Acoustics and Ultrasonic (1) PDF

# ACOUSTICS ## INTRODUCTION - In earlier days, the consideration of acoustics was not common at all. Sometimes a building was found to be so defective acoustically, that hardly a word could be audible. The Fogg art museum hall of Hobart university when built was found to be defective acoustically...

# ACOUSTICS ## INTRODUCTION - In earlier days, the consideration of acoustics was not common at all. Sometimes a building was found to be so defective acoustically, that hardly a word could be audible. The Fogg art museum hall of Hobart university when built was found to be defective acoustically that the speaker could make his word understandable to his audience. - Acoustics is the science of sound which deals with the properties of sound waves, their origin, propagation and their action on obstacles. - Acoustics find wide applications in many fields of engineering like electro-acoustics, design of acoustical instruments and architectural acoustics. - Architectural acoustics deals with the design and construction of music halls and sound recording rooms to provide best audible sound to the audience. ## CLASSIFICATION OF SOUND - Sound is vibration in an elastic medium with definite frequency and intensity which can be heard by the human ear. - On the basis of frequency 'f' sound waves are classified into three types: - Infrasonic - Audible sound - Ultrasound - The sound waves having frequencies less than 20 Hz are called as _infrasound_. These sounds are not audible. - The sound waves having frequencies between 20 Hz to 20 kHz are called as _audible sound_. - The sound waves having frequency greater than 20 kHz are called as _ultrasound_. The ultrasound is also not an audible sound. ## CLASSIFICATION OF AUDIBLE SOUND - There are two types of audible sound - Musical sound - Noise - **Musical Sound:** The sound which produces pleasing effect on ear is called musical sound. - **Example:** sound produced by musical instruments like sitar, violin, flute, piano etc. - **Properties of musical sound:** The musical sound waveform are regular in shape, have definite periodicity and there is no sudden change in amplitude shown in figure. - **Noise:** The sound that produces a jarring effect on the ear and unpleasant to hear is called noise. - **Example:** sound produced by aero plane, road traffic, crackers etc. - **Properties of noise:** The noise waveform are irregular in shape, do not have definite periodicity and there is a sudden change in amplitude as shown in figure. ## CHARACTERISTICS OF MUSICAL SOUND - The characteristics of musical sound are: - **Pitch:** is related to frequency - **Loudness:** is related to intensity of sound - **Timbre:** is related to quality of sound ### PITCH - It depends upon the frequency. - It is a sensation which separates two sounds having same intensities and different frequencies produced by same musical instrument. (Pitch helps in distinguishing a note of high frequency and low frequency sound of the same intensity produced by the same musical instruments.) - A sharp (shrill) sound is produced by a sound of high frequency. - The voice produced by ladies and children are of high pitch type as frequency is high. - Similarly, the sound produced by a mosquito is of high pitch as frequency is high. - Sound produced by lion is of low pitch as frequency is low. - Thus, greater the frequency of a sound the higher is the pitch and vice versa. - The pitch of sound changes due to Doppler's principle, when either the source or the observer or both are in relative motion. ## LOUDNESS - Loudness is the characteristics which is common to all sounds whether classified as musical sound or noise. - Loudness is a degree of sensation (awareness) produced in ear. - The loudness depends upon intensity and also upon the sensitiveness of the ear. - Loudness varies from one listener to another. - Loudness is a physiological quantity. - Units of loudness are phon and sone. - Loudness and intensity are related to each other by the relation. $L = k logI$ where k is constant - From this relation it is seen that, loudness is directly proportional to the logarithm of intensity, and is known as Weber – Fechner law. - From the above equation, $\frac{dL}{dI} = \frac{k}{I}$ where $\frac{dL}{dI}$ is called as sensitiveness of ear. - Therefore, sensitiveness decrease with increase of intensity. ## TIMBRE - It is the quality of sound which separates two sounds having the same loudness and pitch. - It helps us to distinguish between musical notes emitted by different musical instruments and voices of different persons. - Sound of same intensity and same frequency coming from tabla and violin can be separated due to timbre. ## INTENSITY OF SOUND - Intensity I of sound wave at a point as the amount of sound energy Q flowing per unit