbles attain a volume at which they can no longer absorb energy, they collapse violently during a high pressure cycle.

This phenomenon is termed cavitation.

During the implosion very high temperature(approx 5,000K) and pressures (approx 2,000atm) are reached locally,

The implosion of the cavitation bubble also results in liquid jets of up to 280m/s velocity.

Acoustic Grating

The ultrasonic waves generated with the help o a quartz crystal inside the liquid in a container sets up standing wave pattern consisting of nodes and anti-nodes.

The nodes are transparent and anti-nodes are opaque to the incident light.

In effect the nodes and anti-nodes acts like grating(a setup of large number of slits of equal distance) similar to that of ruling in diffraction grating. It is called as acoustic grating or aqua grating.

At nodes the density of the liquid is maximum and at antinodes density is minimum.

This arrangement is very much similar to the diffraction grating and is called acoustic grating.

Hence, by using the condition for direction, we can ind the wavelength of ultrasound and thereby the velocity of sound in the liquid medium.

When ultrasonic waves are generated in a liquid kept in rectangular vessel, the wave can be reflected from the walls of the vessel.

The direct and reflected waves get superimposed, which causes a standing wave to be formed.

The density o the liquid at the node will be more than the density at an antinode.

Under these conditions, if a beam of light is passed through the liquid at right angles to the wave the liquid acts as a diffraction grating. Such a grating is known as an acoustical grating.

Here, the antinode acts as the transmitting slit and the node acts as the opaque part. Thus resembling a normal ruled diffraction grating.

This is obvious because the nodes have points o minimum density and hence allow more amount o light to pass through them than those at antinodes.

Thus, the nodes act like slits.

Diffraction Grating

A Diffraction grating is an extremely useful device.

It consists of large number of narrow slits side by side.

The slits are separated by opaque surfaces.

When a wavefront is incident on the grating surface, light is transmitted through the slits and obstructed by the opaque spaces.

Such a grating is called transmission grating.

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Properties of Ultrasonic waves and Production of Ultrasonic waves

Piezo Electric Crystals – Principle, Construction, working

Determination of Ultrasonic Velocity in Liquid(Acoustical Grating Method): Principle, Construction and working

Ultrasonic Scanning Methods A, B and C Scan Displays

Sonogram Recording of movement of Heart: Principle and working

Characteristics of sound and Classification of Sound

PH8201 Notes r2017 notes

Piezo Electric Crystals

Piezo Electric Crystals:

Piezo Electric Crystals Principle, Construction, working, Advantages and Disadvantages

The crystals which produce piezo-electric eect and converse Piezo electric effect are termed as Piezo electric crystal. Example: Quartz, Tourmaline, Rochelle Salts etc.

Piezo Electric Crystals

The crystals which produce piezo-electric effect and converse Piezo electric effect are termed as Piezo electric crystal.

Example: Quartz, Tourmaline, Rochelle Salts etc.

At typical example or a piezo electric crystal (Quartz) is as shown

It has an hexagonal shape with pyramids attached at both ends. It consists of 3 axes. Viz.,

(i) Optic Z axis, which joins the edges of the pyramid

(ii) Electrical axis(X axis), which joins the corners of the hexagon and

(iii) Mechanical axis, which joins the center or sides of the hexagon as shown

X-cut and Y cut crystals

X-Cut crystal:

When the crystal is cut perpendicular to the X-axis, as shown in the figure 1.4.2, then it is called X-crystals.

Generally X-cut crystals are used to produce longitudinal ultrasonic waves.

Y-Cut Crystal:

When the crystal is cut perpendicular to the Y-axis, as shown in the figure 1.4.3, then it is called Y-cut crystal.

Generally, Y-Cut crystal produces transverse ultrasonic waves.

Piezoelectric Effect

Definition: When a mechanical stress is applied to the mechanical axis with respect to optical axis, a potential difference is developed across the electrical axis with respect to optic axis

Inverse Piezoelectric Effect:

Definition: When an alternating electric field is applied to electrical axis with respect to optical axis, expansion or contraction takes place in the mechanical axis with respect to optical axis.

Production of Ultra sonic waves – Piezo Electric Effect

Principle:

This is based on the Inverse piezoelectric effect.

When a quartz crystal is subjected to an alternating potential difference along the electric axis, the crystal is set into elastic vibrations along its mechanical axis.

If the frequency of electric oscillations coincides with the natural frequency of the crytal, the vibrations will be of large amplitude.

