T/S parameters .. what does Qts have to do with dipole?

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JinMTVT

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#1
I would like to have links of more information
about the thiele smalle parameters definitions and such

i know that it has been discussed over and over
but i can't seem to find any complete reference to the parameters with the search function!!

Then,
i hear everybody talking about the importance of Qts
when choosing a driver or Dipole operation ..
but then i find a website telling us how to calculate Qms and Qts from electrical/mathematical pov..

so i don't understand at all why Qts has an important to driver operation in OB mode ..please explain

Also is there any "list" wich we can read that states what kind of parameters we should be looking for in drivers?

IS there any information about the driver's quality in the parameters?

thanks all again for your time
😀
 
#3
let me know...

if you find any good breakdown of how the T/S data all interacts. This i really important to allow you to choose a suitable driver. Would not mind you double posting such info on the ripole/dipole driver thread as well. Regards Moray James.
PS: as with any box speaker a driver with a higher Qts (above about 0.4) will give you a bit of extra hump in the response at roll off, so 0.707 yields the smoothest and most extended roll off and higher Qts bumps the response even more. This is a good thing for OB and dipole designs if you do not want to include any bass EQ to equalize the roll off caused by front to back cancelation. If you want to keep the box as small as possible then lower Qts drivers with EQ are the way to go it would seem. But I am not the guy to ask.
 
#4
Hi,

A High Qts driver (say 2) can extend the bandwidth of an open baffle.

This is common in older equipment, (and open backed guitar
conmbo's), the bass peak compensates for some of the baffle
roll-off, but bass falls sharply below Fs.

The technique is also used in the Quad ESLs, they have a Qtc ~ 3.
Which is why bass power handling is very limited.

Low Qts drivers really on the box to raise it to a sensible Qtc.

Now a lot of large cheap drivers have high Qts (e.g. 1.2) which means
they will have higher Qtc in a box and will boom whatever you do.

However they will work very well on a open baffle.

🙂/sreten.
 
#5
Every dipole speaker has a 6 dB/oct. dipole drop-off. Where it drops depends on the baffle size. You can compensate for this roll-off, if your driver has a rising frequency response in the same area. The price you pay is a much steeper drop-off to the lowest frequencies.
Maybe a picture can help you understand how different qts work. On the horizontal axis 1.00 is the fr of the driver, 2.00 is 2xfr, 0.5 is 1/2xfr and so on:
 

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#6
so basically
the only point of getting a higher QTS driver for dipole
is $$$$...

since one would be better using a higher quality driver with a lower Qts and use a filter/EQ to play with the response of the system .and still have a better low/low end ??


What does Qts basically reflects?
the quality of the craftmanship of the driver?
quality of its electrical properties?

i don't quite get it yet on how it is related to the output of the driver and the quality

we often hear about that cheaper drivers have a higher Qts

so is a good 12" or 15" with a .25 or .30 Qts of higher quality than a driver witha .75Qts ?
 
#7
Hi,

for a given cone mass etc high Qts implies low efficiency,
i.e. low Qts indicates a strong motor / cone mass ratio.

But if you put a low Qts driver on open baffle bass efficiency will be
poor (and bass power handling high) due to the overdamped bass,
therefore you need loads of bass boost and a powerful amplifier.

It depends on the application which approach is best. With open
baffles and low bass distortion at high excursion is a major issue.

But then two 15" are a cheap route to 30hz open baffle bass
without amp EQ, wired as a 0.5 way to extend bass one octave :

Typical spec with 2 cheapo 15" polypropylene drivers :

Qts = 0.92, Fs = 30Hz, sensitivity = 95dB - baffle loss (4 ohm).

Note that Q=0.9 only gives a mild bump of less a dB.

🙂/sreten.
 
#8
so basically
the only point of getting a higher QTS driver for dipole
is $$$$...
Click to expand...

No, it really is the type of application which dictates the prefered driver Qts. The Qts value for a driver is really a definition of the damping inherent in the driver at resonance and determiens the low frequency roll off "shape". There are two sources of damping, the mechanical damping Qms and the electrical damping Qes. Adding these two damping terms provides the Qts value.

Qts = 1/(1/Qms + 1/Qes)

The Qts is really an indication of the low frequency extension of the driver around resonance. Looking above at Rudolf's curves you can see that a low Qts will have a rolled off response near resonance while a high Qts will have a peaking response. The maximum flat low frequency response has a Qts of 0.707.