area in unit time when the surface is held normal to the direction of the propagation of sound wave. $I = \frac{Q}{AT}$ - If A = 1 m² and t = 1 sec. then I = Q, where Q is sound energy. - The intensity is a physical quantity which depend upon the different factors like, amplitude 'a', frequency ' f' and velocity 'v' of sound together with the density of the medium p. - The intensity I in a medium is given by $I = 2 \pi^2 f^2 a^2 \rho v$ - The unit of intensity is Wm-2. - The minimum sound intensity which a human ear can sense is called the threshold intensity. And its value is 10-12 watt/m². - If the intensity is less than this value then our ear cannot hear the sound. - This minimum intensity is also known as zero or standard intensity. - The intensity of a sound is measured with reference to the standard intensity. ## INTENSITY LEVEL (RELATIVE INTENSITY) IL - The intensity level or relative intensity of a sound is defined as the logarithmic ratio of intensity I of a sound to the standard intensity Io. $IL = k log_{10}(\frac{I}{I_0})$ - Let I and Io represent intensities of two sounds of a particular frequency; and L1 and Lo be their corresponding measures of loudness. - Then, according to Weber-Fechner law, $L_1 = k log I$ $L_0 = klog I_0$ - Therefore, the intensity level or relative intensity is $IL = L_1 - L_0$ $IL = klog I – klog I_0 = k (log I – log I_0)$ $IL = klog (\frac{I}{I_0})$ - If k = 1, then IL is expressed in a unit called bel. $IL = log (\frac{I}{I_0})$ - From equation (3), it is seen that, 10 times increase in intensity i.e. I = 10 Io corresponds to 1 bel. - Therefore, bel is the intensity level of a sound whose intensity is 10 times the standard intensity. - Similarly, 100 times increase in intensity, i.e. I=100 Io corresponds to 2 bel and 1000 times increase in intensity, i.e., I=1000 Io corresponds to 3 bel and so on. - In practice, bel is a large unit. Hence, another unit known as decibel ‘dB' is more often used 1 dB = 1/10 beli.e. one decibel is 1/10 bel i.e. $IL = 10 log (\frac{I}{I_0}) dB$ ## PHYSICAL SIGNIFICANCE OF 1dB - From equation (4), the intensity level in dB is $IL = 10 log (\frac{I}{I_0})$ - Using the above equation, let us determine what change in intensity is produced when the intensity level is altered by 1 dB. ### CASE - 1 - If IL = 0 dB then $0 = 10 log (\frac{I}{Io})$ - As 10 ≠ 0, $log(\frac{I}{I_0}) = 0$ $\frac{I_1}{Io} = 10^0 = 1$ $I_1 = Io$ ### CASE - 2 - If IL = 1 dB then $1 = 10 log (\frac{I}{Io})$ $1 = 0.1 = log(\frac{I}{Io})$ $\frac{I_2}{Io} = 10^{0.1} = 1.259$ $I_2 = 1.259 I_0$ - Thus, the difference between equation (5) and (6) gives, I = 1.259 Io - Therefore, it is observed that, a change in intensity level 1 dB alters the intensity by 26%. - The threshold of audibility is 0 dB and the maximum intensity level is 120dB. The sound of intensity level 120 dB produces a feeling of pain in the ear and is therefore called as the _threshold of feeling_ or _threshold of pain._ **NOTE:** - The units used to measure loudness are phon and sone. - **Phon:** the measure of loudness in phons of any sound is equal to the loudness in decibel of any equally loud tone of frequency 1000 Hz. - **Sone:** defined as loudness of a 1000 Hz tone of 40 dB intensity level. OR It is equal to the loudness of that particular sound having a loudness level of 40 phon. ## DIFFERENCE BETWEEN LOUDNESS AND INTENSITY | Sr.\ No.|Loudness|Intensity| |:--:|:--:|:--:| |1|It is a degree of sensation produced on the ear. |It is the quantity of sound energy flowing across unit area in unit time.| |2|It varies from listener to listener. |It is independent of listener.| |3|It is a physiological quantity. |It is a physical quantity.| |4|Its unit is sone. |Its unit is Wm-2| |5|$L = klog I$ |$I = E/At$| ## ABSORPTION COEFFICIENT 'a' - The sound absorption coefficient 'a' of a material is defined as the ratio of sound energy absorbed by it to the total sound energy incident on it. $Absorption coefficient, a = \frac{sound\ energy\ absorbed\ by\ material}{ total\ sound\ energy\ incident\ on\ it}$ - In order to compare the relative efficiency of different sound absorbing material Sabine assumed a standard absorbing material. - Sabine chose an area of 1 m² open window to be the standard unit of absorption. - The unit of absorption coefficient is Sabine or also called as O. W. U. (open window unit) ## REVERBERATION AND REVERBERATION TIME - The persistence or prolongation of sound in a hall even through the source of sound is cut off is called reverberation. - This is because, a sound produced in a room undergoes multiple reflections from the walls, floor, ceiling and any other reflecting material before it becomes inaudible. - Thus, a person in the room receives the successive reflections of progressively diminishing (decreasing) intensity continuously. - Therefore, the