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THE MOST COMMONLY ASKED QUESTIONS ABOUT BUILDING ENCLOSURES
Many JBL users build their own loudspeaker enclosures. Their audio
skills range widely from novice to expert. From the thousands of
letters and calls we have received addressing the subject of
loudspeaker enclosure construction, we have determined the most common
questions and present the following Questions and Answers. The
particular questions listed attempt to answer as many questions as we
feel are necessary to provide enough information to build an enclosure
which will allow your JBL loudspeaker to operate to its potential. The
questions selected here concentrate on vented "bass reflex" enclosures,
since low frequency horns are fairly complex, and many good tested
designs exist. Also, it is often more econonomical to buy a bass horn
enclosure than to build one. Vented box enclosures are by far the most
popular enclosure type. Vented boxes are finding increasing use by
touring sound companies, displacing existing horn enclosure designs
because of the greater low frequency power output and extended low
frequency capability they offer when used in arrays. In addition to
their simple design requirements, vented loudspeaker enclosures offer
flexibility of design in shape, weight and component complement, and
usually produce the best results obtainable from modern loudspeaker
drivers at the lowest cost.
[1]
Q: What makes a good vented enclosure?
A: Basically, an enclosure serves to partition the front and rear of
the driver's cone, preventing the opposing air pressure changes
produced by cone motion from cancelling, and allowing the radiation of
sound from the front of the driver only. In addition, vented
enclosures allow the compressibility of the air inside the enclosure to
work as a more active part of the "system" consisting of driver and
enclosure. Beyond these two basic functions, a low frequency
loudspeaker enclosure should do absolutely nothing, that is, it should
add no effects of its own--no vibration, no tonality, no motion--
nothing to interfere with or absorb acoustic energy produced by the
driver.
[2]
Q: Is it possible to get low, punchy bass from a small enclosure?
A: Yes, if the driver in the enclosure is designed for low bass
operation in a small enclosure. Unfortunately, it's usually a small
driver that can work properly in a small enclosure, and that dictates
that lower sound levels will result from the small amount of air such a
small driver can move. Larger boxes (with larger bass drivers) produce
more bass, smaller boxes produce less bass. It's a fact of life, like
the fact that it takes a bass viol, a tuba, longer piano strings, or
very large organ pipes to produce bass energy in the air. Low bass
requires that more air move, and bigger boxes contain more air that can
be put to work making low bass.
[3]
Q: Can I get more bass from my enclosure by installing a bigger driver?
A: A given enclosure will not automatically produce more bass when a
larger driver is installed, in fact the opposite is often the result.
[4]
Q: What about putting two drivers in the enclosure to increase bass?
A: Placing two bass drivers in an enclosure designed for one will
usually produce less bass and more midrange output, and will upset the
operation of the driver-enclosure system because each driver will
behave as though it is installed in an enclosure which has only half
the internal volume of the original enclosure (with one driver).
[5]
Q: What should I do to use two drivers (for more bass)?
A: There are two alternative possibilities. When using two identical
drivers, you can build an enclosure with twice the internal volume of
the original enclosure that contained one driver, or you can duplicate
the original enclosure and stack the two. As the latter alternative
suggests, when building the double enclosure, it's necessary to treat
the enclosure as if it were two enclosures--you must double the porting
used on the single smaller enclosure--although it is not necessary to
divide the volume of the double enclosure unless two different driver
models (e.g. E130 and E155) are used and their interaction would be
undesirable. A usable example of this might be a 227 liter (8 cubic
foot) enclosure divided into two chambers so that the E130 occupies 57
liters (2 cubic feet) and the E155 occupies 170 liters (6 cubic feet).
In this case, the ports tuning either chamber to the same desired
frequency will be quite different.
[6]
Q: What does port or enclosure "tuning" mean?
A: In exactly the same way the resonant note from a bottle can be
raised and lowered by adding or pouring out liquid to change the
bottle's air volume, enclosure tuning is affected by the ratio of air
volumes in the port (the bottleneck) with its attendant flow
resistance, and the enclosure interior volume. Tuning of loudspeaker
enclosures is a result of manipulating the differences in effective air
mass between the enclosure interior and the air in the port. The
bottle-like nature of a vented enclosure is known as a "Helmholtz
resonator." The ports or ducts in a vented enclosure work only over a
narrow band of frequencies near the chosen tuned frequency, producing
the same effect noted when blowing across a bottleneck--a single
distinct pitch.
