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Physical Characteristics
The Fostex NF-1A active monitor’s enclosure is a two-way, bass reflex
design with dual, flared elliptical ports. The enclosure is made up of 7/8-inch
thick vinyl covered MDF on the sides, while the black painted baffle includes
an additional 1/2-inch of thickness at the woofer. Transducers are recessed
in the baffle and secured with wood screws. The interior sides and top of
the cabinet are lined with 1/2-inch fiberglass, while the back and bottom
each include “hyperbolic paraboloid” deflectors to damp standing
waves and any internal back waves reflecting inside the enclosure. The amplifier
compartment is isolated from the rest of the enclosure by the 7/8-inch thick
MDF.
The visually impressive woofer design of this nearfield monitor blends new
and old technology: The hyperbolic paraboloid cone design, which Fostex
labels HP, is based on well-established structural engineering principles.
Harry Olson used these principles to design a similar complex-topology loudspeaker
cone for the RCA LCA-1A studio monitor and JVC also introduced a speaker
system using a similar woofer cone in the 1970s. Today, in order to minimize
weight but retain tensile strength, soft drink companies manufacture plastic
bottles with hyperbolic paraboloid bases. The HP cone offers extended bandwidth
(no puckering on the extreme low frequencies and reduced breakup on the
high end) and the cone and surround allow this transducer to operate well
past 5 kHz.
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On-axis
and 30° off-axis frequency response. Neither on-axis nor off-axis
responses are flat. Note narrow notch at 750 Hz.
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On-axis
impulse response: Impulse response shows decent alignment of woofer
and tweeter, but not great. Good decay, but not the smoothest.
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Distortion
vs. frequency. Very good distortion figures: less than 1% THD above
50 Hz, and mostly below 0.5%. Key: THD+N = ³ trace; 2nd harmonic
= m trace; 3rd harmonic = q trace.
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This
spectral contamination distortion test compares 12 non-harmonically
related input tones (tall spikes) to the speaker output. The resulting
non- linear distortion products (intermodulation, crossmodulation)
are the lowest we’ve tested (about -60 dB).
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The foam surround is
also well thought out. Fostex calls it a “UDR” (up down roll)
tangential edge, which comprises alternating up half roll, down half roll
sectors, divided by a tangential wall. (Tangential crease or pleat surrounds
have long been used on tweeter and compression driver diaphragms in order
to minimize diaphragm rocking modes.) Other key features on this cast
aluminum frame woofer are double stacked flat 3 3/4-inch diameter spiders,
a 1 1/2-inch diameter aluminum bobbin voice coil assembly and a vented
pole piece. The conventional, ceramic magnet motor structure is magnetically
shielded by a bucking magnet and shielding can. Terminations are 0.205-inch
and 0.110-inch male tabs.
The faceplate to the Fostex tweeter is a black cast aluminum horn that
loads the diaphragm by means of a wide dispersion waveguide. The 21mm
soft dome is a “UFLC” (Urethane Film Laminated over Cloth) diaphragm,
which is attached to a Kapton bobbin voice coil assembly. The conventional
ceramic magnet motor structure is shielded with a bucking magnet. The
T-yoke is topped off with a hemisphere of felt to absorb the back wave
off of the diaphragm. Terminations are also 0.205-inch and 0.110-inch
male tabs.
Powering the transducers are dual 60-watt amplifiers with a 6dB/octave,
minimum-phase active crossover at 5 kHz. The power supply includes an
oval, low-hum field transformer. Inputs can either be balanced (XLR) or
unbalanced (1/4-inch). User controls include a continuously variable power
input adjustment and a continuously variable tweeter level adjustment
(±3dB). Three-way switches modify LF and HF output (+3dB, 0, -3dB)
and there is also a mid frequency attenuator switch (-3 dB). For the measurements,
all user controls were set to neutral or off.
Acoustical Characteristics
Measurements on this system were made using a SYSid test system,
Earthworks M30 microphone and Lab 101 mic preamp. The speaker was supported
on a K&M 21300B Crank It Up speaker stand. Our measurements showed
that the frequency response of the Fostex NF-1A is not “flat,”
despite its very low distortion measurements. From 200 to 2k Hz, the on-axis
response rises 3 dB, with a narrow band valley centered at 750 Hz. From
2 to 3 kHz, the response falls off 4 dB, and then climbs 8 dB to around
7.5 kHz. After 7.5 kHz, the response falls off again by 4 dB up to 15
kHz. The 30° off-axis response follows that of the on-axis until 1
kHz, where it falls 3-dB/octave to that of the on-axis. At 10 kHz, the
off-axis response equals that of the on-axis then falls off sharply after
that.
The impulse response shows decent alignment in time between tweeter and
woofer, but not perfect. The decay is damped, but not smooth.
Now the good news. Besides the cone technology, the major highlight of
this monitor is its very low distortion. The Fostex NF-1A studio monitor
is the lowest distortion speaker we have tested to date. Above 50 Hz,
the THD is less than 1%, with most of the spectrum under 0.5%. The multi-tone
spectral contamination test looks at the system’s intermodulation
and crossmodulation products (“self noise”). Once again, the
spectral contamination is the lowest we have ever tested, with the noise
floor being around 60 dB down from the input tones.
The Fostex NF-1A has likely been designed to emulate commodity loudspeakers
used in portable combo systems and is probably best suited to producing
a reasonable impression of what mixed material will sound like in the
real world. But with the judicious use of a parametric equalizer, the
NF-1A can put out both a flat response and super clean sound.
John Schaffer and Rob Baum are engineers associated with Menlo Scientific,
an independent test facility in Berkeley, Calif. For information on testing
methodology, visit www.mixonline.com.
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Reprinted with permission from Mix Magazine, August, 2000
© 2000, Intertec Publishing, A Primedia Company All Rights Reserved
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