Speed of Sound: Pitch, Amplitude, and characteristics of sound waves

Sound waves, essential for human perception, exhibit distinct characteristics. Frequency determines pitch, amplitude influences volume, and wavelength relates to the spatial aspects of a wave. The interplay of these features, along with speed, phase, intensity, pitch, and quality, defines the rich and varied world of acoustic experiences.

Description of Sound Waves: Frequency, Amplitude, and Speed Essentials

• Sound waves can be described by its 
  • • a) Frequency, 
    • b) Amplitude and 
    • c) Speed.
• Frequency: It refers to the number of oscillations or vibrations per unit of time. 
  • In the context of sound, it determines the pitch of the sound.  
  • If we can count the number of the compressions or rarefactions that cross us per unit time, we will get the frequency of the sound wave.
  • Unit: Hertz (Hz) is used to measure frequency, which represents one cycle per second.
  • High Frequency Sounds: It has a high pitch, like a whistle or a bird’s chirp. 
  • Low Frequency Sounds: It has a low pitch, like the rumble of thunder. 
  • Example: The frequency of a typical conversation is about 300 to 3,000 Hz.
• Amplitude: It refers to the maximum displacement or distance moved by a point on a vibrating body or wave from its equilibrium position during one complete vibration. 
  • In the context of sound, it determines the loudness or intensity of the sound. 
  • Unit: The unit of amplitude is usually measured in decibels (dB) in acoustics.

Heinrich Rudolph Hertz was born on 22 February 1857 in Hamburg, Germany and educated at the University of Berlin. He confirmed J.C. Maxwell’s electromagnetic theory by his experiments. He laid the foundation for future development of radio, telephone, telegraph and even television. He also discovered the photoelectric effect which was later explained by Albert Einstein. The SI unit of frequency was named as hertz in his honour. 

• Example: A loud shout will have a greater amplitude compared to a soft whisper.
• Speed: Speed of sound is the distance travelled per unit time by a sound wave as it propagates through a medium.
  • Unit: The speed of sound is typically measured in metres per second (m/s).
  • The speed of sound varies depending on the medium. 
  • In air, at a temperature of about 20°C, it’s approximately 343 metres per second.
  • Example: In dry air at 20°C, sound travels at around 343 metres per second.

Various Elements associated with Sound: Wavelength, Frequency, and Sonic Phenomena

• A sound wave in graphic form represents how density and pressure change when the sound wave moves in the medium. 
• Compressions are the regions where particles are crowded together and represented by the upper portion of the curve. 
  • Thus, compressions are regions where density as well as pressure is high.
• Rarefactions are the regions of low pressure where particles are spread apart and are represented by the valley, that is, the lower portion of the curve.. 
• The distance between two consecutive compressions (C) or two consecutive rarefactions (R) is called the wavelength. 
• The wavelength is usually represented by λ (Greek letter lambda). 
  • Its SI unit is metre (m).
• Frequency: The number of the compressions or rarefactions that cross per unit time indicate the frequency of the sound wave. 
  • It is usually represented by ν (Greek letter, nu). 
  • Its SI unit is hertz (symbol, Hz).
• Time Period: The time taken by two consecutive compressions or rarefactions to cross a fixed point is called the time period of the wave. 
  • It is represented by the symbol T. 
  • Its SI unit is second (s). 
  • Frequency and time period are related as follows: = 1 v T

• Pitch: How our brain interprets the frequency of an emitted sound is called its pitch. 
  • The faster the vibration of the source, the higher is the frequency and the higher is the pitch. 
  • A high pitch sound corresponds to a greater number of compressions and rarefactions passing a fixed point per unit time.
• Amplitude: Objects of different sizes and conditions vibrate at different frequencies to produce sounds of different pitch. 
  • The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave. 
  • It is usually represented by the letter A. 
  • For sound its unit will be that of density or pressure.
  •  The loudness or softness of a sound is determined basically by its amplitude. 
• The amplitude of the sound wave depends upon the force with which an object is made to vibrate. 
• A sound wave spreads out from its source. As it moves away from the source its amplitude as well as its loudness decreases. 
• Louder sound can travel a larger distance as it is associated with higher energy.
• The quality or timber of sound is that characteristic which enables us to distinguish one sound from another having the same pitch and loudness. 
  • The sound which is more pleasant is said to be of a rich quality. A sound of a single frequency is called a tone. 
  • The sound which is produced due to a mixture of several frequencies is called a note and is pleasant to listen to.

• Noise is unpleasant to the ear whereas music is pleasant to hear and is of rich quality. 
• The speed of sound is defined as the distance at which a point on a wave, such as a compression or a rarefaction, travels per unit time. 
• We know, speed, v = distance / time = λ/T.
  Here λ is the wavelength of the sound wave. 
• It is the distance travelled by the sound wave in one time period (T) of the wave. 
• Thus, 

v = λ ν (1/T = v)
        or v = λ ν
That is, speed = wavelength × frequency. 

• The speed of sound remains almost the same for all frequencies in a given medium under the same physical conditions. 

• The amount of sound energy passing each second through the unit area is called the intensity of sound. 
• We sometimes use the terms “loudness” and “intensity” interchangeably, but they are not the same. 
• Loudness is a measure of the response of the ear to the sound.

Speed of Sound in Different Media: Medium Properties and Temperature Impact   

• Sound propagates through a medium at a finite speed. 
• The sound of a thunder is heard a little later than the flash of light is seen. 
  • Hence, sound travels at a speed which is much less than the speed of light.
  •  The speed of sound depends on the properties of the medium through which it travels.
• The speed of sound in a medium depends on the temperature of the medium. 
  • The speed of sound decreases when we go from solid to gaseous state. 
  • In any medium as we increase the temperature, the speed of sound increases. 
• Example: The speed of sound in air is 331 m s–1 at 0ºC and 344 m s–1 at 22 ºC.

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