October 1957 Radio & TV News
Wax nostalgic about and learn from the history of early
electronics. See articles from
Radio & Television News, published 1919-1959. All copyrights hereby
Audio crossover networks have
the same fundamental mission as RF multiplexer filters in radio systems, which is
to separate and steer specific bands of frequencies into two or more signal paths.
While simple in concept, implementation in hardware can be a major challenge depending
on requirements for channel separation, feedthrough, phase and group delay, amplitude
equalization, distortion, and other factors. This article discusses some of the
decisions used by crossover network designers when considering where to make band
breaks, while leaving actual circuit design rules to other authors. I built a set
of custom speakers many moons ago and went through the frustrating process of deciding
where to place the breaks and which speakers to use (based
on their rated frequency responses). In the end, I got an acceptable result,
but I never was convinced that all my work - including paying for fairly expensive
(on USAF, A1C pay) coils for the bass part of the
crossover network - justified the trouble. While I appreciate a truly fine audio
system, 'good enough' is a fairly low standard for me when trading off quality and
Choosing Your Crossovers
By Norman H. Crowhurst
What crossover frequencies and circuits should you use in your 2-, 3- and 4-way
hi-fi speaker systems?
When you set out to choose your hi-fi loudspeaker installation, you probably
do not choose the crossovers as a separate entity, it is true. But different systems
offer quite a variety of different crossover frequencies and rates of roll-off,
etc., so you will want to know what it is that governs this choice. How many crossovers
(two-, three- or four-way system), what frequencies are used for crossing over,
and how sharply do the crossovers act are questions to which each system gives different
On the question of how many crossovers, there are two extreme schools of thought.
One of these favors no crossovers at all - a single extended range loudspeaker unit,
that responds to all the frequencies in the audio range. According to protagonists
for this approach, you will avoid all the phase shifts and all the problems that
crossovers "get you into." What you don't avoid, however, is the problem of getting
one loudspeaker unit to respond to a frequency range covering the ratio of 1000
to 1, from 20 cycles to 20,000 cycles. Even if you are content with a slightly less
ambitious range, say from 40 cycles to 10,000 cycles, this is still a tremendous
range of frequencies to cover with one vibrating system.
To radiate the low frequencies satisfactorily, it must inevitably have a large
surface area exposed for radiation. To radiate the upper end satisfactorily it must
be extremely light and rigid, to avoid the kind of break-up that causes erratic
response to consecutive frequencies.
Another requirement is that it shall not introduce any distortion. If there is
any distortion in the way it handles the lower frequencies with the large movements
they involve, this will also modulate the higher frequencies, besides causing distortion
components to the low frequencies themselves. This intermodulation, as it is called,
gives a "dithery" rendering of programs when low frequencies and high frequencies
occur at the same time, and can be particularly noticeable on material like organ
It is practically impossible to make a loudspeaker unit with perfectly uniform
response over a range of even 250 to 1 (let alone 1000 to 1) and also with absolutely
no distortion, particularly of the lower frequencies. The use of crossovers proves
to be a boon in helping out, both to achieve a uniform frequency response and minimize
intermodulation distortion. This leaves us with the question, how many crossovers,
Here the protagonists of the opposite extreme come in by pointing out that serious
intermodulation distortion can only appear when more than one octave is handled
by the same loudspeaker. Consequently it would be good to have 10 loudspeaker units,
each covering an octave and thus completing the range from 20 cycles to 20,000 cycles.
The first loudspeaker unit would cover from 20 to 40 cycles, the second 40 to 80,
the third 80 to 160, the fourth 160 to 320, and so on, up to 20,480 cycles. This
would necessitate nine crossovers between the 10 units.
Of course, there is no comparison between the effect of these two extremes on
the budget requirement. However good a single extended-range loudspeaker unit is
made, it would never approach the cost of 10 separate units, each made for its own
frequency range, with nine crossovers. So it might appear that the number of crossovers
you use depends on your budget. But before our millionaire readers proceed to order
10-way loudspeaker systems, it should be pointed out that this extreme is not the
Fig. 1. Above curves show the division of the entire audio spectrum into ten
octaves. This may be regarded as the very "ultimate" in the number of crossovers.
While, if well designed, it would certainly do a wonderful job of providing a
smooth frequency response and freedom from IM distortion, this is not all that is
required of a system. In fact, from some aspects, it is not even the most important
thing required of a system.
