Michael Sabolish has an
excellent write up.
Building your own speaker cabinets can be a
challenging task, however the rewards, both financially and personally, can be
great. Perhaps you just aren't pleased with the offerings from commercial
companies, or have distinct requirements or tastes that don't line up with
what's available for sale at your local music store. Or perhaps you just enjoy
woodworking as a pastime...regardless of the reason it is the intention of this
article to assist inexperienced cabinet designers to design and construct a
speaker cabinet for use with their guitar or bass.
Design
Parameters
Before embarking on this task, it is important to
decide what you are looking for out of this project. Some people wish to save
money, and refuse to pay the inflated prices for a Marshall 4x12" stack.
Others may want more power handling then what they can buy in the store. Or
maybe you want to make an artistic statement...the possibilities are endless!
However having an idea what to expect out of the
finished product is important, so you have goals to work towards. If your only
concern is saving money, you can rule out vintage Celestions because they'll
set you back around £75-$100 a piece (in low quantity). Or if weight is your
primary concern, you can avoid certain types of construction materials. Perhaps
frequency response is the most important. In general, have the following
parameters in mind:
* Cost
* Size
* Power Output
* Frequency response
* Weight
* Appearance
The theorem that says, "There's No Free
Lunch" absolutely applies with regards to speaker design. If it was
possible to design a speaker cabinet that weighed a pound, cost $1, had a
response of 1Hz to 50 kHz, and could handle a zillion watts, then someone would
have done it (and been rich!). In reality, a lot of these variables are
mutually exclusive: if you want increased frequency response, the power output
will decrease, and the cabinet will be larger (all other things being equal).
If you want maximum power output, be prepared to pay more for the drivers and
sacrifice some frequency response.
Keep these things in mind as you read over the next
sections.
A
Note on Speaker Selection
Don't skimp too much on the speakers: they'll
dictate the volume, durability, and overall tonality of the cabinet. However
you can have all of the above qualities without spending a fortune.
A word to the wise: if you absolutely MUST have
Celestions, then by all means buy and use them. However, like most older things
in the music industry, they have a "magic" premium attached to them,
and hence you must pay more to get them.
I have found that you can get speakers from
well-known companies like Eminence, Dayton, and JBL for MUCH less money, and
they sound just as good.
Try and stay away from speakers with cloth accordion
or treated paper surrounds for any designs other than open backed. The surround
on these speakers can be a large source of air leaks in sealed enclosures,
which results in lowered efficiency.
Also look for the highest SPL in dB @ 1W. Remember,
each 3-dB increase in efficiency results in twice the apparent sound level at a
given power input. Most speakers for instrument use are very high (in the high
90's, and some are over 100) for the wattage they accept.
Guitar
Cabinets: Open Back (or infinite baffle)
Open back guitar cabinets are extremely easy to
design, and typically require the least amount of construction material to
manufacture. They have a very bright and full tone, and make great recording
and live cabinets due to the fact that they have wonderful "room
filling" properties.
One does not need to worry much about internal
volumes, calculations, and other design criteria. Designs are mostly centered
around form and weight.
To design these types of cabinets, start with the
number of drivers. Most open backed cabinets contain 1, 2 or 4 speakers, but
choose any number that meets your needs. Remember that twice the number of
drivers doesn't necessarily equate to twice the sound output, due to acoustic
coupling and standing wave cancellation in the room. However, more speakers is
definitely louder due to the increase in cone surface area.
The design starts with the baffle. Make sure it is
big enough to hold the speakers while giving at least an inch or two of
clearance on all sides. So for a 2x12" design, the length of the baffle
would need to be at least 2*12"+4 = 28 inches. The width would need to be
at least 12"+2 = 14 inches.
Guitar
Cabinets: Sealed (or acoustic suspension)
Sealed guitar cabinets are the main staples of the
4x12" stack cabinet manufactures like Fender and Marshall (what guitar
player doesn't want a full stack?). Sealed enclosures can (in general) handle
more power, decrease the frequency response (due to a natural built-in roll-off),
and increase the design time.
Sealed enclosures are designed by selecting an
overall speaker and cabinet Q (called Qtc). The concept of a system Q should be
familiar to anyone who has studied electronics and filtering theory. Q can be
thought of as an overall system response, accounting for resonance of the
system. In general, systems with a low Q can recover from resonance quickly,
and the response deviation band is fairly narrow. In terms of audio properties,
lower Qtc values can respond to transient signals better (or more appropriately
they have better transient response), at the cost of slightly reduce power
handling capabilities. As the Qtc is increased, overall low frequency output
power is increased, along with increased power handling abilities.
