- The acoustic analog of the electrical capacitor or the mechanical spring is a cavity, or a tank (Morse, 1981, p. 234).
- In the low-frequency limit, an applied external pressure will compress the enclosed air, which then acts like a spring because of its elasticity. Assuming a cavity volume
*Q*, an increase in applied pressure*P*will decrease this volume by an amount -*dQ*. - The ratio of change in volume to original volume is called volume strain or dilation and is given by
- For small dilation, Hooke's law for fluids provides an accurate approximation to the relationship between the increase in applied pressure, the resulting strain, and the bulk modulus
*B*,

- The bulk modulus for sound waves is nearly adiabatic and is expressed in terms of the density of air and the speed of sound
*c*as . - Using Eq. (6) and writing the change in cavity volume, -
*dQ*, in terms of the sinusoidal volume velocity*U*as

(7)

(8) - The impedance of a mechanical spring is equal to
*k*/s, so by analogy the acoustic cavity has an equivalent ``spring constant'' equal to

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