hifi/stereo - micro transmission line speaker system

HiFi/Stereo - Micro Transmission Line Speaker System
Warren Gregoire and Associates LLC
FOR THE DISCRIMINATING AUDIO ENTHUSIAST
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SYSTEM DESCRIPTION

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 line.

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.

SPECIFICATIONS

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 their name.

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 3.75" deep.

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 13kHz.

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 150Hz.

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 charge).

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.