IMPEDANCE IN AUDIO TECHNOLOGY(cont.)
SERIES/PARALLEL CIRCUITS
When the loudspeakers are wired in combination of series/parallel,
the opposition to current flow is determined by the resultant
impedance that the amplifier sees. Two parallel circuit branches,
each consisting of two 8 ohm speakers in series, become two
16 ohm circuit branches if parallel and the amplifier will
see a load of 8 ohms.
W = 402 ÷ 8 = 1600 ÷ 8 = 200 watts
I = 200 watts ÷ 40 volts = 5 amps = 2.5 amps per parallel branch
I have tried to keep this explanation of impedance
informative while covering basic rules governing the power
generated by the amplifier. When discussing a technical subject
such as impedance, it is necessary to employ mathematics to
illustrate the relationship between voltage, current, impedance,
and the resultant power produced in the circuit. I realize
that a lot of people don't' like math. You don't need the math
if you are going to just be a roadie or stage technician, but
if you desire to truly understand how sound equipment functions,
you must accept the fact that the math works. If you want to
design systems and specify equipment, then you will need to
understand the math involved.
If you now understand the relationship of the speaker load
to the power produced, you may think that the lower the impedance
of the speaker load, the more current will flow and maximum
power will be produced by the amplifier. However, in reality
the amplifier can develop only so much current flow from its
output stage until the point that the maximum sage output current
is reached.
This is why amplifiers have a rated minimum load impedance
limit, i.e. they can only develop their maximum safe power
at the rated minimum load impedance.
If the amplifier were allowed to produce more current than
the rated power required, it would destroy itself. The output
devices would fail, due to the excess heat generated in the
transistors. The more current that flows in a circuit, the
hotter the conductor becomes; this is also the case for transistors
that are amplifying the signal. This is why most power amplifiers
today will begin to current limit in order to protect themselves
when the loudspeaker load goes below the minimum rated load
impedance.
Loudspeakers also have a minimum impedance that is even lower
than the nominal impedance published by the manufacturer. The
actual impedance varies with frequency, and it is for this
reason that many manufacturers publish impedance charts that
will indicate at what frequency the impedance is at its minimum.
IMPEDANCE
Example Impedance Curves
Note that the minimum impedance is lower than the nominal impedance.
Some people who check out a loudspeaker's resistance to direct
current with a volt-ohm meter (VOM) become confused, because
the DC resistance of a speaker is much lower than the stated
nominal impedance. Remember, audio signals are alternating
in their direction of current flow (AC). A typical DC resistance
measurement can be 20% lower than the nominal impedance rating.
POLARITY OF LOUDSPEAKERS
I haven't discussed the electrical polarity of the
loudspeaker yet. Most loudspeaker manufacturers produce speakers
that move OUT when a positive referenced voltage is present
at the red terminal. All of our Scorpion and Black Widow loudspeakers
respond to a positive voltage at the red terminal by moving
forward.
The correct wiring of loudspeakers with regards to proper polarity
is shown below:
 For
your information, in case you aren't aware of this, at least
one manufacturer's loudspeakers move IN when a positive referenced
voltage is present at their red terminal. In that manufacturers'
loudspeaker systems, the loudspeaker leads are reversed to
place the woofers or cone loudspeakers "In-phase" with
their compression drivers mounted on their high frequency horns.
Their compression drivers have what is considered normal polarity-they
move out when the positive voltage appears at their red terminal.
This fact about polarity is ver important when putting one
manufacturer's loudspeaker in a system in conjunction with
another manufacturer's components, as in adding a subwoofer.
You must verify that a positive voltage, placed on what is
supposed to be the positive speaker lead wire, will indeed
cause that speaker to move out.
This can be accomplished with a simple nine-volt transistor
radio battery. With the positive terminal of the battery placed
on the positive speaker lead and the negative terminal on the
negative speaker lead, the speaker should move OUT. If the
speaker moves IN, the leads need to be reversed either at the
loudspeaker itself, at the input jack, or at the power amplifier's
output terminals.
The reason for the difference in the direction of the loudspeaker
cone's movement is that some loudspeakers have opposite magnetic
polarity. If you try putting two identical types of loudspeakers
together where the magnets are back to back, they will repel
one another. If , on the other hand, you take a Black Widow
and JBL and place them back plate to back plate, they will
attract one another and may be difficult to pull apart. If
you reverse an electro-magnetic system's magnetic polarity,
you are also reversing its electrical polarity. They are opposite
sides of the same coin.
Since I have brought up the subject of magnetic/electrical
polarity, let me tell you one situation that I have experienced
on a couple of occasions in my twenty-seven years of active
involvement in audio. If the magnet or motor structure has
been accidentally placed upside down in the magnetizer, it
will be charged to the opposite magnetic polarity. This speaker
may then be placed in a system with other similar loudspeakers,
but it will move opposite to them, causing the speaker system
to sound thin. The wiring color coding may appear to be correct,
but it is the mis-magnetized motor structure that is the culprit.
When wiring loudspeakers, you must orient the leads correctly.
