Advertisement
Directivity
The directional characteristics of the
TL550D were measured in Electro-Voice's
large anechoic chamber; the test signal was
one-third-octave filtered pink noise at the
frequencies indicated. A full spherical mea-
surement system was used, which is com-
patible with the AcoustaCADD
aided design program. All directional infor-
mation was measured at 20 feet.
Figure 2 illustrates the horizontal and verti-
cal polar responses. Figure 3 shows the hori-
zontal and vertical beamwidths. Beamwidth
is the angle at which the horizontal and ver-
tical polar responses have decreased in level
by 6 dB when compared to the axial fre-
quency response.
Figure 4 illustrates the total directivity of the
TL550D. The directivity factor R
relative value, at a point, of the TL550D
when compared to an ideal spherical
response. The directivity index, D
lated by D
= 10 log R
.
i
q
Power Handling Capacity
To our knowledge Electro-Voice was the first
U.S. manufacturer to develop and publish a
power test closely related to real-life condi-
tions. A random noise input signal is used
because it contains many frequencies simul-
taneously, just like real voice or instrument
program. The signal contains more energy
at extremely high and low frequencies than
typical actual program, adding an extra mar-
gin of reliability. The test combines not only
the overall long-term average or continuous
level—which our ear interprets as loud-
ness—but also short-duration peaks which
are many times higher than the average, just
like actual program. The long-term average
level stresses the speaker thermally (heat).
The instantaneous peaks test mechanical re-
liability (cone excursion). Note that the sine-
wave test signals sometimes used have a
much less demanding peak value relative to
their average level. In actual use, long-term
average levels exist from several seconds on
up. We test for several hours, adding an-
other extra level of reliability.
Specifically, the TL550D is designed to with-
stand the power test described in EIA Stan-
2
TL550D Low-Frequency Speaker System
dard RS-426A. The EIA test spectrum is
applied for eight hours. The spectrum is ob-
tained by filtering white noise (a particular
type of random noise with equal energy per
bandwidth). The filter applies a 6-dB-per-
octave slope below 40 Hz and above 318 Hz.
When measured with a one-third-octave con-
stant-percentage analyzer, this filter produces
TM
computer-
a spectrum whose 3-dB-down points are at
100 Hz and 1,200 Hz with a 3-dB-per-oc-
tave slope above 1,200 Hz. This shaped sig-
nal is fed to the power amplifier with the
continuous power set to provide 1,200 watts
into the 4.8-ohm EIA equivalent impedance
(75.9 volts rms).
Amplifier clipping sets instantaneous peaks
at 6 dB above the continuous power, or 4,800
watts peak (151.8 volts peak). This proce-
dure provides a rigorous test of both thermal
and mechanical failure modes.
(Q) is the
q
The TL550D has also been power tested with
a two-hour sine wave, at minimum imped-
, is calcu-
ance, so that competitive comparisons can
i
be made using consistent methods. Continu-
ous program power is defined as 3 dB above
(double) the continuous sine-wave power
rating.
Step-Down
Step-down is a method of extending the low-
frequency response by increasing the power
input to the system instead of the enclosure
volume. In step-down mode, the enclosure
is tuned at a lower-than-normal frequency.
This increases the output at the new tuning
frequency and reduces the output at the origi-
nal tuning frequency. This smoothly falling
amplitude response can be equalized to ob-
tain a new 3-dB-down point in the region of
0.7 that of the original. To obtain a similar
response without step-down would require
an enclosure with at least twice the volume.
Step-down can be instigated by using the
supplied port cover. The enclosure tuning
will be lowered from 42 to 30 Hz.
If an appropriate low-frequency boost-and-
cut equalization is applied, the normal f
40 Hz is reduced to 28 Hz. It is suggested
that the Electro-Voice XEQ-2 or XEQ-3 ac-
tive crossover be used to provide the required
equalization. The suggested equalization is
an underdamped second-order high-pass fil-
ter tuned to 29 Hz with a Q of 2. This pro-
vides a boost of 6 dB at the tuning frequency
and a 12-dB-per-octave roll-off below. This
filter is directly available on the XEQ-2 or
can be generated in the XEQ-3 by using the
optional EB29/35 EQ module.
Subpassband Speaker Protection
Below the enclosure tuning frequency, cone
excursion increases rapidly. Since acoustic
output is also falling rapidly, there is no util-
ity in driving the system with signals much
below tuning frequency. While such signals
may be in the program material, they are of-
ten extraneous, such as a dropped micro-
phone. The step-down equalization described
in the Step-Down section provides the re-
quired protection. If step-down mode is not
used, the Electro-Voice EX-24, XEQ-2 and
XEQ-3 electronic crossover/equalizers can
also provide subpassband protection. The 3-
dB-down points are 30 Hz (EX-24 and XEQ-
2) and 16 Hz or 32 Hz (XEQ-3).
Other high-pass filters are available and one-
third-octave equalizers can also be effective
at providing the required protection.
Use In Multiples
Cone loudspeakers may be stacked for
greater acoustic output and a narrower
beamwidth. (It is assumed that all cones are
operating in unison or "in phase.") This prin-
ciple is already employed in the dual-woofer
TL550D, and is responsible for the higher
sensitivity and narrower vertical beamwidth
(with the system long axis vertical) relative
to similar single-woofer systems. In addition,
two TL550D's can be used side-by-side and
their combined performance will be differ-
ent from that of a single TL550D in the ways
outlined below.
At relatively low frequencies, below about
150 Hz for typical TL series dimensions,
stacking produces additional acoustic output
without altering dispersion. When a common
signal is applied, a 6-dB increase in maxi-
of
3
mum acoustic output occurs. The cones "mu-
tually couple" and act as one cone with twice
the area (therefore twice the efficiency) and
twice the power capacity. The additional
cone area provides 3 dB more output and the
Advertisement
