Re: Subwoofers from Scan

["Rick Hughes" <rhughes@xxxxxxx>]

If anybody is interested in building a Sub-Bass, I have a LOT of
information on design, construction etc.
I built an active sub bass with electronic variable filter using the
NHT-1259 bass driver.  Probably the best sub-bass driver available
anywhere at any price short of a small house.

For those interested I have pasted in the FAQ for this diver below.  If
anybody wants more information send me a personal email.

Rick



==============NHT 1259 FAQ Version 0.53 - 9/26/96 =====================

This is the unofficial NHT 1259 list of frequently asked questions.
This version is PRELIMINARY.  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.  Mr. Ken Kantor of NHT has contributed to
this document and corrected errors, but is not vouching for accuracy.

=>What has changed since Version 0.5?

The URL for the bass mailing list has been updated.
Some T/S parameters have been updated based on measurements from NHT.
Caution on cutting the right size mounting hole added.

=>What has changed since Version 0.4?

The equation for driver impedance as a function of K and E constants
is included.  Effective cone diameter has been changed from 20cm to
25.4cm.  Moving mass has been added.

=>What is the 1259?

In brief, the 1259 is a cast-frame 12" woofer with a polypropylene
cone, vented voice coil, large magnet structure, rubber surround, and
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 bass mailing list, you get a
special price of $115 each for this driver from A & S.  Normal price
is $149 each.

=>What are its characteristics?

Effective cone diam.    D:     25.40 cm           10.00 in.
Maximum linear excursion Xmax:      1.30 cm            0.51 in.
Bl product   Bl:      9.63 TM
Voice coil dc resistance   Re:      3.40 Ohms
Mechanical mass  Mmd:    115.09 gm
Total Moving mass  Mms:    121.66 gm
Mechanical compliance  Cms:      0.521 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:     25.40 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:     2.600
Electrical Q  Qes:     0.560
Total Q  Qts:     0.461
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

To convert those coefficients to an impedance:
Z = KRM*0.001*(2*Pi*F)^KXM + j ERM*0.001*(2*Pi*F)^EXM
The LEAP KRM value is in milliohms and the ERM value is in mH.
That's why there is the factor of 0.001 in the above formula.

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

=>What are its dimensions?

Outside diameter:	12.22"
Inside diameter (mounting hole):	11.12"
Frame lip height:	0.375"
Maximum magnet structure diameter:	5.71"
Depth, mounting face to back of magnet:	5.00"
Motor height:	2.29"
Total height:	5.47"
Screw holes:	Eight
Screw holes centerline diameter:	11.563"
Screw hole diameter:	0.25"
Electrical terminal width:	0.205"

Caution: Cut the mounting hole very accurately.  If you try to mount a
1259 in an oversized hole, you will not have enough wood left to hold a
screw.  This goes for wood screws, but even more so for T-Nuts or
Well-Nuts.

=>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 fiberglas insulation.

In the NHT 3.3, the first 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).

The best material for speaker boxes is 1" MDF.  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 NHT/SW3p uses 820g of acoustic
polyester stuffing with the 1259, but your enclosure may do better with
slightly different amount.

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. 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
200 uF
Non     NHT
Polar    1259
Electro-
lytic

-In------------*--------

=>Is this woofer really great?

This is a very good woofer.  It is tailored to excellent product
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 consistency 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 suitable 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.

=>How can you shield an NHT 1259 for use near a TV/monitor?

The only practical way to reduce the stray magnetic field from the
speaker is with a magnet glued to the back of the speaker itself,
sized and placed for optimal cancellation.

Aluminum, copper, and tin shields will have NO effect on static
magnetic fields.  Steel or mu metal shields will redirect static
magnetic fields and can reduce them somewhat if well placed, but
it is impossible to completely surround the speaker with metal,
so quite a bit of stray field will get through the best shield.
Don't expect very much improvement with these shields.

=>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.

A & S may make available their $145.00 12" dual voice coil
subwoofer
to the bass-list for $90.00.  This is designed for a bass-reflex
alignment with an FS of 20hz and a Qt of .22  It has a cast frame, flat
spider, dual magnet, rubber surround and a doped paper cone.

=>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, Fifth Edition
by Vance Dickason (C) 1995 - ISBN 1-882580-10-9
$34.95 + $4.45 S&H from:
Old Colony Sound Lab
PO Box 243
Peterborough NH 03458-0243 USA
603-924-9464
$30.00 + approx. $3 shipping from:
Madisound
8608 University Green; Box 4283
Madison WI 53711 USA
608-831-3433

Read the rec.audio.* FAQ, which is available on many usenet sites.
It is also available on the world-wide web at:
http://hydra.unik.no/~robert/hifi/faq/
It has an entire part on speakers with many tips on speaker
construction.

Subscribe to the bass mailing list to learn more about subwoofers
and DIY loudspeakers in general.

There are two versions of the bass mailing list:
* "bass" the real-time list.
* "bass-digest" the digest version.

To subscribe to bass, send "subscribe bass Your Name" in the body
of a message to "listproc@xxxxxxx".  The message
subject is ignored.  If you wish to receive the list in digest
format, add the following line: "set bass mail digest"

Look at these web sites to see speakers made by others, some
of which use the 1259:
http://reality.sgi.com/employees/pdc_engr/bass/
http://www.cen.uiuc.edu/~nb10440
http://bundy.hibo.no/~rpd/Speaker/
http://www.psy.cmu.edu/~stark/audio/DIYaudio.html
http://invalid.ed.unit.no/~dunker/horns.html
http://rsphysse.anu.edu.au/~bill/sp.html

And the official bass mailing list and NHT1259 archive site:
http://www.ece.utexas.edu/~speakers/index.html

=>Where can I send comments on this FAQ?

Please address all discussion of the NHT 1259 to the bass mailing
list.  If you have a suggestion or correction for this specific FAQ,
please send it to Bob Neidorff <neidorff@xxxxxxx>.  I can't promise
that I will read every article in the bass list, so please send
specific corrections directly.

=>Who wrote this FAQ?

Credit goes to many people, including:
Paul Close
Ken Kantor
Doug Purl
and many other members of the bass mailing list, may it live on forever.
========================================================================

--
Bob Neidorff; Unitrode Corporation    Internet: neidorff@xxxxxxx
7 Continental Blvd.                   Voice   : (US) 603-429-8541
Merrimack, NH  03054-0399 USA         FAX     : (US) 603-429-8564

--
_______________________________________________________________________

Rick Hughes
\   Customer Services and Support Programme Manager
\\   Newbridge Networks Limited
\\\   +44 1633 772327
+44 831 680524 (mobile)

_______________________________________________________________________

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