TO DESIGN A SPEAKER
by Steve Deckert
is a good reason why most speakers sound like speakers. It's because
most designers follow a specific sequence of mistakes that lead
to mediocre sounding results.
face it, if you're going to design a do-it-yourself speaker the
first and obvious problem is going to be getting the woofer(s) and
the cabinet tuned for flat extended frequency response. Some will
choose a cabinet and then model different drivers in it, some will
select a woofer and then model different cabinets for it. Both will
then design the crossover based on the published specs of the drivers,
choosing the best looking spot for the crossover frequencies and
then design a network to these predetermined ideals.
will then order the parts and assemble it. Some will be amazed at
the best sounding speaker they've heard, some will be a little unhappy
and start tweaking the design. This procedure for speaker design
is a recipe for mediocrity. Anyone can do it, almost everyone falls
into the trap. To reiterate, you can't design your speaker based
on published specs. Some of them are usually never accurate, and
others too complex to mean anything in your specific situation.
example, considering polar dispersion plots of tweeters and midrange
drivers as you decide what type of crossover and so on is futile.
Drivers all have signatures that defy known specifications, but
are in fact the sum of those specifications. You may have all known
data from NASA grade test gear on a particular driver, but no accurate
way to compile that data in your mind so as to hear what the driver
will actually sound like. You actually have to listen to it.
to it is the only accurate way to draw a subjective description
of it's sound or signature. And of course the front baffle size,
shape, material and angle will also change the signature of drivers,
especially midranges and tweeters. So even if you have the drivers
in your hand and have listened to several frequency sweeps, the
signature you determine the driver to have is false because it's
not yet in the cabinet. Once you have the driver in the cabinet,
the new signature you determine the driver to have is also false
because you haven't installed any crossover components yet. Suppose
you installed a cap on the tweeter, and determine the drivers new
signature, you still have no idea what the final signature will
be because you haven't added the midrange, or woofer. Any drivers
and crossover components you add to the circuit will effect every
other part of the circuit in the way the current and frequencies
divide. So in the end, all the math in the world will not predict
accurately how a speaker will sound, and if it will image or not.
Although if followed will get you close, but also become a limiting
factor of how far you can go.
to these variables, as you try to first determine the signatures
of each driver, is the room acoustics which will effect the perceived
result by at least some 40%. And I've almost forgotten to make it
clear that the proper way to design a speaker
is by identifying the individual signatures of each component and
combining them in a way that is complementary.
This can only be done by human ears. A tweeter that has a signature
you've determined to be a little sharp on the very top end may be
less than ideal when measured against itself, however suppose your
midrange drivers is a little bloomed out and overly warm sounding.
Depending on the overlap and slope of your crossover, the two can
create complementing signatures and actually sound great.
way to design a mediocre speaker is to use all the math you can
and model everything from the crossover to the cabinet to create
a perfect result.
you want a great sounding speaker, you can't design one this way,
for several reasons. The first is that published specs on drivers
are never accurate. You will have to pick the drivers with somewhat
of a casual attitude and then buy them. Once you have them, you'll
have to measure them yourself and see exactly what the specs actually
are. Be prepared to repeat this process. That means you may be setting
some of the drivers you've purchased on the shelf and purchasing
new or different ones hoping for better luck.
basic steps are as follows:
you have the drivers in your hands, and after you've measured the
Thiel&Small parameters of at least the woofers, you are ready
to start designing a crossover. Now that you can plainly see the
discrepancies between the published specs and actual measured specs,
you can be glad you haven't built the crossovers yet. The first
step in designing a crossover is becoming familiar with the characteristics
and signatures of each driver. To do this, you simply hold it in
your left hand and using a frequency generator, start sweeping the
frequencies to extremes of each drivers bandwidth and LISTEN.
will be listening for smoothness of frequency response. Use your
ears, not measurement equipment because the signature of each driver
will be superimposed on the frequency response yielding unique sonic
results that can't be seen in computer modeled or measured plots.
example, on the woofer, you will find it's FS just by feeling and
looking at the woofer while you sweep it. As you sweep, pay close
attention to the sound. Sweep very slowly and hunt for peaks and
noises. This process should be documented on a piece of paper by
listing the results. For example:
~ 300 Hz - very nice
~ 600Hz - good
~ 2300Hz -good
means that around 300Hz there was a change, either a peak, or a
noise of some kind. Another one at 600Hz and a noticeable roll off
after 2300Hz. Do the same test on your other drivers (mids and tweeters).
Circle the Zone of response that had the sweetest sound. When you
find a peak or noise, play with the driver by squeezing it, angling
it, shaking it, tapping on it, anything you can do without damaging
it. The objective simply to see if anything you do changes the characteristics
of the peak or noise. This process usually leads to tweaking the
drivers before they're ever installed in the cabinet. For example:
stamped frame speaker frames resonate or ring at certain frequencies
that depending on size and mass hover between 200Hz and 1200Hz.
Adding damping to the frame can reduce or eliminate these types
of frequency response glitches.
you're using dome tweeters and or midranges, you will have to install
them in the cabinet and sweep them again. Expect large changes in
frequency response based on the interactions of the baffle refraction's.
