[GregU]

Alright, so I'd like to be able to look at the Q's of a sub and get an automatic understanding of how the sub performs. I understand the rest of the T/S parameters but the Q's still dont sink in. I've been getting better at it but I know I still have some misconceptions about them and reading definitions does not help. Can someone put them in other terms or elaborate on them more?

What I understand them to be:

Qts: Total Q, overall. But still have no clue what it is actually representing.
Qes: Electrical structure: Performance of the coil, motor, etc but still have no clue what its representing. How is this defferent from BL?
Qms: Mechanical Q: How stiff the spider is, surround and cone also have an effect. But what is it actually saying?

So what does it mean if the Q's are higher or lower? What I'm thinking is like for Qms, the higher the number the stiffer things are? I noticed on more SQ oriented subs the Qms is generally around 4, so does that mean it's looser? Can someone help out?

 

[Brian Steele]

Alright, so I'd like to be able to look at the Q's of a sub and get an automatic understanding of how the sub performs. I understand the rest of the T/S parameters but the Q's still dont sink in. I've been getting better at it but I know I still have some misconceptions about them and reading definitions does not help. Can someone put them in other terms or elaborate on them more?

What I understand them to be:

Qts: Total Q, overall. But still have no clue what it is actually representing.
Qes: Electrical structure: Performance of the coil, motor, etc but still have no clue what its representing. How is this defferent from BL?
Qms: Mechanical Q: How stiff the spider is, surround and cone also have an effect. But what is it actually saying?

So what does it mean if the Q's are higher or lower? What I'm thinking is like for Qms, the higher the number the stiffer things are? I noticed on more SQ oriented subs the Qms is generally around 4, so does that mean it's looser? Can someone help out?

Greg, get a copy of Dickason's "Loudspeaker Design Cookbook". It's the best investment you can make if you're really interested in this sort of stuff.

A simple explanation - Q is basically a measure of a system's "tendency to resonate" at its resonance frequency (the higher the Q, the greater the tendency), and t/s param-based design is centered around using that info about the driver to produce a combination of box size and tuning to achieve a desired frequency response.

Qes typically has a much more significant impact on a woofer's low-frequency response than Qms.

 

[GregU]

Greg, get a copy of Dickason's "Loudspeaker Design Cookbook". It's the best investment you can make if you're really interested in this sort of stuff.

A simple explanation - Q is basically a measure of a system's "tendency to resonate" at its resonance frequency (the higher the Q, the greater the tendency), and t/s param-based design is centered around using that info about the driver to produce a combination of box size and tuning to achieve a desired frequency response.

Qes typically has a much more significant impact on a woofer's low-frequency response than Qms.

Cool, thanks!

 

[GregU]

Greg, get a copy of Dickason's "Loudspeaker Design Cookbook". It's the best investment you can make if you're really interested in this sort of stuff.

A simple explanation - Q is basically a measure of a system's "tendency to resonate" at its resonance frequency (the higher the Q, the greater the tendency), and t/s param-based design is centered around using that info about the driver to produce a combination of box size and tuning to achieve a desired frequency response.

Qes typically has a much more significant impact on a woofer's low-frequency response than Qms.

Which edition?

 

[sandt38]

Qms - loss of the compliance, set by the Rms (mechanical impedance) of the suspension.
A mechanical / pneumatic "quality" number that truthfully doesn't lend a great deal of insight to helping you choose a subwoofer for most applications.
It is affected by physical properties that you can look at more accurately (IMO) through other specs, such as moving mass (look at Mms), suspension compliance (look at Vas, or a Cms curve), etc.

Qes - loss of the motor. Set by Mms, BL, Re, and Cms.
An electrical "quality" number... think of it instead as a good indicator of motor strength. Do you need a subwoofer with a strong motor? You might be surprised to learn that high strength isn't always a good thing. (hint: Look at EBP... or check out Qts. )
The smaller the number, the stronger the motor.
(also, the smaller the number, the smaller the enclosure, quite often - but it is just one factor in that equation )

Qts - Q of the driver - combinatiom of Qms and Qes.
A calculated number, using Qes and Qms - for a "total quality" number. I sort of downplayed Qms's importance earlier... and even in this calculation, Qes so far outweighs Qms in this calculation, that Qts actually ends up quite close in value to Qes - and small adjustments to Qes affect Qts much more than even large adjustments to Qms would have.
Anyway - this is an important one. One of the "main 3" you need to calculate enclosure size/type/performance. (along with Fs and Vas).
Qts mainly can tell you what sort of application your subwoofer is compatible in:
High Qts value? Very compatible in a sealed enclosure, or in some cases an excellent IB candidate.
Low Qts value? Very compatible in a vented enclosure.
Qts around 0.45 (middle of the road)? Design compromise to work OK in either, rather than outstanding in one and bad in the other.
It's very helpful that way.

