Cone movement vs. frequency

[Nichol1997]

Thank you everyone, I understand the relationship between cone movement vs frequency now. But, now I have a different question.

Here is my setup, I have an 18" subwoofer in a ported enclosure that is tuned to appoximately 14 Hertz with 500 Watts running to it. THe driver has an Xmax of 27 mm and a power handling of 800 watts. I am trying to understand why I can't get the cone to move peak to peak at higher frequencies. By higher frequencies I mean anything from 40 to 90 Hertz. I have a test tone cd and the subwoofer will really start to move when I play anything less than 30 Hertz. I understand the concept of Hertz. For instance, I know that at 60 Hertz the cone has to move back and forth 60 times in one second.

My question is, why can't I get the cone to move peak to peak with higher frequencies? One would think that by just giving it more power it will move more. On that thought, am I limited by the amount of power the voice coil can handle?

[Ron E]

WHat are the T/S parameters?

Model it and you will see that the excursion drops very fast. It reduces by 4 for each octave you increase, which means that you would need 16 times the power to reach the same excursion., then 16*16 or 256 times the power the octave above that.

[Nichol1997]

Originally posted by Ron E
WHat are the T/S parameters?

Model it and you will see that the excursion drops very fast. It reduces by 4 for each octave you increase, which means that you would need 16 times the power to reach the same excursion., then 16*16 or 256 times the power the octave above that.

The driver is a Ascendant Audio Avalanche 18.

Okay, so the only thing that is limiting the driver from moving peak to peak at 60 Hertz is the amount of power I have and the amount of power the voice coil can handle, correct?

I thought there might be some kind of physical constraint like a cone can only move so fast.

[Pano]

I think it gets back to Newton's second law again, which is what Ron was talking about above

At 60 Hz the cone has to move twice as fast as it does at 30 Hz. Right? Soooo, you are going to need twice as much power to move it the same distance twice as fast.

You can throw more and more power at it, but there comes a point when the voice coil starts to heat up and you get power compression. You either run out of power or the coil burns out.

There may be increased resistance from the cone suspension with increased speed - I don't know about that. Ron probably does.

[Svante]

Originally posted by Nichol1997
Thank you everyone, I understand the relationship between cone movement vs frequency now. But, now I have a different question.

Here is my setup, I have an 18" subwoofer in a ported enclosure that is tuned to appoximately 14 Hertz with 500 Watts running to it. THe driver has an Xmax of 27 mm and a power handling of 800 watts. I am trying to understand why I can't get the cone to move peak to peak at higher frequencies. By higher frequencies I mean anything from 40 to 90 Hertz. I have a test tone cd and the subwoofer will really start to move when I play anything less than 30 Hertz. I understand the concept of Hertz. For instance, I know that at 60 Hertz the cone has to move back and forth 60 times in one second.

My question is, why can't I get the cone to move peak to peak with higher frequencies? One would think that by just giving it more power it will move more. On that thought, am I limited by the amount of power the voice coil can handle?

The 800 watts that the driver can handle is limited by the heat in the voice coil. If you feed the driver more than 800 watts, there is a risk that the glue holding the coil together is damaged due to the heat. But there is another limit too, and that is xmax for the driver, if the cone moves more than xmax permits, the sound is distorted*.

Now, since the cone excursion varies with frequency, these two factors limit the maximum output level in different frequency ranges. The black curve is the response of the driver. The thin gray curve is the maximul output level of the system. The red curve (which is partially hidden under the thick gray curve) shows the margin from the response until xmax is reached. The black dashed cuve shows the same for the power dissipation in Re.

The resulting margin, for which neither of Xmax or Pmax is exceeded is the thick gray curve. If the thick gray curve is added to the response, this results in the maximum output level, the thin gray curve.

So, it is understandable that you cannot reach xmax for higher frequencies, since the maximum input there is limited by (available and sustainable) power and not xmax.



*There are of course gray zones here, but for the sake of understanding this is a good model.

[sreten]

Hi,

Or in other words if you could xmax it above 50Hz you would then be
very likely to have overexcursion / distortion / endstop thumping lower
down the frequency range, especially with a port frequency of 14Hz.

Also note Svante curves are typical for a sealed box, vented looks different.

/sreten.