Problems with port tuning (Noob)

[BURRometer]

I'm working on a ported enclosure for my RS225. UniBox suggests a 25cm x 3" port for a 50L with an Fb of 29.5hz. I've used ThomasW's port measuring procedure shown below. Odd results (for me anyway), so I tried a few variations on port lengths out of curiousity. Here's what was measured:

Hz-----25cm-----10cm---4cm(box wall only)

50-----1.5-----1.5-----1.5
45-----1.5-----1.0-----1.4
40-----1.2-----0.7-----1.5
35-----0.7-----0.7-----1.6
30-----0.5-----0.7-----1.7
25-----0.4 ----0.7-----1.6
20-----0.3-----0.6-----1.3
15-----0.2-----0.3-----0.6
10-----0.0-----0.0-----0.0

Where is the rise? Any thoughts would be greatly appreciated.
-Frank

To test the tuning frequency of a ported sub:

Wire the resistor in series with the (+) lead going from the amp to the (+) woofer input terminal. Wire the (-) lead normally from the amp to the (-) speaker terminal.

At low volume send 50Hz sine wave into the speaker.

Set the DMM to "AC" and measure the voltage across the +/- terminals of the woofer.

Set the output from the amp so the multi-meter reads between 1&2 volts.

Now lower the frequency in steps 5Hz at a time. Each time write down the voltage reading from the meter.

As you go down in frequency there will be a point where the voltage bottoms, then starts back up again with the next lower frequency. The lowest meter reading is the port tuning frequency.

For example:

50Hz-1.2V
45Hz 1.3V
40HZ 1.2V
35Hz-1.1V
30Hz-1.0V
25Hz-.75V
20Hz-.5V
15Hz-.75V
10Hz-1.1V

In this example the port tuning is 20Hz. That's where the meter reading shows the lowest voltage.

[Dennis H]

I think you've got your test wiring wrong somehow. They shouldn't be dropping to 0.0 volts at 10 Hz.

[BURRometer]

hmmm,

I rewired the system and got the same results. The resistor in series with the pos. lead to the speaker. Measurements are taken from the speaker terminals.

The resistor tested fine.

Could the problem be the cheap Technics receiver I'm testing with?

[ThomasW]

The brand of amplifier shouldn't make any difference unless it's somehow bandwith limited, is it a digital switcher?

As Dennis said there should be some measurable voltage at 10Hz.

What DMM are you using?

[BURRometer]

No, the amp isn't digital and the DMM is a just a generic unit.

I found the Manual:AC Voltage

Responce: Calibrated in RMS of a sine wave.
FREQ. Range: 45-450hz

[BURRometer]

Are the above specs the problem? I've looked at 5-6 DMMs and they all contain the same AC specs.

[ThomasW]

Don't know....

Since you have the projected tuning from the software, try measuring down in 1 or 2 Hz incriments from 35Hz to 25Hz, and see what you get.