hi,i need to test a bass reflex tuning,and want to prepare a little test circuit.i dont understand why i need a resistor in the circuit.i m not trying to test real impedence,just the port frequency.thanks for any help.
In the test circuit your talking about the voltage across the resistor is measured with the meter. If you don't want to use a resistor, you could always use the meter series connected in the circuit to measure the current. The results are suposedly less accurate but it will probably tell you what you need to know.
I seem to remember writing about about this a while ago, but here it is again. In the '70's before I had any analyzing gear, I did have a Audio generator and a multimeter. On a lot of early reflex cabs I did, the basic port opening was made one size. I had made a series of ports all with different lengths. Using the rule of achieving two impedance peaks of equal amplitude, I would sweep a signal through, and basically, the needle on the meter would move up and down. With a box with a hole in it, this happens twice. Adjustments were made until the two peak readings became the same. I then assumed, with my limited knowledge, that the cab was perfectly tuned for the speaker and the volume of the box.
i did read your previous post,which gave me the idea.i just wasnt sure if i needed a resistor in line.the reason for asking is that i live in the middle of nowhere,up a mountain far away from the nearest electronics supplier.i could try and wind my own if it would help the results.thanks for the replies,ill try tonight, minus resistor,and see what happens.(and the 'wait for cone to stay still test' as well)
Matching the two impedance peaks sets the port tuning to the same frequency as the resonance of the driver. That doesn't mean however that the tuning is optimised for the application.
Using a series resistor is by far the easiest way to determine the impedance curve and thus port tuning.
I tend to set up a spreadsheet which plots the impedance curve as I go so that I can see where I need to take more measurements. Most of the time you can just do 5Hz steps but close to the peaks you need to do much smaller steps to plot the impedance accurately.