In the previous tutorial, sound waves and their properties were discussed. Now, let's learn about acoustic waves.
Sound waves typically refer to frequencies audible to humans, which are in the range of 20 Hz to 20 kHz. However, waves with frequencies greater than 20 kHz are ultrasonic waves. And those in the gigahertz (or higher) range are called hypersonic waves.
Generally, the term “acoustic wave” is used in reference to a sound or vibration of any frequency. This makes sense because, in physics, acoustics is the study of any mechanical wave in a solid, liquid or gaseous medium. These are longitudinal waves, which move in the same direction of vibration as their direction of travel. The source of these waves is the vibration produced in any medium or sound source.
Since sound waves are also generated by the vibration of a medium, it follows that acoustic waves are a type of sound waves that travel on a surface, whether liquid or gas.
Here are some representations…
Acoustic waves in the air
By studying sound as an acoustic wave, we can better understand its mechanical properties.
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
Let’s review each one…
1. Acoustic or sound pressure
The pressure generated by any surrounding sound waves. Sound pressure is the deviation in equilibrium atmospheric pressure due to a sound wave and is measured in pascals (Pa) or N/m2.
When a sound wave travels through a medium, it creates a disturbance in the pressure of that medium – increasing its total pressure.
This is expressed by an equation:
Ptotal = P1 +Ps
Where…
Ptotal = Total pressure in the medium
P1 = Pressure generated by the sound wave
Ps = Static or equilibrium pressure
The increase in pressure can be measured using a microphone in the air or a hydrophone in the water.
2. Particle speed
The speed of a wave particle in a particular medium, which is measured in m/s. Imagine that a sound wave travels through a liquid medium. In this case, the speed of the particle is the speed of the liquid, as the sound wave causes the particles to move back and forth according to the vibration of the wave.
However, the speed of the particles is not the same for the liquid and the sound wave. A sound wave travels much faster than a particle in this medium.
Particle velocity can be calculated using this equation:
v = dδ/ dtδ
Where…
v = Particle speed
δ = Particle displacement
3. Acoustic or sound intensity
Together, sound pressure and particle speed constitute sound intensity. It is defined as the transfer of energy per unit area. This area is always perpendicular to the energy transferred.
Therefore, sound intensity is the power transferred by a sound wave per unit area and is measured in watts per square meter or W/m2.
Sound intensity can be calculated as follows:
Sound intensity, I = p*v
Where…
p = Sound pressure
v = Particle speed
4. Particle displacement
The displacement of a particle from its original position is called when sound travels through a medium. Particle displacement is measured in meters .
When a sound wave travels through air, particles in the air undergo displacement, depending on the speed of the particle. The displacement can occur in the direction of the sound wave or opposite to it.
The displacement of the particles is calculated by this equation:
δ =
Where…
v = Particle speed
dt = Period of time
A basic understanding of these mechanical properties of sound is useful when working with 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, we'll take a look at microphones, including how they work and how they are classified.
