Product Review -
Eminent Technology LFT - VIII Speakers - March, 1997
By John E. Johnson, Jr.
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
[email protected].
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
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