# Qts, what does it mean?



## buchaja (Nov 10, 2007)

For car audio applications, I mean. 

Let's say I'm looking for a speaker to mount in my doors. A Mid/midbass, 8 incher would be nice. Over in this site http://lenardaudio.com/education/05_speakers.html , about two thirds of the way down the page under the heading Resonance Q, you will see a graphical depiction and explanation of the relationship between Q and resonance. 

Now, all the really expensive, high-end speakers I've looked at recently (http://www.madisound.com/catalog/product_info.php?manufacturers_id=148&products_id=8261) seem to have a very low Qts. Does this mean they would not sound good in my theoretical application? 

Since I'm new to forums, and this is the first one I've joined -- kudos everyone, this is a nice place to hang out and learn -- I am assuming IB means infinite baffel. Is this correct? And if so, is that the acceptable approximation for an in-door mounting like the one I'm considering.

Hope the links show up as HTML. 

Thanks,

Jim


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## solacedagony (May 18, 2006)

"Qts
A unitless measurement, characterizing the combined electric and mechanical damping of the driver. In electronics, Q is the inverse of the damping ratio. The value of Qts is proportional to the energy stored, divided by the energy dissipated, and is defined at resonance (Fs). Most drivers have Qts values between 0.2 and 0.8."
-http://en.wikipedia.org/wiki/Thiele/Small

Car doors are usually considered infinite baffle (IB).

I can't explain the why and how behind it, but low QTS drivers work better in vented enclosures and high QTS drivers work well in IB installs. This isn't to say you couldn't use a low QTS driver in an IB install.


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## buchaja (Nov 10, 2007)

Hmm, thanks. I read that very wikipedia definition last night. 

With all the talk about the Scan Revs and similiar drivers here, I was wondering how everone was using them. 

Now I wish I could sort drivers at Madisound by Qts.


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## Fellippe (Sep 15, 2006)

solacedagony said:


> "Qts
> A unitless measurement, characterizing the combined electric and mechanical damping of the driver. In electronics, Q is the inverse of the damping ratio. The value of Qts is proportional to the energy stored, divided by the energy dissipated, and is defined at resonance (Fs). Most drivers have Qts values between 0.2 and 0.8."
> -http://en.wikipedia.org/wiki/Thiele/Small
> 
> ...


The Efficiency Bandwidth Product (EBP) is a ratio of fs to qts.

Ratios > 90 tend to favor vented apps
Ratios < 40-50 tend to favor sealed apps
Ratios between 40-90 can go either way

Low qts woofers very often will exceed 90 for the EBP. A woofer with a low fs of 32hz, and a qts of .32 will have an EBP of 100. Raise the fs and this # only increases. 

As for the "why" behind all of this, I'm not 100% sure myself but I'd venture to guess that the combination of a sealed enclosure (or IB install) and a peakier driver is roughly similar to a vented enclosure and a smoother driver. 

With a great percentage of home audio speakers featuring vented enclosures/passive radiators, it becomes evident why a majority of the raw drivers are low qts.


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## npdang (Jul 29, 2005)

You guys are making this too complicated. If I asked you what grass is for example, there's obviously going to be alot of different levels of abstraction... the important thing is to find which one is most useful to you. Is knowing the biological or chemical composition going to be very useful? Or is knowing that it's green, has a certain texture and shape, is photosynthetic, etc. more useful?

Imho, Qts is most useful when trying to evaluate a driver's low end rolloff. If you know Qts, and you know Fs, you now have a good idea of what the low end sensitivity will look like. Qts describes the shape of the knee toward rolloff, and Fs describes the point toward which the driver's low end sensitivity drops (bearing in mind that this approximation holds only with small drive levels, as you increase the input signal driver behavior becomes non-linear).

Here is a chart of the rolloff comparing various Q's. A Q of .707 would give you the flattest response before rolloff (gold line). Something like the blue line would be around .5, and yield perfect transient response, but tend to have a shallower, earlier rolloff toward resonance. A high Q such as the black line (>1), has a peak in the response, but the quickest rolloff toward resonance afterward.

Obviously, when you put the driver in a box both Fs and Qts of the system are raised. For example, compare a driver with a Q of .5 and an fs of 30hz, to a driver with a Q of .3 and an fs of 25 hz. It's possible for the final system Q of the second driver to be critically dampened at .5, with an fc of perhaps 35hz. The first driver when used in a sealed box, would never be able to achieve that target response. And, if you know the Vas of a driver, you will also be able to eyeball the box requirements of the driver. 

Now why are high Q drivers recommended for IB? Because the low end sensitivity is more optimized in high q alignments to provide an optimally flat response (compare the gold curve of a high q driver to that of the blue curve of a low q driver). That's not to say that you can't EQ that response though, which is why I don't think it's that big of a deal. I would worry more about the things you can't fix, like non-linear distortion.


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## Thumper26 (Sep 23, 2005)

awesome thread. i just learned a lot!


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## Steak (Mar 16, 2006)

well I just didn't understand a lot... I guess I'll go back to bcae1 to try to figure out what NP is trying to explain


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## ws6 beat (Jul 14, 2005)

me too. npdang is the man.


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## solacedagony (May 18, 2006)

Steak said:


> well I just didn't understand a lot... I guess I'll go back to bcae1 to try to figure out what NP is trying to explain


To put it very simply: Look at the graph NP posted. The different lines represent different values for Qts. Each different value for Qts gives a different low end rolloff.

Npdang: So the driver's Qts combined with the enclosure's Q would result in your final total system Q? Also, wouldn't low Q drivers in general work well in small (high Q) boxes when going for flat FR?


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## npdang (Jul 29, 2005)

solacedagony said:


> To put it very simply: Look at the graph NP posted. The different lines represent different values for Qts. Each different value for Qts gives a different low end rolloff.
> 
> Npdang: So the driver's Qts combined with the enclosure's Q would result in your final total system Q? Also, wouldn't low Q drivers in general work well in small (high Q) boxes when going for flat FR?


Yah, you can think of it that way. 

The two very important thing I think people forget though are:

1. These are SMALL SIGNAL parameters. In other words, once you start putting real power on your driver all bets are off.

2. In evaluating low end sensitivity Qts by itself is near useless... you need to know Qts AND Fs. People always seem to get so hung up on one or the other.


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## Tommythecat (Apr 6, 2006)

Boxes dont have Q values. You're valuing the wrong things here. Q is a descriptor of what's really going on. Boxes provide either "restoring force" (aka air spring) or some sort of resonant device (port, 1/4 pipe or t-line, etc.) or both.

A small volume of air will not give you a Q, it will give you a springy restoring force which will combine with the speaker's natural "springiness" (or the suspension - spider, surround, etc.) and thus affect the resonance frequency of the speaker. 

Which is why low Q drivers can typically be used in smaller enclosures, the speaker doesn't have much inherent restoring force and relies on the box to provide it.


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## Oliver (Jun 25, 2007)

Excellent !! Nguyen


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## Foglght (Aug 2, 2007)

So, you are saying that these Exodus Audio 12's that I have now that are a .9 QTS are perfect for IB since they should have a pretty stiff suspension, and don not require the box to provide the rebound?


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