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Audio Filters - Understanding Acoustic Waves - 2/8

Written By: 

Diksha
In the previous tutorial, sound wave and its properties were discussed. Now it's time to understand Acoustic Waves. Generally, the term sound wave is used to refer the waves having frequency range audible to humans that is 20 Hz to 20 KHz. The waves having frequency greater than 20 KHz are called Ultrasonic waves and waves having a frequency range in Giga Hertz or higher are called Hypersonic Waves. The term Acoustic Wave is a more general term to refer sound or vibration of any frequency.  
 
In physics, Acoustic is the branch of science which deals with the study of any mechanical wave in the solid, liquid and gaseous medium. Acoustic Waves are longitudinal waves. That means, they have the same direction of vibration as the direction of their travel. The main source of these waves is the vibration which is produced in any medium. The sound waves are also generated by the vibration so it can be said that acoustic waves are a kind of sound waves which travel on a surface, liquid or in a gaseous medium. 
 
Acoustic Waves in Air
 
Acoustic Waves in Air
 
 
Acoustic Waves in Water
 
Acoustic Waves in Water
 
 
Acoustic Waves on Surface
Acoustic Waves on Surface
 
So, studying sound as an acoustic wave could help understanding its mechanical properties. The acoustic waves are characterized by the following physical properties - 
 
1.Acoustic or Sound Pressure 
2.Particle Velocity 
3.Acoustic or Sound Intensity
4.Particle Displacement 
 
These properties will help understanding the mechanical nature of the sound. 
 
1.Acoustic/Sound Pressure
 
This is the pressure which is generated by the sound wave in the surrounding. The deviation in the equilibrium atmospheric pressure due to sound wave is called as sound or acoustic pressure. This is measured in Pascal or N/m2. When a sound wave travels through a medium, it creates the disturbance in the pressure of that medium. This increases the total pressure of the medium. This can be expressed by the following equation -  
Ptotal = P1 +Ps
Where, 
Ptotal = Total pressure in the medium 
P1 = Pressure generated by sound wave
Ps = static or equilibrium pressure
This extra pressure can be measured by using a microphone in air medium and by a hydrophone in water. 
 
2.Particle Velocity 
 
Particle velocity is the velocity of the wave particle in a particular medium. It is measured in m/s. If a sound wave is traveling through a liquid medium, the particle velocity is the velocity of the liquid as the sound wave causes the liquid particles to move back and forth as per the vibration in the wave. The particle velocity is not same for the liquid and sound wave. The sound wave travels much faster than a particle in the medium. The oscillation of particle is slow about their position as compared to the sound wave. The Particle Velocity can be calculated by the following equation - 
v = dδ/dt
Where, 
v = Particle Velocity
δ = Particle Displacement
 
3.Acoustic/Sound intensity 
 
The sound pressure and particle velocity together make the sound intensity. The Sound intensity or Acoustic Intensity can be defined as the power transfer per unit area. Here, the area is always perpendicular to the energy which is transferred. So sound intensity is the power which is transferred by the sound wave per unit area and it is measured in watt per square meter or W/m2. The sound intensity can be calculated as follow - 
Sound intensity, I = p*v
Where, 
p = Sound Pressure 
v = Particle Velocity 
 
4.Particle displacement 
 
The displacement of the particle from its original position, when the sound wave travels through a medium is called as Particle Displacement. This is measured in meter.
The sound wave when travels in the air, the particle in the air experiences the displacement as per the particle velocity. This displacement can be in the direction of sound wave or opposite to it. The Particle Displacement can be calculated by the following equation - 
δ =
Where, 
v = Particle Velocity
dt = Time Period 
 
Understanding these mechanical properties of sound will be useful in understanding the working of Audio Transducers. The most common audio input transducer is a microphone and the most common audio output transducer is a speaker. In the next tutorial, working and classification of microphones will be discussed. 
 

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