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.