This month is third in a series
about speakers. Before I move on to discussing Bass Guitar
speaker systems I want to address two subjects that I ran
out of room to discuss last month.
You probably have heard people refer to speakers as being
in or out of phase, but what exactly does this mean? First
of all, phase is a relative term meaning that for something
to be in phase or out of phase it must be referenced against
something else. Therefore it is not possible for a single
speaker to be out of phase (well, with some exceptions
that we will get back too).
Let's first examine a speaker cabinet containing two speakers
that we want to wire up to use. On the back of each speaker
is two terminals, one marked + (positive) and one marked
- (negative)
By convention the + terminal is placed on the left when
viewing the speaker from the rear. Remember from our previous
discussion that these terminals are connected to each end
of the voice coil winding. Since sound is alternating current,
it doesn't matter which terminals you hook the amp too.
As long as both are connected you will hear sound.
Remember we also discussed that a speaker has a front
wave from the cone moving forward and a rear wave from
the cone moving backwards and that if the speaker is not
in a cabinet, which prevents the rear wave from mixing
with the front wave, the two opposite waveforms will cancel
each other out especially in the lower frequencies. This
same effect will happen if the two speakers in the cabinet
are wired opposite to each other. If the positive wire
from the amplifier is connected to the plus terminal on
one speaker and the minus terminal on the other speaker
and the negative wire from the amplifier to the minus terminal
on one speaker and the plus of the other speaker, the speakers
will now move in opposite directions when they both receive
the same signal from an amplifier. There are now two opposite
waveforms created that cancel each other just as the front
and rear waves do on a speaker being played without a cabinet.
This will cause a really thin sound to be heard but also
brings up another problem. Even though the sound is canceled
acoustically, there is still the same amount of power electrically
being input to the speakers. And what do you think happens
next? When the player hears this thin lower volume sound
he tries to compensate by turning up the volume and the
bass EQ. He is now asking the amp to deliver more power
and the speakers to receive it, even though it is still
not sounding right. This often leads to speakers blowing
from overpowering, failure of the amp's output stage or
both.
So why did I mention before that a single speaker could
be out of phase? The accepted convention is that with a
positive voltage the speaker moves forward and backwards
on a negative voltage.So if I decide to add another amp
of the same model and play them together for more volume
and lows you will experience a problem if the speakers
are not in phase. Using different models of amps means
you will have to experiment because there are multiple
phase inversions of the signal as it makes its way through
the audio circuitry of the amp and there is no guarantee
that the signal exits the amp in the same phase as it enters,
so two different model amps could possibly be out of phase
with each other even if the speakers are wired correctly.
Aside from the above, does phase
really matter with a single speaker? There are recording
guys who may argue that point in relation to multi track
recording and I remember Ceaser Diaz, who was the amp
tech for the Stones, saying that he needed to reverse
the phase of their speakers when they were playing South
of the Equator(closer to the Earth's magnetic South Pole)
to make the amps sound right, but I think that was just
one more of his crazy theories. One other interesting
phase idea is the phase switch on an acoustic guitar
amp. This permits flipping the phase of the speaker in relation to the vibrations of an acoustic
guitar top as a feedback canceling aid.
SERIES OR PARALLEL WIRING
Another thing to consider when wiring speakers is the
difference between wiring speakers in parallel or in series.
Yes I know that it is necessary to do this to match impedance
but what other factors are in play here?
DAMPING
Damping is a measurement of how well the amplifier can
control a speaker cones movement. In our discussion of
how a speaker works we also compared it to a microphone.
To review, putting a signal into a speaker voice coil will
cause the coil to push and pull against the magnetic field
and move. In a dynamic microphone,air pressure from sound
will push and pull the diaphragm attached to the coil in
and out of the magnetic field inducing a voltage in the
coil. It is this effect that occurs also in the speaker.
As the speaker is moved by the signal it still has momentum
when the signal lessens. Since the coil is still moving
in the magnetic field, it creates a voltage that is output
out of the coil and back to the amplifier. This causes
the amplifier to have less control over the speaker. When
we put two voice coils in series since the amplifier is
not directly controlling each speaker and the damping factor
is reduced. This manifests itself in a looser flubbier
low end that may or may not be desirable.
BRANCH INDUCTANCE
Another parameter that a speaker has is inductance. Any
coil of wire has the ability to store energy at least temporarily,
which is called inductance. Another fact about inductors
are they pass lows more easily than highs forming a filter.
Therefore if we have a multiple speaker setup to be used
with an amp there are several wiring differences that will
cause the inductance of the system to vary changing the
resulting tone. We can use as an example a classic style
Marshall 4 x 12. In its stock configuration it contains
four twelve inch sixteen ohm speakers. The speakers are
divided into two sets. Each set is wired in series resulting
in 32 ohms (16+16=32),increasing the total inductance and
then both sets are wired together in parallel bringing
the total cabinet impedance to 16 ohms (32/2=16). The amount
of branch inductance created here is more favorable to
lows as they pass easier than highs. And as we said above
the damping is reduced creating less control over the lows.
We can wire the cabinet a second way. This would be to
put all four sixteen ohm speakers in parallel creating
a four ohm cabinet. In comparison to the stock cabinet
above, the branch inductance is lower, passing more highs
and the damping is higher tightening up the lows. There
is one last possibility. If you wire each set of speakers
in parallel yielding 8 ohms (16/2=8) and put it in series
with the other set it still results in a sixteen ohm cabinet
(8=8=16). But the resulting branch inductance and damping
fall somewhere in between the stock series/parallel cabinet
and a full parallel cabinet, resulting in less of a tone
change than a full parallel cabinet would have when compared
to the stock cabinet.
How much difference does wiring the cabinet differently
make really depends on what the player is looking for.
If you are pretty satisfied with your tone and looking
to tweak it a bit, there may be value here. If you are
nowhere near what you want to hear there are probably other
variables in your system that if changed will bring you
closer to you goal. One easy way to listen to the differences
in wiring is to find a 4 x 12 stereo cabinet that has switching
built into it. These cabinets permit you to easily switch
between 16 ohm series- parallel and four ohm mono. Plus
they can be configured as an eight ohm stereo with two
right and two left speakers at 8 ohms per side.You can
fairly easily listen to the differences between the different
wiring schemes without several hours of rewiring.
Next month I will continue the discussion on speakers
moving on to bass guitar speaker systems
Steve
Weiss is the owner and main technician of Steve Weiss
Electronics Inc. He is experienced in the repair
of analog and digital musical equipment. This includes
everything from Vintage Tube Amps and Pro Audio equipment
to Digital Keyboards There is also a guitar repair
shop staffed by some of the areas top guitar repair
techs. He is authorized for warranty work on most
major brands. Steve Weiss Electronics is located
inside of Sam Ash Music at 5460 West Sample Road
Margate, FL 33073 954-975-3390 Ext 272. Steve has
also spent 25 years on the road as a performing guitarist
and is the designer of Primal Guitar amps that can
be seen at Primal
Audio.com Steve can also be reached at stevew@metromusicmayhem.com |
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