The ports were tested at 15, 20, 25, 30 and 35hz, ( power and excursion permitting )

See raw data page for actual measurements at the different frequencies

The following table summarises the relevant results:

xlim - excursion limit reached
plim - power limit reached

Using a separate graph for each of the port diameters shows how slope and limiting velocity vary with frequency.

103mm diameter ports

The 103mm ports show the variations in slope and limiting velocity.

Below 25hz, the limiting velocity doesn't fall much further. The slope appears to stabilise at 20hz.

86mm diameter ports

The 86mm port results include good data for 35hz

As with the 103mm ports, below 25hz, the limiting velocity doesn't fall any further.

The slope also appears to stabilise at 20hz.

The 152mm port could only be tested at velocities that reveal slope. The lower frequencies were difficult to measure because of bad structural resonances. The 160 litre box produced the highest SPL's, and at 350w, major items in the room were producing a lot of noise.

The 15hz slope is based on a single measurement, which gives a low level of accuracy, so will not be used for analysis

Measuring the variation in limiting velocity

Reading the limiting velocities from the above graphs gives the following table:

The percentages indicate how much the limiting velocity changes from the 30hz figure:

The change appears to be independent of the port diameter, being solely determined by the frequency.

At 35hz, the limiting velocity is higher than the 30hz figure by +23%.

At 25hz, the limiting velocity is lower than the 30hz figure by -33%

Measuring the variation in slope

Reading the slopes from the above graphs gives the following table

Noting the Velocity at Area Ratio = 2 allows a comparison.

The percentages indicate how much the slope changes from the 30hz figure:

Usable velocities for AR=2

Again, the change appears to be independent of the port diameter, being solely determined by the frequency.

Since the 35hz figure for the 103mm port is based on a single reading, it's best to play it safe and use the more conservative value based on the multiple readings for the 86mm ports

For 25hz, the average is around -28%

For 20hz, the 152mm port was difficult to measure, so we'll give more weight to the 86mm and 103mm results

At 35hz, the usable velocity is higher than the 30hz figure by +29%

At 25hz, the usable velocity is lower than the 30hz figure by -28%

At 20hz, the usable velocity is lower than the 30hz figure by -37%