If the frequency of the electric field is in the ultrasonic frequency range, the crystal produces ultrasonic waves.

Construction:

The circuit diagram is shown in the figure 1.5

It is base turned oscillator circuit.

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

This is coupled to the electronic oscillator through the primary coil L₃ of the transformer.

Coils L₂ and L₁ of oscillator circuit are taken for the primary of the transformer. The collector coil L₂ is inductively coupled to base coil L₁.

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

Working:

When the battery is switched on, the oscillator produces high frequency oscillations.

An oscillatory e.m.f is induced in the coil L3 due to transformer action.

So the crystal is now under high frequency alternating voltage.

The capacitance of C1 is varied so that the frequency of oscillations produced is in resonance with the natural frequency of the crystal.

Now the crystal vibrates with larger amplitude due to resonance.

Thus high power ultrasonic waves are produced.

Condition for Resonance:

Frequency of the oscillator circuit = Frequency of the vibrating crystal

Where ‘l’ is the length o the rod

‘E’ is the Young’s modulus o the rod

‘ρ’ is the density of the material of the rod.

‘P’ = 1,2,3 …. Etc for fundamental, first overtone, second overtone etc respectively

Advantages:

Ultrasonic frequencies as high as 500MHz can be generated.
The output power is very high. It is not affected by temperature humidity.
It is more efficient than the Magnetostriction oscillator.
The breadth of the resonance curve is very small. So we can get a stable and constant frequency of ultrasonic waves.

Disadvantages:

The cost of the quartz crystal is very high.
Cutting and shaping the crystal is quite complex.

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Properties of Ultrasonic waves and Production of Ultrasonic waves

Principle and working of SONAR – Sound Navigation and Ranging

Determination of Ultrasonic Velocity in Liquid(Acoustical Grating Method): Principle, Construction and working

Ultrasonic Scanning Methods A, B and C Scan Displays

Sonogram Recording of movement of Heart: Principle and working

Characteristics of sound and Classification of Sound

PH8201 Notes r2017 notes

Properties of Ultrasonic waves and Production of Ultrasonic waves

Post author By Mr. Padeepz
Post date January 25, 2018
No Comments on Properties of Ultrasonic waves and Production of Ultrasonic waves

Properties of Ultrasonic waves and Production of Ultrasonic waves

The properties of Ultrasonic waves and Production of Ultrasonic waves are explained here

Properties of Ultrasonic waves

The human ear is sensitive to sound waves in the frequency range from 20-20, 000 Hz.

This range is called Auduble range. Sound waves of frequency more than 20,000Hz are called Ultrasonics. These frequencies are beyond the audible limit.

These waves also travel with the speed of sound(330m/s)

Their wavelength are small.

Production of Ultrasonic waves

Principle:

When a rod of ferromagnetic material like nickel is magnetized. Longitudinally, it undergoes a very small change in length.

This is called Magnetostriction effect.

Construction:

The circuit diagram of magnetostriction ultrasonic generator is as shown in the figure1.3.2.

A short permanently magnetized nickel rod is clamped in the middle between two knife edges.

A coil L₁ is wound on the right hand portion of the rod.

C is a variable capacitor. L₁ and C₁ form the resonant circuit of the collector-tuned oscillator.

Coil L₂ wound on the LHS of the rod is connected in the base circuit.

The coil L₂ is used as a feed back loop.

Working:

When the battery is switched on, the resonant circuit L₁C₁ sets up an alternating current o frequency.

This current lowing round the coil L1 produces an alternating magnetic iels of frequency f along the length of the nickel rod.

The rod starts vibrating due to magnetostrictive effect.

The vibrations of the rod create ultrasonic waves.

The longitudinal expansion and contraction of the rod produces an E.M. in the coil L₂.

This e.m.f is applied to the base of the transistor.

Hence the amplitude of high frequency of high oscillations in coil L1 is increased due to positive feedback.

The developed alternating current frequency can be turned with the natural frequency of the rod by adjusting the capacitor.

Condition for Resonance:

Frequency of the oscillator circuit = Frequency of the vibrating rod

Where ‘l’ is the length o the rod

‘E’ is the Young’s modulus o the rod

‘ρ’ is the density of the material of the rod.

The resonance condition is indicated by the rise in the collector current shown in the milliammeter.

Advantages:

Magnetostriction Oscillators are mechanically rugged.