[7]
Q: Is it always necessary to use a port for good bass?
A: JBL uses vented enclosure designs because they are superior to
sealed enclosure designs in several important ways--as long as it is
possible to tightly control the loudspeaker driver parameters in
manufacturing as JBL does. Vented designs produce lower distortion at
the lowest operating frequencies, afford the driver protection against
mechanically destructive large cone excursion, and better enable the
driver to absorb and utilize its full power rating from an amplifier
when operating at low frequencies. It is important to keep in mind
that porting and tuning an enclosure provides air loading for the bass
driver down to frequencies just below the Helmholtz frequency, but does
not provide any loading for the driver at frequencies below that, such
as subsonic turntable rumble, record warp or microphone wind pickup.
If you intend to operate a sound system at high power levels, we highly
recommend an electronic high-pass filter to eliminate subsonic input to
the power amplifier(s). This will substantially increase the available
useful power from the amplifier which will then only operate in the
audible frequency range. Such a filter is the UREI model 501 Sub Sonic
Processor, or the built-in sub-sonic switches of the JBL Electronic
Frequency Dividing Network model 5234A.
[8]
Q: Where should I locate the port(s) with respect to the woofer?
A: Bass reflex enclosures are usually designed to tune from about 100
hertz and down. The length of sound waves at these low frequencies is
over 11 feet, so port placement is not critical. Ports may be located
anywhere on the baffle with no change in bass performance; some designs
even locate ports on the back of the enclosure which works well as long
as the enclosure is not close to a wall (a couple of port diameters
away) and there is an unobstructed air path between the woofer and the
port. Overall, it's safest to locate the port somewhere on the baffle
with the woofer(s) far enough away from side walls to avoid interaction
between port and enclosure wall or the fiberglass insulation on the
wall.
[9]
Q: What should the ducts be made of? Is round better than rectangular?
A: Port ducts may be made of anything rigid, such as paper cardboard
with about a 1.5 mm (1/16") or larger wall thickness. They can be any
shape, square or rectangular (such that port area remains constant) and
made of wood or other suitable material. It is not necessary to use
PVC pipe for port tubing, particularly when most carpet stores throw
away large amounts of heavy carboard tubing of between 3 and 4-1/2
inches inside diameter.
[10]
Q: What is the relationship of duct length to port area?
A: When port area is increased, independently of other factors,
enclosure tuning is raised. If duct length is increased, independently
of other factors, enclosure tuning is lowered. To keep the same tuning
(Helmholtz frequency) you will need to increase duct length as you
increase port area.
[11]
Q: How big should the port be?
A: The bigger, the better. Any port causes some resistance to air
movement, and so introduces unavoidable losses in output to the system
as a whole. The ratios of port area and length and enclosure volume
determine the Helmholtz frequency tuning. Mechanical reactance
elements, stiffness and air mass, control the effective air mass
ratios. At very low operating levels, where air in the port does not
move very fast, a small short port will behave the same as a large
longer port as far as enclosure tuning is concerned. At high power
levels however, the restricted air flow of the smaller port will
produce output level losses, some de-tuning and at high enough levels a
small port will cause the enclosure to behave like a sealed enclosure
with little or no contribution from the port. To minimize resistive
losses, the largest practical port should be used. Computer listings
of port choices calculated to limit air velocity inside the port duct
will list duct sizes which are normally impractical. A 380 mm (15 in)
diameter port is not an unreasonable choice for a 380 mm bass driver,
however the necessary length would dictate that such a port might
itself have a volume of many cubic feet, sometimes equal to or larger
than the original enclosure. A good rule of thumb would be to avoid
ports whose circular area is smaller than at least 1/3 the diameter of
the driver such as a 127 mm (5 in) diameter port for a 380 mm (15 in)
driver. This will usually provide sufficient port area so that the
port will not "whistle" when the system is operated at high power
levels near the helmholtz frequency--a sure indication of severe system
losses and potential power compression and low-frequency output
limiting.
[12]
Q: Can I use several smaller ports instead of one big one?
