Chapter 12––>Sound

Introduction

Have you heard the following sounds  – Sitar, Violin, Veena, your favorite song playing in your iPod, temple bells, chirping of birds, vehicles horning in traffic, dogs bark, pressure cooker whistling, crackers bursting, rattling and breaking of objects etc. While some sounds may be pleasant and soothing, some could be annoying. In this chapter, we will explore more Sounds.

Sound is produced by a vibrating body

• Sound is always produced by a vibrating body. Sometimes, the vibration is clearly visible and the sound is strong, whereas in some cases, the vibration is small and the sound is feeble.
• The to and fro or back and forth motion of a body is termed as vibration or oscillatory motion
• When a tuning fork is struck hard against a rubber ball, it vibrates producing a sound.
• When the strings in a violin vibrates, it produces sound.
• When the diaphragm of a drum is struck, it vibrates and produces sound.

Sound can propagate through solids, liquids and gases

• Sound can propagate through solids, liquids and gases. In fact, it needs a medium to propagate.
• The matter or substance through which sound propagates is called Medium.
• Sound can propagate through Solids: The doctor checks us using the stethoscope. He can hear the sound from within our body because sound travels through solids.

• Sound can propagate through liquids: In fact, this is how aquatic animals communicate with each other.

• Sound can propagate through gases/air: This is the reason why we are able to hear a person standing near us speak. Generally, air is the most common medium for sound propagation.

Sound cannot propagate through vacuum

Sound cannot propagate through vacuum. Here is an experiment to prove this.

• Take an airtight glass bell jar and an electric bell.
• Connect a vacuum pump to the bell jar.
• Suspend the bell inside the jar as shown
• Now ring the bell. The bell sound will be heard very clearly.
• Gradually start pumping out the air from the jar. Simultaneously, also ring the bell. The bell sound becomes feebler and feebler as the air is pumped out.
• When the air is completely pumped out, the sound will not be heard at all.
• This proves that sound does not travel through vacuum.

• neighboring third particle and then again comes back to its position of rest. This process continues until the vibration reaches the listener and he is able to hear the sound.
• As we see, the actual particles of the medium don’t travel from the point of generation to listener. It is only the vibration that travels through the medium. This is called Wave motion.
• A wave is a disturbance that moves through a medium when the particles of the medium set neighboring particles into motion. Since sound waves are characterized by the motion of particles in the medium, they are Mechanical Waves.

• Sound propagates in Compressions and Rarefactions.
• Compression : When a vibrating body moves forward, it pushes or compresses the air ahead of it thereby creating a region of high pressure and high density. These are called Compressions. They are represented by the upper portion of the wave curve.(C)
• Rarefactions: When a vibrating body moves backward, it releases or loosens the air ahead of it thereby creating a region of low pressure and low density. These are called Rarefactions. They are represented by the lower portion of the wave curve.(R)
• The peak of the wave is called Crest and valley of the wave is called Trough.
• Sound waves are longitudinal waves. The individual particles of the medium move in a direction parallel to the direction of propagation of the disturbance. The particles do not move from one place to another but they simply oscillate back and forth about their position of rest thereby propagating only the disturbance from one place to another.

Wavelength, Frequency, Time Period

• Wavelength: The distance between 2 consecutive crest or troughs is called Wavelength. It is represented by the symbol  λ(lamda). Unit is m.

• Time Period: The time taken for one complete oscillation in the density of the medium is called the Time Period of the wave. It can also be defined as the time taken by 2 consecutive compressions or rarefactions to cross a fixed point. It is represented by the symbol T. Its unit is in seconds.

• Frequency : The number of oscillations per unit time is called the Frequency of the sound wave. The number of compressions or rarefactions that cross a given point in per unit time is the frequency of the wave. It is denoted by μ and is unit is Hz (Hertz).

 
A sound of a single frequency is called Tone. A sound which is a mixture of different frequencies is called Note and is very pleasant to listen to.

Frequency   =    No of oscillations  
                            Time (in seconds)

 μ    =   1/T

Speed of Sound

• Speed of Sound : It is the distance which a point on a wave, travels in unit time.

The speed of sound depends on the following factors.

• Medium: The speed of sound decreases as we go from solid to gaseous
• Temperature : As temperature increases, the speed of sound increases. (The speed of sound in air is 331m/s at 0 degrees and 344m/s at 22 degrees)
• Pressure of the medium

     We know that, Speed   =  Distance / Time

     We know that any point of the wave travels a distance λ in time T. So, we have

     v = λ/T  = λμ =   (Because μ = 1/ T )

Light travels faster than sound in air. Hence we see the lightning before we hear thunder.

Pitch, Amplitude, Loudness, Intensity

• Pitch: Frequency determines the shrillness or the pitch of the sound. Higher the frequency, higher the pitch or it is said that the sound is shrilling. Lower the frequency, lower the shrill or pitch. Eg: Babies voice has more frequency and thereby more shrilling than an adult voice.

• Amplitude: It is a measure of the loudness or softness of the sound. The magnitude of the maximum disturbance in the medium on either side of the mean value is called Amplitude of the wave, represented by the letter A.

• Loudness: Loudness is the response of ear to the sound. Loudness is proportional to the square of the amplitude of the vibration body producing sound. The more the amplitude of vibration, the sound is the loud. The lesser the amplitude of vibration, the sound is feeble.

