Index to Articles

 

Product Review - Eminent Technology LFT - VIII Speakers - March, 1997

By John E. Johnson, Jr.

Divider

Click for larger photo
Click to see
large photo

Eminent Technology LFT-VIII Hybrid Linear Field Transducer Loudspeaker; Tweeter and mid-range (separate) are aluminum etched onto Mylar sheet; One 8" cone woofer; Frequency response 25 Hz - 20 kHz 4 dB; Sensitivity 83 dB/w/m; Power requirements 75 w minimum; 8 Ohm nominal impedance; Size 60" H x 13" W x 1" D (woofer enclosure is 15" D); Weight 60 pounds each; Light oak trim; $1,500 pair; Eminent Technology, Inc., P.O. Box 6894 (225 East Palmer Street), Tallahassee, Florida 32314 (32301) (shipping address in parentheses); Phone 904-575-5655; Fax 904-224-5999; E-Mail brucet@eminent-tech.com.

There are several basic types of speakers. We are all familiar with the most common type, the cone speaker, where a voice coil is wound around a former which is situated at the base of a cone. The coil moves inside a magnet, and the cone moves air back and forth, which we hear. The other types use thin sheets of plastic or metal which move the air. One of these is the electrostatic speaker which has a conductive plastic sheet, with a bias voltage applied to it. The plastic sheet is suspended between two perforated metal "stators". The music signal voltage is applied to the stators, and the charged plastic sheet moves back and forth as the voltage on the stators changes [
click here to see diagram]. This is a "push-pull" speaker, since one stator pushes the charged plastic sheet, and the other pulls (they alternate being + and -, depending on which way the current is flowing in the transformer). A second type in this category is the Planar Magnetic speaker. In this case, there is also a plastic sheet, onto which a coil of fine wire is glued. Magnets on one side attract or repel the coil depending on the direction of current flow (the music signal alternates in direction of flow). Because the magnets are all on one side, this type of speaker is called "single-ended" [click here to see diagram]. A ribbon speaker uses either a metal foil (true ribbon speaker) or a foil glued onto a plastic ribbon (quasi-ribbon speaker), about a half inch wide. It is suspended between magnets which are situated to either side of the ribbon [click here to see diagram].

The Eminent Technology LFT-VIII (modified recently from original version) is similar to a planar magnetic speaker. However, magnets are placed on both sides of the plastic sheet (making it push-pull), and the voice coil is photo etched onto the sheet rather than being a wire that is glued on. Eminent calls this "Linear Field" [
click here to see diagram]. Because the coil is so thin (0.00033"), Eminent feels this gives better transient response than planar magnetic speakers, since the final weight of the sheet is less [click here to see photo of mid-range panel] [click here to see photo of tweeter]. Also, because the magnets are on both sides of the sheet, it (the sheet) is always within a strong magnetic field, whereas with planar magnetics, the sheet moves away from the magnetic field, in one of the directions. Compared to ribbon speakers, the linear field speaker moves more air, since the sheet is larger than the ribbon (the LFT-VIIIs have a mid-range sheet of 1262", and the tweeter is 102"). Finally, compared to electrostatic speakers, the linear field speaker does not use a transformer and does not need its own power supply. Of course, these are all engineering differences and opinions of the manufacturer. The designers of other speaker types have their own opinions as to what is best. The final analysis is the sound - Does it perform well or not?

The LFT-VIIIs arrived in several packages. The panels and woofer enclosures were separate. Putting them together was not simple, but only because the instruction manual was written for engineers rather than consumers. Therefore, I will mention some things here that will clarify assembly, for those of you who decide to purchase a pair. (After you put the speakers together, the instruction manual can be used for its extensive list of specifications and other valuable information.)

First, this is a two-person job. After unpacking all materials, place one of the woofer enclosures in the middle of the room. Then lay it over on one side and attach the metal rod feet with two metal screws (two for each metal foot) so that the front has a section protruding farther than the section protruding from the rear [
click here for photo]. Next, turn the enclosure upright, and have a second person hold one of the panels along the front, slightly lifted, so that you can attach it to the woofer enclosure with the screws that are provided. Now attach the front and back grille cloth frames (they adhere to the panel with Velcro). Finally, attach the wires on the back so that the woofer and panels are connected together at the binding posts (unless you are bi-wiring or bi-amping), and the panel wires to the appropriate screw terminals. The mid-range has two wires, and the tweeter has one that can be attached to one of three screws: high, mid, or low [click here for photo]. The high position gives the most high frequency output, and the low, the least (depending on how much high frequency output you like). Be careful when moving the assembled speakers so that you don't poke your fingers into the plastic sheets.

We found the LFTs to sound best (in our opinion) with them situated slightly toed in and raised at the front. Metal spiked feet are provided so that the speakers can be lifted from the floor entirely. We used the spikes at the front, and placed metal discs under the spikes so that they would not damage our carpet or pierce the cables that we have under the carpet. We tested them with several components, all power amps being at least 100 w/ch.