A: Yes, however there is a phenomenon associated with air resistance
resulting from air drag on the internal surfaces of port ducts and
turbulence at the ends of the ports that requires a duct length
correction when several ports are used. For example, when using four
100 mm (4 in) tubes instead of one 200 mm (8 in) tube (which has the
same port area but one-quarter the internal surface area), the length
needed will be slightly less than that needed for the single 200 mm
tube, perhaps 5% to 10% less, depending on overall enclosure volume.
These effects exhibited by port ducts is exaggerated by proximity of
the duct to enclosure interior surfaces or any other type of boundary
that may cause air turbulence near the end of the duct, therefore it's
important to keep duct ends away from the rear of the cabinet or other
obstructions by an amount at least equivalent to or larger than the
dimension across the port. If you are using a rectangular port that
has as one of its sides, an enclosure wall, you might have to use some
correction.
[13]
Q: Is there a simple mathematical way of designing proper enclosures?
A: Yes, a JBL scientist, D.B. Keele Jr., simplified the work of A.
Neville Thiele and Dr. Richard Small so that anyone with a pocket
calculator and a ruler or straight edge can design the right enclosure
volume and choose the right port or duct for a given loudspeaker
driver. JBL offers, at no cost, a four-page "kit" containing detailed
step by step instructions, written specifically for non-mathematicians,
showing how to use published Thiele-Small driver parameters in
enclosure design. Examples are shown with their results graphically
represented. An enclosure design flow chart and enclosure venting
nomograph are included.
[14]
Q: Should the enclosure's baffle be removable?
A: This is a question of mechanical strength and rigidity. All
enclosures, particularly those intended for rough portable use, should
be constructed with all sides permanently fixed by glue and screws, and
sealed air-tight by virtue of well cut and glued joints. It is
preferable to mount loudspeakers from the front of the baffle board to
eliminate the possiblity of reflections from the inside of the
loudspeaker mounting hole, thus it becomes unnecessary to provide for
removing the baffle. Woofer openings are usually large enough to reach
through in order to work inside the box, for example, to install other
components.
[15]
Q: Is there a preferred shape for loudspeaker enclosures?
A: There are a number of shapes that improve performance and some that
cause distinct degradation in performance. For single, full-range
drivers (e.g. JBL's LE8T) a sphere is the ideal shape for an enclosure
because the curved surfaces avoid the diffraction effects of cabinet
edges, which bend sound waves in a manner dependent on frequency. For
multi-way loudspeaker systems, spheres are usually impractical because
of the large size needed and because of the precise orientation
required for optimal listening. Conventional enclosures work best
mounted flush into a wall where diffraction is controlled by virtue of
the wall surface, and for free-standing enclosures, tilting, angled and
curving surfaces may be employed to help reduce or control edge
diffraction. The overall shape of the enclosure is relatively
unimportant except where the shape makes it difficult to build a rigid
enclosure. It is best to avoid enclosure dimensions that are multiples
of each other, such as 1 X 2 X 4 ratios, and strive to use dimensions
that have somewhat unrelated ratios such as 1 X 1.23 X 1.41.
[16]
Q: What is the best material to use for building enclosures?
A: For home and permanent installation use, high density particle wood
is the most cost-effective material for general enclosure construction.
The best wood to use for portable enclosure construction is 14 to 20
ply per inch Finland birch type. Birch plywood is very expensive
however, and a carefully braced enclosure made of high grade void-free
fir plywood can do the job just as well in most cases. The thicker you
can make the cabinet walls, the better the results will be because of
reduced wall vibration and resonance, but the tradeoff is cost and
weight. Enclosure walls should be cut so that edges form an air-tight
seal when glued together. Cleats and caulking can also be used if
needed to insure a good fit and tight air seal.
[17]
Q: Is bracing necessary? How much should be used?
A: Bracing should be added to the enclosure interior to minimize
enclosure wall vibration. Enclosure walls simply cannot be stiff
enough since wall vibration indicates that energy is being wasted to
move enclosure panels rather than moving air. 25 X 76 mm (1 X 3 in)
pine bracing fixed on edge with glue and screws to the enclosure walls
will help provide the minimum necessary stiffening without affecting
the internal volume significantly. If you are building large subwoofer
enclosures, bracing with two-by-fours works better, though you should
take the bracing volume into account since a 3 m (10-foot) length takes
up 12.9 liters (0.36 cubic foot) of enclosure volume.