sound lasts for some time even after the source has stopped emitting the sound. - This effect is called reverberation. - The time taken by the sound to fall below the minimum audibility level after the source stopped emitting sounding is called reverberation time. - Unit of reverberation time is second. - **Sabine's Formula:** Sabine defined the reverberation time as, the time taken by the sound intensity to fall to one millionth of its original intensity after the source stopped emitting sound. - The reverberation time is given by $T= \frac{0.167 V}{A}$ where V is the volume of the hall and A is total absorption in the room $A = \sum aS = a_1S_1 + a_2S_2 + a_3S_3 + ... + a_nS_n$ where 'a' is the absorption coefficient and S is the surface area. ## IMPORTANCE OF REVERBERATION TIME - The reverberation time always has to be maintained at an optimum value. - This is because, if the reverberation time is too small the loudness will be inadequate (not in proper amount) and the sound will die away instantaneously. Such a hall is said to have dead effect. - On the other hand, if the reverberation time is to large, the greater will be the confusion due to mixing or overlapping of sound. - Therefore, the reverberation time has to be maintained at an optimum value. ## DETERMINATION OF ABSORPTION COEFFICIENT OF A MATERIAL - The absorption coefficient 'a' of a sound absorbing material can be determined by measuring the reverberation time described as follows: - The reverberation time T1 in a hall is measured without placing the sound absorbing material in the hall. Thus $T_1 = \frac{0.167 V}{A}$ where A is total absorption. - The sound absorbing material of absorption coefficient a1 having surface area S1 is placed in the hall and the reverberation time T2 is measured. Thus, $T_2 = \frac{0.167 V}{A + a_1 S_1}$ - From above equations, $\frac{1}{T2} - \frac{1}{T1} = \frac{a_1 S_1}{0.167 V}$ $\frac{1}{T1} - \frac{1}{T2} = \frac{1}{0.167 V}$ $:a_1 = \frac{S_1 (T1 - T2)}{T_1 T_2}$ - Hence, knowing the values of reverberation time, surface area of material, and volume absorption coefficient of the material can be determined. ## SOUND ABSORBING MATERIALS ### PROPERTIES OF SOUND ABSORBING MATERIALS - They should be highly porous. - They should be cheap and easily available. - They should be easy to fix and good looking. - They should be light in weight and durable. - They should be water proof and fire proof. - They should be efficient over a wide range of frequencies. ### CLASSIFICATION OF SOUND ABSORBING MATERIALS - **Porous absorbent:** When sound wave strike a porous material, a small part of it gets reflected, while major part of sound enters into the porous material and gets converted into heat energy hence it becomes inaudible and do not create interference. - **Example:** fiber board, soft plastic, rock wood, wool wood, glass silk etc. - **Cavity resonator:** A chamber with a small opening is known as a cavity resonator. The sound waves which enter the cavity gets multiple reflections become inaudible. These materials are used in air condition plants. - **Resonant absorbent/panel absorbent:** When sound energy is incident on these materials, it gets converted into heat due to flexural vibration of the panel. - **Example:** gypsum board, hard board panels, wood board, suspended plasters, rigid plastic board panels. - **Composite absorbent:** When the functions of all the three types described above are combined in a single unit, it is known as Composite type absorbent. When the sound energy strikes the panel, it passes through it and is damped by resonance of air in the cavity. - **Example:** empty jars and bottles, perforated cardboard etc. ## NOISE POLLUTION - The unwanted sound is called noise. Pollution cause be noise is known as noise pollution. - Noise produce unpleasant effect and create adverse effects to human health. ## NOISE CONTROL IN MACHINES - Insulating the source with sound reducing houses. - Providing a dynamic balance to vibrating machine. - Using large work area consisting of good sound absorbing materials. - Reducing structure born noise using double doors or walls with air spaces between them. - Providing sound reducing ear muffs and plugs to all the workers. - Isolating machine area from offices, show rooms etc. ## FACTORS AFFECTING ACOUSTICS OF BUILDING AND THEIR REMEDIES - The various factors affecting the acoustics of building such as - Reverberation time - Echelon effect - Loudness - Resonance - Focusing - Noise - Echo - Above factors with their remedies are explained in brief. ### REVERBERATION TIME - Reverberation is the persistence of prolongation of sound in a hall even after the source stopped level. - The reverberation time is the time taken by the sound to fall below the minimum audibility level after source stop