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I built a sub box but I forgot the exact port length, I had modified it twice in the building stage and never wrote it down. I was shooting for a 20hz tune so it's probably close enough but I was wondering is there an accurate way to really know ?


I tried to reverse engineer it- but the calculator on HTS and my own are way off each other and I can't actually remember the length with certainty. I don't want to remove a driver to peek inside to measure either.


Sorry if my question is noob. Thanks in advance for replies.
 

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If you are able to measure the impedance that will tell you.
 

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Quote:
Originally Posted by augerpro  /t/1526773/is-it-possible-to-measure-port-tune-on-a-built-box#post_24593128


If you are able to measure the impedance that will tell you.
If not place the cab on its back, with the driver facing up. Put some sugar or salt on the cone, do a slow sine wave sweep. The sugar/salt will 'dance' with the excursion of the cone. At the tuning frequency the cone movement is at a minimum, and the dancing will pretty much cease, increasing again as you go either up or down in frequency from that point.

Another method is to measure the port output close mic'd; port output will peak at Fb.
 
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Quote:
Originally Posted by augerpro  /t/1526773/is-it-possible-to-measure-port-tune-on-a-built-box#post_24593581


Genius Bill!
Perhaps, but I can't take credit, this trick has been around almost as long as I have.
 

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Quote:
Originally Posted by Bill Fitzmaurice  /t/1526773/is-it-possible-to-measure-port-tune-on-a-built-box#post_24593694


Perhaps, but I can't take credit, this trick has been around almost as long as I have.

They had woofers before electricity?
 
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Originally Posted by Mfusick  /t/1526773/is-it-possible-to-measure-port-tune-on-a-built-box#post_24596047


What is good way to get exact frequency tones ?
You should be able to find tone generator freeware on line. For that matter the W7 version of WinISD has one.
 
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Can someone explain the physics of this to me? I have some ideas that are terribly well-formed, but I feel like understanding is within reach if my puny human brain had a nudge in the right direction.


I can understand how the air-mass of the port has an impedance peak, but I can't reconcile how that frequency is also the peak of acoustic output from the port (is that a misconception?).
 

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I appreciate the links. I avoided saying very much when I posed the question to avoid leading responses, but most of that is reasonably familiar to me. (I actually was Dr. Nave's student about a decade ago, I love hyperphysics) The idea that the acoustic output of the port is the back wave filtered through the resonant system of the port is a new-ish idea for me, so I'll need to let that marinate.


Here's where it breaks down for me, I think. For a given voltage across the voice coil, I expect a certain displacement, governed by the combination of the driver's T/S parameters and the pneumatic parameters of the cabinet. In order for the displacement to be small, there should be high impedance somewhere - in this case in the cabinet and port design (that's the whole point of this thread right? Find the peak in impedance.) My problem is that my intuition says that air should move freely (with low impedance) to get high output. ...now that I say it, I see the problem with it. The energy has to be pressure; and pressure exists as a result of resistance to flow, in a sense (in the same way that you can't have voltage without a way to keep the charge separated). The impedance leads to the pressure buildup and oscillation which is the acoustic output.


Edit: still not right.
 

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Quote:
Originally Posted by HopefulFred  /t/1526773/is-it-possible-to-measure-port-tune-on-a-built-box#post_24596510


I can understand how the air-mass of the port has an impedance peak, but I can't reconcile how that frequency is also the peak of acoustic output from the port (is that a misconception?).
There are two peaks in a vented box, one above and one below the port frequency. At the port frequency impedance is at a minimum value. When you use an impedance sweep to identify the tuning frequency you look for that minimum impedance frequency, which also coincides with where the cone has minimum excursion.
 

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Quote:
Originally Posted by Bill Fitzmaurice  /t/1526773/is-it-possible-to-measure-port-tune-on-a-built-box#post_24597047


When you use an impedance sweep to identify the tuning frequency you look for that minimum impedance frequency, which also coincides with where the cone has minimum excursion.
Why is excursion low? What provides the force that opposes motion in the motor?
 

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It's easier to think about it as a mechanical resonance, which in fact is what it is; the fact that the moving air also creates sound is incidental.


The vent air mass on the box air spring has its resonant freq at Fb.


The phasing relative to the driver cone is such that the vent air is moving inward and compressing the box air at the same time the cone is also trying to move inward; its motion is reduced by the increased pressure.


The driver impedance dips here because the restricted motion lowers back-EMF.
 

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( Because it isn't being impeded and it's between 2 resonant peaks )

http://www.youtube.com/watch?v=AVQRABgkPNk
http://www.youtube.com/watch?v=LuSS9tgpoEI

http://www.acoustics.salford.ac.uk/feschools/waves/shm.php


"The frequency at which the box/port system resonates, known as the Helmholtz resonance, depends upon the effective length and cross sectional area of the duct, the internal volume of the enclosure, and the speed of sound in air.


When this vent air mass/box air springiness resonance is so chosen as to lie lower in frequency than the natural resonance frequency of the bass driver, an interesting phenomenon happens: the backwave of the bass driver sound emission is inverted in polarity for the frequency range between the two resonances. Since the backwave is already in opposite polarity with the front wave, this inversion brings the two emissions in phase (although the vent emission is lagging by one wave period) and therefore they reinforce each other. "

From
http://en.wikipedia.org/wiki/Bass_reflex
 
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