The Scientific Essence Of Sound: Getting To The Basis Of Everything We Hear
No one becomes a scientific guru of sound overnight. However, your time will be well invested, because this information is truly at the heart of everything you do when working with sound.

Volume Velocity. The volume velocity (U) in its most simple sense can be considered to be the amount of air that is moved by an acoustic source, such as a loudspeaker, or the amount of air that causes a transducer to move, such as a microphone diaphragm.

The units of volume velocity are m3/s. The “volume” here does not refer to level or loudness, but to occupied space as measured in cubic units.

At the boundary of the vibrating object, the acoustic particle velocity will be the same as the physical (moving) velocity of the object itself.

The vibrating air particles results in an acoustic medium flow perpendicular to the vibrating object.

The magnitude of the acoustic medium flow will depend on the size of the vibrating object.

The volume velocity is defined by the equation below.

Equation 5
where,
U = volume velocity, m3/s
u = particle velocity, m/s
S = surface area of vibrating object, m2

The volume velocity is directly proportional to particle velocity and the surface area. Increasing either one will increase the volume velocity. The volume velocity has the same sinusoid variation and phase as the particle velocity.

Equation 5 readily shows us that if we want to move a given amount of air, such as with a loudspeaker, we can either use a single cone (large S) with a relatively low amplitude surface (particle) velocity or use a smaller cone operating with a higher amplitude surface velocity. (Figure 4)

Figure 4: Concept of equal value velocity (U): loudspeaker at left has larger surface area (S) and smaller particle velocity (u), while loudspeaker at right has smaller surface area (S) and larger particle velocity (u).

Of importance to volume velocity is its relationship to acoustic impedance (Z). The units of acoustic impedance are Pa s/m3 and can be considered analogous to acoustic ohms. The acoustic impedance is defined by the equation below.

Equation 6
where,
Z = acoustic impedance, Pa s/m3
p = acoustic pressure, Pa
U = volume velocity, m3/s

We will see in a future article the acoustic impedance is comprised of acoustic resistance and acoustic reactance, similar to electrical impedance.

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Comment (1)
Posted by LeeT  on  03/14/11  at  04:54 PM
Wow you guys really know your stuff to the core. Very impressive.
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