Sound

The persistence of sound because of repeated reflection is called Reverberation.

In a big hall while speech or a program sound persist because of repeated reflection from the walls until it is reduced to a value where it is no longer audible. This is highly undesirable and cause disturbance and irritation among public sitting in the hall. This happens because of Reverberation.

Reduction of Reverberation

Reverberation is reduced by covering of the roof and walls of the auditorium with sound absorbent materials like compressed fireboard, rough plaster or draperies. The seat materials are also selected on the basis of their sound absorbing properties, such as coir is used in the cushioning of chairs.

Uses of Multiple Reflection of Sound

(1) Megaphones or Loudhailers, loudspeakers, horns, etc. are used to address people or produce sound attached with music players, etc.

Figure: Megaphone¹⁸

Figure: Megaphone¹⁹

Figure: Horn²⁰

These devices are designed to send sound in a particular direction without spreading in all directions.

In these devices a tube is followed by conical openings. The conical openings send the sound in one direction, i.e. towards people. Because of conical openings sound wave from source are reflected successively and guided to forward direction towards audience.

Figure: Trumpet²¹

Figure: Shehnai by Rajender Prasanna²²

Reference: By Berthrand55 - Own work, CC BY-SA 3.0, Link

Musical instruments, such as trumpet and shehanais are also designed to send the sound from source after multiple reflections towards forward direction.

(2) Stethoscope

Stethoscope is a medical instrument which is used to listen sound of heart beat, sound coming from lungs while taking brethe, pulse, etc. so that doctor can judge ailments if any.

Figure: Stethoscope²²

Figure: Multiple Reflection of sound through Stethoscope²³

In Stethoscope a diaphragm is attached with a tube and two ear–piece. Tube is designed to send the sound to the ear using multiple reflections.

(3) Curved Ceiling of Conference Hall

Ceiling or floor or both of concert and/or conference halls are made curved. The curved ceiling or floor sends the sound to all direction after reflection. Sound reflecting to all direction reaches to audience sitting in hall to hear clear sound.

Figure: multiple reflection of sound in a concert hall ²⁴

Sometimes a curved board is used behind the stage to send the sound to the audience after reflection for clear hearing.

Figure: multiple reflection of sound using a curved sound board ²⁵

Range of Hearing

Human being can hear the sound of about 20 Hz to 20000 Hz frequency. This range of frequency between which human being can hear the sound is called Audible Range.

Children under the age of 5 can hear sound upto 25000 Hz. But with age ear of people becomes less sensitive to hear such higher frequency of sound.

Some animals, such as dogs can hear the sound upto 25000 Hz.

Infrasonic Sound Or Infrasound

Sound of frequencies below 20 Hz are called Infrasound or Infrasonic sound. Infrasonic sound falls below the audible range of hearing and thus, human being do not hear such sound.

Incapability of not hearing the infrasonic sound is a kind of boon to human being. Because, if human being were able to hear infrasonic sound, they would be got disturbed all time by hearing the sound of heart beat, breathing, churning of food in abdomen, sound of flow of blood.

Sound of vibration of a pendulum is below the audible range of sound, i.e. is infrasonic sound or Infrasound.

It is said, that Rhinoceroses communicate using infrasound of frequency as low as 5 Hz. Whales and elephants produce sound in the infrasound range.

It is assumed that earthquakes produce low frequency infrasound before main shock waves begin because of which many animals get alerts possibly. And it is observed that some fish and some animals get disturbed before earthquake.

Ultrasound Or Ultrasonic Sound

Sound of frequencies higher than 20 kHz are called Ultrasonic Sound or Ultrasound. Many animals such, dolphins, bats and porpoises produced ultrasound.

Bats produce ultrasound to detect its prey. Moths of certain families have very sensitive hearing ability, and can hear the high frequency ultrasound produced by bat flying nearby. Because of this powerful hearing ability, these types of moths are able to escape from getting captured by bat.

It is known that rats also produce ultrasound.

Human beings are unable to hear Ultrasound or Ultrasonic Sound.

Application of Ultrasound

As ultrasounds or Ultrasonic sounds are able to travel along well defined path even in the presence of obstacles. This is the cause that Ultrasound or Ultrasonic Sound is used for various purposes in industries and in medical fields.

(a) Use of Ultrasound for cleaning purpose

Many times it becomes difficult to clean parts located in hard to reach of many types of machinery. In those cases ultrasound is used to clean those parts.

For cleaning such objects are kept in a solution, called cleaning solution. And ultrasonic waves are sent into solution. Because of high frequency of ultrasound, the particles of unwanted materials, such as grease, dirt, etc. detached and drop out. The object thus gets cleaned thoroughly.

(b) Use of Ultrasound in Detection of Cracks

In the construction of big structures, such as bridges, buildings, machines, etc. big metallic blocks are used. Cracks, flaws and holes may be present in such big metallic blocks which are difficult to detect out. These cracks and flaws or holes reduce the strength of metallic blocks.

