Driver cooling VS amount of port ?!?!?!

[Kyblack76]

I use my fluke. It has a thermal couple I can reach in, and touch the motor and even the coil on the 2 driver side ones.

I hear unwind in my coil is over 500 degrees.

[Krakin]

It plays a pretty big role in it.

if you don't get adequate port area the air can't flow in the way it needs to without being obstructed. That right there is known as port compression.

This is a response curve of just a port, with the spike in response the tuned frequency. If the problem is severe enough, the port becomes useless as airflow is almost completely restricted and response starts to look a lot more like a sealed box as the clean spike in the graph above is leveled flat and badly distorted.

This in turn makes it hard for the speaker to be cooled off, and in car audio people like to through gigawatts at speakers, and as the power affects the speaker you get what is known as power compression. Power compression affects any and all speakers driven at extreme amounts of heat, which has become very common with new low efficiency/high xmax subwoofers that have massive moving mass. As the voice coil heats up the Thiele/Small parameters change and with low port area, which causes poor air movement for cooling, you get drastically worse change in the parameters of the speaker.

This is a graph of a speaker with its voice coil that stays cool, yellow, and a graph of one that becomes affected by power compression in orange.

Although these are two separate problems, they both get viciously worse near tuning with the output of the enclosure being sacrificed.

[timc31610]

Could you slide it into the vent on the back of the sub?

[timc31610]

I like the visuals Krakin, it would make sense that heat would build up closer to tuning since there would be less cone movement.

[Kyblack76]

Gets worse at tuning?

Correct me if im wrong, but, at/around enclosure tune, the driver moves LEAST right?

[Krakin]

Could you slide it into the vent on the back of the sub?

You don't want anything that would cause any vortices. This would also mean near/in any vent.

As the air flows around the object it is forced into what is known as a vortice where the air keeps trying to go back to where it originally was.

This is why wires in the port are an awful thing.

[timc31610]

Gets worse at tuning?

Correct me if im wrong, but, at/around enclosure tune, the driver moves LEAST right?

Maybe I didn't phrase it right, at tuning you would have less cone movement which would mean less airflow and less cooling. Right?

[Kyblack76]

Thats how im seeing that also.

[Krakin]

Gets worse at tuning?

Correct me if im wrong, but, at/around enclosure tune, the driver moves LEAST right?

It does move the least, but this is also where the port, theoretically, is working at 100%.

I'm just going to copy/paste from a paper I wrote for my own reference.

5a: Above the resonance Frequency

Above the resonant "frequency of the enclosure the mass reactance of the vent becomes high, and the cabinet behaves as though it were completely closed, presenting a purely stiffness reactance to the rear of the cone. At some frequency the combined stiffness reactance of the cone suspension system and the enclosure will become equal to the mass reactance of the cone. At this frequency a further resonance will occur, and again the amplitude will be considerably less than the cone resonance.

Due to the mass of the air inside of the enclosure and the port, or a passive radiator for that matter, can't react instantly to the pressure created by the active driver. Above the enclosures resonance frequency it is too slow to take away any energy from the drivers, and does almost nothing, refusing to move when the direction of force changes too fast.

GIF 1*

5b: At the Resonance Frequency

At the resonance frequency of the enclosure it begins to suck so much energy so forcefully through the internal pressure, that it is loading the driver in use. The cone of the driver then becomes the fulcrum to bear all the force from its own acceleration, which in turn limits the active driver's excursion.

GIF 2*

5c: Below the Resonance Frequency

Below the resonant frequency of the enclosure the stiffness reactance becomes high, and the system behaves as though the air mass in the vent were coupled directly to the mass,of the cone. At some frequency the reactance of this combined mass will become equal to the stiffness reactance of the suspension system of the cone. A resonance will occur at this frequency, the amplitude of which will be considerably lower than that of the initial cone resonance, and the radiation from the vent will be in antiphase with that from the cone.

Below the resonance the reflex of the system backs off from loading the driver. Since it is only venting back-pressure from the driver it is acting no more than a hole in the baffle, which in turn causes cancellation with the speaker's front output. This in turn rolls off the output at an insane 24 dB/octave, or 200 times less output every 1/2 of the last frequency. The port also no longer limits excursion or suspension that would normally be provided at or above resonance. This can easily cause the driver to bottom out, or smack against the t-yoke.

GIF 3*

Edit: top speaker is the subwoofer, bottom speaker is the port/PR.

Edi#2: You can also experiment with he pictures I have shown in real life. Just get a string and attack a weight in the middle and a weight at the bottom of the string. you can simulate the speaker/port by taking your hand and holding the top of the string and then taking your other hand and slowly moving the speaker weight back and forth while increasing the frequency. Doing this you will be able to see the affects of playing below tuning, playing at tuning, and playing above tuning.

[timc31610]

Krakin, are you some sort of mad scientist? LOL

I m going to run through my test tones and find the one where my subs seem to move the least and measure the velocity, and compare it to 5+ and 5- test tones. What you wrote makes perfect sense and the diagrams help but I would like to see it in person.