Speaker Design Basics
O.K. So you would like to design some speaker cabinets
for yourself. Easy. We don't expect to turn you into an acoustic engineer,
but you may be wondering what all those funny symbols are about. The design
applet we have provided allows you to design cabinets for a single transducer.
The issues surrounding multi-speaker designs, crossover design and so forth
are well covered in other references.
Experience high fidelity audio with the best home theater speakers available.
A speaker is an electroacoustic system which combines the characteristics
of the loudspeaker (or transducer) itself and the cabinet in which it is
mounted. Most of the work involved in building a good sounding speaker
cabinet involves trading off various parameters of a desired speaker's
performance within the limits of acceptable box size, low end frequency
response, maximum acoustic output (perceived loudness) and efficiency.
Types of speaker cabinets.
Closed and Vented Cabinets.
There are a number of different types of cabinets. The simplest is
the closed-box. This is simply a box with a hole in it where the transducer
is mounted. A lot of hi-fi cabinets are closed box designs. A rough rule
of thumb is that the bigger the box, the lower the frequency response of
the system. This is only true up to the point where the transducer can
no longer produce lower frequencies. Many old speaker designs used closed
cabinets, as the frequency response was fairly good, despite almost no
design work needed. It is possible to predict the frequency response of
a closed-box, but our applet will not do this.
For a long time it has been known that the bass response from a speaker
cabinet may be improved by introducing a port. A port is
just a tube or similar that looks like a hole in the speaker cabinet. By
choosing the port dimensions to match the speaker and cabinet, an
extened bass response is possible (for a fixed box size). A well designed
ported (or vented) cabinet will usually outperform a closed box,
but more design work is necessary. Choosing port dimensions was
until semi-recent times (1970s or so) a trial-and-error affair. Two engineers
by the names of Neville Thiele and Richard Small devised an analysis method
that may easily be used to design vented cabinets well. These days most
speaker manufacturers will provide a set of parameters for their speakers
known as Thiele-Small parameters from which cabinets may
be designed. All those funny symbols in the applet (and on your speaker
brochures) such as Qes, Xmax, Vas and so forth are
You can find out the Thiele-Small parameters for a transducer you have
no information for if you have the appropriate test equipment. This should
not usually be necessary.
Our design applet allows you to use the Thiele-Small parameters for a speaker
to design a vented cabinet that is tuned correctly. There are a number
of different tunings possible for a vented cabinet. We have chosen the
so-called Butterworth B4 maximally-flat amplitude alignment (tuning).
This just means that smooth frequency response is our main design criteria.
If you buy a woofer with known Thiele-Small parameters then you can plug
the parameters into the applet, then choose box dimensions and the applet
will calculate the vent dimensions. Just add a tweeter and crossover and
voila, your very own speaker design.
If you are using a tube or some such other thing for the port, then
you can enter its diameter into the duct diameter text field (in
place of the floating word), and the designs will all use that diameter
for the port. Note, that the minimum recommended diameter is that
shown when the text field is left as floating.
If you want to design speakersfor hi-fi use, then a vented box is probably
your best bet.
When you are building speakers for public address (P.A.) use, a closed
box or vented box is probably not very good. There are a number of reasons
for this. Firstly, they are not very efficient. Typical closed and vented
cabinets are less than 5% efficient, so you generate a lot more heat than
sound volume. Also, the maximum acoustic power that may be produced
is limited. What that means is that although your speaker may be rated
at 100 watts continuous (RMS), it may be unable to produce any noticeable
increase in sound output above a much lower power level. i.e. your speaker
may be producing its maximum sound output at say 5 watts, and any increase
in power really just ends up as distortion, not more noise! So, a 100 watt
continuous rating means nothing other than the speaker won't blow up until
you start putting more than 100 watts into it.
How can this be? Well, one of the Thiele-Small parameters is known as Xmax.
This is a measure of the maximum displacement (front-back movement) that
the transducer can exert before ten percent (10%) distortion sets in. This
is because the voice-coil in the transducer is beginning to leave the strongest
part of the magnetic field in the transducer's magnet (that big heavy lump
of metal on the back of the speaker).
Yet another problem with closed and vented cabinets for P.A. use is that
the size of the room plays an important part in the frequency response
of the speaker. Why? Imagine that the entire room is a cabinet mounted
to the front of the speaker - which acoustically it is. It is usually said
that you need a big room for low frequency sound reproduction, and that
is the reason why.
So what's a budding rock star/disc-jockey to do? Build a horn-loaded
cabinet of course.
A horn-loaded cabinet is a lot like sticking a trumpet on the front of
your transducer. The effect is similar to cupping your hands around your
mouth when yelling to someone at a distance. It makes it louder. Why? Well,
a horn is like an acoustic tranformer. What happens is that a big horn
(or funnel if you like) effectively makes the transducer drive the entire
area of air at the end of the horn. So, say the horn opening is ten times
the size of the transducer, then the driven area is ten times greater.
The transducer is said to be better coupled to the air. You know
those big black boxes at the bottom of a rock band's P.A. system? They
are horns. The transducers inside the cabinets themselves are not very
big compared with the box.
Why bother, when you can just use more power? A horn-loaded cabinet designed
for maximum efficiency can be as much as seventy-five percent (75%) efficient
- so you'd need about 15 times as many speakers and amplifiers in a vented
system to match such a horn. But that's not all - remember Xmax?