[18]
Q: How should I mount drivers on the baffle?
A: Mount drivers on the front of the baffle whenever possible to avoid
the reflections from inside the mounting hole. Heavy drivers should
normally be front-mounted using Tee-nuts and machine screws or JBL's
MA15 clamps. If Tee-nuts are used, apply a bit of Bostic or Pliobond
type rubber glue to the inside of the nut flange to help avoid losing
the Tee-nut inside the enclosure when installing the driver. Baffle
board construction is much easier if all baffle parts are assembled
prior to final box assembly.
[19]
Q: Do I need fiberglass inside the enclosure?
A: JBL uses a 25 mm (1 in) padding of 1/2-pound density fiberglass
stapled to the enclosure interior on all surfaces except the baffle.
You should use 100 mm (4 in) thick dacron or 25 mm (1 in) fiberglass on
at least three of the surfaces of parallel interior walls. Keep sound
absorbing materials away from the port(s) as the air velocity inside
the port can be sufficient to tear off bits of the material and squirt
them out of the enclosure. It is not necessary to cover the inside of
the baffle, but doing so will rarely degrade system performance. The
enclosure exterior may be covered with your choice of any suitable
finish or decoration; this will not affect bass performance and in some
cases (as with Formica) may help stiffen the enclosure walls.
[20]
Q: Does Fiberglass significantly affect enclosure tuning?
A: No, not unless the enclosure is stuffed full of fiberglass, in which
case the apparent volume of the enclosure increases by 12% to 20% as
seen from the point of view of the bass driver. Stuffing the enclosure
full with fiberglass is not recommended because it introduces system
losses, is expensive and interferes with port operation. The exception
to this would be a sealed "air suspension" type system enclosure where
more virtual volume is needed and actual volume is not available,
and/or where box dimensions which are multiples of each other can't be
avoided and the fiberglass stuffing will help absorb the internal sound
reflections.
[21]
Q: What is needed to mount a midrange on the baffle with the woofer?
A: For cone-type midrange drivers, a sealed sub-chamber should be used
to prevent interaction with the enclosure's bass driver. JBL drivers
suitable for sealed-chamber midrange use require only 10 to 40 liters
(.3 to 1.0 cubic foot) of chamber volume to operate at typical midrange
frequencies, above 200 hertz. Subchambers should be constructed
solidly and liberally lined with fiberglass. As in the case of
enclosure shapes, avoiding multiples of dimensions, subchambers should
be built so as to avoid square and cube shapes in favor of non-related
numerical ratios.
[22]
Q: Is there any special procedure for mounting a horn in an enclosure?
A: Use of a horn/compression driver does not require any subchamber
since these devices form their own air-tight seal. JBL horns such as
the 2344, 2370, MI-291 and 2380 horn family also seal their own cutout
opening in the enclosure when properly mounted on the baffle. Better
compression drivers are quite heavy, so a brace should be provided to
cradle the driver to prevent driver movement during shipping. In
combination with the length of a horn as a lever, driver mass can cause
the assembly to tear off the baffle or break the horn if the enclosure
is handled roughly or dropped. Driver mass can also tear off the horn
throat if cabinets are dropped on their backs.
CONVERSION CONSTANTS and USEFUL DATA
____________________________________
LITERS FEET^3 INCHES^3 METERS^3 MILLIMETERS INCHES METERS
___________________________________ _____________________________
1.00 = .03531 = 61.0 = .001 1.00 = .039 = .001
28.32 = 1.00 = 1,728 = .02832 25.40 = 1.000 = .0254
1000.00 = 35.31 = 61,016 = 1.00 1000.00 = 39.370 = 1.000
TO FIND SOUND WAVE LENGTH: divide velocity of sound by frequency (Hz)
(SOUND VELOCITY = 344 m/s, 1130 ft/s or 13,560 in/s)
AREA OF CIRCLE = 3.14 x (radius squared) Note: radius = 1/2 diameter
TO FIND THE DIAMETER OF A CIRCLE WITH EQUIVALENT AREA:
2 x square-root of (area divided by 3.14)
example: area of 9" tube = area of 8" square duct calculated:
(area) 64/3.14=20.37, square root = 4.51 x 2 = 9.03 (diameter)
VOLUME OF CYLINDRICAL DUCT = circular area x length
VOLUME DISPLACED BY JBL LOUDSPEAKERS: 8" = .05 cu ft, 10" = .1 cu ft,
12" = .15 cu ft, 15" = .2 cu ft, 18" = .3 cu ft.