sounding. - In order to have a good acoustics effect, the reverberation time has to be maintained at optimum value, if the reverberation time is too small, the loudness becomes inadequate. As a result, the sound may not reach the listener. And if reverberation time is too large, sound will get mixed. - **Remedies:** - By providing window and openings. - By having full capacity of audience in the hall or room. - By using heavy curtains with folds. - By covering the floor with carpets. - By decorative the walls with beautiful pictures. - By covering the ceiling and walls with good sound absorbing material like, fiber board, roofing etc. - The reverberation time is depends on the size of the hall and the quality of sound. ### LOUDNESS - Loudness should be uniform throughout the room. The loudness may get reduces due to the excess of sound absorbing materials used inside the hall or room. - **Remedies:** - If the loudness of sound is not adequate, the loudness can be increased by adopting the following methods. - By using suitable absorbents at the place where the feel loudness to be high. As a result, the distribution of loudness becomes uniform. - By constructing low ceilings for the reflection of sound towards the listener. - By using the large sounding board behind the speaker and facing the audience. - By using public address system like loudspeakers. ### FOCUSING EFFECT - Focusing is produced due to curved surfaces present in the room. - Sound is focused at focus point. - Intensity is maximum at points were constructive interference occurs. - Intensity is minimum at points were destructive interference occurs. - The quality of original sound is affected due to this effect. - **Remedies:** - Avoid curved surface inside the hall - Keep sound absorbing material on the curved surface. ### INTERFERENCE EFFECT - Interference is produced due to superimposition of waves. - Intensity is maximum at points were constructive interference occurs. - Intensity is minimum at points were destructive interference occurs. - The quality of original sound is affected due to this effect. - **Remedy:** Keep sound absorbing material in the hall. ### ECHO - An echo is heard due to reflection of sound from a distant sound reflecting object. - If the time interval between the direct sound and reflected sound is less than 1/15th of a second, the reflected sound is helpful in increasing the loudness. But, those sounds arriving later than this cause confusion. - **Remedy:** An echo can be avoided by covering long distance walls and high ceiling with suitable sound absorbing material. This prevents the reflection of sound. ### ECHELON EFFECT - Echelon effect is produced due to reflection of sound from regular reflecting surface present in the room. Due to this a new separate sound due to multiple echoes is generated. This echelon effect affects the quality of the original sound. - **Remedies:** - Avoid regular reflecting surfaces. - The remedy to avoid echelon effect is to cover such surfaces with sound absorbing materials. ### RESONANCE - Resonance occurs due to matching of frequency. In case, if the window panels and sections of wooden portions have not been tightly fitted they may start vibrating creating an extra sound in addition to the sound produced in the hall or room. - **Remedy:** The resonance may be avoided by fixing the window panels properly. Any other vibrating object which may produce resonance can be placed over a suitable sound absorbing material. ### NOISE - The hall or room should be properly insulated from external and internal noises. In general, there are three types of noises. - **A. airborne noise** - noise enters the room due to air medium. - **B. structure borne noise** - noise enters the room due to structure. - **C. inside noise** - noise is produced from inside the room. - **Remedy:** The hall can be made air conditioned. By using doors and windows with separate frames having proper sound insulating material between them. ## CONDITIONS FOR GOOD ACOUSTICS - A hall or an auditorium is said to be acoustically good if they satisfy the following conditions. - Resonance effect should be avoided. - Focusing effect should be avoided. - Interference effect should be avoided. - There should not be echelon effect. - Noise should not enter the room. - The loudness of the sound is uniform throughout the hall. - The hall should have a proper reverberation time. - The quality of the sound is uniform throughout the entire hall. - There should not be any overlapping of sound. - The presence or absence of audience can not affect the quality of sound. # ULTRASONICS ## INTRODUCTION - The human ear is sensitive to sound wave in the range of 20 Hz to 20 kHz. Waves are coming in the range of greater than 20 kHz are called ultrasonic waves. And which are coming in the range less