Well, one of the really nice things about a horn-loaded cabinet is that
since the driven air is 'leveraged' by the horn, the cone of the transducer
doesn't move as far to produce the same power output. In fact, it is easy
to design a horn so that the maximum electrical power to the speaker (i.e.
the continuous power rating) can be fed to the speaker before it
reaches the limit given by Xmax. So, when you feed in 100 watts into such
a design, the sound continually increases up to the maximum power the speaker
can handle. That's why nearly every P.A. you see uses horn-loaded cabinets.
Some exceptions to this are unusual designs like the Bose 802 speakers
- but Metallica would need a hell of a lot of them!
O.K. So we've solved the efficiency problem and the useful power handling
problem by using a horn. Now what about the room? Well, yet another advantage
of the horn is that it really works from the transducer out, and effectively
decouples the transducer from the room. So, what happens is that
the frequency response of the horn is essentially independent of the room.
Not completely, but significantly better than any other type of cabinet
If horns are so good - I mean efficient, loud, and almost room-independent,
why isn't every speaker a horn? Ahh, the obvious question. And the answer?
Size. A horn-loaded cabinet has to be large to have smooth frequency response.
This is of course dependent on the lowest frequency response the cabinet
needs to reproduce. One of the old rules of thumb said that you need a
horn with an opening of 10 metres (30 feet) in diameter to reproduce frequencies
down to about 30 Hz with smooth response - a touch larger than your average
vented cabinet. In fact, those humungus boxes at the rock venue are usually
useless below about 50 Hz or so - even though it feels like you're going
to shake apart.
Oh dear, I guess we can't build small horns that go low. Well, yes and
no. A common ploy is to build a horn that is unusually small and use active
equalisation to flatten out the frequency response. This works O.K. as
a compromise, and a lot better than you'd expect. Active equalisation is
used in many systems, even the Bose 802s mentioned above, and they aren't
even horns! Another approach was developed by Paul Klipsch and is used
in some Klipsch speakers. He uses a trick in the area of the horn
near the transducer to produce much flatter frequency response than expected
from the horn dimensions.
Types of Horns
There are a number of different horn-flares that can be used for cabinets.
The flare is the cross-sectional shape of the horn as it goes from the
start near the transducer (the throat) to the end (the mouth).
A few of the better known ones are parabolic, conical, exponential and
hyperbolic. These, in order, go from least efficient to most efficient
(acoustically). If you draw a parabola (for the cross-section of the horn)
and cut a hole in the bottom for the throat then you know what a parabolic
horn looks like. A hyperbolic horn by comparison is pretty much like a
trumpet stretched out - i.e. a tube that flares very little until it gets
to the end where it flares suddenly. The conical and exponential horns
are between these two extremes.
So, if a hyperbolic horn is the most efficient, then I guess we use that
flare for our loud P.A., right? Wrong. The problem with such a tight flare
is that as sound pressures increase, the restricted passage for the air
causes it to begin to compress. This causes distortion - not good. You
don't want to compress the air, cause it gets hot, etc, etc - and the sound
starts to sound really bad. So, what flare type do we choose? Probably
the best flare trading off high-efficiency versus low-distortion is the
exponential flare. This is what a very large number of commercially produced
Hey, my woodworking skills aren't good enough to build an exponential horn,
and I'm not about to build a fibreglass mold! O.K. Ok! An easy to build
flare is of course the conical flare. In fact, it is the easiest - not
as efficient, but a lot easier to build. Just four angled bits of wood
will do it. These sort of horns are sometimes seen as the mid-range part
of a P.A. system.
Parabolic horns are so inefficient that they are really of no practical
So, there you have it. If you're adventurous, you may want to build an
exponential horn, but a conical one may be good enough - it's up to you.
The applet will allow you to design both styles.
Cool, so I'll just replace the vented woofer box with a loud horn - yeah?
Well, not exactly. The improved efficiency of the horn is gained at the
loss of bandwidth. So, it may be louder but it will drop off in frequency
at the upper end before the tweeter takes over if you're not careful. How
do you work this out? The throat of the horn (the size of the little hole)
can be as big as the transducer cone, or smaller. Maximum efficiency
is achieved when they are the same size, but the bandwidth is also the
smallest. To increase the bandwidth, you must shrink the hole. The ratio
of the cone to the throat is known as Sd/St. Of course,
as you increase Sd/St (or reduce the throat size) the efficiency
drops. To find out what value of Sd/St to use, you first enter the
Theile-Small parameters for the transducer, then go to the Horn Response
page and look at the plot for 'Speaker Limits'. Those graphs show what
your transducer is capable of. You can also use this approach to evaluate
the suitability of a transducer for horn-loading.
Well, no doubt you've heard cars driving by you going thump-thump-techno-techno
or similar. Many of these are using a so-called band-pass cabinet to produce
huge amounts of bass from a small cabinet. This is a design where the entire
speaker (or speakers) are inside the box and the only path for sound
is out through a port like the one in a vented cabinet. These designs
have very limited bandwidth (hence band-pass), but produce a large low-frequency
acoustic output compared with a normal vented cabinet of the same size.
They are fairly inefficient as well. Our applet does not contain any design
abillity for such cabinets, but if you want to build one for your car (or
home hi-fi subwoofer), a good place to get information is to write to Polk
Audio in the U.S. and ask for details on their HVCD subwoofers. Theres
is a patented band-pass design that works very well. You can also call
them in the U.S. on 800/638-7276 and ask for Dave Lytwynec at extension
208 or Ken Swauger at extension 891. This is second-hand information, so
please treat them nicely.
So there you have it. Lots of information is out there if you want to learn
more, try some of the references we've included on our references page
if you like. Happy designing!
Back to the speaker page.