Air-spring non-linearity, causes?

[thadman]

Following some research on acoustic resonances, it was brought to my attention that the enclosure air-spring becomes non-linear beyond a certain level of compression. Therefore the Maximum SPL (below an arbitrary distortion threshold) attainable by a system is thus dictated by the volume of air within the enclosure.

Dennis H over on HTGuide provided a useful formula for determining distortion relating to air compression:

Distortion % = 140*(one-way driver displacement)*(1/enclosure volume)

What are the causes for this form of distortion? Is it due to the temperature decay or the pressure imbalance between sides of the diaphragm?

How can we decrease this phenomenon?

Thanks,
Thadman

[Steve Eddy]

Originally posted by thadman What are the causes for this form of distortion? Is it due to the temperature decay or the pressure imbalance between sides of the diaphragm?

It's due to air being not entirely linear when it comes to compression and rarefaction.

How can we decrease this phenomenon?

Don't use air.

se

[Fast1one]

Originally posted by Steve Eddy


Don't use air.

se

Hehe! Or don't use a box

[thadman]

Originally posted by Fast1one
Hehe! Or don't use a box

How about a transmission line? How would its attributes affect air non-linearity?

[Ron E]

In most cases, the air in the box is more linear than your driver's suspension. It is when the driver volume displacement exceeds 10% of the box volume that it will become significant. Still, whether it is more significant than driver nonlinearity depends on the driver.

Transmission lines pressurize a box, so they are affected.

[thadman]

Originally posted by Ron E
Transmission lines pressurize a box, so they are affected. [/B]

Of course they do. It's rather elementary to describe the distribution of pressure in a line with a Cos wave. However, for the same volume of air and driver displacement, does the TL (driver at one end, opposing end open) wrt a closed line (driver at one end, opposing end closed) possess the same degree of air non-linearity?

[Ron E]

It is really rather elementary stuff. Given the same SPL at the same frequency, the degree of pressurization will be the same in a sealed, vented, PR, TL, etc.... The proof is left to you as an exercise...

[Mark Kelly]

Originally posted by Steve Eddy


It's due to air being not entirely linear when it comes to compression and rarefaction.

Don't use air.

se

I'm not so sure that it is due to non linearity of air. Even with an ideal gas which follows the completely linear ideal gas law PV = nRT, if we assume constant temperature (so nRT = C (constant))

P = C/V

thus dP/dV = -C/V^2

Since dP/dV is the spring constant this means that even an ideal gas is a non-linear spring.

[thadman]

Originally posted by Mark Kelly


I'm not so sure that it is due to non linearity of air. Even with an ideal gas which follows the completely linear ideal gas law PV = nRT, if we assume constant temperature (so nRT = C (constant))

P = C/V

thus dP/dV = -C/V^2

Since dP/dV is the spring constant this means that even an ideal gas is a non-linear spring.

A potential solution could then be to make the drivers suspension force the inverse of the air spring force. As far as I understand, the individual non-linear forces should sum to be linear as the total restoration force.

Is it then possible to accurately determine an enclosures air spring force and model it? Even with acoustic foam or rigid fiberglass present?

[thoriated]

You may be able to do that to some extent with some drivers by adding a small DC offset to the signal to compensate with the driver suspension linearity. Not really recommended.