Measurement of Sound

Common Instruments used to measure sound

MEASUREMENT OF SOUND

INTRODUCTION

Acoustics is the science of sound. Noise is any unwanted sound, and is a subjective concept. Acoustics covers two areas, those of room acoustics and control of noise.

Noise can be defined as any unwanted or damaging sound, that is, sound which interferes with what people are trying to do, or sound which has an adverse effect on health or safety. Sound is a disturbance, or wave, which moves through a physical medium (such as air, water or metal) from a source to cause the sensation of hearing in animals. Sound is the sensation of the medium acting on the ear. The source can be a vibrating solid body such as the string of a guitar or the membrane of a drum, but it can also be a whistle. The medium may be either a fluid or a solid. The average human ear can perceive sound of frequencies between 20 and 16,000Hz. This range varies with individuals, age and other subjective factors. The human ear is less sensitive to sounds of higher intensities and this is important in preventing damage.

The instrument in measurement of velocity of sound in open Air, measurement of pitch, and the measurement of intensity of sound are the basic focus of this write up

The word sound conveys a double meaning. It refers to

(i)The mental sensation perceived by the ears and

(ii)The cause responsible for that perception namely, the physical phenomenon external to the ear

CHAPTER ONE

Measurement of velocity of sound

(1)Determination of velocity of sound in Open Air

The earlier experiments for the determination of the velocity of sound in open air consisted in observing directly the time interval between the seeing of the flash light due to the firing of a distant gun and the time taken by sound. The velocity of light is so great, that the time taken by it travel over ordinary terrestrial distance is negligibly small and the interval therefore gives practically the time taken by the sound to travel from the source to the observer. Hence if this distance can be determined at once.

The method is simple, but liable to a number of errors given below.

(a)Effect of wind: the velocity of sound is affected by wind. When the wind is blowing the velocity relatively to the ground is not the same as its rate of propagation through still air. The wind may be blowing in the direction of sound propagation or against it.

(b)Temperature and humidity of the air: the velocity of sound is also affected by temperature and humidity.

(c)Personal equation: it is not possible to record the time at the exact moment when the flash is seen or the sound is heard and hence some error always creeps in.

(2)A Modern Outfit for the Determination of Velocity of Sound in Open Air:

Two microphones M¹ and M² about 8 kilometers apart, are separately connected to two separate primary coils P¹ and P² through separate batteries B¹ and B². The two primary coils are surrounded by a single secondary coil ‘S’ connected to a sensitive mirror galvanometer G. when the gun is fired a slight change in the current in the microphone ‘M¹’ takes place, which in turn induces a current in ‘S’ and a ray of light reflected from the mirror M of galvanometer is deflected drum D which is rotated at constant speed. The filmalso receives time signals from a standard turning fork. On the arrival of the sound at the second microphone M²again an induced current is produced in ‘S’ and the ray of light is deflected once more. The interval between these two deflections being read accurately from the film, the velocity of sound in air is calculated.

(3)Measurement of Velocity of Sound by Organ Pipes:

For this purpose organ pipes of adjustable length are needed. Such a pipe is turned to a fork of known frequency. This can be easily done by the observation of beats. Having done this, the length of the pipe is noted. The pipe is then increased in length till it gives the same note in the next higher mode of vibration. The length of the pipe is again noted the difference between the two lengths is. Find in g in this

way and knowing n, the frequency of the fork, and velocity of sound in air can be calculated.

(4)Velocity of sound in a Gas by See beck’s Tube:

The points of silence are observation with the given gas inside the see becks tube.

For the first minimum B =λ, note that the length to be measurement from the point ‘C’ which is the centre of the side tube L. No correction for the open and is to be considered and so v= n λ = 4n BC.

This method has the advantage that a small volume of gas is used, and so its temperature and degree of humidity can be accurately determined.

SEEBECK’S TUBE

(5)Hebb’s Method, velocity of sound in Free Air. (1905)

In 1905 HEBB performed an accurate experiment to measure the velocity of sound in free air which utilized a method of interference. He carried out his experiment in a large hall to eliminate the effect of wind, and obtained the temperature of the air by placing thermometers at different parts of the room. Two parabolic reflectors, R¹, R², are placed at each end of the hall, and microphones, M¹, M², are positioned at the respective foci, S, S, to receive sound reflected from R¹, R². by means of a transformer, the currents in the microphones are induced into a telephone earpiece, P, so that the resultant effect of the sound waves received by M¹,M² respectively can be heard.

A source of sound of known constant frequency is placed at the focus S¹. The sound waves are reflected from R¹ in a parallel direction and travel to R² where they are reflected to the focus S² and received by M². The velocity of sound can be calculated from the relation V= f λ, as F and λ are known.

HEBB’S METHOD

(6)Kundt’s Tube (1866)

As early as 1866 kundt designed a small scale apparatus for determining the velocity of sound in solids,

Liquids and gases contained in tubes. His experiments on the measurement of the velocity of sound in gases under all conditions have provided valuable information on the molecular grouping of their constituents. The principal of the method is that a column of gas is enclosed in a tube and a note of fairly high frequency and therefore of short wavelength produced with the help of longitudinal vibrations of a rod is sent along it and the length of this column adjusted until resonance occurs and stationary vibration are set up. Then the length would be an integral multiple of half wavelength, since each end must be a node.

λ r = 2lr

Where λ, is the wavelength in the rod.

