testing reflex tuning with multimeter

What's going on, it's 0050 here but the post I've just made has a timestamp of 0143!

Edit: Ah, just noticed the forum time is wrong. That would explain it.

Chris, when you mention application. What application are you thinking about when it comes to a bass reflex cab.

Hi Tony, I simply meant that tuning the cab to the driver resonance might not give the response you want. For example a small mid-top cab you will often tune the port much higher than the driver resonance.

OK chris, I see what you mean. We all have our own approaches, and I have only ever considered tuning Bass cabs that are big enough. When we didn't have the equipment we have today, it was harder to be flexible. With simple programs that we use today such as Bass Box, you can drag the frequencies where you want, because you can see where you're going, which wasn't the case years ago. The equal peaks thing comes straight out of the text books where it explains the single peak of a sealed cab, and how basically putting a hole in the cab which produces two, is still only a compromise, but still better than one. Because of the fact that above a certain frequency, tuning can be pointless, I would never tune a mid cab, but there are plenty of people who use a full range 2 way system as a mid top, and end up using a bass speaker as mid. I guess this is why you see smallish cabs that are tuned. Manufacturers only ever assume them to be bass speakers when tuning. That's always been my approach anyway.

On a very small direct radiating mid-top I might use a port up at about 70-80Hz but never any higher than that.

I don't know how much you have studied the actual physics of reflex cabs and Helmholz resonators.

For those that haven't, a driver in free air or in a closed box has a single impedance peak at the mechanical resonance of the cone/suspension/air mass. This is caused because at this frequency the motion of the cone is also at a maximum and therefore so is the generated back EMF in the voice coil.

Adding a port creates another resonant system which opposes the motion of the cone at it's resonance and causes a dip in the impedance. Superimposing the two gives two peaks either side instead of one. If the two resonances are at the same point then the peaks are symmetrical. If the lower peak (frequency wise) is larger, the port is tuned higher than the driver resonance and if the higher peak is larger then the port is tuned below the driver resonance.

I personally like to avoid tuning the port below the driver resonance because you end up with an even larger excursion peak and often a dip in response immediately above the port tuning.

In my opinion the optimum result is often acheived when the driver your using has it's resonant frequency at the same frequency you want to tune the port and thus you can coincide the two.