Audible and Inaudible Sound

Sound is a form of energy which is generated by a vibrating body. It requires a medium for its propagation. The transmission medium could be gaseous, solid or liquid. The waves in which the direction of propagation of the wave is the same as the direction of vibration of the particles of the medium are known as longitudinal waves. The waves in which the direction of propagation of the wave is perpendicular to the direction of vibration of the particles of the medium are known as transverse waves. In order for a sound to be generated, a source is required. An example of a source of sound is a speaker in which the diaphragm of the speaker vibrates to generate sound.

Now let us see how a source of sound works. When a sound source vibrates, then the particles of the medium surrounding it vibrate. The vibrating particles move further away from the source of sound as the vibration of the medium due to the vibrating particles continues. The propagation of the vibrating particles away from the source takes place with the speed of the sound. This is how a sound wave is formed. The velocity, displacement and even the pressure of the medium vary in time at a distance. One thing which should be kept in mind is that the vibrating particles of the medium do not actually travel along with the sound waves. The vibration of those vibrating particles further passes on the vibrations and make other particles vibrate and the process continues. There are three phenomena that can take place during the propagation of the sound wave. Either the sound waves will be reflected or attenuated or else refracted by the medium. 

There are Three Factors Which Will Affect the Nature of the Propagation of Sound. These Factors are

The Relationship Between Density and Pressure: the relationship between density and pressure will affect the speed of sound in the medium. This relationship is further affected by temperature.

State of the Medium Through which Sound is Propagating: If the medium through which sound is propagating is moving, then the speed of sound will increase if the motion of the medium is in the same direction as that of propagation of sound and it will decrease if the motion of the medium is in the opposite direction of propagation of sound. 

Medium Viscosity: The rate at which the quality of sound will be attenuated is determined by the viscosity of the medium through which it is propagating.

Properties of a Sound Wave

There are some key features of a sound wave. They are frequency, wavelength, intensity, the pressure of the sound, amplitude, direction of propagation and speed of sound.

Underneath is the description of each characteristic of the sound wave. 

1. Frequency - The total number of waves that are produced in one sec is known as frequency. It is also defined as the total number of vibrations counted in one sec. The frequency is obtained when the velocity of the wave is divided by the wavelength of the wave. It is measured in hertz (Hz).

2. Wavelength - The distance between the adjacent same parts that is the distance between two consecutive troughs and crests of a sound wave is known as the wavelength. "Metre" is the SI unit of wavelength. The wavelength of the audible frequency range lies between 17mm to 17m.

3. Intensity - The power that is carried or produced by the sound wave per unit area is known as the intensity of the sound wave. The power carried is in the perpendicular direction of that area. The SI unit of the sound intensity is watt per metre square (W/ m^2). 

4. Pressure - The deviation of the local pressure from the equilibrium or the average atmospheric pressure is known as the pressure of the sound wave. It is also known as acoustic pressure. The SI unit of this pressure is Pascal (Pa).

5. Amplitude - The maximum distance travelled by the vibrating particles from their mean position when a sound wave is propagating through a particular medium is called the amplitude of a sound wave. It can also be described as the loudness of the sound wave. It is measured in decibels (dB).

6. The Direction of Propagation - The direction of propagation of the sound wave is decided by the direction of vibration of the vibrating particles. The sound wave shows to and fro motion. When sound travels, the vibrating particles form the regions of compression and rarefaction. The sound wave propagates as a longitudinal wave in the air or other mediums.

7. Speed - The distance travelled by the sound wave per unit time is known as the speed of the sound. In other words, the speed of the sound wave is equal to the product of the frequency and the wavelength. The SI unit of speed is metres per sec (m/ s). The speed of sound in the air is 343 metres per sec.

Basically, Based on the Information About the Frequency, Sound Can be Classified Into Two Categories. They are

Audible Sound: All the frequencies residing between the limit of 20Hz and 20KHz can be perceived by human beings. Therefore, these sound waves having frequencies within the range of 20Hz and 20KHz are known as audible sounds. But the frequency that we can hear is often dependent on several other factors like our environment. Frequent exposure to loud noises can affect the hearing capacity of an individual. The frequencies in the higher frequency range get difficult to perceive. This often happens with old age.

Inaudible Sound: The frequencies residing below 20Hz and those residing above 20KHz cannot be perceived by the human ears. Hence all the frequencies below 20Hz and that above 20KHz are known as inaudible sound. Infrasonic sound is the term used for the frequencies below 20Hz and that above 20KHz in the higher range of frequencies are known as ultrasonic sound. Animals like dogs can perceive frequencies lying above 20KHz. Hence dogs are trained in police forces with the help of whistles which act as a source of frequencies greater than 20KHz, which are audible to dogs and not to human beings. These higher ranges of frequencies have many applications. They are used in the medical field, they are used in technologies. They have applications in tracking and researching diseases and are often used in curing them.

Perception of Sound: Different sounds are perceived differently by the human brain. The sense of hearing is important for all organisms including human beings. The sound is used to detect danger, to hunt, to navigate the way and most commonly, it is used to communicate with other beings. Nowadays, many technologies have been invented which allow us to record sounds. Not only records but sounds can also be generated and transmitted with the help of technology.

The sounds which are unpleasant to hear are is termed as noise. Noises are unwanted sounds. In more technical terms, noises are those factors which hinder the desirable components. To study and analyse a sound wave, six factors are to be considered. These factors are pitch, loudness, duration, location in space or three dimensional or spatial location, sonic texture, and timbre. 

Pitch: The periodic nature of the vibrations which builds a sound wave is known as a pitch. The pitch could be a high pitch or a low pitch depending upon the frequency. 

Duration of Sound: The duration of sound is identified when we can first hear the sound or when the sound starts till the time when we cannot hear the sound anymore or when the sound stops. The duration of sound could be long or short. 

Loudness of Sound: When we hear a sound, the nerves which help us in hearing or the nerves which are present in our auditory system, are stimulated. Loudness is what is defined as the total number of such stimulation of the auditory nerves over a cycle or time period.

Timbre: The quality of various sounds is referred to as a timbre. Examples are the thudding sound that a hard solid like a rock makes when it hits the ground.

Sonic Texture: The different number of different sources of sound and the way they interact with each other is termed as sonic texture.

Spatial or Space Location: Spatial location is defined as the location of the sound waves in space, considering the different geometrical axes, x y, and Z-axes.