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-   -   Tweeter Qms (http://www.diyaudio.com/forums/full-range/223107-tweeter-qms.html)

smigza 8th November 2012 07:14 AM

Tweeter Qms
 
Hi All,

This is my first post on DIYAudio - so please don't be nasty :o . Just have a question about choosing a tweeter for a project of mine

At the moment I have three candidates, which are:

SB Acoustics SB26STCN - Qms= 1.6, Qe=1.77, Qt=0.84
VIFA bc25sc06-04 - Qms= 3.45 Qe=1.76, Qt=1.17
Tang Band 28-1177SB 1 - Thiele Parameters not given

Looking at the Q factors, I can make the deduction that the SB Acoustics tweeter has a better response time than the Vifa, but the Vifa has a better dynamic range. I would like to know if the better dynamic range in the vifa tweeter would be outweighed by the improvied transient response in the sbacoustics tweeter.

For further discussion, which of these tweeters would you choose and why?

Also, can anyone point out where I can find tangband thiele parameters. I have tried their website and emailed them with no luck. The website just has N/A listed against all of the q factors for all of their tweeters lol!

Thanks :cool:

Steve

chris661 8th November 2012 12:57 PM

Quote:

Originally Posted by smigza (Post 3233091)

Looking at the Q factors, I can make the deduction that the SB Acoustics tweeter has a better response time than the Vifa, but the Vifa has a better dynamic range.

I'm curious: how did you deduce this?

Tweeters generally shouldn't be operated near Fs, so the Q factors (which describe how the fundamental resonance is damped) are largely irrelevant, so far as I can see.

What's the project?

Chris

smigza 9th November 2012 03:16 AM

Hi Chris,

Thanks for responding. The project is some bookshelf monitors which will be driven by a gainclone that uses a logitech squeezebox as its source.

I read an article online, which said that tweeters which high mechanical Q generally have more flexible diaphraghm, and therefore create have a better dynamic range as it is easier for the driver to move the diaphraghm. The Q can be used to predict the tweeter's impulse response. Generally systems with a low Q have a faster impulse response as damping factor =1/2Q, eg higher damping, which explains less energy stored per cycle as Q is smaller.

So you can see here that a high Qms will give a good dynamic range but a slow transient response.

However, if you have a low Qe, this means that the driver is fast, and can therefore improve the transient response of the tweeter.

Overall since both Q paramaters combine to give a Qt, a high Qm and low Qe should give a reasonable Qt as 1/Qt=1/Qe + 1/Qm, hence my deduction.

So by looking at the thiele paramters I posted, I was trying to explain that it looks like the vifa with higher Qm and higher Qe will have a more flexible diapgraghm but slower motor, and the SB Acoustics with lower Qm and Qe will have less dynamic range buy a faster motor, as reflected in the Qt values as well.

Please correct me if my logic is wrong, I'm an electrical dude not mechanical and am making those deductions on my knowlede of a system's transient response.

Anyway, looking at the Q values would you expect much dynamic range and transient performance difference between the vifa and the Sb acoustics tweeters?

Also Tangband got back to me and gave me the following values for those who are interested: Qe=2.64, Qm=2.38, Qt=1.25

Thanks,

Steve

fastbike1 10th November 2012 01:34 PM

Of the things to consider for speaker design, tweeter qms is none of them. Think about Chris' answer a bit more.

Speaker design can't be reduced to consideration of a single parameter for a driver. Another thing you haven't considered is whether any of these differences can be heard. The room will dominate your sound.

smigza 12th November 2012 12:09 PM

I have used a number of parameters to find a candidate tweeter, those being distortion, frequency response, Qe, Qt and CSD.

Both tweeters have similar Qe, CSD, distortion and frequency response. The SB Acoustics has a better Qt as it is closer to 0.7 (slightly underdamped at 0.8 ish)

If room response dominates the sonic property of a tweeter, then what parameters would a designer use to choose a speaker? Can you give me an answer, as I honestly don't know.

At the end of the day, I'm really after advice as to which one should be the better option. If the Thiele parameters have no effect on sound than I would clearly choose the cheaper driver as all other properties are equal. However, from all of the information I have read, the authors generally say that Thiele parameters are used to describe a speaker's characteristics, I'm guessing these are its sonic characteristics (impulse response characteristics.) Since both tweeters have a different Qt, this surely must describe how they sound different to some degree.

chris661 12th November 2012 02:45 PM

The balance for dome tweeters seems to be as follows:

find a small enough one so that its dispersion remains wide until >10kHz (for example)
while ensuring its big enough to have plenty of power handling around the crossover frequency.

A 1" dome will handle lower XOs or louder volumes than a smaller dome, at the expense of dispersion. Pick your poison.
I'd say it'll depend on which driver you're trying to mate the tweeter to - bigger drivers will need lower XO points.


Tweeters are generally (and with good reason - power handling) operated at least one octaves above their resonance. Some really good tweeters will handle <1 octave, but most will complain and/or melt.
The Q factors describe the driver's properties at resonance.

With a 12dB/octave slope (about the minimum slope I'd use a tweeter with), the resonant frequency is 12dB down, which is 1/16th the power of the rest of the range.

With this in mind, do you think that the tweeters resonant properties are that important?

Chris

smigza 13th November 2012 12:46 PM

Makes a lot of sense. The SB Acoustics seems to have much better dispersion performance than the VIFA, and is a lot more easier to crossover, and can be matched more easily to the woofer I have chosen.

So the SB tweeter seems to be the way to go. Thanks for the info


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