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# Properties of Sound Waves: Speed, Reflection and Echo

In physics, sound is defined as the vibration in a medium that can be heard. This vibration can be due to various reasons like, solids vibrations or when an object moves back and forth rhythmically. In daily life, we only experience sound through air, but it can also travel through liquids and solids. The study of sound waves is known as acoustics. According to new study, sound waves carry a little mass with them as they travel. Elephants can communicate with the sound waves which have lower frequency than human beings range. Moreover, flies can’t hear any sound. Here we will learn the properties of sound waves like its speed through various mediums, reflection, refraction and scattering.

### What is Sound?

Sound is a mechanical wave, which is longitudinal (motion of particles of medium and wave direction have angle of zero degree between them) in nature. When sound is produced from some source then it produces disturbance in surrounding medium, due to which we can hear it. Since sound is a mechanical wave, so it needs a medium to travel, that’s why, sound can’t travel through vacuum as there is no medium to carry sound vibrations. Simply saying, sound is a pressure variation. Region of sound wave with greater pressure are called compression and regions with less pressure are rarefactions.

### How is sound produced?

Sound is created when the vibrations of an object travel through a medium until they reach the human eardrum. Sound is created in the form of a pressure wave. When an object vibrates, it causes the air molecules around it to vibrate as well, causing a chain reaction of sound wave vibrations to spread throughout the medium.

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### Components of sound wave

Amplitude: It is the maximum or minimum distance from the mean position i.e. magnitude of expansion by the medium. The points with grater magnitude indicates loud sound while points with low magnitude show less sound. It has units of pressure (Pascal), as sound is caused by pressure variations.

Wavelength: A sound wave is caused by the pressure variations. The areas of high pressure are called crests and areas of less pressure are troughs. The distance between 2 consecutive crests or troughs is called wavelength of sound waves.

Pitch: Frequency or pitch of sound wave is defined as the rate of vibration i.e. number of waves passing per unit time. Its SI unit is hertz. It is related to the speed of sound as:

Velocity = wavelength x frequency

### Types of sound waves

Sound is of 2 types: Audible and Inaudible.

Inaudible sounds: These are the sounds that human ear cannot hear. The frequency range in which humans can hear is between 20Hz to 20 KHz.  Sounds with frequency below 20 Hz are Infrasonic Sounds. Elephants can hear sounds in this range.

Sounds with frequency greater than 20 KHz are Ultrasonic Sounds. Dogs, Bats can hear high frequency sounds.

### Properties of sound waves

Properties of sound waves include its speed and the ways by which it interacts with some medium.

### Speed of sound

Speed of sound in a medium depends upon the elasticity and inertia of that medium.

Inertial properties include density of medium, if density of medium is greater than definitely, speed of sound will be greater. If a medium is more elastic i.e. less flexible, then sound will travel faster through it. Since solids are denser than liquids and gases, that’s why sound travels faster in solids than gases and liquids. The speed of sound in air at 20 degree Celsius is 343 m/s, while speed of sound in solid is 6000 m/s. However, speed of sound in water is about 1500 meters per second.

Absorption of sound: It is the property of sound by which energy is transferred to the medium (through which it is travelling) as heat. Higher frequencies are absorbed more quickly, so their intensity decreases.

Reflection of sound: This is the property of sound wave by which it comes back to the same medium, in which it was travelling before striking some tissue. The amount of reflected portion depends upon the impedance of tissues at interface. This acoustic impedance can be reduced by using gel in ultrasound. It minimizes the sound wave reflection.

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Refraction of sound: Some part of an ultrasound wave is reflected when it crosses interface between 2 tissues, the remaining portion enters the material, due to which its velocity changes, which further causes refraction. Angle of refraction is dependent on the velocity change.

Scattering of sound waves: If the wavelength of tissue is less than that of incident sound wave, then sound wave is scattered. On the other hand, reflection occurs when object has larger wavelength than incident wave. Scattering increases if acoustic impedance mismatch increases.

Echo of sound: Echo is usually considered as the reflection of sound, but actually there is a little misconception. Echo is actually that sound which is heard after reflection when actual sound has stopped i.e. sound produced due to reflection. If you shout in a large vacant area, then you can observe it.