The construction cost is low.

They are capable of producing large acoustical power with fairly good efficiency.

Limitations

It can produce frequencies up to 3MHz only.

They frequency o oscillation depends upon the temperature.

Breadth o the resonance curve is large. It is due to vibrations of elastic constants of ferromagnetic material with the degree of magnetization.

So we cannot get a constant single frequency.

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Piezo Electric Crystals – Principle, Construction, working

Principle and working of SONAR – Sound Navigation and Ranging

Determination of Ultrasonic Velocity in Liquid(Acoustical Grating Method): Principle, Construction and working

Ultrasonic Scanning Methods A, B and C Scan Displays

Sonogram Recording of movement of Heart: Principle and working

Characteristics of sound and Classification of Sound

PH8201 Notes r2017 notes

ABSORPTION COEFFICIENT

The ABSORPTION COEFFICIENT we know that all the sound waves when pass through as open window passes through it.

ABSORPTION COEFFICIENT

Thus, we can say that the open window behaves as a perfect absorber of sound and hence the absorption coefficient can be defined as the rate of sound energy absorbed by a certain area of the surface to that o an open window of same area.

Definition:

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

For example if 2m² of a carpet absorbs the same amount of sound energy as absorbed by 1 m² of an open window, then the absorption coefficient of the carpet is 1.2=0.5.

The absorption coefficient is measured in open window unit (O.W.U) or Sabines.

Average absorption coefficient

The average absorption coefficient is defined as the ratio between the total absorption in the hall to the total surface area of the hall.

Measurement of sound absorption coefficient

Let us consider a smaple for which the absorption coefficient (a_m) is to be measured.

Initially without this material the reverberation time in a room and again the reverberation time is measured and let it be T₂.

Then rom Sabine’s ormula

For Case (1) i.e. without the sample

Here, by knowing the terms on the right hand side the absorption coefficient of the given sample can be determined.

FACTORS AFFECTING THE ACOUSTICS OF BUILDING

We know, when sound waves are produced in a hall, it reaches the observer directly as well as after reflections from walls, floors, ceilings, etc.

Thus there is a possibility for causing interference between these waves, which in turn affects the originality of the sound produced.

The actors affecting the acoustics (sound) of building are as follows.

Unoptimised reverberation time
Very low or very high loudness
Improper focusing of sound to a particular area, which may cause interference
Echoes or echelon effects produced inside the buildings
Resonance caused due to matching of sound waves.
Unwanted sound rom outside or inside the building, so called noise may also affect the acoustics o buildings.

OPTIMUM REVERBERATION TIME AND ITS REMEDY

We know Reverberation time is the taken for the sound to fall to one millionth o its original sound intensity, when the source of sound is switched off.

This reverberation time is high then it produces, echoes in the hall and if the reverberation time is very low, the sound will not be cleary heard by the audience.

Therefore, for clear audibility, we should maintain optimum reverberation.

The optimum reverberation time can be achieved by the following steps

By having the full capacity o audience in the auditorium.
By choosing absorbents like felt, fiber, board, glass etc inside the auditorium and even at the back of chairs.
Reverberation time can be optimized by providing windows and ventilators at the places wherever necessary and using curtains with folds or the windows.
The reverberation time can also be optimized by decorating the walls with beautiful pictures.

The optimum reverberation time will not be constant for all types of building; it varies from one building to another as follows.

For concert halls, the speech should have the optimum reverberation time of 0.5 seconds and music should have the optimum values o 1 or 2 seconds
For auditorium, or theatres, the optimum reverberation time should be between 1.1 to 1.5 seconds for smaller area and between 1.5 to 3 seconds for larger area.

Loudness and its remedy:

We know loudness is the degree of sensation produced on the ear; it varies from observer to observer.

But, it is found that for a single observer the loudness varies from one place to another in the same auditorium.

This defect is caused due to the bad acoustical construction of buildings.

The loaudness will be very low in some area and will be very high in some areas. It can be optimized by the following remedies.

Remedies

Loudspeakers should be placed at the places where we have low loudness.
The loudness can also be increased by making reflecting surfaces, wherever necessary
Loudness can be increased by constructing low ceilings
Absorbents are placed at the places where we have high loudness.

Thus, the loudness should be made even, all over the auditorium, so that the observer can hear the sound at a constant loudness at all the places.

