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What is Acoutic Impedance?

could anyone explain this to me?
If 2 materials have a large difference in impedance, then most of the energy is reflected(the intensity of the reflected wave will be high). If the impedance of the 2 materials is the same then there is no reflection.

and um.. what is Impedance?
thanks a lot

Posted from TSR Mobile
Original post by christie.leung
could anyone explain this to me?
If 2 materials have a large difference in impedance, then most of the energy is reflected(the intensity of the reflected wave will be high). If the impedance of the 2 materials is the same then there is no reflection.

and um.. what is Impedance?
thanks a lot

Posted from TSR Mobile


Hello and welcome to TSR.

This is an interesting question :smile: and one which does not come up very often on these forums.

In very simple terms, acoustic impedance is how much work needs to be done in order for a substance to change the state of mechanical vibration of its constituent molecules.

A material with a high acoustic impedance needs far more work to change its state of mechanical vibration than one with a low acoustic impedance.

In other words acoustic impedance describes resistance to change. Use of the word 'impedance' is because the resistance to change is frequency dependent. i.e. resistance changes as a function of frequency.

So when there are two dissimilar materials with widey different acoustic impedances, sound pressure waves travelling in a low impedance medium, will not readily transfer energy into the higher impedance material.

The pressure wave bunches up at the boundary and rebounds because the energy cannot easily continue in its direction of travel but must still be conserved. Some of the energy will cross the boundary but most will stay within the original medium.

Think of it like a billiard table analogy: The free moving billiard balls representing the molecules of a low impedance medium, meets the table edge (high impedance medium) and rebound. (Reflection and refraction.)

Conversely if the impedances are similar, then most of the sound energy will transfer easily across the boundary and hence very little will be reflected. For maximum energy transfer with no refection, the acoustic impedances must be matched.

In quantitative terms:

Acoustic impedance Z = acoustic pressure / (sound velocity x surface area)

Z = p/vA

measured in rayls:

1 Rayl = 1 Newton-second per cubic meter (SI base units kgs-1m-2)
(edited 10 years ago)
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christie.leung
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wow! what a thorough explanation, thank u very much

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