Thanks Zilch and Rudy for all of your help.
Your responses made me really question my test results. So, it forced me to go back to “The Book”. “The Book” in this case is “Testing Loudspeakers” by Joseph D’Appolito.
The excel chart that I included in my last post was created using a voltage divider technique described in Joseph’s book. For test equipment, I used a freeware computer program called SigJenny and an inexpensive digital multimeter. I have tested accuracy of the SigJenny program, using oscilloscope and I have found it to be very accurate. So, I know that the SigJenny program is not at fault here. Since, I don’t have my computer hooked up to my stereo. I used the SigJenny program to create my own DIY Port Testing CD with each track spaced just 1 hertz apart.
Getting back to my excel chart, all of the data contained in it is fairly accurate. But, I have completely misunderstood it meaning. After taking some additional impedance measurements, this time with the ports closed, I have now gathered all of the information necessary to properly calculate the enclosure tuning. The missing bit of information is the closed-box resonant frequency of my enclosures, which just happens to be 48 Hertz.
Fb = box tuning frequency
Fc = the closed-box resonant frequency
Fh = the frequency of the higher-frequency impedance peak
Fl = the frequency of the lower-frequency impedance peak
Fm = the frequency of the minimum impedance between the two peaks
Fb = square root of ( (Fh x Fh) + (Fl x Fl) – (Fc x Fc) )
So, plugging in the data values for my left enclosure into the formula above, the box tuning works out to be:
38.74 Hz = square root of ((58 x 58) + (21 x 21) – (48 x 48))
And for the right enclosure tuning works out to be:
37.23 Hz = square root of ((57 x 57) + (21 x 21) – (48 x 48))
These numbers come in fairly close to BB6P predicted values for an enclosure with a “heavy” amount of fill. Since, my project has a dog box, there is larger amount of surface area relative to its volume, which may explain why my project comes in with a heavier amount of fill in it.
Normally, the Fm value is very close to the Fb value. But, in my project, the Fm value appears to be much lower than the Fb value. And that is where I got really confused. :confused:
Over this last week end, I did try to observe at what frequency the cone movement was at its minimum. But, with separate test tones for each frequency, I found this test method next to impossible to judge accurately. So, I think I will stick with the voltage divider method for now.
Thanks again Zilch and Rudy, you both deserve a cold frosty one. :cheers:
Baron030
Final port tuning info????
I'm curious what the final result on the port length ended up at.
Ron