October 1957 Radio & TV News
These articles are scanned and OCRed from old editions of the Radio & Television News magazine. Here is a list of the Radio & Television
News articles I have already posted. All copyrights are hereby acknowledged.
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 cost.
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 answers.
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
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 music.
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, and where?
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 ideal either.
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
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 looks flat.
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 question.
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
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
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
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 programs.
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 is advisable.
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
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
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
Posted August 28