than 20Hz frequencies are coming in the range of infrasonic (subsonic) range. - We can describe the sound waves as the vibrations or oscillations of molecules (particles) in solids, liquids and gases. - Hz (hertz) is the unit of frequencies. - 1 Hz = 1 Cycle per second - 1 kHz = 1 thousand Hz - 1 MHz = 1 million Hz - Humans cannot sense the ultrasonic waves, but dogs and other animals has ability of hear the high frequency sounds. The wavelengths of ultrasonic waves are very small that's why they have very wide applications. - They are widely used in marine applications, medical diagnostics and non-destructive testing of new products. - Dolphins and Bats can generate ultrasonic waves and find their path. ## PROPERTIES OF ULTRASONIC WAVES - Ultrasonic waves have very high frequency. - Ultrasonic waves have very low wavelength. - They have very high penetration power because of higher energy as compared to audible waves. Ultrasonic waves can penetrate through metals and other materials which are opaque to e.m. waves. - They have high energy content. - They can travel over long distances as a high directional ray and without loss of energy. - With increasing of frequency, the speed of propagation in medium of ultrasonic waves is increases. - In ultrasonic waves the diffraction effect is negligible because of the wavelength of ultrasonic is very small - Optical laws like reflection, refraction, diffraction etc. are observed with ultrasonic waves. - Ultrasonic waves produce 'cavitation' effect when made to pass through some solids. - The velocity of the ultrasonic waves depends on the temperature of the medium through which waves propagate. ## GENERATION OF ULTRASONIC WAVES - There are two methods for generation of ultrasonic waves. - Mechanical generator (Galton whistle generator) - Electrical generator (Magnetostriction and piezoelectric generator). ### MAGNETOSTRICTION OSCILLATOR (PIERCE OSCILLATOR) **Principle:** Magnetostriction Effect - When we put a rod of ferromagnetic material like nickel or iron, in a magnetic field parallel to its length, there is change in length of rod and it depends on the magnitude of the field and nature of the ferromagnetic material. This phenomenon is called magnetostriction effect. This method was discovered by Joule in 1847. Nickel produces the very large magnetostriction effect in compare to other ferromagnetic materials. - If we put a rod in an alternating field of frequency f, in each half cycle rod changes its length. As a result, there is some setup of vibrations in the rod whose frequency is twice the magnetic field frequency. The amplitude of the vibration is small, but when the frequency of the field is set equal to the natural frequency of the rod, resonance occurs and amplitude of the vibrations will be larger. If the frequency of the field comes in ultrasonic range, an ultrasound of frequency 2f will be generated in the medium that is in contact with the ends of the rod. - The frequency of rod is given by $f = \frac{1}{2l} \sqrt{\frac{Y}{\rho}}$ where L is length of the rod, Y is the Young's modulus, p the density if the rod. **Construction:** - G. W. Pierce was the first to design an ultrasonic oscillator basing on the phenomenon of magnetostriction. Schematic diagram is shown in Figure. - A nickel rod is clamped from the center. Two coils L1 and L2 are wound at the ends of the rod. Coil L1 is connected to variable capacitor and form tank circuit. One end of tank circuit is connected to collector of transistor via milliammeter and other end is connected to emitter of transistor via battery. Ends of L2 are connected to base and emitter of transistor and form feedback loop. **Working:** - When battery is switched on, due to tank circuit alternating current (AC) is generated with frequency $f = \frac{1}{2 \pi\sqrt{L_1 C}}$ - Due to this current rod gets magnetized and induce emf in coil L2. This emf is given back to tank circuit to maintain oscillation. - Also due to this current rod vibrates with frequency $f=\frac{1}{2l}\sqrt{\frac{Y}{ \rho}}$ where l is length of the rod, Y is the Young's modulus, p the density if the rod. - With the help of variable capacitor, we can vary the frequency of AC. When frequency of AC becomes equal to natural frequency of the rod i.e., resonance condition is achieved, rod vibrates vigorously and ultrasonic waves are generated. During this milliammeter shows maximum reading. The frequency of generated ultrasonic waves is $f = \frac{1}{2\pi\sqrt{L_1 C}} = \frac{1}{2l}\sqrt{\frac{Y}{\rho}}$ **Advantages:** - The construction is very simple. - The cost of the generator is low. - At low ultrasonic frequencies, a large power output is possible without the any risk of damage of the oscillator circuit. - It can generate ultrasonic of frequency till 3 MHz. **Disadvantages:** - It cannot