From my own experimentation, lower Q values have a cleaner
tone, whereas higher values result in a darker and warmer tone. For guitar
cabinets, good values of Qtc are between 0.707 and 1.0. Anything below 0.707
lacks the punch in the midrange tones, and anything above 1.0 lacks detail and
adds a ~2db response step in the low-mid range, which will have to be corrected
with external EQ.
Once an overall system Qtc is selected, you can find
the necessary volumes using equations.When doing guitar cabinets, try to select
speakers with appropriate Qts values that will put the F3 below the lowest
fundamental tone produced by the instrument you are playing. For 6-string
guitar, lowest fundamental is the low E string that vibrates at 81 Hz.
If your F3
is below this then you won't experience low end roll-off in the frequency range
of your instrument. Careful examination of the equations reveals that in order
to get lower F3's, you'll need to find drivers with the lowest possible Qts.
Again, you must juggle all of these parameters and decide what is most
important.
As a case in point: I was asked to design a full
stack for the guitarist in a heavy metal band. I asked him about his preferred
requirements, and all I got was "make it damn loud". I also inquired
as to what key he played in, and the answer was mostly dropped D.
As a translation into design parameters, he's
probably looking for more bass output and power handling: this means a higher
Qtc is in order. The F3 should be below the lowest fundamental, which in this
case is about 73 Hz(low D). I also had an idea to try and get the response peak
to be in line with the D power chord. In this case, I try and make the Fc (or
box resonance frequency) at the lower A string (or about 110 Hz). I ended up
selecting a Qtc of 1.0. As a side, note, I managed to build these cabinets for
around $210 a piece, or about $500 for a full stack! An amazing price
difference from commercial offerings.
The result was a cabinet stack tailored to the exact
needs of the player. The 2.7 dB peak at 110 Hz added a hard punch to the
natural 5th of the D power chord, which sounded incredible. These are the kinds
of things you can do with your own designs!
Bass
Cabinets
Bass cabinets are a whole other beast. The design
parameters are somewhat different, and the approach is very different.
Bass guitars don't produce a lot of mid range
frequency energy. Even the highest 4th or 5th harmonics of the highest notes
(like the 24th fret of the top G string) don't reach much above 1 kHz. If you
want to really be safe, plan on a high end roll-off of about 2 kHz.
A lot of bass cabinets have folded horn tweeters,
like those from Eden or SWR. I find that amplifying frequencies this high above
the fundamentals results in a tone that is a bit shrill to my tastes, but to
each his own. I find that bass guitar speakers available commercially roll-off
at much too high a frequency. A lot of the commercial woofers designed for car
audio and/or home theatre fit the bill nicely, however it all depends on the
type of sound you like. I like a lot of low bass and low midrange punch, which
indicates a high-end roll-off of 1 kHz or so.
Bass
Cabinets: Open back
I don't like open back bass cabinets: they are
relegated to mostly practice amp duty. They lack low frequency content and
require huge speakers. Stay away from them unless you are looking for something
small to hook to a small 10-watt practice amp.
Bass
Cabinets: Sealed
Sealed bass cabinets can be done, however realizing
a low enough F3 is extremely difficult given the majority of woofers available
for sale. In order to get decent transient response, you must find drivers with
extremely low Vas levels. Remember that the low E string on a bass rings out at
41 Hz, and if you want to be able to use a 5 string, the low B rings out at 30
Hz. It is very difficult to realize these F3's without huge cabinets, but it
can be done. Be very careful of F3’s here: cone excursion goes up as a square
with increasing frequency. So if you end up with a cabinet F3 of around 60Hz,
and you pump a lot of low frequency power into it (say from the low B string at
31Hz), you will most likely exceed cone excursion limits and damage the
speakers! Always try and stay within the limits imposed by your design.
Bass
Cabinets: Ported (or vented)
Vented bass cabinets yield the lowest F3's of all
the designs. These designs use one or more vent tubes that radiate most of the
low frequency energy as you approach the vent frequency of the design. You can
test this with any vented subwoofer: stick an object into one of the ports and
you'll notice that the very low frequency energy decreases dramatically.