FOCUSING AND INTERFERENCE EFFECTS

In some places of a hall, the sound will not be heard properly and that place is said to be a dead space, which is due to presence of convex or concave surfaces in the hall as shown in the figure.

Sometimes the sound waves will have interference pattern because of ceiling surfaces which will create maximum intensity of sound(due to constructive interference) in some places and minimum intensity of sound(due to destructive interference) at some places and hence causing uneven distribution of sound intensity in the hall and hence causing uneven distribution of sound intensity in the hall.

Remedies

By avoiding curved surfaces (or) covering the curved surfaces by suitable absorbents the focusing can be avoided.
By evenly polishing and decorating with absorbents the interference effects can be avoided.

ECHOES AND ECHELON EFFECT

In some halls, the walls o the halls will scatter the sound waves rather than reflecting it, thus way create nuisance effect due to echoes.

The echoes are formed when the time interval between the direct and reflected sound waves are about 1/15^th of a second.

This effect occurs due to the reason that the reflected sound waves reaches the observer later than the direct sound.

If there is a greater repetition of echoes of the original sound to the observer then the effect is called as Echelon effect.

Remedies

The echo can be avoided by lining the surfaces with suitable sound absorbing materials and by providing enough number of doors and windows.

RESONANCE

Resonance occurs when a new sound note of frequency matches with standard audio frequency.

Sometimes, the window panel, sections of the wooden portion is thrown into vibrations to produce new sounds, which results in interference between original sound and created sound.

This will create disturbance to the audience.

Remedies

The resonance effect can be avoided by providing proper ventilation and by adjusting the reverberation time to the optimum level.
Nowadays the resonance is completely eliminated by air conditioning the halls.

NOISE

Noise is an unwanted sound produced due to heavy traffic outside the hall which leads to displeasing effect on the ear. There are three types of noises.

Air Borne noise
Structure Born Noise
Inside Noise

All these three noises pollute the area at which it has been produced and create harmful effects to the human beings.

Fortunately human beings have the capability to reject the sound within certain limits with conscious efforts and to carry on his normal work.

But sometimes the noises are strong which results in the following effects.

EFFECTS PRODUCED DUE TO NOISE POLLUTION

It produces mental fatigue and irritation.
Diverts the concentration on work and hence reduces the efficiency of the work.
It sometimes affects the nervous system and lowers the restorative quality of sleep.
Some strong noises leads to damage the eardrum and make the worker hearing impaired.
The noises which are produced regularly will even retard the normal growth of infants and young children.

AIR BORNE NOISE

The noise which reaches the hall through open windows, doors, and ventilations are called as air borne noise.

This type of noise is produced both in rural areas natural sound of wind and animals and in urban areas noise that arises from factories, aircrafts, automobile, trains, Flights etc.

REMEDIES

By making the hall air conditioned, this noise may be eliminated
By allotting proper places or doors and windows, this noise can be reduced.
It can be further by using double doors and windows with separate rames and by pacing the absorbents in-between them

STRUCTURE BORNE NOISE

The noise that reaches the hall through the structure of the building is termed as Structure Borne noise.

Those types of noise produced inside the building, which may be due to the machinery operation, movement of furniture’s footsteps etc and these sounds will produce structural vibration giving rise to the Structure Borne Noise.

REMEDIES

By properly breaking the continuity of the interposing layers by some acoustical insulators this type of noise can be avoided.
By providing carpets, resilent, antivibration mounts etc., this type of noise can be reduced.

INSIDE NOISE

The noises that are produced inside the halls is known as inside noise.

Or example in some offices the sound produced by machinery, type writers ect produces this type of noise.

REMEDIES

By placing the machineries and type writers over the absorbing materials or pads this type of noise can be reduced.
It can be reduced by covering the floors with carpet.
By fitting the engine on the floor with a layer of wood or elt between them this type of noise can be avoided.

FACTORS TO BE FOLLOWED FOR GOOD ACOUSTICS OF BUILDING

To have a clear audibility of sound have an optimum level

The reverberation time should have an optimum level
The sound must be evenly distributed to each and every part of the building.
There should not be any focusing of sound to any particular area.
Each and every syllable of sound must be herd clearly and distinctly, without any interference.
There should not be any echoes, echelon effects and resonance inside the buildings.
The building should be made as sound proof building, so that external noises may be avoided.
Generally to say the total quality o sound should be maintained all over the building to all the audience.

