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ggidzinski 8th February 2019 12:07 AM

Measuring driver parameters vs the data sheet values
I'm making the transition from building established designs to starting from scratch. I'm currently working on a low cost full range/neo tweeter design for a computer (learning at a low cost point) and I just measured the free air parameters of a Fountek FR88EX full range driver using DATS2. I calibrated the leads and the 1K ohm resistor measurement and was careful with the weighted VAS measurement. I am surprised at how different my measured numbers are vs. the data sheet. Is this typical or have I likely done something wrong?:




wintermute 8th February 2019 12:26 AM

Hi George, It is worth having a look on Fountek's site to see if they have any information on how they do their measurements. Not all manufacturers do it the same way. SB-Accoustics, and Seas both have information on their sites about their testing method. I found out recently that Seas uses 2V for their drive signal which is much higher than what something like DATS uses I believe.

break-in and even pre-measurement warm up procedures may affect the results.

Does the DAT's use MLS signals or swept sine for doing the measurement? I found that some (and only some) drivers give widely different results with mls compared to swept sine. Other drivers will give almost identical results with either.

Your fs above is the biggest discrepancy, break in may be affecting this, but fs was the thing that I found with some drivers was way off with MLS testing.


hollowboy 8th February 2019 01:21 AM

Check both your drivers. It might be that you've been unlucky and have a faulty one.

I once got four "full range" drivers from P Audio, and found that 3 were nicely on spec, but one had a weak magnet.

Midrange (cone driver) horns; low relevance of magnet strength and Qts

ggidzinski 8th February 2019 01:28 AM

I just checked the other driver and the numbers are similar. Fs is even off more at 144Hz.


wintermute 9th February 2019 02:04 AM

Try running them at the manufacturers stated fs at close to max excursion for 5 minutes before doing your T/S measurement and see whether you get any difference.


ggidzinski 9th February 2019 02:40 AM

Is that a one time break in step or an every time warm up step? Just a sine wave at Fs?


wintermute 9th February 2019 04:06 AM

Hmm I was thinking that I picked that up from SB accoustics tech note and that it was to be done before each measurement.. however it seems maybe they are suggesting that it will actually break in the driver (I thought it would take longer than that).

They do state it should only make 10-15% differnce, which seems in your case to not be enough to account for the differences.

This is the link to Seas info on their T/S measurements DATASHEETS

As you can see Seas and SB are using different drive levels (which will definitely give different results). a woofer measured at 1V will almost certainly show different T/S params to one measured at 2V.

I guess the other thing I should ask is what your mounting method is for the driver?

Clamped securely (in some sort of frame) in free air away from any nearby objects is ideal. Don't sit the driver on it's magnet, especially if it has a vented pole piece.


DRONE7 9th February 2019 04:22 AM

Life is a lottery... Specs are Puffery.... ;) Work with what you have..... measure over several days and variables=driven/ambient/up/down/flying around those magnificent men in... whoa...!!!
Then average and build... Don't let the math get in the way of the enjoyment... :)

Entropy rules... Devices 10 years on a shelf will not measure as manufacturers specs... work with it

planet10 9th February 2019 04:47 AM

I hav eposted this before but can’t find where:


T/S parameters, extremely useful though they are, are not perfect: they are a snapshot of a driver's behaviour under a given set of circumstances; it does not necessarily follow that these hold good under other conditions (they rarely do), so it is advisable to make allowances for these variations in the design stages.
Most people look at T/S and see a set of scalar numbers, but in reality they are a function of current (or voltage) -- they change with weather too. If one does not take care to account for these changes you end up with something that doesn't work all that well. For instance a typical tightly tuned bass reflex, with a driver with a non-flat T/S curve, will only be correctly tuned for one drive level. Not only does this mean that they will be in tune at only one volume level, given the dynamics in music, they will only be spot on at one level in the music and be progressively out of tune with changes in the dynamic levels.

You have no doubt noticed that Scott & my designs favour fairly high ratio vent aspect ratios (or volume fill damping). This adds an R to the vent and, analogous to a grid-stopper, and gives them a greater tolerance to non-flat T/S curves. Nothing models this, and requires an experienced eye to extropolate from what is modeled.

These are the numbers i measured:
Re: 6.17
Fs: 107.14
Qes: 0.90
Qms: 3.12
Qts: 0.70
Vas: 1.23
dB/2.83v: 85.1
Sd: 2762
BL: 3.00
Mms: 1.95
Cms: 1130

We used the factory numbers to design the Fountek Five and that worked out well.


TBTL 9th February 2019 06:58 AM

Qes, Qms, fs, Vas are related to the suspension stiffness which is not constant. Try comparing BL, Re, mms, sensitivity if you want to know if the values stated in the data sheet are fair.

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