Measuring a Tweeter

[mikee55]

measuring a tweeter

Hello, I just learnt, with the help of richie00boy, how to measure a woofer. I now want to make a small pair of bookshelves to go on my pc. I will be using an Audax 6" fibreglass driver, which I have measured and I want to use a tweeter which its T?S parameters are unknown. With a 2 way passive in mind, what do I need to measure on the tweeter, to mate it to the mid/bass unit.

Cheers Mike

 

[ocool_15]

The rear chamber is sealed on tweeters your likely to look at and likely t/s specs are not listed. A frequency response graph and an impedence sweep would be a good start. The lowest crossover for a tweeter is normally stated as 2*fs or sometimes 3*fs.

 

[mikee55]

To find Fs

Hi ocool_15, is it a simple matter of finding Fs? And would I use the same technique that I would of used for a woofer?
I used the technique from Readresearch (richie00boy)
And then use that in a crossover calculator?

Sorry for all the questions

Cheers Mike

 

[ocool_15]

Finding fs is the same for a woofer or tweeter, I just use a series resistance and measure the voltage drops. The peak voltage drop on the tweeter is then the Fs. A graph like below showing the impedence and frequency response will give you a good start.

You can use a crossover calculator but may find some experimentation is needed. With the woofer especially, make sure you are using the impedence at the crossover point or compensate for the voice coil inductance with a zobel.

Example Dayton Tweeter

 

[mikee55]

Doh, confused again

Hi ocool_15,

I'm confused again, can you explain this a little clearer. I have to admit because its worded different,( no offence) its harder to understand. I wire up my system like this

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

With the woofer especially, make sure you are using the impedence at the crossover point or compensate for the voice coil inductance with a zobel.

Can you explain this any easier.

Cheers Mike.

 

[richie00boy]

Use the same technique Mike. Start at 3kHz and sweep roughly to get you in the ball park, then you can sweep with finer resolution once you have honed in on a smaller area.

 

[mikee55]

bitin my nails

Hi Rich, I've been worried in case you tell me off for using your picture. I'll set up tommorrow and let you know what I find.

Cheers

Mike.

 

[richie00boy]

No worries Mike! You are more than welcome and thanks for the acknowledgement where you got it from

 

[mikee55]

You're Welcome

Noobs click this way.

http://www.readresearch.co.uk/

 

[mikee55]

Hello again, I have used my tools and found that I have

Fs=1146.9hz
Re=6.9 ohms

What else do I need to find out to implement this into my design?

Cheers Mike

 

[adason]

sensitivity...

 

[mikee55]

Bloomin hell, SPL?

Hello adason, thanks for replying. Do I need an spl meter to do that, or do I need more T/S parameters and another sum? Of course spl being a T/S parameter also.

Cor it ain't easy is it?

Cheers Mike

 

[tade]

if you are matching drivers all you need is a reference spl, not an absolute one, which means calibration is unnecessary. Measure your woofer. If it measures 80db at one volume level at the crossover, so should the tweeter. If the tweeter measures 86db, pad it by six db.

 

[adason]

exactly...

 

[ocool_15]

Re: mikee55

Since a speakers impedence will vary with frequency the values of the components will also vary.

Given a hypothetical speaker
8 ohms nominal
6 ohms dcr
voice coil inductance of .3mH

If you calculate the value for a 6dB/oct low pass on a 8 ohm speaker at 3200hz you will get a .398mH inductor value.

If that same woofer has a inductance of .3mH the impedence will be approximately = speaker dcr + 6.032 ohms at 3200hz. A 12.032 ohm load will actually require about a .598mH inductor, not the .398mH value calculated above.

That is part of the reason why you may want to implement a zobel as detailed below.

Here is some information on a zobel. Note the 2 calculators will provide different answers.
Definition
Zobel Calculator

Better Definition and formula

Your method for finding impedence may vary from mine but thats ok as long as you get the same result.

 

[mikee55]

Thanks ocool_15

Hello, How can I find the Le parameter?

Cheers mike

 

[mikee55]

Hang on!

Hi there, I'm still in the early stages and just found Fs of the tweeter. I'm now looking at a Zobel network for my Audax HT170FO fibreglass 6" woofer, per other thread.I seem to have 2 going, sorry.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=100569

Please still continue to help, thanks Mike

 

[Vikash]

I'm not impressed that you have two threads with the same title running. I've merged both together.

 

[mikee55]

Thanks

Thanks Vikash, I forgot I had the other going, I'm getting bogged down with not knowing what I'm doing. My appoligies.

Mikee55

 

[mikee55]

Oops, I get told off

I've been asking how to Measure a Tweeter with respect to designing a passive cross over, Knowing that going active is audibly better, whats involved with the crossover calculations now?
Does it become less of a hassle, or do I still have to compensate with a zobel, etc?

Cheers, Mike