Originally Posted by LTD02
have you put these in the torture chamber yet? it would be interesting to see measurements of the value of this heat removal strategy vs. stock sealed enclosures.
jbl has made claims about the benefits of cooling strategies in some of their transducers. one measurement they use to validate the theory is power compression test. page 5 of this document provides an example:http://www.jblpro.com/pub/technote/tn_v1n18.pdf
in the cooling chamber, were you planning to have just a pipe, or is the entire "wind tunnel" a small-finned heat sink coupled to the pipe for max cooling?
From page 1: "Heat is drawn away from the voice coil primarily by radiation to the neighboring metal structures..."
The idea is to draw the heat away from the metal structure through conduction and convection. There is little doubt that fins, which simply add to the area of heat sink, will increase efficiency. But, with 200 square inches of heat sink already incorporated, additional heat sink is not necessary in my designs.
The JBL paper linked to relies strictly upon air flow. As excursion decreases (frequency increases), convection becomes less effective and conduction becomes more effective.
In a sealed box, heat is not only radiated to the motor structure, it's radiated to the trapped air. The result is that the voice coil loses its ability to shed heat as the temperatures rise resulting in the increased temperatures of both the voice coil and the motor .
One of my theories as to why most people run their subwoofers hot is based on the principal that applies to the pro sound industry, where they bump the input to compensate for the loss in output from heat.
Steve C popped in to call ThruVent cooling a "compromise". I view it in the opposite. Conventional wisdom says "Build a bigger box. That way there is more air to heat and less input power to heat the motor and air". That, IMO, is a compromise that carries a high price in labor, materials, bracing, weight, less placement options, less headroom and more real estate.
I can design a subwoofer that looks as good as it performs, but if I increase the size of that design to the size of a refrigerator, it ceases to look good and just looks radically out of place for the application. And, reducing the input power by 3dB doesn't solve the heat problem and reduces output potential above the knee, which is why, in the extreme case of the IB, multiple drivers are used to compensate.
How much does a cooling system add? Depends on the source, listening level and duration of the listening session. I've said 3-4dB, which was the original design goal. I no longer torture my subs because I'm not JBL with the funds and time required to torture hardware and critically measure the results, so I'll leave that to someone else.