TL606DMT Low
-Frequency Speak
er System
TL606DMT Low-Frequency Speaker System
2
Power Handling Capacity
To our knowledge, Electro-Voice was the
first U.S. manufacturer to develop and pub-
lish a power test closely related to real-life
conditions. First, we use a random noise in-
put signal because it contains many frequen-
cies simultaneously, just like real voice or
instrument program. Second, our signal con-
tains more energy at extremely high and low
frequencies than typical actual program, add-
ing an extra measure of reliability. Third, the
test signal includes not only the overall “long-
term average” or “continuous” level—which
our ears interpret as loudness—but also
short-duration peaks which are many times
higher than the average, just like actual pro-
gram. The long-term average level stresses
the speaker thermally (heat). The instanta-
neous peaks test mechanical reliability (cone
and diaphragm 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 sec-
onds on up, but we apply the long-term av-
erage for several hours, adding another ex-
tra measure of reliability.
Specifically, the TL606DMT is designed to
withstand the power test described in EIA
Standard RS-426A. The EIA test spectrum
is applied for eight hours. To obtain the spec-
trum, the output of a white noise generator
(white noise is a particular type of random
noise with equal energy per bandwidth in Hz)
is fed to a shaping filter with 6-dB-per-oc-
tave slopes below 40 Hz and above 318 Hz.
When measured with the usual constant-per-
centage analyzer (one-third-octave), this
shaping filter produces a spectrum whose
3-dB-down points are at 100 Hz and
1,200 Hz with a 3-dB-per-octave slope above
1,200 Hz. This shaped signal is sent to the
power amplifier with the continuous power
set at 800 watts into the 3.5 ohms EIA
equivalent impedance for the TL606DMT
(52.9 volts true rms).
Amplifier clipping sets instantaneous peaks
at 6 dB above the continuous power, or
3,200 watts peak (105 volts peak). This
procedure provides a rigorous test of both
thermal and mechanical failure modes.
SubpassbandSpeaker Protection
Below the enclosure tuning frequency, cone
excursion increases rapidly. Since acoustic
output is also failing rapidly, there is no util-
ity in driving the system with signals much
below the tuning frequency. While such sig-
nals may be in the program material, they
are often extraneous—such as from record-
surface irregularities (strong 5- to 25-Hz
components) or a dropped microphone. The
DL15MT very-low-frequency reproducer is
ruggedly designed and has a high maximum
excursion before damage (±0.5 inch). How-
ever, high-output subwoofer systems such as
the TL606DMT should be protected by a
high-pass filter with a 3-dB-down corner fre-
quency of about 0.8 the enclosure tuning fre-
quency. Below the corner frequency, a roll-
off of 12-dB-per-octave is usually sufficient.
Without protection, subpassband signals may
“bottom” the DL15MT’s. Damage will prob-
ably result, especially after repeated occur-
rences. Even if bottoming does not occur,
the subpassband signals waste amplifier
power and modulate (distort) the frequen-
cies which are within the TL606DMT’s op-
erating range. Much “woofer distortion” or
“muddy bass” can be attributed to lack of
subpassband protection.
The Electro-Voice EX-24, XEQ-2 and
XEQ-3 electronic crossover/equalizers pro-
vide 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 ef-
fective at providing the required protection.
Use in Multiples
TL606DMT’s may be used in multiples to
increase acoustic output. In the following dis-
cussion, it is assumed that all speaker cones
are operating in unison (in phase) when a
common signal is applied. A 6-dB increase
in maximum acoustic output results when
two speakers are located side by side. For
operation at very low frequencies, the woofer
cones “mutually couple,” acting as one
speaker with cone area and power-handling
capacity twice that of a single speaker. The
doubling of cone area doubles efficiency,
providing a 3-dB increase in sound pressure
level. The second 3 dB comes from the dou-
bling of power capacity.
Mutual coupling occurs when the frequency
is such that the center-to-center distance be-
tween the two speaker cones is less than
about one-quarter wavelength. For a given
center-to-center distance, the highest fre-
quency at which mutual coupling will occur
can be calculated from the following for-
mula:
3,000
f = —————,
D
max
where D
max
is the distance in inches and f is
frequency in Hz. When D
max
is greater than
one-quarter wavelength, as would occur if
two TL606DMT’s were widely spaced, the
level increase tends to be limited to the
3-dB power-handling increase.
More than two TL606DMT’s can be used
for increased output. In general, maximum
acoustic power output ability increases as the
square of the number of mutually coupled
cones. For example, four cones would pro-
vide 4
2
or 16 times the power output of a
single cone, an increase of 12 dB (10 log
10
16 = 12 dB). Note that the associated in-
creased efficiency (2.9% X 4 = 12%) ap-
proaches that of a fully horn-loaded design,
but in a much smaller enclosure.
System Positioning
Subwoofer systems such as the TL606DMT
are often located on the floor. This is both
convenient and can provide a desired high
acoustic impact when the speakers are, for
example, placed near the periphery of a dance
floor. In other installations, such as a theater
or auditorium, the audible location of a sub-
woofer operating at a sufficiently low cross-
over frequency (below about 125 Hz) will
not be particularly evident. The other sys-
tem elements operating above the subwoofer
range can be positioned for the desired
locational cues and uniform audience cov-
erage.
Floor location provides the acoustic half-
space environment associated with the 5.8%
system efficiency noted in the Specifications
section. Location at a floor-wall junction
(acoustic quarter space) doubles efficiency
