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NHT1259
FAQ
This is
version 0.2 of the
PRELIMINARY, unofficial NHT 1259 list of Frequently Asked Questions. The
accuracy of data in this FAQ is unconfirmed and this FAQ is not sanctioned by
NHT, the bass mailing list, or any other person(s). Mr. Ken Kantor of NHT has seen
this document and corrected one error, but is not vouching for accuracy.
This document
is now many years old. Some of the details such as driver availability may no
longer be correct. However technical details for the NHT1259 driver itself
should still be valid.
What is the 1259?
In brief, the 1259 is a
cast-frame 12" woofer with a
polypropylene cone, large vented voice coil, large magnet structure, rubber
surround, and extremely long linear throw. The 1259 is a 6 ohm speaker with a
single voice coil.
The 1259 is a custom
woofer designed by Bill Bush, NHT Chief Engineer, for
use in NHT products and made for them by Tonegen of Japan. NHT has made the
driver available to builders through a retailer called A & S Speakers (Also
known as Just Speakers).
A & S Speakers
4075 Sprig Drive
Concord CA 94520 USA
Voice 510-685-6400
FAX 510-603-2724
As of January, 1996, if you
mention the internet, you get a special price
of $115 each for this driver from A & S. The regular price is $149 each.
Madisound also sells the 1259 for $149.
What are its characteristics?
Effective cone diam. D: 20.00 cm 7.87 in.
Maximum linear excursion Xmax: 1.50 cm 0.59 in.
Bl product Bl: 7.50 N/A
Voice coil dc resistance Re: 3.40 Ohms
Mechanical mass Md: 32.00 gm
Mechanical compliance Cms: 1.52 e-3 m/N
Mechanical losses Rms: 1.55 kg/s
Nominal impedance 6 ohms
Maximum music power 300 watts
Frequency range 19-1000 Hz
Sensitivity 90 dB SPL @ 2.83 VRMS @ 1 meter
Voice coil inductance 1.2 mH
Voice coil diameter 50 mm
Voice coil height 34 mm
Air gap height 8 mm
Magnet weight 59 oz
THIELE/SMALL PARAMETERS
Effective cone diameter d: 20.00 cm
Maximum linear excursion Xmax: 1.30 cm
Resonant frequency F0: 19.00 Hz
Equivalent volume Vas: 190.00 L 6.71 cu. ft
Mechanical Q Qms: 2.60
Electrical Q Qes: 0.56
Total Q Qts: 0.46
Reference efficiency n0: 0.22 %
Output level< SPL: 85.52 dB SPL
Effective area Sd: 507.00 sq. cm
Maximum displacement Vd: 408.41 cu. cm
Krm: 28.89
Kxm: 26.67
Erm: .584
Exm: .647
PERFORMANCE COMPARISONS CLOSED REFLEX
Enclosure volume Vb: 88.62 299.21 L
-3dB frequency F3: 32.43 15.56 Hz
Peak response ripple Rh: 0.00 0.04 dB
Maximum acoustic output Par: 0.15 0.03 W
Maximum SPL SPL: 100.88 94.14 dB SPL
Maximum electrical input Per: 68.57 14.52 W
FREQUENCY RESPONSE
Intervals per octave 6.0
CLOSED REFLEX
Starting frequency
15.9 -12.66 -2.21 dB
17.8 -10.78 -0.77 dB
20.0 -8.99 -0.27 dB
22.4 -7.29 -0.13 dB
25.2 -5.73 -0.09 dB
28.3 -4.36 -0.07 dB
31.7 -3.20 -0.05 dB
35.6 -2.27 -0.03 dB
40.0 -1.56 -0.02 dB
44.9 -1.05 -0.01 dB
50.4 -0.69 -0.00 dB
56.6 -0.45 0.00 dB
CLOSED BOX DESIGN
Desired system Q Qtc: 0.71
Recommended box volume Vb: 88.62 L 3.13 cu. ft
Enclosure damping 3.00 (0 - 5)
Tuning ratio a: 1.91
System resonance Fc: 32.43 Hz
Actual system Q Qtc: 0.71
-3dB frequency F3: 32.43 Hz
Peak response ripple Rh: 0.00 dB
Maximum acoustic output Par: 0.15 W
Maximum SPL 100.88 dB
Maximum input power Par: 68.57 W
VENTED BOX DESIGN (NOT RECOMMENDED!!!)
Recommended box volume Vb: 299.21 L 10.57 cu. ft
Tuning ratio a: 0.64
Enclosure resonance Fb: 16.50 Hz
Recommended vent diameter Dv: 20.05 cm 7.89 in.
Desired vent diameter Dv: 5.08 cm 2.00 in.
Vent length Lv: 3.73 cm 1.47 in.
-3dB Frequency F3: 15.56 Hz
Peak response ripple Rh: 0.04 dB
Max. acoustic output Par: 0.03 W
Maximum SPL 94.14 dB SPL
Maximum input power Per: 14.52 W
How can I use it?
The 1259 works best in a sealed enclosure of
approximately 3 cubic feet (85 liters), stuffed with approximately 3 pounds of
acoustic polyester fiber or fiberglass insulation.
In the NHT 3.3, the
first speaker design in which the 1259 was incorporated,
the internal volume used is 2.7 cubic feet (76.44 liters). In addition, the NHT
3.3 makes use of a sub-divided enclosure with resistive damping between sections
to achieve an even lower 3dB rolloff frequency (called F3).
One of the best
material for speaker cabinets is 1" MDF (Medium Density
Fibreboard). It is more expensive than particle board, but much better damped.
