QES and Compression

[Beatin' 10+ year member]

Do high motor force subs (those with low QES) suffer less from power compression then low motor force subs?

Isn't this the reason why high qts subs should be put into sealed boxes because they suffer from excessive compression trying to resonate a port?

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[T3mpest 10+ year member]

Do high motor force subs (those with low QES) suffer less from power compression then low motor force subs?
Isn't this the reason why high qts subs should be put into sealed boxes because they suffer from excessive compression trying to resonate a port?

Depends on the type of compression There are 2 main types of compression, hopefully going over both will explain the issue well enough..

First type is thermal compression. As the voicecoil of a woofer heats up, the impedance of the coil rises, the reduces the wattage that the amplifier can produce. So the strength of your motor has no real effect here.. Essentially the bigger a a coil is, the more energy it takes to heat it up, the better it will handle power compression. It's why when you get over 1500 watts, I'd MUCH rather have a 4inch coil than a 3inch one. Regardless of all the cooling tricks you can put in a motor, you may be able to stop the coil from frying, but stopping it from heavily compressing from 1500 watts to say the 2500 rms of a top line 3inch coil, not so much. A 4inch coil is just better at taking high power.

The second type of compression, less talked about is BL compression. As the coil begins to leave the gap, their is less force acting on the coil, so BL drops and the Q of the system actually raises when this happens. So for this type of compression a higher Q and compression go hand in hand, but that's not the static T/S parameter you see on a spec sheet, this is what happens to those T/S parameters when the speaker actually sees power. The key to controlling this isn't necessarily starting with a low QES, but having a high xmax. Xmax is a measurement of how far the cone can move before the BL product drops to 70% of it's resting value (if it's klippel measured xmax anyways).

High QES subs generally don't go in ported boxes because they produce irregular frequency response. They have weaker motors that do have trouble resonating the port. This leads to a very wide frequency range that they see gain in, but at a very reduced magnitude. Instead of the port producing lots of extra output where it's tuned, you just get a bit more output everywhere. Pretty hard to design a ported box around that, especially when it unloads below tuning. This is great for a 4th order bandpass where you can tune it higher and have the sealed side to keep the cone under control below tuning, but for a standard ported box, just leads to a rough response.

 

[Beatin' 10+ year member]

So basically, if a low and a high qts sub have the same sensitivity and x-max, the low qts sub will have less spl compression at higher power levels than the high qts sub due to a stronger motor?

 

[T3mpest 10+ year member]

That's going to be application depenedent... I mean if you put them both in sealed boxes and played a 20hz tone the low q driver will suffer more compression because it won't be as sensative in a sealed box down low and will need more.power.

 

[Beatin' 10+ year member]

That's going to be application depenedent... I mean if you put them both in sealed boxes and played a 20hz tone the low q driver will suffer more compression because it won't be as sensative in a sealed box down low and will need more.power.

You see, this is what I can't understand.

At high power levels, surely the low qts sub will have much closer linear spl increase compared to the high qts sub due to the stronger motors being able to deal with the high pressures involved with the cone going in and out.

Let's say we have 2 subs, high and low qts, both rated 1000wrms.

When reaching the maximum rated power, you don't typically get +3db anymore for every doubling of power. So would it be safe to assume that the high qts sub going from 500watts to 1000watts would theoretically get a +1db increase, whereas the low qts sub would get more than a 1db increase because of the stronger motor?

 

[T3mpest 10+ year member]

You see' date=' this is what I can't understand.
At high power levels, surely the low qts sub will have much closer linear spl increase compared to the high qts sub due to the stronger motors being able to deal with the high pressures involved with the cone going in and out.

Let's say we have 2 subs, high and low qts, both rated 1000wrms.

When reaching the maximum rated power, you don't typically get +3db anymore for every doubling of power. So would it be safe to assume that the high qts sub going from 500watts to 1000watts would theoretically get a +1db increase, whereas the low qts sub would get more than a 1db increase because of the stronger motor?[/quote']

Nope, unfortunately that's an assumption we CANNOT make.. Imagine the low QTS sub uses a powerful motor, but light and fairly short coil, but good cooling.. Many pro audio drivers are built like this. That will give you a low xmax.. Now imagine the high Q driver is a heavier longer coil, but with a wider gap and weaker overall motor as well as no cooling tricks, so they are each rated at 1k.. If you get each motor moving to say 10mm, and that's near xmax for the lower Q driver, it will then begin to experience more compression than the high Q driver. Partially because of being near it's xmax limits where BL is really going to be dropping off and also partially because the lighter coil will suffer more power compression..

One thing to rememeber is decibels are logarithmic. Going from 172.2-175.2db is MUCH harder than going from 30-33db, but they are both 3db gains.

The main gains you'll see from a low Q sub is in it's effeciency near tuning. It'll produce a larger more proncounced peak right at tuning. It may or may not hold up at higher power, you'd need more than Q of the driver to really know.. Many drivers start off as low Q, until softparts get moving. Suddenly the suspension is tightening down and the coil is leaving the gap rather quickly and the voice coil doesn't have adequate cooling.. A driver like that is going to look like an SPL driver at low drive levels, but the response won't hold up as power increases.

A low Q driver is more effecient at tuning. This does generally mean it will experience less compression at tuning compared to a higher Q driver, at the same SPL levels.. At the same power level, it will come down to how they handle power as the low Q driver is more effecient and will likely be moving the coil much further.. Still, switch things to a sealed box and now the high Q driver will have the advantage down low.

I think it's better to think of it as less effecient overall than to think of it as suffering from more compression. Compression is a non linear effect that tends to get worse as power and excursion increase, kind of a wild card type of thing.. Low effeciency is an across the board issue, that wont' change as power as applied so much. A high Q driver is less effecient at moving a port, period.. However it can sometimes make up some of the difference as power is applied, because high Q drivers sometimes at least have long coils and high xmax values.. When looking at compression you really have to look at the entire design..

XBL^2 designs for example are often high Q designs. However their "compression" issues are generally very low within useable limits. The notch they put in the gap lowers it's BL at rest, while linearizing it over stroke. The BL force is stable through the entire stroke of the motor. If you look at an XBL^2 design you'll see a rollercoaster looking BL curve. Lower in the center then it goes up toward the middle and then back down toward the ends... Then at the very end it drops like a ROCK, just past xmax. This means if the driver has 30mm of xmax, it's going to experience VERY little compression until you get it to xmax. However past xmax compression will be ridiculous, because BL is dropping so much. This makes XBL^2 drivers very hard to damage mechancially, you'll amost always fry the coil before they bottom out, because it suffers a lot from BL compression, but not until it leaves linear limits, until then, it suffers almost none. So looking at a compression plot of drivers like this, you'll get the 3db per doubling of powe you expect until you either A. get near the powerhandling limits of the driver, or B. Get right up the excursion limits.. A standard driver with the same 30mm xmax might start having minor compression once you get around 10of xmax and get worse from there.. At 35mm they would look the same, but at 25mm, the xbl^2 driver would do much better.

That also being said, since the xbl^2 driver is high Q, it would still **** at driving a port, it would just **** just as bad, regardless of power level lol.