generate ultrasonic of frequency above 3 MHz. - The frequency of oscillations depends on temperature. At higher temperatures the o/p from the oscillator will not be very stable. ### PIEZOELECTRIC EFFECT <start_of_image> - When pressure is applied to a pair of opposite faces of crystals like quartz, tourmaline, Rochelle salt etc., cut with their faces perpendicular to its optic axis, equal and opposite charges appears on the other faces as shown in figure. This phenomenon is known as piezoelectric effect. - The sign of charges gets reversed if the crystal is subjected to tension instead of pressure. ### PIEZOELECTRIC OSCILLATOR **Principle:** Inverse piezoelectric effect - When an alternating voltage is applied on a pair of opposite faces of piezoelectric crystal (quartz), it starts vibrating at the frequency of the applied voltage. - Vibrations occur at maximum amplitude at the natural resonant frequency of the piezoelectric crystal, which is determined by physical dimensions. The frequency of the vibrations is calculated by equation given below $f = \frac{n}{2l} \sqrt{\frac{Y}{\rho}}$ where L is the length of crystal plate, Y is the Young's modulus along the appropriate direction, p is the density of the crystal and n (number of modes) = 1, 2, 3, ..... **Construction:** - P. Langevin developed a method for producing ultrasonic waves using the piezoelectriceffect in 1917. **Working:** - The X-cut quartz crystal is placed between two metallic plates A and B which are connected to the coil L3. Coils L1, L2 and L3 are inductively coupled. Coil L1 is connected with variable capacitor C to form tank circuit. One end of tank circuit is connected to emitter of transistor and other end is connected to base of the transistor. One end of L2 is connected to emitter of transistor via battery and other end to collector. - When battery is switched on, due to tank circuit alternating current (AC) or alternating voltage is generated with frequency $f = \frac{1}{2\pi\sqrt{L_1 C}}$ - Due to this AC, an alternating emf is induced in coil L3 by transformer action. This emf is imparted on metallic plates. Due to this quartz starts vibrating with frequency, $f = \frac{n}{2t} \sqrt{\frac{Y}{\rho}}$ where, n = 1, 2, 3.. etc. for fundamental mode (first mode), first overtone (second mode), etc. E = Young's modulus of the material, p = density of the material and t = thickness of the crystal plate. - With the help of variable capacitor, we can vary the frequency of AC. When frequency of AC becomes equal to natural frequency of the quartz i.e., resonance condition is achieved, quartz vibrates in one of the modes of vibration and ultrasonic waves are generated. The frequency of generated ultrasonic waves is $f = \frac{1}{2\pi\sqrt{L_1 C}} = \frac{n}{2t} \sqrt{\frac{Y}{\rho}}$ **Advantages:** - Through this oscillator we can generate ultrasonic waves up to 500 MHz. - It is more efficient then magnetostriction generator. - The ultrasonic generator output (o/p) is very high. - Output does not depend on environmental changes such as temperature and humidity. **Disadvantages:** - The cost of the natural piezoelectric crystals is high. - The cutting and shaping of piezoelectric materials is very complex and involve tedious mechanical processes. ## DETECTION OF ULTRASONIC WAVES - The presence of ultrasonic waves is detected in a room by the phenomena of stationary wave method using the Kundt's tube, or the sensitive flame method or the thermal detectors or the quartz method as was done by Langevin. As ultrasound is beyond audibility, the above indirect methods are used. ### KUNDT'S TUBE METHOD - The Kundt's tube method for the detection of ultrasonic waves is limited to low frequency waves. When ultrasonic waves go through the glass tube filled with light weight power (lycopodium powder) spread in the tube, the powder gets blown off at antinodes and forms heaps at nodes. - The distance between any two nodes is equal to half of the wavelength, i.e. d=us/2. From this equation. The wavelength of ultrasonic is estimated. This method, however, fails as the wavelength of ultrasonic is very small as equal to few mm. ### THERMAL DETECTOR METHOD - This method is also used to detect the ultrasonic waves in which a fine platinum wire is moved through the medium. The temperature changes at the nodes and thereby there is a change in the resistance of the platinum wire, whereas the resistance remains constant at the antinodes as the temperature remains constant there. With the help of bridge circuit, the change in resistance of the platinum wire is measured. ### SENSITIVE FLAME METHOD - Ultrasonic waves are detected by moving a sensitive flame in the medium. The stationary positions of flame (at antinodes) and the shining positions (at nodes) will help in finding the exact location of nodes and