Subsection 6.1.1 Sound
Sound is a longitudinal wave that can be produced by vibration. The speed at which sound waves travel through a medium depends on the density and stiffness of the medium, whether condition, temperature, moisture, etc. It is a mechanical wave and creates pressure on a layer of medium when propagates, hence it is also called a pressure wave. Sound wave is longitudinal in nature. When such energy reaches to our eardrum it begins vibrating and we get sensation of hearing. The physical character of sound depends upon the listener’s audible perception. The physical quantity that defines the loudness is amplitude (sound pressure) of the wave and the quantity that defines the timbre (or quality) of sound is a frequency \(f\text{.}\) A sound of frequency \(20\, Hz\) to \(20\, kHz\) is audible to healthy human ear. Loudness increases with the increase of pressure amplitude at a given frequency. Loudness (or Intensity level) is a sound energy received by a unit area of listener’s eardrum (or detector) in unit time. Loudness is a listener’s perception. Intensity is an energy carried by a wave per unit area per unit time. Intensity is a property of sound producing device. Loudness is an intensity received by a listener.
Subsubsection 6.1.1.1 Reverberation and Echo
Echo and reverberation are two distinct phenomenon of sound waves which occur due to its reflection from any surfaces. Sound endures in our brain for longer than 0.1 second. If sound is reflected from any surface reaches to our ear within 0.1 second of the initial sound, then it seems that the sound is prolonged. The reception of reflected sound within 0.1 second causes the reverberation(or) the prolonging of a sound effect. The speed of sound in air is about \(340\, m/s\) at room temperature. If the wall of a room is \(17 \) meter away from the source of sound then it reflects back from the wall and reaches to the ear within 0.1 s
\begin{equation*}
t=\frac{d}{v} = \frac{2\times 17}{0340} = 0.1 \,s.
\end{equation*}
Hence, causing a prolonged sound effect, called a reverberation. Echoes are also the effect of reflection of sound waves perceived by the ear. When a reflected sound wave reaches the ear after 0.1 seconds of the original sound was heard, then echoes are produced. If the elapsed time between the arrivals of the two sound waves is more than 0.1 seconds, then the sensation of the first sound will have died out. In other words, echo is a sensation of second sound wave after the effect of original sound wave will vanish. It is not a prolonging of the original sound wave as in reverberation.
Subsubsection 6.1.1.2 Doppler Effect
The Doppler effect is the change in perceived frequency of a source due to the relative motion between the source and the receiver. It applies to all sorts of waves. Doppler effect can be experienced as the change in pitch (or frequency) received. When the ambulence is approaching, the pitch of siren is getting higher than the actual frequency of siren. When the ambulence passes by, the frequency sounds the same as the actual emitted frequency and when the ambulance is moving away, the pitch is getting lower than the actual emitted frequency. When the source of the waves is coming towards the observer, observer receives more waves every second compared to being at rest. That is waves bunch together for the observer giving impression for the decrease of wavelength and causing an increase in the frequency. Conversely, if the source of waves is going away from the observer, observer receives less waves every second compared to being at rest. That is waves ”spread out” for the observer giving impression for the increase of wavelength and causing a decrease in the frequency. Doppler Effect
3
physics.bu.edu/~duffy/HTML5/doppler.html