If you were building a closed box speaker and wanted the volume to be small you might select a driver with a Qts of 0.2. Then by adding a box to the back of the driver, you are putting an air spring in parallel with the drivers suspension and raising the system Q to 0.707 a maximally flat alignment. Also recognize that you are raising the resonant frequency of the system. If you tried to build a clsed box with a high Qts driver the result would be a grossly underdamped system with a boomy ringing one note bass sound. The type of cabinet and the driver Qts are strongly linked to the optimum system performance.

For an OB application, there is no box so the Qts and fs of the driver dictate the low frequency response. If you use a low Qts driver you will have a rapidly rolling off bass resposne due to the shape of the bass for a low Qts driver and the low frequency roll off of the open baffle itself. You can adjust this response with a filter to boost the low end or a resistor to drop the midrange both of which force the driver to behave more like a higher Qts driver.


since one would be better using a higher quality driver with a lower Qts and use a filter/EQ to play with the response of the system .and still have a better low/low end ??
Click to expand...

The build quality of a driver and the Qts are not related. Either a low Qts driver/filter or a higher Qts driver will work on an OB. I have done both and prefer the second approach.


What does Qts basically reflects?
the quality of the craftmanship of the driver?
quality of its electrical properties?
Click to expand...

The damping property of the driver around resonance, it has nothing to do with craftmanship or quality.


i don't quite get it yet on how it is related to the output of the driver and the quality

we often hear about that cheaper drivers have a higher Qts

so is a good 12" or 15" with a .25 or .30 Qts of higher quality than a driver witha .75Qts ?
Click to expand...

A high Qts driver will tend to be less expensive only because it does not require as big a magnet. Magnets are heavy and expensive. Cheap drivers are cheap drivers, Qts has nothing to do with quality. If you look you can find cheap crappy drivers for any value of Qts.

Bottom line. If you are going to build an OB system with a SS amp I would recommend looking for a quality driver with a Qts of about 1.0 for the bass section. If you are going to build an OB system with a tube amp, look for a quality driver with a slightly lower Qts between 0.7 and 0.8.
 
#9
Linkwitz seems to fly in the face of what most people on here say:

Q34 - What is the optimum Qts for the drivers of a dipole woofer?

A34 - The low frequency roll-off of a woofer and its associated group delay are optimal, from what I have observed, when they follow the response of a 2nd order highpass filter with Q = 0.5. When a driver is mounted in a dipole W-frame or H-frame its mechanical resonance frequency Fs decreases to Fd, due to air mass loading, and Qts increases by a similar percentage to Qtd.
For example, a driver with very strong motor, Fs = 18 Hz and Qts = 0.2 might have Fd = 16 Hz and Qtd = 0.22 as determined from an impedance measurement of the baffle mounted driver. With Qtd < 0.5 the low frequency behavior of the woofer is characterized in the complex s-plane by real axis poles at -69 Hz and -3.7 Hz and by 3 zeros at the origin. One of these zeros is due to the front-to-back dipole cancellation with its 6 dB/oct low frequency roll-off. The frequency response of this 3rd order acoustic highpass filter must be equalized to obtain a flat response. A suitable target response could be Fd = 20 Hz and Qtb = 0.5. It is easily realized with two shelving lowpass filters. The first filter with a pole at 20 Hz and a zero at 69 Hz corrects for the low Qts of the driver. The second filter with a pole at 20 Hz and a zero at 400 Hz compensates the 6 dB/oct roll-off due to dipole cancellation. The 3.7 Hz pole is low enough in frequency so that the response is dominated by the 2nd order roll-off below 20 Hz. An advantage of a low Qts driver is the ease with which it can be equalized for an optimum response with Q = 0.5.
A driver with a smaller motor might give Qtd = 0.7 and Fd = 20 Hz, which leads to a pair of complex poles in the s-plane. This can be readily changed to a 3rd order Bessel highpass response by using a shelving lowpass filter with a pole at 20 Hz for the necessary dipole roll-off compensation.
Likewise, if Qtd = 1 and Fd = 20 Hz, then an additional pole at 20 Hz, from the dipole equalization, leads to a 3rd order Butterworth acoustic highpass response. Third order filters introduce more group delay than 2nd order ones. It is therefore advantageous to use Qts < 0.5 drivers, even when they require driver roll-off equalization in addition to the normal 6 dB/oct dipole correction.
Click to expand...
 
