Railway Science (Physics-Sound) (Part-II)

The phenomenon of sound depends on the amplitude of the sound wave. If the amplitude of the sound wave is large, then the sound is said to be loud. Sounds above 80 dB become noise to human ears. Hertz (Hz) - The smallest basic measure of frequency. Humans can listen to sounds in a frequency range from about 20 Hz to 20 kHz. Pitch - A characteristic of sound which is used for differentiating between the shrill and flat sound. Note - A sound that is produced when several frequencies are mixed.

Sound is a vibration that propagates through a medium in the form of a mechanical wave. The medium in which it propagates can either be a solid, a liquid or a gas. Sound travels fastest in solids, relatively slower in liquids and slowest in gases. Sound waves are longitudinal waves. Humans can detect sounds in a frequency range from about 20 Hz to 20 kHz. Loudness of sound is measured in decibels (dB). The number of periods or cycles per second is called frequency. The SI unit for frequency is the hertz (Hz). Audiometer - Tool used to measure a person's hearing capacity.

The speed of the sound is proportional to the temperature. If the temperature increases, the speed of the sound also increases. Compared to winters, the temperature is high in summers because of which sound travels faster in summer. The velocity of sound in air at 20°C is 343.2 m/s which translates to 1,236 km/h.

Persistence of hearing of a human ear means how much time the sensation of any sound stays in our brain. Echo is a repetition or a partial repetition of a sound due to reflection. Echoes are heard in enclosed spaces with hard and tough walls such as wells, caves, or large auditoriums. The speed of sound in air is 331 m s⁻¹ at 0 ºC and 344 m s⁻¹ at 22 ºC.

It is measured in hertz (Hz), where one hertz equals one cycle per second. Sound timbre is known as the characteristic waveform of sound that depends on the material from which it produces. Amplitude- It is defined as the loudness or the amount of maximum displacement of vibrating particles of the medium from their mean position when the sound is produced. Intensity- It is defined as the power carried by sound waves per unit area in a direction perpendicular to that area.

Wavelength is inversely proportional to the frequency, The wavelength will decrease when the frequency increases which gives higher tones. Amplitude (Loudness of sound) - When sound propagates through a medium such as air, the amplitude decreases faster with higher frequencies because higher frequencies are better impaired than lower ones.

Given, the time period = 0.02s.
The frequency (f) of a wave is the reciprocal of time (T),
Frequency (f) = 1/time period = 1/T
Frequency (f) = 1/0.02 = 50 Hz.

The speed can be calculated using the formula: Speed (v) = wavelength (λ) x frequency (f). Wavelength (λ) = 2 m Frequency (f) = 200 Hz Speed (v) = f × λ ⇒ v = 200 × 2 ⇒ v = 400 m/s

Frequency formula in terms of wavelength and wave speed is given as, f = 𝑣/λ.
Where f - frequency, 𝜈 - wave speed, λ - wavelength.
Given f = 25 Hz, λ = 4 m.
𝜈 = f × λ → 𝜈 = 25 × 4 = 100 m/s.
To calculate time. Given distance - 200 m, speed - 100 m/s.
Time = 𝑑𝑖𝑠𝑡𝑎𝑛𝑐e/s𝑝𝑒𝑒d = 200/100 = 2 sec.

Given that, Time(t)= 6 seconds, Speed of sound (v) = 346 m/s.
⇒ Total Distance = speed × time = 346 m/s × 6 s = 2076 m.
⇒ Distance between person and cliff = 𝑇𝑜𝑡𝑎𝑙 𝑑𝑖𝑠𝑡𝑎𝑛𝑐e/2 = 2076/2 = 1038 m.