antinodes and by knowing the mean distance between two consecutive nodes, the ultrasonic wavelength can be determined as usual. However, if the frequency of the ultrasonic wave is known, its velocity through the medium can also be estimated. ### QUARTZ CRYSTAL METHOD - This method of detecting ultrasonic waves depends upon the piezoelectric effect. When one pair of faces of a quartz crystal is exposed to ultrasonic waves, electric charges are developed on the other pair of opposite faces which are perpendicular to the previous one. These charges are amplified and detected by using electronic circuits. ## DETERMINATION OF VELOCITY OF ULTRASONIC WAVES (ACOUTIC GRATING METHOD) - When we propagate ultrasonic waves in a transparent liquid medium, after reflection it will superimpose with incident wave and forms stationary waves. At nodes density of the medium becomes maximum and at antinodes density of the medium becomes minimum. Due to this there is periodic variation in density of liquid. It leads to a periodic variation of refractive index of the liquid. Such a liquid column subjected to ultrasonic waves constitutes an acoustical grating. If monochromatic light is passed through the liquid at right angles to the waves, the liquid causes the diffraction of light. Such liquid column subjected to ultrasonic waves is called acoustic grating. - Figure below shows experimental setup for determination of the velocity of ultrasonic waves. Ultrasonic waves are produced in a liquid contained in a glass tube. - The density and hence the refractive index of the liquid is maximum at nodal points and minimum at antinodal points. Therefore, the nodal areas act as opaque regions while antinodal areas act as transparent regions for light. The liquid column thus resembles a ruled grating. - When the crystal is at rest, a single image of the slit is formed in the screen. When the crystal is excited a diffraction pattern is produced. It consist of a central maxima followed is the grating element which is given by d = λu / 2. $\lambda_u = \frac{2n\lambda}{sin\theta}$ - where a is the wavelength of monochromatic light beam, n is the order of the maxima. - By using above equation du can be determined. The frequency f of the waves is known from the frequency of the oscillator. The velocity of waves in the liquid can be found from the equation, $v = f\lambda_u$ - The above method of determining du and v of ultrasonic waves is known as 'acoustic diffraction method'. ## APPLICATIONS OF ULTRASONICS - Ultrasonic is widely used in industry, medicine and marine applications. Here some of the applications is given below, ### ECHO SOUNDER - Ultrasonic waves can be generated in the form of directed beam like light beam. Further, the ultrasonic waves can travel over long distances in water. As a result, ultrasound is widely used in marine applications. The depth of the ocean can be found by an echo sounder. The ship is equipped at its bottom with a source and a receiver of ultrasound of a specific frequency. The source sends out short pulses of ultrasonic waves and receiver receives reflected pulses. Measuring the time interval between the pulse sent and the pulse received, the depth of the ocean can be computed with the help of the formula $d = \frac{vt}{2}$ where, d is the depth, t is the time interval and v is the velocity of ultrasonic wave. ### SONAR - The word sonar means Sound Navigation and Ranging. The ultrasonic waves which are highly directional can be used for locating objects and determining their distance in the seas. Sonar can also measure depth of sea, find underwater obstacles like rocks and shipwrecks, locate shoals of fish and trace their movement, help divers and military applications such as finding submarines, mines, enemy ships etc. ### FLOW MEASUREMENT - There are various ways to measure the flow velocity in a pipe by measuring its effect on the passage of ultrasonic waves transmitted and received at the outside of the pipe, without disturbing the flow by inserting anything into the pipe. ### MATERIAL CHARACTERIZATION - Ultrasonic velocity and reduction are dependent on material properties which in turn may be related to other characteristics that can be of special interest to the engineer (e.g. breaking strength of cast iron, grain structure of still). ### DETECTION OF CRACKS OR FLAW IN METALS - It there is any hidden flaw or crack in metal, ultrasonic waves can be used to identify these defects by non-destructive testing. ### MEDICAL DIAGNOSTICS - Medical diagnostic is another field of application of ultrasonic waves. This field, on the one hand, mainly consist of examining and anatomical structure of soft tissue or its functioning, while on the other hand it is used for checking the normality of blood flow in various vessels. All these examinations are noninvasive.