Vented enclosures are designed not by picking a
particular Qts, but rather by picking a specific alignment. An alignment can be
thought of as a set of design criteria, which yield a set of responses from the
system
Always be sure to select a port size that is roughly
half the diameter of the driver you will be using. This prevents wind noise in
the tube, which is kind of like blowing across the opening of a partially
filled bottle, and hearing the resonate hum. I like to use 10-inch drivers,
with a port tube size of 3 to 4 inches.
One design I finished recently used 4, 10"
Dayton drivers, and used a pair of 4" vent tubes approximately 7 inches
long each. The cabinet dimensions were roughly 24" by 24" wide and
high, and the depth was about 13". Although the cabinet is quite large and
heavy, it has an overall F3 of 29 Hz, which allows a five string bass to really
push out the low B string with authority. Total cost of this entire enclosure
was $175.
Cabinet
Construction and Finishing
The finishing of cabinets can take all of the sound
advice from a multitude of woodworking lore and knowledge. There are myriad
joint types, finishing styles, and bracing styles.
Most cabinets are made from plywood or MDF (medium
density fiberboard). Don't confuse MDF with particle board: particle board is
made from much bigger chunks of wood than MDF. You can tell particle board from
MDF just by looking at it: particle board is made from sand sized chucks of
wood, whereas MDF looks like pressed wood powder.
A tip: don't use particle board! It was never designed
to stand up to a lot of vibration abuse. I have seen 1/2" particle board
speaker cabinets self-destruct in higher power situations. My favourite
materials of choice are a) birch covered plywood in 3/4" thickness, and b)
MDF in 1" thickness. These two materials seem to give the best balance
between strength and weight, and are both relatively easy to work with using
common woodworking tools.
Making
the Circular Speaker Cut-outs
This is by far the hardest part of finishing the
speaker baffle. Making a good circular cut is quite challenging without the
proper tools.
There is always the tried a true method of using a
reciprocating saw by hand. If you are good with the saw, this works fine. Go
slowly and follow the line. If you measured the width of the speaker frame
exactly, allow about 1/16" clearance by making the cutout slightly larger.
The driver needs a slight amount of play in the hole so that when the screws
holding the speaker to the baffle are tightened, the frame of the speaker does
not warp.
If you check with most suppliers of speaker building
equipment, they sell a contraption to which you can bolt a router. It has a
center alignment pin which screws into the center of the speaker cutout, and
you simply drag the attached router in a circle to make the cutout.
Covering
the Cabinet
For covering the cabinet, I use the industrial black
carpet that is backed by a thick coating of a rubber like material. This
covering wears very well, and can hide defects in the wood you are using. It is
very easy to apply with 3M spray contact adhesive. It is also relatively
inexpensive. Take your time and use pieces which are as big as possible. I used
to have problems of the light colored wood showing through the places where the
carpet is cut and edged together: I solved this problem by painting every
corner side black (you can do this the same time as you paint your baffle
wood). This yields invisible seams. I also place small black staples at the
seams to ensure that they will never pull up.
If you like natural wood finishes, you have two
options. The first is to simply stain the birch veneer that covers birch plywood.
If you are careful when you build the cabinet (and use mitred corner joints, as
the end grain on butt joints looks terrible stained) this can look every bit as
good as any quality home furniture. Wood veneers also work well, especially if
you are using very ugly plywood.
You can also use a popular covering called Toulex.
It looks like black that has some surface texture applied to it. Crate and
Marshall cover their cabinets with this. It wears well and looks good, although
the seam work is a lot more difficult to pull off cleanly.
Handles
and Other Hardware
Buy high quality strap handles. Good ones will have
a metal reinforcement bar inside of them. Use a bolt and T nut to hold them in,
because you don't want to have the handle pull out of the cabinet sending your
prized cabinet tumbling down a flight of stairs (spoken from experience).
Marshall uses recessed handles that mount to the
side of the cabinet. I don't like these because they require a large cutout in
both sides of the cabinet, which reduces the strength of the cabinet and is
another potential source of air leaks. To each his own, however. Just make sure
that if you buy some, that they are metal (not plastic).
Buy corner reinforcement! Cabinets tend to get beat
up with use, and the corners are the weakest areas and the first to cave in.
Corner reinforcement spreads impact force along the sides of the cabinet making
it less likely that the corner will cave. I have used both metal and plastic
ones with good success. If you are building a full stack, there are special
ones that interlock and prevent your top slanted cabinet from sliding off the
bottom one.
