Subwoofer Box Tuning Measurements

[///M5 2,833]

So you designed and built a box and think you know what it is tuned to, but there are a lot of reasons to verify that you are right. Some of these are:

1) To make sure you understand the math behind designing the box

2) To check up on the person that did the design for you

3) To check on your building skills

4) To verify where you are so that when you redesign to deal with an acoustic difficulty that you found you are able to.

Whatever the reason, there is a simple way to do this.

You will need the following equipment:

1) Your system installed in your vehicle.

2) A CD with a variety of low frequency tones on it, I

[Acidburn 10]

that is good information to know

i might just have to try this out on my little sealed guys

[dmz2711 0]

///M5, AWSOME tutorial on how to measure box tunning!! I was always wondering about that, now I finally have a conclusive answer on how to do it.

The one question I was wondering about, is how to find box tunning when there are two or more subs(sharing the same enclosure airspace) as well as two or more amps in the setup? Would it be best to wire the subs in series(or parallel?), and just use one of the two amps to determine enclosure tunning(using the same method as you described)? or is there a completely different method for multiple subs and amps? thanks again!!!

[3stagevtec 2]

1) Wire your resistor in series with your driver. For example, take the + output from your amplifier and run it to the resistor, connect the other end of the resistor to the + input on your sub. Leave the

[helotaxi 46]

Nope. The voltage from the amp should be the same and with only one load in the circuit (the driver) all the volatge drops across the one load. By adding the second fixed load in series, you can use the voltage drop across that load to figure out the impedance of the variable load. This is because the current through the circuit is inversely proportional to the impedance of the circuit and the voltage drop into the resistive load is proportional to the current going through it. By measureing the voltage on the fixed load you indirectly determine the impedance of the variable load at that freq.

Sean, something seems wrong with your numbers. The impedance is minimum at tuning for a ported box which would mean highest current which would mean highest voltage drop across the resistor, no? Your numbers work for a sealed box with an Fb of 33 hz but not a ported one unless I'm missing something.

[3stagevtec 2]

Nope. The voltage from the amp should be the same and with only one load in the circuit (the driver) all the volatge drops across the one load. By adding the second fixed load in series, you can use the voltage drop across that load to figure out the impedance of the variable load. This is because the current through the circuit is inversely proportional to the impedance of the circuit and the voltage drop into the resistive load is proportional to the current going through it. By measureing the voltage on the fixed load you indirectly determine the impedance of the variable load at that freq.

Sean, something seems wrong with your numbers. The impedance is minimum at tuning for a ported box which would mean highest current which would mean highest voltage drop across the resistor, no? Your numbers work for a sealed box with an Fb of 33 hz but not a ported one unless I'm missing something.

mind if i challenge you abit.. i was just playing with WinISD alpha, and according to that program, the lowest impedance is actually a little below the tuning frequency of the enclosure.. not sure how accurate the program is though..

have a look, both enclosures tuned to the same 40Hz.. the lowest impedance on the red graph is highlighted at 37.75ohms.. (the lowest on the grey graph is 38.44Hz)

[helotaxi 46]

Can't see the picture, filtered by the server here. Double check what the progaem actually says that the tuning is. A lot of times with WinISD you'll change one parameter of the box and it will alter a few others for you. But to answer your question, by definition min impedance for a ported enclosure is at resonance. This is where cone movement is minimal and thus the reactive part of the impedance is minimized and Re acounts for the majority.

[3stagevtec 2]

by definition min impedance for a ported enclosure is at resonance. This is where cone movement is minimal and thus the reactive part of the impedance is minimized and Re acounts for the majority.

i agree with this.. but WinISD shows slightly different everytime.. and i don't want to fully doubt the program either..

(from my 1st example)

i had double checked the tuning which was set at 40Hz, and minimum impedance seen was at 37.75 and 38.44Hz for 2 different subs.. (both enclosures chosen at 5cf)

tried it again with another sub.. JL 13W7, box 4cf, Fb 35Hz

Impedance at 35.1Hz - 0.840 ohms

Lowest Impedance seen 0.820 ohms at 33.64Hz

actual lowest impedance is shown again at around 1-2Hz below tuning frquency.. my guess is WinISD is accounting for some other 'parameter' which shifts the mim impedance a bit lower that tuning...

[dmz2711 0]

///M5, AWSOME tutorial on how to measure box tunning!! I was always wondering about that, now I finally have a conclusive answer on how to do it.

The one question I was wondering about, is how to find box tunning when there are two or more subs(sharing the same enclosure airspace) as well as two or more amps in the setup? Would it be best to wire the subs in series(or parallel?), and just use one of the two amps to determine enclosure tunning(using the same method as you described)? or is there a completely different method for multiple subs and amps? thanks again!!!

Its been a long time, but I am still wondering about this question?? Now that there have been some new recent replies, would anyone know the answer? thanks.

[helotaxi 46]

It won't matter how the subs are wired, just as long as the resistor is wired in series with the whole of them.