I also like to keep FS in mind when looking at QTS to determine enclosure compatibility too.

I hope that helped.

 

[Neil]

I highly disagree that a low Qes denotes a high motor strength; often there is a correlation, but not a causal relationship necessarily. If motor strength is what you want to determine, take BL^2/Re.

Greg, the most recent edition will be fine. There are lots of answers here and elsewhere (google) if you look around. The Wikipedia page on Thiele/Small parameters shows how the qualities are mathematically derived, which I think makes it easier to understand what they represent.

 

[sandt38]

QES is the electrical dampening. In other words when you pass a signal through the voice coil, the QES lets us know how well the energy is converted to usable electromotive force within the magnetic field.... I would say that is a good indicator of motor strength. He was interested in simple definitions, so I made it as simple as I knew how.

 

[Neil]

Well, I think you can easily explain Qes without saying that it is an indicator of motor strength. It is quite likely to have a driver with a higher Qes only because it has a bit more mass, though the motor strength is identical. In fact, we see it quite regularly within a particular line, where every model has the same motor strength, but the Qes rises from 10" to 12", 12" to 15", etc. Easy is always good, as long as it is correct.

One argument you could make is that a lower Qes driver has a stronger motor relative to it's mass, suspension stiffness, and DC resistance than a higher Qes driver. It is critical, though, to make this distinction.

 

[sandt38]

One argument you could make is that a lower Qes driver has a stronger motor relative to it's mass and suspension stiffness than a higher Qes driver. It is critical, though, to make this distinction.

I did indicate it is set by Mms, BL, Re, and Cms . Although I guess I should have clarified.

 

[npdang]

Imo, knowing what these things represent doesn't help you understand a sub's performance.

It's more useful to simply think of them as "parameters" you can plug in to design enclosures.

If you really want to get a better understanding of driver performance, there's some great threads in the tutorial section. You want to be concerned with LARGE SIGNAL behavior, not parameters associated with typically very small input levels.

 

[Andy Wehmeyer]

Q isn't a very straightforward parameter and none of the technical explanations or definitions make it easy to understand.

Q is a unitless measure that indicates the amount of overshoot the motor and suspension allow or cause near resonance and it's useful in determining how much more overshoot we should design into the box/woofer system to reach a target response. Overshoot means the woofer keeps moving after a signal goes away. Before you freak out and think that the best woofer doesn't allow any and that it perfectly reproduces the signal...not so fast...

Here's how it works:

Qes is an indication of how much overshoot the motor allows--that number is usually very small--for car audio woofers intended for sealed boxes, it's often less that 1.

Qms is an indication of how much overshoot the suspension causes and the number is always much larger than 1.

The equation for Qts is a simple product over sum and treats Qms and Qes like resistors in parallel. It's Qes= (Qms*Qes)/(Qms+Qes). Looking carefully at the equation, it's easy to see that the motor has more influence over the behavior of the speaker than does the suspension. The Qts number is always closer to the Qes number than it is to the Qms number.

The motor controls the motion of the cone and the spider prevents the coil from leaving the gap. The spider is a spring and acts like a super ball. The stiffer the spider, the springier the suspension, the more overshoot it causes and the higher the Q. Putting the woofer in a sealed box doesn't exert more control over the woofer's movement, it causes less control. That seems counterintuitive.

The air in the box is also a spring and the combination of the air and the spider make a stiffer spring--it's like a bouncier super ball.

Since Fs (free air resonance) of the woofer is determined by the relationship of the springiness of the spider (Vas) and the mass of the moving assembly and the air surrounding the cone (Mms), changing the spring changes the resonance. We can make the spring stiffer with a box, but we can't make it less stiff--we can't increase damping (elimination of overshoot) with a box.