#10
The effect of Qts on the OB response is somthing like this:

An externally hosted image should be here but it was not working when we last tested it.


Qts values: Red - 0.3, 0.5, 1, 2, 3 - Blue. Obviously a Qts of between 1 to 2 is suitable for this OB, and this is much higher than what typically is considered good for closed or bass-reflex boxes (~0.2-0.5). The resonant behaviour of higher-Q drivers compensates the bass drop that is inherent in the OB principle.

Edit:

If the Qts value change is caused by a Bl change, the sensitivity will also change and result in these curves (which have the same shapes, but different levels):

An externally hosted image should be here but it was not working when we last tested it.
 
#15
Thanks for all the good answers and good information here. There is a lot to learn.

Also thank you to Rudolf for his nice OB site (in German) and to Brain Steele for his site and nice Excel spreadsheet.

Interesting that Linkwitz takes the low Qts approach, while Martin King likes the high Qts version. The high Q version looks simpler to implement.

Please keep the good info flowing.

BTW, Svante, I downloaded Basta a long time ago - but never used it! Time to try some OB sims in Basta.
 
#16
Thanks all for the seriously nice information! 🙂
very educative!!!

Ok

let's get it on now 🙂

So QTS isn't related to quality, but rather to the bahavior of the driver system
( magnet power VS cone/resistance VS response.. ??)


So then, please explain how to recognise and choose QUALITY DRIVERS ???

Without diminutive comparison, i would never buy
CHEAP drivers that aren't of the top quality i can afford,
since i am doing audio projects because i wanna upgrade or i am looking for the highest quality possible.

So how can i judge the quality of a specific driver by T&S ????

And where did you guys see a 1.0 Qts driver?

all i can see have from .25 to .60 Qts

What are the other important T&S parameters when choosing drivers ??

I was comparing 2174 at McCauley website
( the magnificnent 18" )
and some drivers at www.tr.ca

let's say that i wanna buy drivers from Belisle Acc.
( wich is located 30 mins from my place 😛 )

http://www.tr.ca/2006/english/products/loudspeakers/loudspeakers.htm

looking here
browsing for a Ripole sub in the 12-15" region
how do i chose what i need?

that is the problem i have with T&S parameters,
i think that we need to understand a lot to be able to even determine suitable drivers to consider
 
#18
Thanks Svante. I worked for a company that supplied a European manufacturer and everything had to be in metric units. Confusing at first but eventually we all got the hang of it. We used m, cm, and mostly mm. I had not seen dm before. The only metric unit that I never understood was when they sent me a specification containing kgf (kilograms force) which blew my mind.
 
#19
So how can i judge the quality of a specific driver by T&S ????

And where did you guys see a 1.0 Qts driver?

all i can see have from .25 to .60 Qts

What are the other important T&S parameters when choosing drivers ??
Click to expand...

You cannot tell anything about how a driver will sound from T/S parameters. The only measure of quality is how close the T/S parameters will be for any two units. I once bought some close out drivers from Radio Shack for $5 each. The drivers were not real high performance units but the T/S parameters of the 12 measured were extremely consistent indicating a solid and controlled manufacturing process. When you get the drivers remeasure the T/S parameters yourself, they probably will not be the same as the data sheet. This will also help when optimizing your design.

It is very difficult selecting drivers when they all look the same on paper and have only slight variations in the T/S parameters, some manufacturers have 4 or 5 competing units in a given diameter. It can be paralyzing trying to wade throught the data and pick the "best" one. At some point you just have to make a decision based on what you read on the forums and just gut instinct. Otherwise you never make a move. Chances are if it is a reputable manufacturer and you desing the system correctly it will work well.

Eminence makes a number of 15" woofers with different Qts values that are good for open baffle. I use the Alpha 15A (Qts ~ 1.1) but others have reported good reaults with the Beta which has a lower Qts. I have been very impressed with the quality of the two different Eminence drivers I have purchased, the Alpha 15A is about $55 each which in my opinion is a steal. You could spend a lot more but i am not sure what you will be getting that justifies the increased cost.
 
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