I really like to put high quality casters on my
cabinets. You can get 3 inch locking casters which support up to 500 lbs. from
Home Depot for about £3 a piece. Your back will thank you for it. Keep in mind
that you only want to put them on the bottom cabinet of your stack, as you
don't want the top one rolling off in the middle of a gig (although buying
locking casters alleviates this problem).
For grill cloth, there are several options. I like
to use commercial grill cloth because it is cheap and easy to use. I have also
successfully used screen door wire available at home improvement stores. It
comes in a nylon/plastic variety as well as metal. The metal stands up to abuse
more than either the plastic variety or cloth, however if it is stretched too
tight it will hum and make rattling noises.
I have also been informed that commercial sheet
metal companies sell an industrial grate used in making tractors and other
heavy equipment. It is very heavy, but can withstand just about any abuse
thrown at it (e.g. boots, guitars, and beer bottles). You can buy it by the
linear yard, although I'm not sure of the cost. It is very reminiscent of what
SWR uses on their Goliath cabinets.
A note on acoustic stuffing: I highly recommend that
all sealed and vented
enclosures have 50% of their internal volume stuffed
with acoustic stuffing. You can buy this by the pound from Part's Express, or
you can use the fiberglass insulation made for insulating homes. Be warned: the
pink fiberglass is a nose, eye, and throat irritant so take the proper
precautions. Stuffing actually increases the apparent volume of the cabinet,
and helps compensate for volume lost by driver baskets, handles, crossover
components, etc. It also helps break up standing waves inside the cabinet.
Wiring
Use high quality silicon wiring of at least 16 gauge
for connecting up the speakers. Make sure it is of the braided type, because it
has higher current handling capabilities, and is easier to bend around internal
obstructions.
Also use a high quality speaker jack on the back of
the cabinet. My favourites are the heavy duty binding post type terminals,
where the speaker wire actually fits in a hole in the post, and there is a nut
to lock it to the terminal. Another favourite is the locking 1/4" jacks by
Neutrix. Whatever you do don't use either a) a regular 1/4" guitar jack,
or b) the spring-loaded push button type terminals. I absolutely guarantee that
these types will pull out during a gig! Also solder all wiring connections. A
lot of speakers come equipped with push on connectors, but don't use them. If
it vibrates loose you'll have to tear the cabinet apart to get at them again.
Also be sure to seal the terminal with caulk or
Liquid Nails to eliminate air leaks.
Also be sure your speakers are phased properly
before you button up your cabinet. A quite way to check is to take a 9V
battery, and touch the positive and negative leads to the respective speaker
leads (e.g. where you plug in your amp). All of the drivers will move to a
fixed offset. Make sure they all move the same direction. If one woofer sucks
in while the other speakers push out you have it out of phase. Fix it now or
the sound quality will suffer.
Also BE SURE YOUR OUTPUT IMPEDANCE IS CORRECT!
Many a would-be speaker builder has mis-wired the speakers and ruined perfectly
good amplifiers. Keep in mind that solid-state amps and tube amps react very
differently to mismatched speaker loads.
A solid-state amp will still work with a higher
impedance load that it is rated to drive. So if you wire your cabinet as 8-ohm
impedance and your amp is expecting 4 ohm, all that will happen is the output
volume will suffer. Due to the fact that solid state have a very low output
impedance, if you try to put a 2 ohm load on your amp from a mis-wired cabinet,
it will try to source too much current and something will give (usually the
fuse, but maybe your power BJT's?).
Tube amps are a different story. Efficiency is
assured by the correct output impedance being reflected onto the primary of the
output transformer, which in part determines the amount of current flowing
through the output tubes. If you put on a wrong impedance (EITHER higher or
lower), the output tubes "see" an impedance mismatch and actually put
out LESS power into the cabinet. It can also cause transformer flyback and tube
socket arching leading to a quick death to either the output tubes, cathode
resistors, or the output transformer (maybe all!). In short, DON'T mismatch
cabinet impedance with a tube amplifier!
For two speaker systems, it's common to get drivers
with twice the expected impedance of your output amplifier, and wire them in
parallel. So if your amp needs 4 ohms, buy two 8-ohm drivers and wire them in
parallel to yield 4 ohms. For four speaker systems, I have found it is best to
wire in a series-parallel configuration, where two separate pairs of speakers
are wired in series, and the resulting pairs are wired in parallel. In this configuration,
buy drivers with the same impedance as you need (for an 8-ohm cabinet, buy four
8 ohm drivers). There are other wiring possibilities as well (such as strict
series).
Michael Sabolish