The Ultrasonic waves are allowed to pass through the metal blocks. If there were even a small defect, the ultrasound waves get reflected back indicating the defects. A detector attached with device detects the problem after catching the transmitted ultrasound waves.

Figure: detection of cracks and flows using ultrasound in a metal block ²⁶

Ordinary sound waves cannot be used for such purposes as it will bend around the corners of the defective location.

(c) Echocardiography

Echocardiograph is a medical equipment which is used to form image of pattern of heart beats. The image or graph formed by echocardiograph is called echocardiography.

In echocardiograph, ultrasonic waves are made to reflect from various part of the heart which forms the image.

(d) Ultrasound Scanner

Ultrasound scanner is a medical device used to form images of human internal organs, such as liver, gall bladder, uterus, kidney, etc.

Figure: Ultrasound Scanner ²⁷

Reference: By The original uploader was Drickey at English Wikipedia. - Transferred from en.wikipedia to Commons., CC BY-SA 2.5, Link

Forming of image of human internal organs is generally known as Ultrasonogrphy. In ultrasound scanner ultrasound waves are sent to the body and get reflected from a region where there is a change of tissue density. Reflected waves are detected using detector and image is formed on monitor. This image is printed on film.

Using ultrasonography doctors detect deformity if any in internal organs.

(e) To break Kidney and Gall Stones

Ultrasound is used to break small stones formed in kidneys or gall bladder into fine grains. After breaking of stone in fine grains, latter get flushed out or removed.

SONAR

SONAR stands for Sound Navigation And Ranging. SONAR is a device used to detect the distance and direction of objects under water.

SONAR is usually fitted on ships. Transmitter of SONAR sends the ultrasonic waves in the water which reflect after striking with the object. The reflected wave is detected by sensor called detector.

Figure: SONAR²⁸

Reference: By NOAA - http://www.nauticalcharts.noaa.gov/hsd/learn_survey.html, Public Domain, Link

The distance of object which reflects sound wave is calculated using the speed of sound in water and time interval between transmitted and reflected wave received.

Let time interval between transmission and receiving of reflected wave = t

And speed of sound in sea water = v

Let the total distance travelled by sound wave = 2d

Now, we know that, distance = speed × time

Thus, 2d = v × t

⇒ d = vt/2

Thus, using above method and calculation, distance of object under water is calculated using SONAR .

Figure: Principle of an active SONAR²⁹

Figure: Reference: By Georg Wiora (Dr. Schorsch) - Self drawn with Inkscape, CC BY-SA 3.0, Link

This method of detecting the object under water using SONAR is called echo ranging.

The SONAR technique is used to determine the depth of the sea and to locate underwater hills, valleys, submarine, icebergs, sunken ships, etc.

Catching of Prey by Bats using Ultrasound Waves

Bat is a nocturnal animal. Animals which become active in night are called Nocturnal. Thus, bats come out to search in prey during night.

Figure: Detecting of object by bat³⁰

Reference: By Petteri Aimonen - Own work, Public Domain, Link

Bats have unique technique to catch prey. Bats produce ultrasound wave. These ultrasound waves are reflected after striking with object and received by ears of bat. After receiving of reflected ultrasound waves, bats sense the direction, position, size and type of prey. The nature of reflection of ultrasound waves tells the bat where the obstacle or prey is and what it is like. After detecting, bats catch prey.

Porpoise, a type of aquatic animal similar to other fish, also use ultrasound waves for navigation and location of food in dark.

The scientific name of bat is Chiroptera

Structure of Human Ear

Ear is a sense organ which enable us to hear. Ear converts the pressure variation in air because of sound waves into electrical signals. These electric signals are sent to brain via auditory nerves. Brain interprets the electric signal in sound.

Figure: Human Ear³⁰

Ear is divided into three parts, External Ear, Middle Ear and Inner Ear.

External Ear

External ear is situated outside the body. External ear is called Pinna . In many animals, such as fish, birds, etc. Pinna is absent. Pinna is found in mammals only. Actually Pinna is one of the characteristic of mammals.

The function of Pinna is to collect the sound waves from surrounding. After catching of sound waves from surroundings, Pinna send them to the middle ear via ear canal. This ear canal is called Auditory canal.

Middle Ear

The middle ear starts with the end of external ear. At the end of auditory canal there is a thin membrane called Ear Drum . Ear drum is called tympanic membrane also. There are three bones found in middle and are called ossicles collectively. Name of these bones are hammer, anvil and stirrup.

Pressure variation because of sound waves coming through auditory canal vibrates the ear drum. Moving of ear drum, vibrates the ossicles bones.

Ossicles are the smallest bones in the human body.

Inner Ear

In inner ear cochlea and three semi circular canals are found. Cochlea turned the pressure variation crated by ossicles into electrical signal. These electrical signals are sent to brain via auditory nerves.

Brain then interprets the electrical signal into actual sound and we become able to hear the sound.

The semi circular canals found in inner ear help us to balance our body also.