JBL LOUDSPEAKER MOUNTING HOLE AND BOLT CIRCLE DIMENSIONS:
mounting holes:
8" = 7-1/16" 10" = 9" 12" = 11-1/16" 15" = 13-31/32"
18" = 16-13/16"
bolt circles:
8" = 7-5/8" 10" = 9-3/4" 12" = 11-9/16" 15" = 14-9/16"
18" = 17-3/8"
BIBLIOGRAPHY of RECOMMENDED AUDIO REFERENCES
____________________________________________
FOR AUDIO NOVICES:
BOOKS:
David B. Weems, "Building Speaker Enclosures," Radio Shack
publication, stock# 62-2309
"The CAMEO Dictionary of Creative Audio Terms," Creative Audio & Music
Electronics Organization, 10 Delmar Avenue, Framingham, MA 01701
F. Alton Everest, "The Complete Handbook of Public Address Sound
Systems," Tab Books #966, Tab Books, Blue Ridge Summit, PA 17214
David B. Weems, "Designing, Building & Testing Your Own Speaker
System," Tab Books #1364 (this is the same as the Weems book above)
Abraham B. Cohen, "Hi-Fi Loudspeakers and Enclosures," Hayden Book Co.,
0721
Alex Badmaieff and Don Davis, "How to Build Speaker Enclosures," Howard
W. Sams & Co., Inc., 4300 West 62nd Street, Indianapolis, IN 46268
Bob Heil, "Practical Guide for Concert Sound," Sound Publishing Co.,
156 East 37th Street, New York, NY 10016
PAPERS:
Drew Daniels, "The Most Commonly Asked Questions About Building
Enclosures," JBL Professional, 8500 Balboa Blvd., Northridge CA, 91329
Drew Daniels, "Using the enclosure design flow chart," JBL
Professional, 8500 Balboa Blvd., Northridge, CA 91329
FOR EXPERIENCED AUDIO PRACTITIONERS AND HOBBYISTS:
BOOKS:
Jens Trampe Broch, "Acoustic Noise Measurement," Bruel & Kjaer
Instruments, Inc., 185 Forest Street, Marlborough, MA 01752 (617) 481-
7000
Howard M. Tremaine, "The Audio Cyclopedia," 2nd Edition 1969, Howard W.
Sams & Co., Inc., 4300 West 62nd Street, Indianapolis, IN 46268
Arnold P. Peterson and Ervin E. Gross, Jr., "Handbook of Noise
Measurement," General Radio, 300 Baker Avenue, Concord, MA 01742
Martin Colloms, "High Performance Loudspeakers," A Halstead Press Book,
1978 John Wiley and Sons, New York and Toronto.
Harry F. Olson, "Modern Sound Reproduction," 1972, Van Nostrand
Reinhold Co., New York.
Harry F. Olson, "Music Physics and Engineering," Dover Publications,
180 Varick Street, New York, NY 10014
Don and Carolyn Davis, "Sound System Engineering," Howard W. Sams &
Co., Inc., 4300 West 62nd Street, Indianapolis, IN 46268
F. Alton Everest, "Successful Sound System Operation," Tab Books #2606,
Tab Books, Blue Ridge Summit, PA 17214
PAPERS:
Drew Daniels, "Notes on 70-volt and distributed system presentation,"
for the National Sound Contractors Association Convention, September
10, 1985, JBL Professional, 8500 Balboa Blvd., Northridge, CA 91329
Drew Daniels, "Thiele-Small Nuts and Bolts with Painless Math,"
presented at the 70th Convention of the Audio Engineering Society,
November 1981 AES preprint number 1802(C8).
FOR ENGINEERS:
BOOKS:
Harry F. Olson, "Acoustical Engineering," D. Van Nostrand Co., Inc.,
250 4th Street, New York 3, NY 1957 (out of print)
Leo L. Beranek, "Acoustics," Mc Graw-Hill Book Co., New York 1954.