Loudness  =  k.(Amplitude)2  (where k is the proportionality constant)

The loudness is expressed in a unit called decibel (dB).

• Intensity : The amount of sound energy passing each second through unit area is called the Intensity of Sound.

Reflection  of Sound

When sound waves hit a polished or rough surface, it bounces off. This is Reflection of Sound. (Similar to Reflection of Light)

Law 1 : The incident ray of sound, the normal at the point of incidence and the reflected ray of sound all lie in the same plane.

Law 2 : Angle of incidence is equal to the angle of reflection

Echo

When we shout near a mountain, we will hear the same sound again and again with small time interval gaps. This is called an echo. Echoes may be heard more than once due to successive or multiple reflections

The sensation of any sound persist in our ear for about 0.1 s, which means that to hear a clear echo, the reflected sound must be atleast 0.1 seconds after the original sound.

• The sensation of any sound persist in our ear for about 1 s
• The speed of sound in air at 22 degress is 344m/s
• Therefore, Distance = Speed * Time = 344m/s * 0.1 s = 34.4m
• That is the full distance ie from the source to the wall and back to the source. So, half the distance ie 2 metres is the minimum distance of the obstacle to hear a clear echo.

Reverberation : Multiple Reflections

In a big hall, where there a musical concerts/dance shows etc, there will be repeated reflections from walls all round. The sound will persist for a very long time due to this multiple reflections. The repeated reflection that results in this persistence of sound is called reverberation.

In a big hall, reverberation is required so that the entire hall can hear the sound. At the same time, if the reverberation is very high, the sound will persist for too long even if the source of sound has actually stopped. So, this is undesirable. The reverberation effect should be controlled. Several acoustic techniques are deployed to do this.

Uses of Multiple Reflection

• Some musical instruments have a tube followed by a conical opening. This architecture helps to reflect sound again and again such that most of the sound waves move in the forward direction. Eg: Horn, Trumpets, Loud speakers etc

• Stethoscope is used by a doctor to hear the patient’s heart beat. The sound from the heart reaches the ear of the doctor by multiple reflections as shown.

• The walls and ceiling of concert halls are curved because of which sound after reflection reaches all corners of the hall. Also, roof and walls and seats of the auditorium are generally covered with sound-absorbent materials like compressed fireboard, rough plaster and draperies for a pronounced sound effect

Audible and Inaudible sounds

• The human ear can detect sounds in the range of 20Hz-20KHz. These are called Audible Sounds. Eg: Music, Instruments, Crackers.
• Frequencies below 20Hz and greater than 20KHz cannot be heard by the human ear. These are called Inaudible Sounds. Eg: It is believed that bat calls are in the inaudible ultrasonics range. Dogs can hear frequencies greater than 20KHz, so they are used by the police to respond to such whistles. Ultrasound scans produce sounds in this range.

Applications of Ultrasounds

Ultrasounds are high frequency waves, which can travel in defined paths even in the presence of many obstacles

Medical applications

• Ultrasonic waves are used to capture the images of the heart and its features and problems associated with the heart, if any (for treatment purpose). This is called echocardiography.
• Ultrasound scans/Ultrasonography are very commonly used to get images of internal body organs such as liver, kidney, uterus. It helps the doctor to diagnose and treat problems in the body of the patient. In this method, ultrasound waves are made to travel through the body to the organ under consideration. They get reflected and these are converted to electrical signals which can be monitored on a screen.
• Ultrasonography is used to observe the growth of the fetus inside the uterus. It can also used to monitor the abnormalities.
• It is used to break kidney stones into fine grains which later get flushed out through urine.    

Industrial Applications

• Ultrasound is used to clean machine parts located in places which are not easily accesible – electronic components, internal parts, spiral parts etc. The objects are placed in a cleaning solution and waves are passed through it. Because of the high frequency of the waves, the dust particles just fall out.
• It is used to detect defects, flaws, cracks in machine parts, bridges, building etc. Ultrasound waves are passed at one end and monitored using detectors. If there are laws or cracks, then the ultrasound waves are reflected back indicating the presence of a defect.

SONAR (SOund Navigation And Ranging)

• SONAR is a device that uses ultrasonic waves to measure distances which are practically impossible to measure – depth of ocean, distance and direction of underwater objects.
• SONAR comprises of a ultrasound transmitter and detector fitted in a ship.
• The transmitter produces ultrasound waves which travel, hit the bottom of the ocean(or any object) and return back and are detected.
• The time taken for the ultrasound to return (t) and speed of ultrasound in water (v) is now known. So, we have distance d = (v * t)/2 (Divided by 2, because the wave travels twice the distance, ie from transmitter to object and then object to detector)

The Human Ear

• Pinna : Pinna is the outer ear. It acts like a funnel to send the sound waves inside the ear.
• Auditory canal: Sound travels from the outside through auditory canal
• Eardrum: The sound then falls on the eardrum which is at the end of the canal.
• Bones Hammer, Anvil, Stirrup : The sound vibrations are then passed on to these bones in the middle ear.
• Cochlea: The sound waves are converted to electrical signals.
• Auditory nerve: It sends the signals to the brain.

Brain: The brain interprets the signals and that’s how we hear.