KUNDT’S TUBE (1866)

(7)Quincke’s Tube Measurement of velocity of sound in a tube:

QUINCKE devised a simple method of obtaining permanent interference between two sound waves. He used a closed tube SAEB which had openings at S, E, and placed a source of sound at S

A wave then travelled in the direction SAE round the tube, while another wave travelled in the opposite direction SBE.

If the tube is pulled out farther, the sound increases in loudness to a maximum; the path difference is then λ. If K is the distance moved from one position of minimum sound. MN say, to the next position of minimum sound PQ say, the 2k = λ.

CHAPTER TWO

Measurement of Pitch

(8)Measurement of Pitch by Cagniard de la Tour’s siren

Early methods of finding the frequency of a note employed savart’s toothed wheel and the disc siren. In the case of a savart’s wheel when a card piece is held against the teeth and the wheel is turned, firstly separate taps are heard; as the speed of the wheel increased, these blend into a single note.

A better instrument is Cagniard de la Tour’s siren it is an apparatus in which the wind chest, the disc with holes, and the counter all are put together.

(9)Measurement of Pitch by Sonometer.

The frequency of the fundamental note of a vibrating string is given by n=½ℓ√t/m, where ℓ is tuning length of the string in cm, T the stretching force in dynes and in the mass per cm of the string in grams.

A string is stretched on a Sonometer box with a known load and a portion of it between two bridges is tuned to unison with the given fork by the method suggested.

The Sonometer pulley must be quite free from friction.

(10)Measurement of the Absolute Frequency of a fork by the Falling plate.

The apparatus consists essentially of smoked glass plate arranged to fall past the tuning fork whose frequency is to be determined. The fork ‘F’ with a light aluminum style attached to its prong mounted on a vertical pillar resting a base board.

no = n+ b = a √ g + b

(11)Measurement of Pitch by the Revolving Drum Method i.e. (Chronographic Method).

When a smoked paper is drawn under a vibrating tuning fork with a light style attached to one of its prongs, a wavy line is traced on the paper. It is only necessary then to count the number of waves in any interval of time and divide it by the interval to get the frequency of the fork.

The method is very accurate as it is independent of the rate of rotation of the cylinder but merely depends upon the time of vibration of the pendulum which can be determined accurately by the Astronomical method.

(12)Measurement of Pitch by the Tonometer

Scheibler devised his Tonometer which consists of a series of tuning- forks extending over an octave, the interval between any two consecutive forks are about four beats per second.

(13)Stroboscopic Method of determining Frequency

A stroboscope is a device by means of which a moving object can be made to appear stationary.

It is generally a metallic disc provided with one or more circles of equal sized and evenly spaced dots.

Vibration Meter 2050
This is our industry-standard Vibration Meter for assessment of machinery vibration, with digital display for spot measurement of Vibration in Acceleration/ Velocity/ Displacement/ Bearing Spike. Suitable for overall Vibration Severity Assessment. Supplied with a rechargeable battery. / Vibration Meter 2040
This is a general purpose low cost Vibration Meter for assessment of machinery vibration, with digital display for spot measurement of Vibration in Velocity/Displacement. Suitable for overall Vibration Severity Assessment measurement as per IS11724.

CHAPTER THREE

Measurement of Intensity of Sound

(14)Rayleigh’s Method of measuring the intensity of just audible sounds:

Here we measure the energy generated by the source and the distance at which the sound is just audible.

Rayleigh carried out the experiment on a still day with a whistle of frequency as source. The whistle was placed in the open and was blown with air at a pressure of 9½cm of water using a volume of 196cm³ of air per sec.

E= pg h x v

= 1x9.80x9½x196

(15)Topler and Boltzmann’s interferometer Method:

It makes use of the principle of optical interference to measure the pressure amplitude at a node in an organ pipe.

(16)Direct measurement of displacement amplitude of air vibrations: by Andrade and Parker

The displacement amplitudes have been directly measured by Andrade and Parker in case of very loud sounds by placing minute dust particle in the acoustic field and observing their displacements by a high power microscope. They used smoke particles of maintained by a diaphragm excited by a valve amplifier at resonance frequencies of the tube

I =

Where v is the velocity of sound.

(17)The hot wire Microphone Method:

With the help of the hot wire microphone the amplitude of air vibrations may be calculated by a method due to Rechards. The performed the experiment with the hot wire grid attached to the prong of a vibrating fork.

y= a sin w t.

(18)Radiation Pressure Method:

This fact forms the basis of an absolute method of measuring the energy density of sound and hence its intensity.

Hence the intensity of sound I

Where y is the ratio between the two main specific heats of the gas.

Vibration Analyzer cum Portable Balancer 6050
This is a microprocessor based Vibration Measuring, analysis and balancing instrument with memory for Analysis results and PC data transfer for trending the spectral information and printing Magnitude v/s frequency plots. It also has inbuilt balancing programs for 1-plane and 2-plane balancing. / Vibration Analyzer 5050
This is a general purpose Vibration Analyzer suitable for measuring and analyzing vibration on machinery. It can work as a portable balancer also with the addition of the optional Stroboscope. One can find out the likely causes of vibration by observing the various frequencies encountered, and then proceed to balance it in-situ if required.

Group 6 Olajubu O.O Arc/03/1939 Olukolu Q. A Arc/03/1945, Onanuga O.A Arc/03/1948,