Some have used 3/4" MDF successfully, and have made up for the thinness with
extra braces. MDF is hard to find, but many lumber yards can special order it
from their normal distributors. Often, the lumber yard doesn't even know that
they can get it, because so few people ask for it.
When calculating volume,
allow for the space taken up by the speaker itself
and all internal braces. Use lots of braces spaced irregularly. They will not
hurt and will probably help. Use cleats at the joints, shelf braces, cross
braces, and even battens across the larger panels. Box L vs W vs H is not
critical at all (nor in any other closed box in which the shortest wavelengths
to be reproduced are appreciably longer than the shortest box dimension).
Exact enclosure volume
is not critical, and stuffing can be added or
subtracted to fine tune the response. Mr. Ken Kantor recommends adjusting the
stuffing by monitoring the impedance versus frequency of the sealed box system.
Add stuffing to lower the frequency where the impedance is highest. When that
impedance peak starts to rise in frequency, you have added too much.
The 1259 is built
to be mounted facing outwards. Do not use it facing
downward or upward. The suspension is not suitable for this use and distortion
and/or damage will result from the sagging cone.
Is there a dual voice coil version available?
No. Some people use two
1259 subwoofers for stereo and claim that the performance is superb. There are
many advantages to dual subwoofers, such as better stereo image, fewer problems
with room interactions, and of course, more bass. Others only use one 1259. If
you use just one, you should get a separate power amplifier just for the 1259
and use that amplifier to mix stereo down to mono.
How low will this driver go?
In an optimum cabinet, it will go flat to
30 Hz, and will generate appreciable energy down to 20 Hz, thanks to the more 6
db/octave rolloff of an acoustic suspension cabinet (versus the 12 db/octave
rolloff of a tuned port enclosure). Depending on the shape and dimensions of the
listening room (i.e., the combination of room gain and room modes), a proper
1259 subwoofer can produce satisfying response well down towards 20 Hz. The 1259
is designed to work well in a near cubical enclosure placed flush against the
wall with the driver pointed out into the room.
How high will the 1259 go?
The woofer will operate effectively up to 1
kHz, but the signal applied to the 1259 should be down by at least 12 dB at 1
kHz. Optimum crossover frequencies range from 100 to 250 Hz.
What about a crossover for it?
In the NHT 3.3, the 1259 is driven
through the following low-pass filter: This filter has a cutoff frequency of
approximately 100 Hz.
12 mH
+In-----L1-----*--------
Iron | |
Core C1 |
200uF |
Non NHT
Polar 1259
Electro- |
lytic |
| |
-In------------*--------
Is this woofer really great?
This is a very good woofer. It is tailored
to excellent home audio in a reasonably sized sealed box, and at a fair price.
In addition, the NHT Company puts extremely high demands on Tonegen, the speaker
manufacturer, for consistancy and accuracy in the driver. NHT tests each 1259 to
10% nominal Vas, Mms, and Qts, and +/- 1 dB response, at their factory before
shipping to A & S. This is much tighter tolerance than home builders get in
other drivers, and has many benefits.
You can spend more money
and get a larger woofer, a bigger magnet, etc. Such
a driver may be more suitablet for specific applications, like theater sound or
outdoor sound, but may actually be inferior for home use. Large magnets improve
efficiency, but reduce the low-frequency radiation of woofers by acting as a
brake on the counter-voltage the voice coil generates as it cuts through the
flux of the magnet. Larger woofers suffer from cone breakup and distortion for
large excursions. Finally, more expensive woofers may require larger or more
complex enclosures.
To our knowledge, the
NHT 1259 is one of the most effective deep-bass woofers
that have been offered to the DIY constructor. The 1259 has been designed
specifically for a small sealed box (acoustic suspension). Used in a vented box,
the 1259 might produce more bass, but at the expense of some combination of
increased distortion, looser bass, less even frequency response, less maximum
output, and at the risk of damage. Vented boxes are not recommended for the NHT
1259. The NHT 1259 is a very high compliance woofer which relies on the air in a
sealed enclosure to provide back pressure that linearizes cone motion and
protects the driver from damage or destruction by excessive excursion below the
frequencies at which a vented box loads the woofer cone.
One additional attribute of the
NHT 1259 is that a key developer of the
driver, Mr. Ken Kantor, has helped members of the bass list use the driver to
its full potentials, offered candid advice, and even revealed some NHT
proprietary details for using the driver. This level of support for a DIY driver
is unheard of today.
Are there any other great woofers available for DIY subwoofing?
Several
manufacturers offer excellent woofers, including Eton, Peerless, JBL, McCauley,
Madisound, and Dynaudio. Members of the bass/DIY loudspeaker list often discuss
their experiences with other woofers. More than a hundred members of the list
have bought the NHT 1259, some who appreciate its unique balance between
reasonable price and superb performance, and others who went by its reputation.
Where can I learn more?
Buy and read the Loudspeaker Design Cookbook by
Vance Dickason. This is the best treatise on DIY speakers that is friendly and
readable, yet technical and accurate.
The Loudspeaker Design Cookbook, by Vance Dickason ISBN 1-882580-10-9
Old Colony Sound Lab
PO Box 243
Peterborough NH 03458-0243 USA
603-924-9464
Madisound
8608 University Green; Box 4283
Madison WI 53711 USA
608-831-3433
Who wrote this FAQ?
Credit goes to many people,
including:
- Paul Close
- Ken Kantor
- Dan Hildebrand
- Doug Purl
and many other members of the bass mailing list, may it
live on forever.
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