Audax HT300G0 - compliance 3x too low - how long a break-in needed?

[capslock]

Quick and dirty measurement of static compliance on the new driver yielded about 0.62 mm/N. Then I ran the driver for about 12 h at 30 Hz with about +/- 7.5 mm excursion.

The displacement for various weights can be seen below. The suspension is extremely linear over a wide range, but the compliance is about 0.58 mm/N.

Audax.fr lists the data sheet on their server, but the link does not work. Partsexpress have a data sheet, they state Vas = 77.7 l. Sd is given with 520 cm² in the data sheet for the HT300Z4 (which shares the same basket) from audax.de.

Using the formula Vas = C * Sd^2 * (rho * c_sound^2), and the brackets being equal to 0.141e-6, I get 22.1 l.

In other words, the compliance is 3.5x stiffer than it should be.
- Is the formula wrong?
- Would a MUCH longer break-in reduce stiffness that much?

By the way, the motor force is within 14% of data sheet value.

[FrankDIY]

When I break drivers, I measure them after 24h, 48h, etc and I usually see the parameters stabilizing after 100 hours of breakin.

14% of difference between a measured parameter and the official spec is somewhat "normal"

[daatkins]

Did you measure the value of Vas yourself? I recently measured the TS params of a pair of Audax HT080G0 3" mid/woofs that I've been using for 3 years (ie fully run-in!) and the measured Vas figure is significantly lower than spec:

Driver #1
Fs = 102Hz
Vas =1.07 litres
Qts = 0.61
Qes = 0.77
Qms = 3.06

Driver#2
Fs = 102Hz
Vas = 1.09 litres
Qts = 0.64
Qes = 0.81
Qms = 3.11

Official Audax Spec:
Fs = 80Hz
Vas = 2.0 litres
Qts = 0.53
Qes = 0.7
Qms = 1.96

Nice one,
David.

[capslock]

Francois,

I consider 14% perfectly normal and included this result only to show that my measured compliance value, when combined with my mm/A measurement, yields something that makes good sense.

It is the factor of 3.5 that I am worried about. Have you ever seen that much of a change after break-in?


David,

I did not measure Vas directly, rather I used it as a point of reference because Audax does not specify compliance. What you have observed is similar to my problem.

What amazes me: assuming the got the moving mass roughly right on the initial qualification and are able to keep it reasonably constant from batch to batch , wouldn't you expect the resonance frequency to be almost twice the specified value? After all, having only 0.54x the specified Vas means 1.85x the spring stiffness and for the same mass 1.85x the resonance frequency.

However, your Qms being also 1.57x times too high might just indicate lower frictive losses in the suspension or lower eddy current losses in the VC former, but it might also mean that the reported m oving mass was too small.

1.85/1.57 = 1.175 is actually pretty consistent with the 1.275 factor in resonance frequency you see, indicating they got both Vas and m wrong.

Regards,

Eric

[capslock]

David,

it just occurs to me that my force factor is 14% too low. Your Qes is too high by a similar percentage which indicates your motor is also weaker than the data sheet says it should be.

Isn't it amazing how similar our findings are?

Greetings,

Eric

[daatkins]

Eric,
Here's some more results!

I've just measured a pair of Audax AP100Z0 4" aerogel cone woofers (I'm building a surround system for the PC: 4" aerogel + tweet for the front and the HT080G0 with no tweets for the rear).

Driver #1:
Fs = 93.4Hz
Vas = 2.06 L
Qts = 0.63
Qes = 0.81
Qms = 2.82

Driver #2:
Fs = 91.2Hz
Vas = 2.14 L
Qts = 0.64
Qes = 0.83
Qms = 2.86

Audax spec:
Fs = 64Hz
Vas = 4.72 L
Qts = 0.49
Qes = 0.63
Qms = 2.16

These drivers are not run-in (as in the parcel arrived an hour ago!) but I'm now wondering if my measurement method is flawed.... I use the added mass method (7g in this case) at about 0.5 watt input with Speaker Workshop doing the maths.

I'll run the drivers in overnight and repost more figures tomorrow.

Nice one,
David.

[capslock]

Got some more, too. Still fighting with my computer to get speaker workshop (or anything else, for that matter) to run again. Hooked the HT300 up to the amp for some more break in but this time left the paper pointer in place to set the excursion to +/- 9 mm.

Turns out the resonance frequency must be somewhere between 20 and 25 Hz, probably 22 Hz as excursion is slightly larger on the 20 Hz track of the CD. For higher frequencies, excursion decreases. Will have to check with a scope that the CD booklet indeed does give correct frequencies. If so, this is strange as data sheet claims 36.8 Hz. With compliance 3.5x too high, mass would have to be on the order of 400 g rather than the stated 83 g to explain the resonance frequency. This is hard to believe seeing how flimsy it is.


Good night!

Eric