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Acoustics of buildings

The Acoustics of buildings In day today life sound engineering plays a vital role in film industries, broadcasting of television signals and even in television signals.

Acoustics of buildings

So a new field of science is developed which deals with the planning of a building or a hall with a view to provide best audible sound to the audience and is called Acoustics of building.

Therefore to provide a best audible sound in a building or hall a prime factor called Reverberation.

REVERBERATION

When a sound pulse is generated in a hall, the sound wave travels towards all directionand are reflected back by the walls, floors, doors, windows ceiling etc as shown.

So a sound wave has two to three hundred repeated reflections, before it becomes inaudible.

Therefore, the observer in the hall does not be able to hear a singlesharp sound instead a “role of sound” of diminishing intensity (since part of energy is lostat each reflection)

Reverberation time

The duration for which the sound persist is termed as reverberation time and is measured as the time interval between the sound produced by the source produced by the source and to the sound wave until it dies.

Definition:

It is defined as the time taken for the sound to fall below the minimum audibility measured from the instant when the source sound gets stopped.

In designing the auditorium, theatre, conference halls etc, the reverberation time is the key factor.

If the reverberation time is too large, echoes are produced and if the reverberation time is too short it becomes inaudible by the observer and the sound is said to be dead.

Therefore the reverberation time should not be too large or too short rather it should havean optimum value.

In order to fix this optimum value standard forumla is dervied by W.C.Sabine, who defined the standard reverberation time as the time taken for the sound to fall to one millionth of its original intensity just before the source is cut off.

SABINES FORMULA FOR REVERBERRATION

The relation connecting the reverberation time with the volume of the hall, the area and the absorption coefficient is known a s Sabine’s Formula.

Sabine’s developed the formula to express the rise and fall of sound intensity by the following assumptions.

Distribution of sound energy is uniform throughout the hall
There is Interference between the sound waves.
The Absorption coefficient is independent of sound intensity.
The Rate of emission of sound energy from the source is constant.

Let us consider a small element ‘ds’ on a plane wall AB. Assume that the element ds receive the sound energy ‘E’.

Let us draw two concentric circles of radii ‘r’ and r + dr from the center point ‘O’ of

Consider a small shaded portion lying in between the two semi circles drawn at an angle θ and θ+dθ, with the normal to ds as shown.

Let ‘dr’ be th radial length and rdθ be the arc length

Area of shaded portion rdθdr —- (1)

If the whole figure is rotated about the normal through an angle ‘dϕ’ as shown in the figure, then it is evident that the area of the shaped portion travels through a small distance dx.

To find total energy received by the element ‘ds’ per second, we have to integrate the equation 3 for the whole volume lying within a distance ‘v’ is the Velocity of sound.

It is obvious from the geometry of the figure that,

Growth and Decay of Sound Energy

If ‘P’ is the Power Output (i.e., the rate of emission of sound energy from the source) then we can write.

Here E_m is the maximum energy from the source (which has been emitted) that is maximum energy which is incident on the wall.

Where k is the constant of integration

Growth of Sound Energy

Let us evaluate for growth

Initially during the growth the boundary conditions

Are at t=0 E=0

Thereore equation 8 becomes

Where E_m is the maximum sound energy.

This expression gives the growth of sound energy density ‘E’ with time ‘t’.

The growth is along an exponential curve as shown.

DECAY OF SOUND ENERGY

Let us irst evaluate k or decay.

Here the boundary conditions are at t=0; E=Em

Initially the sound increases from E to E_mand now it is going to decay from Em.

Therefore time is considered as ‘0’ for E=Em. At E=E_mv the sound energy from the source is cut off.

Therefore rate of emission of sound energy from the source=0 i.e., P=0

Therefore from equation 8 we can write

Equation 10 gives the decay of sound energy density with time ‘t’ even after the source is cut off. It is exponentially depressing function from maximum energy(Em) as shown.

The growth and decay of sound energy together is represented in the figure.

PROOF OF REVERBERATION TIME(T)

According to Sabine, the reverberation time is defined as the time taken by a sound to fall to one millionth of its initial value, when the source of sound is cut off.

Equation 13 represents the Reverberation time, which depends on the three factors viz,

Volume of the hall(V)
Surface area(S)
Absorption coefficient(a) of the materials kept inside the hall.

Among these three actors volume is fixed.

Therefore, the reverberation time can be optimized by either varying the surface area of the reflecting surfaces or the absorption coefficient of the materials used inside the hall.

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