The "high" tweeter setting sounded best (to us), and even in this position, the highs were very smooth and slightly laid back, yet sufficiently detailed. This is a design preference of the manufacturer which we found to be quite pleasant, especially with movie sound tracks. The use of dipolar speakers (sound comes out the front and back of the speaker, and air moves out the front while moving in at the back) [
click here to see diagram of speaker motion] is controversial with home theater setups. This is because dipolar (and bipolar) speakers give a more diffuse sound stage than the conventional speaker where sound only comes out the front (a more focussed sound stage). To me, the dipole (and bipole) make great home theater speakers because of the diffuse sound stage. In fact, they do very well in the rear with discrete digital surround (e.g., AC-3), contrary to the common opinion. On the other hand, dipoles are more difficult to position.

Some movie sound tracks have very specific effects coming from the rear. Because digital surround is so discrete, the left rear always comes from one spot, and so does the right rear. This can be distracting. True surround should come from all directions, and thus, IMHO, there should be a mix of a particular sound between at least two speakers, with the sound more prevalent in one. To this end, I would suggest that surround sound processor manufacturers incorporate a digital processing scheme that allows progressive blending of sound from each channel into the two channels on either side of it (e.g., front right blended into center and rear right, rear right blended into front right and rear left, etc.) The blending should be user selectable as to the amount of blending from zero to full, where zero means no blending and full means all channels summed together into mono sound coming from all speakers. This way, all types of speakers could be used for home theater in any mode (Pro Logic, AC-3, DTS, etc.) with complete satisfaction for just about any consumer, and any source (movies or music). In any case, however, dipolar speakers, such as the LFT-VIIIs do very well in the home theater. They sounded great even with a very different center channel speaker. Again, this may be due to their diffuse sound stage. LFT-VIIIs might be overkill in the rear, but more and more, studio producers are sending low frequencies to the rear, even with Pro Logic, and certainly, digital surround will necessitate full range speakers in the rear. So, maybe it isn't overkill after all. Depends on how much you like home theater to produce terrific sound, which the LFT-VIIIs certainly do.

The LFTs use an 8" cone woofer to drive the low frequencies (25 Hz - 180 Hz) [
click here for photo]. The mid-range crosses over to the tweeter at 10 kHz. The blend, to our ears, was seamless. The bass was deep and clean, with no boominess. For movie sound tracks, we found that the use of a good subwoofer helped the relatively small LFT woofers kick out that extra kaboom. For music (CDs), the LFT woofers were fine.

Frequency Response Test Results - 1 meter, left speaker, grille cloths on, SPL set to approximately 80 dB at 1 kHz (Note: these tests are in a live room, not in an anechoic chamber. The results you get in your own room may be different.):

20 Hz - 73.4 dB
25 Hz - 70.1 dB
31.5 Hz - 80.4 dB
40 Hz - 79.7 dB
50 Hz - 87.2 dB
63 Hz - 89.4 dB
80 Hz - 82.7 dB
100 Hz - 88.9 dB
125 Hz - 86.1 dB
160 Hz - 82.3 dB
200 Hz - 88.2 dB
500 Hz - 82.3 dB
800 Hz - 85.0 dB
1 kHz - 80.4 dB
2.5 kHz - 83.1 dB
5 kHz - 83.6 dB
8 kHz - 82.8 dB
10 kHz - 75.9 dB
12.5 kHz - 79.3 dB
15 kHz - 80.1 dB
18 kHz - 74.5 dB

Frequency Response Test Results - 13 feet, left speaker, grille cloths on, SPL set to approximately 80 dB at 1 kHz (Note: these tests are in a live room, not in an anechoic chamber. The results you get in your own room may be different.):

20 Hz - 84.2 dB
25 Hz - 84.6 dB
31.5 Hz - 88.4 dB
40 Hz - 75.8 dB
50 Hz - 79.7 dB
63 Hz - 91.9 dB
80 Hz - 94.1 dB
100 Hz -70.5 dB
125 Hz - 89.2 dB
160 Hz - 84.7 dB
200 Hz - 89.3 dB
500 Hz - 84.7 dB
800 Hz - 79.8 dB
1 kHz - 79.9 dB
2.5 kHz - 88.3 dB
5 kHz - 84.2 dB
8 kHz - 86.0 dB
10 kHz - 80.3 dB
12.5 kHz - 75.8 dB
15 kHz - 83.2 dB
18 kHz - 75.9 dB

As you can see, the room affects the frequency response greatly. Even with the microphone up close, there is sound coming out the back of the speaker that has to be contended with. In the farfield test (13 feet), sound coming out the back is reflected by whatever is behind the speaker and definitely will be picked up by the test microphone. Even so, we found the sine wave tests to be very revealing, in that it is easy to hear harmonic distortion (sine waves are very tough on speakers, and an individual sine wave will sound harsh at mid to high frequency if there is significant harmonic distortion). We could not detect any harshness even at relatively high SPL, and the woofer performed remarkably, with no rattling or audible harmonics when the sine waves were below 20 Hz (usually even good speakers produce some audible harmonics at, say 16 Hz sine wave input - 32 Hz audible harmonic). This represents extremely good test results.

In summary, the Eminent Technology LFT-VIII speakers are very good performers. Clean and precise. You have to like the dipole sound, but if you do, these are definitely winners. At the price, they would be tough to beat.

John E. Johnson, Jr
.


Copyright 1995, 1996, 1997 Secrets of Home Theater & High Fidelity
Return to Table of Contents for this
Issue.