Smooth frequency response, as measured by steady tone testing, is one thing,
but a smooth frequency response, as judged by listening to program material, often
proves to be quite a different matter. This is because our listening is much more
dependent upon the transient response of the system than its response to the steady
tones used for testing.
If you put a loudspeaker inside the voice box of a pipe organ and opened all
the pipe valves, the frequency response of the system, measured with a continuous
gliding tone, would come out pretty close to flat. But can you imagine what the
loudspeaker would sound like? You've guessed it, like a program being heard through
a mass of tuned pipes. A 10-way system, it is true, would not be quite as bad as
this, but it would follow the same general trend.
Each octave bandpass filter will need to have sharp cut-offs at each side if
we are going to take full advantage of the way this system can minimize intermodulation
distortion (Fig. 1). This would mean that tones in each band would set up a kind
of ringing from the loudspeaker on the particular tone played, caused by the characteristics
of the filter. It is true the "ring" would not be so pronounced as with a tuned
pipe, but it would still result in reproduction with considerably more coloration
than the simpler types of systems despite the fact that the steady tone response
So there are disadvantages to both extremes. As a compromise, most loudspeaker
systems now fall into the two-way or three-way classes, with a few going to four-way.
Having narrowed down the number of crossovers to choose, we can now take the next
If you pick a two-way system, the logical crossover frequency will be somewhere
in the region of 600 cycles. Actually anywhere between 400 and 1000 cycles would
be satisfactory. The reason for this is that, whether you consider you need the
frequency response to be from 20 cycles to 20,000 cycles, or from 40 cycles to 10,000
cycles, the middle of the range comes out to about 630 cycles. Since both halves
are 4 or 5 octaves, it is not critical, within half an octave or so, to have the
frequency at precisely 630 cycles. Anywhere between 400 and 1000 cycles will divide
the spectrum approximately into two equal parts (Fig. 2).
Fig. 2. When a two-way system is employed. the use of a crossover in the region
of 600 cycles divides the entire audio spectrum into two equal portions.
If you take a three-way system and consider the spectrum as extending from 40
to 10,000 cycles, which is more like a reasonable extent, because there is very
little audio "intelligence" in the 20 to 40 cycle range and in the 10,000 to 20,000
cycle range, dividing this part of the spectrum into three approximately equal parts
will require crossovers at 250 cycles and 1600 cycles (Fig. 3) .
This choice of frequencies brings an interesting fact to light: 250 cycles is
approximately the frequency of "middle C." Frequencies below this correspond to
the bass part of musical reproduction, while frequencies above this correspond to
treble. So this division gives us one band in the bass and two for the treble. Actually
the range from 250 to 1600 cycles could be regarded as the treble, while the extreme
high-frequency range from 1600 to 10,000 or 20,000 cycles is principally occupied
by overtones and transients that provide definition. In speech the principal components
in this top range are the consonant sounds due to "s," "t," and "d."
Dividing the spectrum four ways would require crossovers at approximately 160
cycles, 630 cycles, and 2500 cycles (Fig. 4).
This consideration has been based entirely on a consideration of the frequency spectrum.
If you consult loudspeaker catalogues you'll find few systems that conform to the
figures just given. This is because there are other factors that complicate the
choice. Wherever you divide the spectrum, by means of a crossover, the lower frequencies
are going to be reproduced by one loudspeaker while the frequencies above this point
come from another. In a musical program, it is inevitable that the fundamental tones
and possibly some of the lower harmonics of certain instruments will be reproduced
by one loudspeaker while other overtones or higher frequencies will come from another
unit. This is one of the undesirable features of multi-way systems.
Fig. 3. If it is desired to divide the more useful part of the entire audio spectrum
into three equal portions, crossovers would be required at the frequencies of about
250 and 1600 cycles. A three-way system would use such values.
Another factor that needs some consideration is the distribution of the dominant
sound energy through the frequency spectrum. This should not be confused with the
apparent loudness of different frequencies. Most curves of average energy spectrums
will show there is not much in either speech or music below 100 or 200 cycles. But
here the word average is important, especially for music. . Bass notes are only
evident in comparatively few passages, which is why an average curve shows them
low. However, when low notes are present, they have considerable energy, and the
system has to reproduce this energy - not an average figure.
At the other end, there is not much energy in speech above 500 to 1000 cycles,
but the small energy present is very important to intelligibility, to carry consonant
sounds, particularly "s," "t," and "d." Musical energy, too, tails off above 1500
to 2500 cycles, except for short bursts from the percussion, which again do not
show up on an average curve. So a system should be able to handle more or less uniform
power peaks over the range from about 40 to 2500 cycles, and should be reasonably
flat in response (at lower levels) from 20 to 20,000 cycles or, as a secondary standard,
from 40 to 10,000 cycles.
