Thermal Compression Design Guidelines

[AbilityBuild]

In a loudspeaker, power compression or thermal compression is a loss of efficiency observed as the voice coil heats up under operation, increasing the DC resistance of the voice coil and decreasing the effective available power of the audio amplifier

from Wikipedia

In addition, an increase in Re will:

• Increase Qes
• Decrease Qms
• Decrease Qts (it depends on the relative size of Qes and Qms but I'm pretty sure Qts would decrease for almost all subwoofers).
• Decrease efficiency
• Decrease sensitivity

I noticed that these changes can strongly impact the behavior of my subwoofer at low frequencies in hornresp. For example, 20C of temperature rise caused my flat response that is within Xmax to change into a peaky response at 20hz that causes the driver to displace beyond Xmax.

What would be a design rule of thumb for taking into account thermal compression? How do I estimate a reasonable temperature rise above ambient?

 

[AbilityBuild]

I guess I should design my subwoofer to play best at the power level I intend to listen.

But how do I estimate the temperature difference? If we try and use newton's law of cooling:
Q = h * A * delta_T
Q = Heat Power = efficiency * P
P = RMS power during a listening session
h = heat transfer coefficient
A = area of the heat transfer surface
delta_T = difference in temperature between air and heat transfer surface.

I suppose P depends on the driver and the type of source input (SPL music, vs movie with a few spikes of bass content).

I suppose h depends on the type of enclosure (ported vs sealed etc).

Not sure if this is the right approach. Seems hard to estimate.

 

[GM]

Indeed! Easier to drive it hard and measure its response or just use the 'Le' heat rise approximation in HR [double click on it to turn it on, changes to red].

 

[chris661]

Decrease Qts (it depends on the relative size of Qes and Qms but I'm pretty sure Qts would decrease for almost all subwoofers).

Since Qts tends to be dominated by Qes in most drivers, Qts will certainly rise.

If you're designing your subwoofer to be run well into thermal compression, then you need to design a bigger subwoofer.

Chris