Once we've decided on a port length and verified the box resonance it's time to measure the nearfield response of the speaker. You measure the nearfield response of a vented system in two steps. First, measure the nearfield response of the driver by putting the microphone in as close as possible to the driver with the capsule centered in the driver (in the center of the dust cap and about 1/2" away). Then, keep the amplifier volume and all settings on the computer unchanged and measure the response at the port by placing the microphone dead center in the port flush with the outer wall (i.e. in the center of the port but not inside the port itself - as close as you can get).

In this way we get two readings - nearfield driver and nearfield port. The following chart shows them.

The port response should be higher than the driver response because in the next step it gets scaled down proportionately to the diameter of the port.

To calculate the overall system response we open the enclosure resource again and select Calculate / Merge Port Response. Select the nearfield and port response datasets and when you click OK a dataset named MyEnclosure.Merged Response is built. This dataset contains the sum of the nearfield driver and port responses (the port response is suitably scaled). You can do this manually by scaling the port response and combining the two responses.

Here's a chart showing the final calculated nearfield system response

Interestingly, the red line is the response calculated by our original Calculate / Vented command. Notice the tight alignment with the actual measurements. Speaker design really is more of a science than an art.

Now we move on to the anechoic frequency response measurements and the tweeter measurements.