Near Field of PL18
refer back to the scaled 1 meter plot of the Near Field measurement
of the PL18 previously taken (Figure 14).
plot, while effective below 100Hz, is not accurate for mids
the deep notch at 1,000Hz.
is at about 2,000Hz whereas Vifa specifies it up to 4,000Hz.
14) Scaled to 1 meter
Gated SPL Sweep
To determine the accuracy of the higher frequencies, a Precision
300 points, Gated SPL sweep is done on the PL18 (Figure 15).
Gated Sweep is a very accurate measurement. It is quasi-anechoic,
in other words, like done in an anechoic chamber but not really
in one. It accomplishes this by opening the meter's "gate"
for the primary signal to pass through, then closing the "gate"
before the first boundary reflection reaches the microphone.
the microphone placed 1 Meter away, Input Signal at 2.83V
and Gate "Meter On" set for 2.3 mSec, the lowest
frequency it can read to is about 500Hz. Note the breakup
15) Gated SPL Sweep of PL18WO09
the Gated measurement done, the two graphs are "Spliced"
together at 500Hz to form one continuous plot (Figure 16).
is interesting to note that the curve below 500Hz was actually
made Near Field, then reference to 1 Meter mathematically.
spliced with the Gated curve made at 1 Meter, the Scaled Near
Field measurements are at the correct SPL. We can conclude
that the Scaling Factor is accurate.
16) Spliced Scaled Near Field with Gated SPL
a more uniform appearance, the new curve is Smoothed by 0.1dB
(Figure 17). This irons out the "jaggedness" of
the Gated measurements.
it is a composite, this graph is a good representation of
the actual frequency response of the PL18 when loaded with
a 18 Liters Box. We are now in a position to compare with
Vifa's Frequency Response Plot (Figure 18) and determine the
effects of box loading.
Effects of Box
Figure 17, we can discount the frequencies below 1,000Hz as
they are relatively flat and bear close resemblance with Vifa's.
higher up, there is an obvious notch at 1,500Hz. This is reflected
in Vifa's plot in Figure 18. I can only deduce that it's inherent
of the PL18.
2,000Hz upwards, we are met by some bumps followed by a horrendous
chunk bitten off at 3,500Hz. This is obviously not from the
PL18 as can be verified from Vifa's plot.
will hazard a guess that the culprits responsible are internal
standing waves and cone breakup causing havoc at the upper
this discovery, I would be greatly concerned if I plan to
cross anywhere between 2,000 - 4,000Hz.
17) Smoothed 0.1dB
18) Vifa PL18WO09
at 1,500Hz, I am quite confident that with a 4th order filter,
these "distortions" will be sufficiently suppressed
so as not to intrude with the tweeter's performance.
shall now proceed to examine the PL27TG35 tweeter in greater