Harry F. Olson, "Elements of Acoustical Engineering," D. Van Nostrand
Co., Inc., 250 4th Street, New York 3, NY (1st ed., 1940, 2nd ed., 1947
both out of print)
Lawrence E. Kinsler and Austin R. Frey, "Fundamentals of Acoustics,"
John Wiley and Sons, New York and Toronto.
N.W. McLachlan, "Loudspeakers: Theory Performance, Testing and Design,
Oxford Engineering Science Series, Oxford at The Clarendon Press 1934,
Corrected Edition, Dover Publications 1960.
PAPERS:
Don B. Keele, Jr., "AWASP: An Acoustic Wave Analysis and Simulation
Program," presented at the 60th AES Convention in Los Angeles, May
1978.
Fancher M. Murray, "An Application of Bob Smith's Phasing Plug,"
presented at the 61st AES Convention in New York, November 1978.
Don B. Keele Jr., "Automated Loudspeaker Polar Response Measurements
Under Microcomputer Control," presented at the 65th AES Convention in
London, February 1980.
R.H. Small, "Direct-Radiator Loudspeaker System Analysis," Journal of
the Audio Engineering Society (JAES), Vol. 20, p. 383, June 1972.
Mark R. Gander, "Ground Plane Acoustic Measurement of Loudspeaker
Systems," presented at the 66th AES Convention in Los Angeles, May
1980.
"Loudspeakers," An anthology of articles on loudspeakers from the pages
of the Journal of the Audio Engineering Society, Vol. 1 through Vol. 25
(1953-1977). Available from the Audio Engineering Society, 60 East
42nd Street, New York, NY 10165 Telephone (212) 661-8528
A.N. Thiele, "Loudspeakers in Vented Boxes," Proceedings of the IREE
Australia, Vol. 22, p. 487 August 1961; republished in the JAES, vol.
19, p. 382 May 1971 and p. 471 June 1971.
Fancher M. Murray, "The Motional Impedance of an Electro-Dynamic
Loudspeaker," presented at the 98th Meeting of the Acoustical Society
of America, November 19, 1979.
Mark R. Gander, "Moving-Coil Loudspeaker Topology As An Indicator of
Linear Excursion Capability," presented at the 64th AES Convention in
New York, November 1979.
Garry Margolis and John C. Young, "A Personal Calculator Program for
Low Frequency Horn Design Using Thiele-Small Driver Parameters,"
presented at the 62nd AES Convention in Brussels, March 1979.
Garry Margolis and Richard H. Small, "Personal Calculator Programs for
Approximate Vented-Box and Closed-Box Loudspeaker System Design,"
presented at the 66th AES Convention in Los Angeles, May 1980.
Fancher M. Murray and Howard M. Durbin, "Three Dimensional Diaphragm
Suspensions for Compression Drivers," presented at the 63rd AES
Convention in Los Angeles, March 1979.
R.H. Small, "Vented-Box Loudspeaker Systems," Journal of the Audio
Engineering Society, Vol. 21, p. 363 June 1973, p. 438 July/August
1973, p. 549 September 1973, and p. 635 October 1973.
JBL TECHNICAL NOTES:
The following are available at no cost from JBL Professional:
Vol. 1, No. 1 - "Performance Parameters of JBL Low-Frequency Systems"
Vol. 1, No. 2 - "70-Volt Distribution Systems Using JBL Industrial
Series Loudspeakers"
Vol. 1, No. 3 - "Choosing JBL Low-Frequency Transducers"
Vol. 1, No. 4 - "Constant Directivity Horns"
Vol. 1, No. 5 - "Field Network Modifications for Flat Power Response
Applications"
Vol. 1, No. 6 - "JBL High-frequency Directional Data in Isobar Form"
Vol. 1, No. 7 - "In-Line Stacked Arrays of Flat-front Bi-Radial Horns"
Vol. 1, No. 8 - "Characteristics of High-Frequency Compression Drivers"
Vol. 1, No. 9 - "Distortion and Power Compression in Low-frequency
Transducers"
Vol. 1, No. 10- "Use Of The 4612OK, 4671OK, And 4660 Systems In Fixed
Installation Sound Reinforcement"
Vol. 2, No. 2 - "JBL/UREI Power Amplifier Design Philosophy"
Instruction Manual - "Motion Picture Loudspeaker Systems: A Guide to
Proper Selection And Installation"
"JBL Sound System Design Reference Manual" ($15)