Consequently a more usual approach is to cover as much as possible of the frequency
range with the mid-range unit. That is, we take as much as the mid-range unit can
comfortably handle without running into difficulties with frequency response and
intermodulation, then the part that the mid-range unit cannot comfortably handle
is delegated, at the low end to the woofer and at the high end to the tweeter.
Fig. 4 With a four-way system, crossovers should be as shown. The lowest channel
would then handle frequencies up to 160 cycles; channel 2 would cover 160-630 cps;
channel 3, 630-2500 cps; and channel 4 would cover all above 2500 cps.
Fortunately this integration problem is not very important at the low-frequency
end. By the time you get down to 250 cycles, the wavelength is four feet. As the
biggest loudspeaker system doesn't usually exceed this dimension in one direction,
you are not going to suffer noticeable lack of integration between frequencies below
and above 250 cycles, or any similar crossover frequency for that matter. So, to
maintain good integration, the usual practice is to use as low a crossover as possible
without running into intermodulation problems.
If better integration can be achieved at the high end by pushing the crossover
up to, say 5000 cycles, then we may raise the lower crossover frequency as high
as, say, 600 cycles. Then the tweeter will just handle frequencies from 5000 cycles
on up, where the mid-range unit runs into serious breakup problems. The tweeter
will maintain the smoothness of response at the high end.
A crossover around 600 cycles is not likely to run into serious integration problems
provided satisfactory transition is achieved in the 600 cycle region. More of this
in a moment, however. If 600 cycles is the crossover point from the low frequency
to the mid-range unit, the range from 20 cycles, or even 40 cycles, up to 600 cycles
is rather wider than any of the other ranges. This is the reason for going to four-way
systems. A unit to handle all the frequencies below 600 cycles can still cause intermodulation.
Such division is not usually necessary in the smaller living rooms with a well-designed
enclosure. Only with the larger systems, where the low-frequency unit has to encounter
considerable diaphragm movements at the bottom end of its range, is this a necessity
to minimize intermodulation.
So much for the question of the different possibilities in frequency of crossovers.
Next we come to the sharpness of the slope. Different crossovers employ different
degrees of separation between the frequencies. The simplest kind of crossover uses
just an inductance or a capacitance for each individual channel. This provides an
ultimate roll-off of 6 db per octave beyond a frequency of about 2 to 1 each side
of the crossover (Fig. 5).
Fig. 5. Here are the amplitude and the phase response curves for crossover networks
having a 6 db/octave slope. Note that at the crossover frequency the phase difference
amounts to 90 degrees.
If the individual units are connected in-phase, so both diaphragms move forward
when the fluctuation from the output transformer in their respective frequency ranges
is in the same direction, the response will come out flat, and the combination of
phase shift from the two will neutralize or cancel, resulting in a uniform response
in frequency and zero phase shift all the way up. Also this simple kind of crossover
does not introduce any measurable degree of transient distortion.
As soon as you get into more complicated crossovers that produce an ultimate
roll-off of 12 db, or more, per octave, there are phase shift problems and also
the transient response is likely to suffer. With a 12 db per octave crossover (Fig.
6), the two voice coils should be connected in opposite phase, otherwise at the
crossover frequency they will be moving in opposite directions and cancel, producing
a "hole" in the frequency response.
This means that phase reversal occurs with this kind of crossover, through the
transition from one side of the crossover frequency to the other. In theory this
could convert a square wave into a triangular one. But demonstrations have shown
that such a change makes no audible difference. The more important difference is
in transient response. We begin to experience the effect described with the 10-way
system. The transition from one unit to the other in the vicinity of crossover is
less likely to be satisfactory.
So why use steeper slope crossovers? The only satisfactory reason is for minimizing
the intermodulation distortion. Another reason that has been advanced is the possibility
of interference between the frequency response from the two units. This will only
occur if the frequency response from one unit becomes quite erratic immediately
beyond the accepted frequency range.
For example, a loudspeaker intended to reproduce up to 600 cycles might show
some erratic peaks and valleys in the region of 1000 cycles. This could seriously
interfere with the over-all response when combined with a separate high-frequency
unit, if the crossover was of the simple type (See illustration Fig. 7).
