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Old 15th May 2008, 08:09 AM   #1
Brownee is offline Brownee  Australia
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Default What’s going on with my port tuning?

Hi

Can anyone give me some advice to a box tuning problem please?

I am building some MTM home theatre speakers using Vifa m13sg-09-16 drivers. I'm trying to put them into 14.5 ltr boxes and want to tune the box to 75hz. I have some 70mm Id ports and the port calcs I get should make the length about 85mm. The problem is if I use this it tunes the box a lot lower. I ended up pulling the port out so I was just left with a hole 80mm dia. and 36 mm deep and still the box is only tuned to 66hz. I have double checked volumes ,calcs and everything I can think of multiple times. What am I missing?

Thanks
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Old 15th May 2008, 09:59 AM   #2
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I had this problem once with a MTM. Sometimes you can get problems with two drivers sharing a volume, especially if there is a high degree of symmetry. You can get unlucky and put the port in a node where the pressure waves from the two woofers cancel, similar to a standing wave. The port doesn't "see" the box. If you are confident your maths is OK, the port dia does look quite big to me, try moving the port location.

Simon
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Old 15th May 2008, 10:05 AM   #3
bjorno is offline bjorno  Sweden
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Hi,

My suggestion is to close the box if you are using two drivers in a 14.5 L box (Qtc=0.718,f-3dB=96Hz)) as the driver Qts is 0.441 and in case of a fairly flat FR ported box you need at least 34 L tuned to 50 Hz (Qt= 0.577,f-3dB about 43 Hz).

b

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Old 15th May 2008, 12:15 PM   #4
sreten is offline sreten  United Kingdom
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Hi,

Shown for two drivers in a box, with specs from here :

http://www.speakerbits.com/net/catal...aspx?id=VF130B

/sreten.
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Old 15th May 2008, 01:47 PM   #5
Brownee is offline Brownee  Australia
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Hi and thanks everyone for their input.



Quote:
Originally posted by bjorno
Hi,

My suggestion is to close the box if you are using two drivers in a 14.5 L box (Qtc=0.718,f-3dB=96Hz)) as the driver Qts is 0.441 and in case of a fairly flat FR ported box you need at least 34 L tuned to 50 Hz (Qt= 0.577,f-3dB about 43 Hz).

b

1(1)
I'm now confused by your post bjorno. 34l

I am new to speaker design but have modeled the drivers in 3 different software packages, multiple times, and with a slight variance always come up with 12 - 16 L depending on alignments. Would it be normal for 2 X 5.5 inch drivers to require 34L? Or am I doing something wrong?

Thanks sreten that model looks more in line with what I am getting. Yes?

Thanks Simon, I think I will try moving the port. There is a shelf brace a little near the port. Could this be interfering?
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Old 15th May 2008, 02:30 PM   #6
bjorno is offline bjorno  Sweden
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Quote:
…I'm now confused by your post bjorno. 34l…
Entering simple data in a simulation program that only relies on data like Qts, Vas and fs is can lead to a design disaster if not previously checking the data consistency.

b

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Old 15th May 2008, 03:59 PM   #7
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Quote:
There is a shelf brace a little near the port. Could this be interfering?
Possibly, if any surface is close enough to the port it will alter the efectiveness of the port a little or a lot, it is hard to predict! Avoid symmetry is my advice.

Simon
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Old 15th May 2008, 04:49 PM   #8
Svante is offline Svante  Sweden
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Default Re: What’s going on with my port tuning?

Quote:
Originally posted by Brownee
Hi

Can anyone give me some advice to a box tuning problem please?

I am building some MTM home theatre speakers using Vifa m13sg-09-16 drivers. I'm trying to put them into 14.5 ltr boxes and want to tune the box to 75hz. I have some 70mm Id ports and the port calcs I get should make the length about 85mm. The problem is if I use this it tunes the box a lot lower. I ended up pulling the port out so I was just left with a hole 80mm dia. and 36 mm deep and still the box is only tuned to 66hz. I have double checked volumes ,calcs and everything I can think of multiple times. What am I missing?

Thanks
Yes, an empty box of 14,5 litres total volume, 70 mm vent diameter, would need a tube length of 81 mm if it is to be tuned to 75 Hz. If the box is filled completely with ideally isothermalizing material, the vent length should be 41 mm. A more realistic amount of box stuffing, corresponding to 40% fill of ideally isothermalizing material results in a port length of 62 mm.

So, this does not explain why you measure 66 Hz with 36 mm port length. So I'll have to look for other things.

How certain are you of the box volume? Is it for both drivers, or is it per driver, ie total 29 litres?

How do you measure the port resonance? Through the impedance curve? Do you connect both drivers doing this, or only one? Both should be connected, in series or parallel does not matter. If it is a 2.5 system bypass the crossover for this measurement, since it feeds slightly different signals to the drivers.

HTH
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Old 15th May 2008, 11:43 PM   #9
Brownee is offline Brownee  Australia
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Hi Svante

.
Quote:
If the box is filled completely with ideally isothermalizing material, the vent length should be 41 mm. A more realistic amount of box stuffing, corresponding to 40% fill of ideally isothermalizing material results in a port length of 62 mm.
The box has 1" of R19 fiberglass on all walls except baffle.


Quote:
How certain are you of the box volume? Is it for both drivers, or is it per driver, ie total 29 litres?
I have checked and re-checked the box volume (doesn't mean it's right though ). The volume is for two drivers, not each. The box internal dimensions are 580 X 139 X 183 which gives 14.75L. Because the baffle is 36mm thick only the driver magnets protrude into the volume and because the holes have a large 22 degree bevel I work out that they don’t take any volume at all.
That leaves the port and small brace to take up .25L.

Quote:
How do you measure the port resonance? Through the impedance curve? Do you connect both drivers doing this, or only one? Both should be connected, in series or parallel does not matter
I used an impedance curve. I have a "Woofer Tester 2" http://www.woofertester.com/wt2product.htm
Both drivers are connected in parallel, no crossover. I ran a sweep and located the min Z between the two peaks. I also checked by placing a sound meter close to a driver and used a sine generator to get the min output freq. Also the Woofer Tester has a measure box feature that gives Fb and box size. All match up
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Old 16th May 2008, 12:42 AM   #10
bjorno is offline bjorno  Sweden
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Hi,

I believe you still are using a too small box for the two Vifa’s M13SG-09-16 and the TS input data to calculate your box is supplied by a careless driver retailer that just copied and transferred some data from the M13SG-09-08 drivers to the 16 Ohm version.

Here is another similar M-T-M design using an internal volume of 0.58 cu. ft.:

http://www.lungster.com/l/speakers/m...mcenter.shtml:

////…As luck would have it, Madisound has a Vifa kit utilizing two M13SG-09-16 and one D25ASG-05. The kit can be purchased complete with all parts and cabinet or as individual pieces. Since I wanted to build the box to suit my taste and decor, I bought the woofers and crossover (I already had the tweeter). Incidentally, the M13SG-09 comes in both an 8 ohm version (-08 suffix) and 16 ohm version (-16 suffix). The 16 ohm version allows two such drivers in parallel to yield an 8 ohm nominal impedance.
The claimed response by Madisound is 40 Hz to 35 KHz vented and 80 Hz to 35 KHz sealed. My own calculations yielded an F3 of above 100 Hz in a sealed cabinet… of 0.58 cu. ft…////

…using the drivers from:

http://www.madisound.com/catalog/PDF...M13SG09-16.pdf

See the simulation using the more trustworthy Madisound TS data of Vifa M13SG-09-16 in a closed 0.58 cu. ft. box: 1(2)-2(2)


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