- Micro Transmission Line Speaker System
Gregoire and Associates LLC
THE DISCRIMINATING AUDIO ENTHUSIAST
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Micro transmission line speaker systems.
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Stereo repairs and stereo modifications.
A one-inch (tweeter-size) driver which reaches all the
way down to below 150Hz? Although it seems unlikely, it
has, indeed, been accomplished in this system by fitting
the driver with a highly flexible surround which yields a
free-air resonance in the 150Hz region. Then the driver
has been mated to a critically damped transmission line
which does not raise the system resonance to a higher
frequency, as would be the case with a sealed or ported
enclosure. The cross-sectional area of the transmission
line is substantially as large as the driver's piston
area, thereby avoiding the "air-stiffening"
action of the sealed or ported enclosures which raises
their resonance frequencies.
The transmission line is a long folded passageway
designed to be 1/4-wavelength long at the free air
resonance frequency of the driver, so that the acoustical
output radiated from the line opening mixes with the
front-wave from the driver, to cause partial cancellation
of the signal, only at the resonance frequency. This
technique leaves the driver free to accelerate and
decelerate with ease, except at the resonance frequency
where its motion is inhibited. This
"frequency-selective damping" quality is
largely responsible for the superior sound quality of
properly designed and implemented transmission line
systems, including this one.
Another useful feature
of the transmission line design is that at one octave
above the 1/4-wave resonance frequency, there is a
reinforcing effect which substantially doubles the output
from the driver, over a fairly broad band of frequencies.
In the case of this particular design, the half-wave
reinforcement action helps to raise the output of the
very small piston, providing linear response to a lower
frequency than would be possible without the transmission
This product's unusual use of the transmission line
back-loading technique (unusual for a mid/tweeter) has
resulted in a midrange drive system with an extremely low
moving mass which is competitive with costly
electrostatic and magnetic planar drivers. It should
provide much of the clarity associated with planar type
speakers, and we think it does! However, it also offers
the dynamic impact of voice coil type drivers, which
results from their ability to focus more magnetic flux on
the driving coil than is physically possible with
magnetic planar designs.
Another advantage of using a single driver to cover 150Hz
through 15kHz+ is the ability to avoid, throughout the
mid-range, the phase distortion which is caused,
inevitably, by even the finest crossover networks.
Therefore, the wide frequency band where most of the
music is (the mid range) may remain untainted by
undesirable crossover network side effects, resulting in
greatly improved precision of the stereo image and
coherence of the sonic presentation.
The sound pressure level from one of these diminutive
systems is necessarily low- only about 81dB (1 Watt/1
meter). But, because of the small size and low cost of
these units (See our order form for generous discounts
for quantity purchases.), it is feasible to use multiple
driver arrays to increase the output to a level which
matches popular woofers and tweeters. For instance, four
units should yield about 87dB, eight units: 90dB.
Arranging the multiple units in a "line-source
array" provides imaging and transparency which
rivals some very costly ribbon and planar drivers, and
equals or exceeds their output level. The Micro TLS also
works effectively to a much lower frequency, and delivers
greater dynamic impact.
DESCRIPTION: High quality oval-shaped
paper cone driver, 35mm X 25mm, with high-compliance
flexible surround, installed in specially designed dual
transmission line labyrinth system. The main transmission
line is a classic 1/4-wave labyrinth, with damping
material behind the driver. A second, much shorter
transmission line with cavity resonator also is
incorporated into the same enclosure, possibly to control
a secondary resonance of the drive unit. The transmission
line enclosure is molded from acoustically inert plastic
material. The enclosure is finished only for functional
purposes, and is intended for use inside some form of
cosmetic exterior treatment (not provided).
MANUFACTURER: The system was
manufactured by a subsidiary of a well known audio
electronics company. We have agreed not to advertise
SUGGESTED APPLICATIONS: "Ultra
wide-band" mid range drive unit to be incorporated
into a high-performance 3-way speaker system, or as a
stand-alone speaker system for applications where
outstanding clarity of sound is necessary, but full
frequency response is not required.
ENCLOSURE SIZE: 4.50" high
(includes mounting flanges) X 1.30" wide X
WEIGHT: 4 ounces (complete system).
FREQUENCY RESPONSE: 120Hz - 18kHz (+ or
- dB not specified, but the unit seems notably free of
high Q resonance's which would cause ringing, and the
resulting "screechy sound"). Response falls-off
rapidly below 150Hz, and gradually rolls-off above about
SENSITIVITY: Approximately 81dB at 1
Watt at 1 meter. Sensitivity may be increased by using
multiple units, i.e. 4 units configured as a "line
source" should yield about 87dB. (See our order form
for generous discounts for multiple unit purchases).
IMPEDANCE: 8 Ohms. (4 units in
series/parallel connection also provide a final impedance
of 8 Ohms.)
POWER HANDLING CAPACITY: Although no
official rating is available, we have tested the
micro-TLS with amplifiers capable of more than 100 Watts
RMS. Sine waves (particularly low frequency signals)
should be limited to a low value, typically 1 watt or
less. When fed with full-range bass signals, a series
capacitor should be used to roll-off the bass below about
TRANSIENT RESPONSE: Although no
objective measurements are available, it is apparent from
listening tests, that the transient response is unusually
quick, probably due to the use of an extremely low-mass,
high quality driver, mated to a properly designed
critically damped transmission line which precisely
controls the main system resonance.
MOUNTING ARRANGEMENT: Top and bottom
mounting flanges with #6 screw holes are molded into the
enclosure for convenient panel mounting. An additional
side screw is available for alternative mounting/support.
CONNECTIONS: Each Micro-TLS is equipped
with approximately 8-inch long red (+) and black (-)
color coded input leads of 24-gauge stranded copper wire.
CROSSOVER RECOMMENDATIONS: Our
preference is to use 6dB per octave crossovers at 300Hz
and 15kHz. Faster crossover rates may be used, as well,
if preferred by the system designer.
STAND-ALONE USE RECOMMENDATIONS: When
using the system as a stand-alone speaker, it may be best
to feed it through a capacitor (perhaps 68MFD,
non-polarized), to prevent the large piston excursions
caused by bass program content which is below the cutoff
frequency of 120Hz. The capacitor may not be necessary,
if driving the speaker from a bass-limited source, such
as a high quality television set's audio output ampflier.
The speaker is very good for TV sound because of its
unusual clarity, coherence, and wide dynamic range.
GUARANTEE: Warren Gregoire &
Associates LLC offers an unconditional 30-day, money-back
guarantee for this product. Try it for a month, and if
not satisfied return it undamaged for a full refund of
the purchase price (less original shipping & handling
LIMITED WARRANTY: Should the product
fail due to a manufacturing defect during the first year
after purchase, we will repair or replace the unit (at
our option) , at no charge to the buyer. We will pay for
shipping the repaired or replaced unit back to the buyer.
Transportation of the faulty unit to our repair facility
is the responsibility of the buyer. All warranty repairs
must be made by our authorized repair facility. This
warranty does not cover misuse or abuse. Consequential
and/or incidental damages are not covered by this
warranty. Conformance of individual production units to
specifications quoted are not guaranteed.
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Specifications, prices and availability
subject to change without notice.
Copyright 2006 Warren Gregoire and Associates LLC
All Rights Reserved.