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Old 28th June 2013, 10:35 PM   #11
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I've been away for a while and it's nice to see that my experiment sparked up some discussion.

Here is a picture of the test setup.
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Old 29th June 2013, 01:04 AM   #12
4Torr is offline 4Torr  United States
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Originally Posted by d1030180 View Post
I've been away for a while and it's nice to see that my experiment sparked up some discussion.

Here is a picture of the test setup.
Niiiice ! The plywood is so lousy here in California.
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Old 29th June 2013, 07:41 PM   #13
4Torr is offline 4Torr  United States
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Sorry, I forgot to mention that the hole on the back side panel is for the connection terminal. So it stays always closed.

Unfortunately I don't have any way to measure the frequency response of the system. At the university where I am studying and working we have a couple of Rion NL-20 and NL-22 sound pressure level meters. Those are used for measuring the SPL levels of concerts. Could those be used for hi-fi measurements too?

Rion Sound Level Meter NL-20 - Rion Sound Level Meter NL-20 Exporter, Importer, Manufacturer, Service Provider, Distributor, Supplier, Trading Company, Mumbai, India
Yes, you can test the frequency response with a sine wave generator and close miking the driver a few centimeters away. This technique should be accurate below 400 hz or so and you can compare curves if you use the same reference level in the high midbass. Better yet, set your reference with a voltmeter at the input terminals at a given frequency. The results should be very interesting.
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Old 29th June 2013, 09:10 PM   #14
4Torr is offline 4Torr  United States
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Yes, you can test the frequency response with a sine wave generator and close miking the driver a few centimeters away. This technique should be accurate below 400 hz or so and you can compare curves if you use the same reference level in the high midbass. Better yet, set your reference with a voltmeter at the input terminals at a given frequency. The results should be very interesting.
I forgot to add that you will need to put a low output impedance amplifier between the signal generator and the driver.
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Old 29th June 2013, 11:36 PM   #15
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I also tried placing stuffed 50x200mm and unstuffed 50x400mm cardboard tubes into the divider. It moved the impedance peak to a lower frequency, around 46-47 Hz (23 Ω) for the 200mm tube and 45 Hz (22 Ω) for the 400mm one.
It it beneficial anyway to lower the system resonance frequency in this way?
Actually inserting the cardboard tubes raises or increases the system resonance frequency, if you consider the type of system that you have as being a type of modified vented system. The Helmholtz resonance frequency is a frequency somewhere in the valley between the pair of impedance peaks. So with the 50 X 200mm tube without wadding in place, Fb equals about 74 Hz. With the 50 X 400mm tube without wadding in place, Fb equals about 70 Hz.

As in your system the vent is subtracting from the acoustic output power of the system, it may be that frequency response won't drop off until below the frequency of the lower impedance peak, but I don't think that that is a foregone conclusion.

With the solid wooden divider in place, what is Qtc of the sealed system? If that Qtc is fairly high, say around 1.5, I think that you might have something quite good, but I think the fairly high Qtc is pretty necessary.

Anyway, if you can make some frequency response measurements, those would be interesting to see.

Regards,
Pete
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Old 30th June 2013, 04:28 PM   #16
gedlee is offline gedlee  United States
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This might be a good learning experience, but it does not hold much hope as a "new" enclosure type that has any advantages. At best it can be used to damp the woofer (damping material and a larger magnet are much more efficient ways to do this.) but at worst in will put a big hole in the response. Bose has used this idea for years, but eventually even they just eliminated it.
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Old 1st July 2013, 05:53 PM   #17
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This might be a good learning experience, but it does not hold much hope as a "new" enclosure type that has any advantages. At best it can be used to damp the woofer (damping material and a larger magnet are much more efficient ways to do this.) but at worst in will put a big hole in the response. Bose has used this idea for years, but eventually even they just eliminated it.
Same conclusion I was thinking. Sims showed increased BL and or isobarik configurations worked better at dampening the extreme low end andor dampening material for midbass/midrange.
Have some 6.5" drivers that if AS would be a 0.5cu' and if BR 1.5cu' where I was running sims, Both are optimal. Having read something eons ago about placing a restrictor plate @ 1/3 volume. Think it was some old AR design from back in the 70's.
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Old 11th July 2013, 08:11 PM   #18
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I finished the first frequency response measurement. This one is for the 20x4 cm cardboard tube stuffed lightly with poly wadding from both ends. The measurement device was set to measure ten seconds and the microphone was about 5 cm away from the woofer cone. Weighting was set to FLAT (other options were A-weighting and C-weighting).
I used sine wave for measurements, 2 Hz increments from 10..100 Hz. And 10 Hz increments from 100..120. Hz

Unfortunately I didn't know how to set the logarithmic Y-axis 'tick marks' to the f/Leq chart in Libreoffice so it's a bit difficult to use.

I still need to measure the "non-ported" version for reference.
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Old 12th July 2013, 02:34 AM   #19
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Originally Posted by d1030180 View Post
I finished the first frequency response measurement. This one is for the 20x4 cm cardboard tube stuffed lightly with poly wadding from both ends. The measurement device was set to measure ten seconds and the microphone was about 5 cm away from the woofer cone. Weighting was set to FLAT (other options were A-weighting and C-weighting).
I used sine wave for measurements, 2 Hz increments from 10..100 Hz. And 10 Hz increments from 100..120. Hz

Unfortunately I didn't know how to set the logarithmic Y-axis 'tick marks' to the f/Leq chart in Libreoffice so it's a bit difficult to use.

I still need to measure the "non-ported" version for reference.
If you have the woofer radiating a continuous sine wave, then the microphone must be placed as close as possible to the cone (dust cover) of the woofer. If not, then presumably your measurements will be incorrect. If interested, see the article by D.B. Keele, Jr., "Low-Frequency Loudspeaker Assessment by Nearfield Sound-Pressure Measurement", April 1974 in the Journal of the Audio Engineering Society.

Maybe you could try placing the microphone about 3 mm away from the cone and see if you get different measurements.

The reference sound pressure level would be at about 200 Hz.
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Old 12th July 2013, 07:25 PM   #20
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Thanks for your advice. This time I placed the microphone exactly 3 mm away from the dust cup and set the sine wave volume in such a way that the dB meter reading was exactly 100 dB at 200 Hz frequency.

The first chart shows the frequency response in a cabinet with a 40x4 cm cardboard tube that was lightly stuffed from one end.
The second chart shows the frequency response in a 34L sealed cabinet.

The data shows decreased sound pressure level around 44-88 Hz area in the "vented" cabinet, while there seems to be a very slight level increase in the 14-34 Hz area. 100-120 Hz output is also higher in the vented cabinet.
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File Type: png FR_34L_sealed.png (91.8 KB, 133 views)
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