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Old 6th February 2007, 09:36 PM   #1
skogs is offline skogs  Norway
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Default vent tuning - interesting article

Hi

http://www.troelsgravesen.dk/vent_tuning.htm
this article really gave me much to think about, and makes the process of tuning a box really hard.
do you guys think we can trust the result of the article blindly? for example multiply the calculated vent length with 0.7? will this give a more accurate result than just using the calculated lenght?

maybe bassbox 6 pro and such design programs have implented these things in their calculations?

and my last question: could i use two ports with different diameter and same length, and use the summed area to calculate the lenght of the vents? this way i could adjust the tuning pretty accuratly by block one or more of the smaller vents.

Řyvind
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Old 7th February 2007, 12:38 AM   #2
Ron E is offline Ron E  United States
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One criticism I have of T.G.'s work is that the minimum in impedance is not always precisely the tuning frequency. Enclosure losses, driver inductance, and perhaps even drive level will affect the shape of the impedance curve enough to change the apparent tuning frequency. The actual tuning frequency is closer to where the diaphragm frequency response null occurs, AFAIK.
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Old 7th February 2007, 12:00 PM   #3
hooha is offline hooha  Canada
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Good article. I think the author achieved what he wanted to communicate to the reader - that perceived port lengths do not equal measured port lengths. What programs like WinISD spit out for numbers should not be contstrued as gospel.

There will always be slight variances in each and every test setup lending itself to error - inductance, volume losses, etc. I think if the reader take that into account, they can be rest assured this test method will achieve a result that is much closer to real life than an equation can ever do.

Mark
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Old 9th February 2007, 11:50 PM   #4
Svante is offline Svante  Sweden
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Basta! manages both the isothermalizing effect of box stuffing and the end correction of cylindrical vents, with some simplifications (the vent is assumed to be unflared and baffled in both ends).
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Old 10th February 2007, 09:53 AM   #5
Geoff H is offline Geoff H  Australia
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Simulators are just that. Good enough to get you into the ball park, but may need fine tuning if measured results differ from what was expected. I always start with a long port, down 10Hz or so, then trim.
I don't go that route much these days.

Geoff.
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Old 10th February 2007, 12:31 PM   #6
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Default My next effort

I will include some features in my next enclosure that make fine tuning easier to do. This will be a MLQW.
The port will be made with a pvc "sweep". It will be a large radius curve.
The mount to the face of the box will be a fitting that is used for attaching pvc conduit to metal enclusres. The lock nut on the fitting will allow the port assemlby to be turned. Turning the assembly will change the location of the inlet of the port within the box.
Flares of ~1 1/2" (37 mm) will be used on both ends.
The test box will have a removable panel so that port length can be changed.


I do need suggestions for the gear I need to measure impedance vs. frequency.

How do I determine the diaphragm frequency response null ???

Thanks for all
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Old 10th February 2007, 12:49 PM   #7
AndrewT is offline AndrewT  Scotland
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Default Re: My next effort

Quote:
Originally posted by Ed Lafontaine
I do need suggestions for the gear I need to measure impedance vs. frequency.

How do I determine the diaphragm frequency response null ???
Hi,
Oscillator (10Hz to 200Hz) and very accurate frequency readout/meter.
DVM with 200.0mVac & 2000mVac scales.
220r & 33r resistors (500mW).

If the oscillator can drive 220r direct, you don't need an amplifier, this is small signal testing!!!.
If the oscillatior needs a buffer then add an amplifier to the list.

Look up wallin jig mk2 for details of a neat way of building the test instrument.
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