Thiele and Small parameters are used to figure out how small the box has to be to increase the springiness of the suspension to increase the overshoot in order to achieve some target response. The woofer in a box can never have a Qtc lower than the Qts of the woofer.

Let's say you have a woofer with a Qts of .5. Without a box the woofer allows very little ovrshoot, but it also makes very little bass. A Qtc of .7 provides the flattest response and the lowest extension possible. In order to get the system to .7, we have to put the woofer in a box that has a volume of air with springiness sufficient to cause a little more overshoot (when the woofer keeps playing it makes more sound) to raise the output near the system resonance to flatten the curve. The parameter that helps us determine the box volume that's required is Vas--which is the volume of air that's equivalent to the springiness of the woofer.

I find that explanation to be a little eaasier to understand than talking about losses and motor strength. Motor strength is related to Q, however. A stronger motor exerts more control, causes a lower Qes, which causes a lower Qts and requires a smaller (springier) box to raise the Q to achieve a certain target response. Don't get hung up on the idea of overshoot and the inaccuracy it suggests. I find that thinking about transient response and critical damping to be detrimental as design criteria. For a minimum phase system, which is what a woofer is, flat response IS transient accuracy--by the Fourier transform, so shooting for a target of .707 with a sealed box is less confusing and yeilds optimum performance.

 

[GregU]

Q isn't a very straightforward parameter and none of the technical explanations or definitions make it easy to understand.

Q is a unitless measure that indicates the amount of overshoot the motor and suspension allow or cause near resonance and it's useful in determining how much more overshoot we should design into the box/woofer system to reach a target response. Overshoot means the woofer keeps moving after a signal goes away. Before you freak out and think that the best woofer doesn't allow any and that it perfectly reproduces the signal...not so fast...

Here's how it works:

Qes is an indication of how much overshoot the motor allows--that number is usually very small--for car audio woofers intended for sealed boxes, it's often less that 1.

Qms is an indication of how much overshoot the suspension causes and the number is always much larger than 1.

The equation for Qts is a simple product over sum and treats Qms and Qes like resistors in parallel. It's Qes= (Qms*Qes)/(Qms+Qes). Looking carefully at the equation, it's easy to see that the motor has more influence over the behavior of the speaker than does the suspension. The Qts number is always closer to the Qes number than it is to the Qms number.

The motor controls the motion of the cone and the spider prevents the coil from leaving the gap. The spider is a spring and acts like a super ball. The stiffer the spider, the springier the suspension, the more overshoot it causes and the higher the Q. Putting the woofer in a sealed box doesn't exert more control over the woofer's movement, it causes less control. That seems counterintuitive.

The air in the box is also a spring and the combination of the air and the spider make a stiffer spring--it's like a bouncier super ball.

Since Fs (free air resonance) of the woofer is determined by the relationship of the springiness of the spider (Vas) and the mass of the moving assembly and the air surrounding the cone (Mms), changing the spring changes the resonance. We can make the spring stiffer with a box, but we can't make it less stiff--we can't increase damping (elimination of overshoot) with a box.

Thiele and Small parameters are used to figure out how small the box has to be to increase the springiness of the suspension to increase the overshoot in order to achieve some target response. The woofer in a box can never have a Qtc lower than the Qts of the woofer.

Let's say you have a woofer with a Qts of .5. Without a box the woofer allows very little ovrshoot, but it also makes very little bass. A Qtc of .7 provides the flattest response and the lowest extension possible. In order to get the system to .7, we have to put the woofer in a box that has a volume of air with springiness sufficient to cause a little more overshoot (when the woofer keeps playing it makes more sound) to raise the output near the system resonance to flatten the curve. The parameter that helps us determine the box volume that's required is Vas--which is the volume of air that's equivalent to the springiness of the woofer.

I find that explanation to be a little eaasier to understand than talking about losses and motor strength. Motor strength is related to Q, however. A stronger motor exerts more control, causes a lower Qes, which causes a lower Qts and requires a smaller (springier) box to raise the Q to achieve a certain target response. Don't get hung up on the idea of overshoot and the inaccuracy it suggests. I find that thinking about transient response and critical damping to be detrimental as design criteria. For a minimum phase system, which is what a woofer is, flat response IS transient accuracy--by the Fourier transform, so shooting for a target of .707 with a sealed box is less confusing and yeilds optimum performance.

Very interesting!

Now, to let this soak in.....