The answer to this argument is that a unit that becomes erratic in its response
so shortly beyond the accepted frequency range is probably not a very good unit
within the accepted frequency range, although its response may look good. Its transient
response will certainly not be as good as the steady tone response.
Not only does the crossover have to take care of delivering the right range of frequencies
to each unit, with uniform coverage, but sometimes adjustment for balance is needed
too. This is because often tweeters are more efficient than woofers or mid-range
units. If the tweeter unit is twice as efficient, then feeding it straight-through
from the crossover will make the tweeter give twice the acoustic output it should
to maintain balance. To care for this, a good crossover should incorporate an attenuator,
or balance control, so the output in acoustic watts can be balanced to take care
of differences in efficiency between units.
Fig. 6. Relevant details of amplitude and phase response for the 12 db/octave
crossover network compared with the 6 db/octave unit (dashed).
This brings up another question - what controls to look for on a crossover. In
turn, this could lead into a much more complicated article, because there is such
a variety of ways adjustments can be made; but let's keep it simple. Only electronic
dividing networks have continuously variable controls, either of crossover frequency
or slope of roll-off, and that's another subject. But some crossovers for use in
loudspeaker circuits have adjustments that can be made in steps, either to change
the slope or the frequency. This may be done by changing capacitor elements, or
by changing taps on inductors, or both. If you buy a unit with these facilities,
make sure it comes with sufficient instructions, so you know exactly what it can
do. Don't be satisfied with a vague promotion statement, such as "this unit is adjustable
to suit your system's exact requirements." Having three or four possible ways of
connecting it will probably insure that one way will sound better than the others,
but it does not insure that any of them are right.
You would do better to buy a unit with only one (right) way of connecting it,
than a "versatile" unit with inadequate information, so you do not know what each
position does. If it comes with complete information you will be able to check that
it does provide facilities for crossing over where you want it to, and for feeding
units at the impedance of your system (4, 8, or 16 ohms).
By and large, then, the recommendation seems to favor using the simpler crossover
and units designed to have a good response, not only within the range for which
they are intended, but also at least an octave beyond the nominal crossover frequency.
Then the over-all response, both to transients and intermodulation distortion tests,
should be quite acceptable.
We have spoken about the question of integration. A few more words of explanation
might be in order here. By integration we mean the radiation of sound as if the
whole frequency spectrum "belongs." This can perhaps best be illustrated by showing
what results without it. If a loudspeaker consists of two units, say one handling
below 600 cycles and the other above, and the low-frequency unit gives a good smooth
response radiated uniformly into the room, while the high-frequency unit tends to
eject its frequencies in a concentrated beam, away from the low-frequency unit,
one can easily get the impression that the high frequencies in the audio spectrum
are "squirted in" as an afterthought from the side.
This is quite unrealistic on musical reproduction, and on speech it can become
distressing. The principal high frequencies in speech are due to the sibilants,
"s's" and so on. Lack of good integration will give the impression that most parts
of the voice come from the low-frequency unit, while the "s's" are added from some
completely different direction. This is even more unnatural than the effect on musical
To summarize, then, the question of choosing a loudspeaker system and what crossovers
to utilize in it, depends to a considerable extent upon the rest of your system.
Fig. 7. These curves show why the response that exists beyond the accepted range
(of the low-frequency unit in this case) can be important to the system.
If you have a larger-than-average living room to supply with sound, or if you
intend to operate at unusually high levels, with an amplifier of 50 watts or more,
then intermodulation is likely to trouble you, and a three- or four-way crossover
For more average sized living rooms and moderate levels of reproduction, a three-way
crossover will certainly be adequate, and for smaller systems a two-way crossover
is quite sufficient.
The best choice of crossover frequency (ies) and the sharpness of the crossover
is dependent upon the types of units used. While there are broad principles, based
on the frequency spectrum itself, these are modified to a considerable extent by
the effectiveness of the units. In most instances, combinations put out by a single
manufacturer usually incorporate the best crossover frequency for that combination.
While sharper crossovers have certain advantages in some circumstances, it is
better all round to choose a crossover with a more gradual transition from one unit
to the other as frequency passes through this region.
Fortunately in this branch of high fidelity we find the same thing we find elsewhere,
that paying a lot more for a more complicated system does not necessarily give us
better performance. In fact, if you shop around, you will be able to find quite
a good performing system at whatever price you are prepared to pay. By paying more
you naturally can get a better system, but just paying more and getting more equipment
into the system doesn't mean it must be a better one.
Posted August 28, 2014