The surface material, i.e.. wood, felt, can largely effect the signature
of the dome drivers as well. Remember that signatures are not found
in specs, polar response plots, or transient response tests. There
is no way to tell from specs the sonic changes that occur between
fabric domes and silk, poly, titanium, or phenolic domes.
drawing notes on paper you can tell the sensitivity of each driver
in relation to each other simply by selecting frequencies that overlap
and swapping drivers. For example, the midrange may well go up to
10K and the tweeter may well drop down to 2K, so setting the frequency
generator around 5K and A/B the mid and tweeter will tell you which
one is louder. The armature trained ear can hear 3dB increments,
so if you can tell one is louder, the gain was at least 3dB.
yourself a crude response plot on your paper for each driver and
as we said, circle the areas that sounded best. This will help you
determine where the best crossover frequencies would ideally be.
Once you've come up with a plan based on the real data you just
measured with your ears taking into account both efficiency and
signature, you can rough in a crossover network. Remember, the midrange
and tweeter have a large overlap, and both can play the same frequencies,
so in the area of overlap, which one has the most pleasant signature?
A question only your ears can answer, and one that must be answered
before designing the crossover.
the crossover can be a successful experience provided you keep it
simple, at least at first. Using your calculator or computer select
the components for either a 6 or 12 dB network that approximately
will hit the ideal crossover points you want. If your speaker is
for high power applications use the 12dB networks, but if your using
lower power amps and higher efficiency speakers you have the option
of using either one. When your caps and coils arrive, start with
the woofer by installing it in the cabinet and sweeping it several
times. Now how smooth has the response become, what changes have
taken place. If you like to measure things, you may be amazed at
how the impedance curve changes once you have the woofer in the
cabinet. Playing with temporarily sealed openings in the cabinet
will make radical changes in the the impedance curve, as will the
final tuning. Assuming your network is either 6 or 12 dB you will
have a coil for the woofer. Once you are satisfied that the woofer
is tuned to the cabinet you should sweep it several times with the
inductor and without. Notice the signature of the woofer changes
completely with an inductor in the circuit. If it makes it sound
worse and you have unusually flat response you can and should consider
throwing it away.
following must be done with music, not test tones.
most systems the mids and tweeters are higher sensitivity than woofers
and will need to be padded. This can get complicated with a calculator
so you're best bet is to assemble a variety of caps and ceramic
resistors, and using alligator clips and test leads, clip together
the 6 or 12 dB network that you purchased the parts for and hook
up the speaker. Play it a low levels and use this first sound as
a starting point. With respect to the polarity of the woofer, the
midrange and tweeter (other drivers) can be either in phase or out
of phase by 180 degrees. If you have more than one driver such as
a midrange and tweeter, you should experiment with changing the
polarity of each one at a time and listening to what happens. You
will notice large shifts in frequency response and presence. The
various combinations possible by doing this can yield either a forward
or laid back sound. The reason this happens is timing, or phase
angle. The phase angle of a speaker changes with frequency, and
in the case of the woofer, effected dramatically by box design and
at this point you have what looks like a pile of spaghetti on the
floor and it's better if you do not know which way is theoretically
correct as far as polarity goes. Just listen. Once you have determined
which polarity combination sounds best it is time to start balancing
the output of each driver in reference to the woofer. This usually
involves padding or shifting of crossover points or both and can
be quickly accomplished with a variety of 1 ohm to 10 ohm ceramic
resistors. The resistors should be used in series with the drivers.
(That's mids and highs only - not the woofer(s) This process should
be done while listening to the speaker and simply involves swapping
out values until you find something you like. Remember that padding
a driver changes it's impedance and that means your pre-calculated
crossover points will also change. This could be good or bad, so
it's nice to have a variety of capacitor values to swap in and out
of the circuit as well.
process can take anywhere from a couple hours to a couple days and
when finished, you'll have a really scary pile of test leads and
parts connected together in front of your speaker. It may have been
difficult to keep track of the circuit as you swap parts in and
out, and that's okay. In fact if the audio gods favor you, you will
find that when you try to draw the schematic you'll discover that
you hooked things up wrong or at least differently then planned.
If this happens and the sound is good, you can smile in the knowledge
that you're probably on to something.
brief paper is just an attempt to deprogram do-it-yourself speaker
designers. Remember your ears are more expensive than the best test
gear, and work better. The reason we favor this design process is
because the actual number of real variables is overwhelming. Things
like Signatures and coloration's in drivers will change your subjective
interpretation of frequency response. A good example is the way
a poly dustcap can superimpose a dryness in the midrange that is
often missinterpreted as better transient response.
more in depth information about this technique, you can visit the
web page where using this process I design a do-it-yourself speaker
that you can build and compare with other speakers to demonstrate
the results. In fact, if you truly want to graduate to a level of
understanding that yields superior results, try this: Design your
crossover before you buy your drivers, like you normally would,
based on the published specs and other people's comments. Then put
it in a drawer somewhere and proceed with the above technique until
your finished. When you build the second speaker, install your pre-designed
crossover and compare the results. You should find the experience
enlightening and if successful a little embarrassing.