3
Polar-to-Listening-Plane Conversion –
To compute the LISTENING-PLANE coverage more precisely, you need to use the exact polar plot of
the speaker (real polar plots directly from test equipment are better than an artist’s redrawing). Polar
plots are usually normalized to the on-axis value, which is usually labeled “0 dB”. For every angle off-
axis, there is a difference-figure between this normalized 0 value and the actual level. To convert to
LISTENING-PLANE coverage, add the “
∆
dB” figure from the following chart for that angle off-axis to
the difference figure from the polar plot (be sure the difference-figure is a negative number).
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Angle
Off-Axis
∆
dB
Correction
Factor
Angle
Off-Axis
∆
dB
Correction
Factor
5
°
-0.0 dB
45
°
-3.0 dB
10
°
-0.1 dB
50
°
-3.8 dB
15
°
-0.3 dB
55
°
-4.8 dB
20
°
-0.5 dB
60
°
-6.0 dB
25
°
-0.9 dB
65
°
-7.5 dB
30
°
-1.3 dB
70
°
-9.3 dB
35
°
-1.7 dB
75
°
-11.7 dB
40
°
-2.3 dB
80
°
-15.2 dB
By using the actual polar plot of the speaker and applying these correction factors from the chart, the
angle which results in a figure of -6 dB is the REAL 6 dB down angle on the listening plane. The new
coverage angle is valid regardless of the ceiling height.
Example -- If we look at the polar plot of a hypothetical speaker with 140
°
coverage, we see that at
70
°
off-axis (140
°
total for both sides) the level is down 6 dB compared to the on-axis level. By
looking at the Polar to Listening Plane Conversion Chart, we need to add -9.3 dB to this -6 dB
figure to find the actual level on the listening plane at this off-axis angle. We find that the level of
this 140 degree speaker (as specified by the POLAR coverage), is actually –15.3 dB (it is NOT
–6 dB) down at 70
°
off-axis. Therefore, listeners located at this off-axis angle will hear sound that
is more than 15 dB down from the level they hear when they pass directly underneath (on-axis) the
speaker. This is a very large difference.
To find the actual 6 dB down point of the speaker for the listening plane, take the actual polar plot
of the speaker and at every increment of 5 degrees off-axis, apply the correction factors from the
polar-to-isobar conversion chart. The 6 dB down angle is that angle at which the figure is -6 dB
(POLAR dB down PLUS correction factor). While it depends on the actual polar plot of the
speaker, speakers with a nominal POLAR coverage of 140
°
can be expected to reach –6 dB between
45
°
and 55
°
off-axis, resulting in an actual LISTENING PLANE coverage of between 90
°
and 110
°
.