2 woofers in single closed box -> metrology -> sound?

[capslock]

Thanks for all the answers, guys. I will redo the test with all connections double-checked, but I doubt there is a problem, after all test #2 looked ok.

Originally posted by GM
[BHmm, if I understand you correctly, this must be a huge cab to get a Qt = 0.23. Also, if the correct voltage divider for a single driver is 7.5ohms, then it seems reasonable to me it should be 3.75ohms for parallel.
[/B]

Yeah, I noticed that one when I went back to my simulation sheet. Fs was where I expected it, Qt should have been around .5 as Rodd pointed out. There is nothing surprising about this low number, though. The fit simply went wrong because of all the spikes.

The reference resistor is simply chosen to be on the same order of magnitude as the impedances I get to measure (which range from 3 to 50 Ohms if the resonance is included). It is perfectly ok for both 4 and 8 Ohm speakers. This arrangement is used by all soundcard based systems I know of.

Regards,

Eric

[capslock]

Ok, here?s a partial explanation:

1) At 48 kHz sample rate, the spikes were less pronounced. Apparently, the input filter of the soundcard is fixed at > 20 kHz. At 6 kHz sample rate, any kind of noise pickup will cause aliasing.

2) When I recalibrated, I sometimes got strange values for the reference resistor. It turned out that inside the rig box, a ground connection of the input attenuator was kind of bad. When it was open, it would pick up dirt from the output leads.

OK, here are some reproducible measurements:

Both drivers in parallel:
50.965 Hz Q 2.388 / 0.896 / 0.652

Q_m is not really as low as the fit suggest. In fact, it is a double peak, consisting of a major peak at about 58 Hz and a slightly weaker one at 43 Hz. So where does this double peak come from?

Each driver measured separately, the other one was shorted (this is in fact a highly damped passive radiator):
top 33.008 Hz 1.745/ 0.661 / 0.479
bottom 34.469 Hz 1.777 / 0.611 / 0.455

The TSP are still in good agreement, albeit slightly less than the free air measurements. Maybe this is because the top driver is about 3 cm away from the top whereas the lower driver has about 20 cm to the bottom.

Measured separately, other driver open (passive radiator):
top 24.000 Hz 9.953 / 1.004 / 0.912
bottom 23.988 Hz 9.559 / 0.964 / 0.876

This is fit is not entirely correct because there is the secondary bass reflex maximum, but agreement between simulated fit curve and measured curve is pretty goog around the main maximum..


OK, now for the remaining questions:
- why do I get a double maximum?
- Will I still have this double maximum when the paralleled drivers are driven from 10 mOhm rather than 7.5 Ohms?
- If not, it is ok not to insert a separator. But how do I determine the Q for the Linkwitz circuit?

[GM]

>The reference resistor is simply chosen to be on the same order of magnitude as the impedances I get to measure (which range from 3 to 50 Ohms if the resonance is included). It is perfectly ok for both 4 and 8 Ohm speakers. This arrangement is used by all soundcard based systems I know of.

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OK, thanks for the info, never used a soundcard based system.

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>- why do I get a double maximum?

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Well, the driver/boundary proximity certainly is an issue. You can sim each driver location in MJK's sealed program to find out approximately how much effect there is.

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>- Will I still have this double maximum when the paralleled drivers are driven from 10 mOhm rather than 7.5 Ohms?

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I don't see why not.

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>- If not, it is ok not to insert a separator. But how do I determine the Q for the Linkwitz circuit?

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Good question, I would be prone to take the average and skip the divider if drivers share the load fairly evenly, but might want to ask 'the man' himself.

GM