Program: TurboBass Express V5.01O.K. So you would like to design some
speaker cabinets for yourself. Easy, I don't expect to turn you into an
acoustic engineer, but you may be wondering what all those funny symbols
are about, well forget them! The software TurboBass Express allows you
to design cabinets for a single transducer (speaker) easily with just 3
parameters from the technical data sheet of the speaker you want to use.
Contact the manufacturer to obtain the TS parameters needed to
successfully calculate your box. Try Google. Enter the product code of
your loudspeaker.
Alternatively CALL THE MANUFACTURE!Remember that you have just
spent a small fortune buying those subs, but to get them working right
and to make them respond to your requirements, they will need a box made
to the correct size, otherwise you are wasting your time with them.
Don't just any old box them TURBOBASS THEM!SEALED CABINET DESIGN
PROGRAMA closed-box design is much easier than a vented-box design.
There are fewer parameters to worry about, and the final system response
is much less sensitive to variations in these parameters. Building a
closed box is easier, because constructing and fitting a vent to a
cabinet can add several hours to the project construction time, vented
boxes also require additional tools. Tuning a vent can be time consuming
and may require some test instruments, although it can be well worth the
trouble with some drivers, others will give excellent performance in a
closed box, and some will function well only this way. To successfully
calculate a sealed box for a given loudspeaker the following parameters
are needed. 1. Fs (the resonant frequency of the driver)2. Vas (the
volume of air having the same acoustic compliance as the driver 3. Qt
(or Q) (total driver Q a measurement of sharpness of resonance VENTED
CABINET DESIGN PROGRAMThe vented cabinet program uses the formulas
developed by D.B Keele,Jr., to find box characteristics that will obtain
good performance with a given driver, you will require the following
parameters 1. Fs (resonant frequency of the driver) 2. Vas (the volume
of air having the same acoustic compliance as the driver) 3. Qt (or Q)
(total driver Q, a measurement of sharpness of resonance) The response
obtained by using this program to design vented boxes is known as the
FOURTH-ORDER BUTTERWORTH, considered by most designers to be optimum. it
is flat, with no peaks or dips, and consequently dose not sound boomy.
Once TURBOBASS has calculated the required size of cabinet use the vent
diameter program to fine tune your cabinet VENT AREA CALCULATING PROGRAM
The following parameters are required to calculate a vent area.1. the
box frequency calculated by vented box program 2. the total loudspeaker
diameter in inches 3. the X-Max, minimum diaphragm excursion in
millimetres.In order to tune the box designed calculated by the vented
box program a vent of a specific size must be installed. The vent must
have an area large enough not to impede air flow, otherwise vent noises
such as whistling and chuffing will be heard. Small considers that
maximum air velocity in a vent dose not exceed 5% of the speed of sound,
vent noise will not be a problem, nor will constriction of air flow
alter the system parameters, Once you have used this program to
calculate the required port size, use the port length calculating
program to fine tune your cabinet VENT LENGTH CALCULATING PROGRAM The
following parameters are required to calculate vent length.1. the box
frequency calculated by ventedbox program2.
the box volume calculated by vented box
program3. the vent area in square inches calculated by vent
are program.Use this program to fine tune the vented cabinet with
Small's method. Note that three inch diameter cardboard tubes are
readily available from such sources as mailing tubes, carpet rolls, and
dry cleaners. However a solid vent is preferable, build the vent with
solid wood (MDF) into and as part of your cabinet, this will give far
superior results.Note that internal porting subtracts the internal
volume from your box . Mounting the duct on the back of the cabinet or
even underneath it, can help attenuate any objectionable noises produced
by the vent.O.K. So you would like to design some speaker cabinets for
yourself. Easy, I don't expect to turn you into an acoustic engineer,
but you may be wondering what all those funny symbols are about, well
forget them! The software TurboBass Express allows you to design
cabinets for a single transducer (speaker) easily with just 3 parameters
from the technical data sheet of the speaker you want to use. Contact
the manufacturer to obtain the TS parameters needed to successfully
calculate your box.Remember that you have just spent a small fortune
buying those subs, but to get them working right and to make them
respond to your requirements, they will need a box made to the correct
size, otherwise you are wasting your time with them. Don't just any old
box them TURBOBASS THEM!Introduction.A speaker is an electro acoustic
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 extended 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 Thiele-Small parameters.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 speakers for hi-fi use, then a
vented box is probably your best bet.Horn-loaded CabinetsWhen 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
transformer. 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
design.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 humongous 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 horns are.Hey, my woodworking skills aren't good enough to
build an exponential horn, and I'm not about to build a fibreglass
mould! 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 use.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. Band-Pass Cabinets 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 ability 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. Turbo Bass. Turbo Bass Express.
Happy designing!