In a series circuit, the connection between loudspeakers always
is made between opposite terminals, i.e., from + red to - black
or vice versa (- black to + red). In paralleled circuits we
always connect black to black (- to -) and red to red (+ to
+).
We have a couple of low frequency enclosures in the Peavey
line that employ what we call a trans-axial loudspeaker loading
technique. One loudspeaker faces inward while the other faces
normal. These two loudspeakers are not on opposite sides of
the same baffle board. There are two separate baffle boards
that are offset to allow the acoustic centers of the two opposite
facing loudspeakers to be in the same plane. In this application
the polarity of the rearward facing loudspeaker is reversed.
Since the speaker is facing backwards, the reversed polarity
causes the two loudspeakers to be acoustically in phase, i.e.,
they are both moving in the same direction at the same time.
There is a difference between loudspeakers that PRODUCE music
(guitar amplifier loudspeakers) and loudspeakers that REPRODUCE
music (sound reinforcement loudspeakers). Guitar amplifier
loudspeakers are actually voiced or designed to have somewhat "soft" cone
breakup, called cone cry by some transducer engineers. Cone
breakup occurs at certain resonant frequencies where the cone
ceases to move as a single linear piston, but moves in segments.
A sound reinforcement loudspeaker should be designed to minimize
all cone breakup modes, and thus perform as linear as possible.
It is acceptable to wire guitar amplifier speakers in series
and parallel configurations. In guitar amplifiers, the damping
factor of the power amplifier is purposely kept low. The speaker
is not controlled or damped well and essentially flops around,
but this is part of the sound.
However, sound reinforcement loudspeakers should NOT be wired
in series. They can be wired in parallel, but they should be
wired in such a manner that each speaker has its own two leads
wired in parallel at the output of the power amplifier. Some
people neglect to do this because it's inconvenient to run
separate speaker lines for each transducer.
Tighter, punchier, more transparent kick drum and bass lines
will result when the loudspeakers are individually wired in
parallel at the power amplifier's output terminals. Tight bass
means control of the loudspeaker or high Damping Factor. More
on this later.
Sound reinforcement loudspeakers can be wired in parallel,
but not internally in the loudspeaker enclosure. Each loudspeaker
should have its own set of speaker wires that may be wired
in parallel at the output of the power amplifier. Most loudspeaker
systems have parallel input jacks on the enclosure. If we didn't
include them and other manufacturers did, some salesman that
didn't know any better would use this against us to sell another
product. More on this later also.
Moving right along, I have even more information to help you
understand impedance. We should learn by other's mistakes so
we don't have to repeat them. Several years ago while working
on some projects in Africa, I encountered a technician who
did not understand the difference between DC resistance and
AC impedance. We stated earlier that a simple definition of
impedance was "the opposition of one thing to another." I
also said that impedance was different and more complicated
than DC resistance (or the opposition to Direct Current flow),
because DC resistance is constant. Impedance varies depending
on the frequency of the signal.
DC resistance is equal to the voltage drop (pressure) across
the device under measurement, divided by the current flow (number
of electrons) passing through the device. DC resistance is
rather straightforward. In dealing with the opposition to current
flow offered by components in an electrical circuit that contains
varying electrical cycles of audio frequencies, the opposition
to current flow is know as the more complex impedance.
There are a couple of different types of impedance. The following
are some definitions of impedance from the Dictionary of Scientific
and Technical Terms by McGraw-Hill:
IMPEDANCE: (PHYS) 1. The ratio of a sinusoidally
varying quantity to a second quantity, which measures the response
of a physical system to the first, both being considered in
complex notation; examples are electrical impedance, acoustical
impedance, and mechanical impedance. Also known as complex
impedance. 2. The ratio of the greatest magnitude of a second
quantity which measures the response of a physical system to
the first; equal to the magnitude of the quantity in the first
definition.
ELECTRONIC IMPEDANCE: Also known as Impedance.
(ELEC) 1. The total opposition that a circuit presents to an
alternating current, equal to the complex ratio of the voltage
to the current in complex notation. Also known as the complex
impedance. 2. The ratio of the maximum voltage in an alternating
current circuit to the maximum current; equal to the magnitude
of the quantity in the first definition.
ACOUSTIC IMPEDANCE: (ACOUS) The complex ratio
of the sound pressure on a given surface to the sound flux
through that surface, expressed in acoustic ohms.
MECHANICAL IMPEDANCE: (MECH) The complex ratio
of a phasor representing a sinusoidally varying force applied
to a system to a phasor representing the velocity of a point
in the system.
If you find these definitions a clear as a Columbian cup of
coffee, or if you feel as if you have been mentally zapped
by a "Star Trek" phasor; read on. Perhaps my further
explanations will help you to better understand. We did tell
you they called it complex impedance.
The opposition to electrical current flow takes two forms,
passive resistance (which produces heat), and an active reaction
when there is capacitance or inductance in the circuit. The
opposition created by capacitance or inductance is referred
to as reactance.
A capacitor consists of two electrical conductors separated
by a dielectric or something that will support or store an
electrical charge. Air itself can support an electrical charge
and is said to have a dielectric of one. A capacitor is said
to have a capacitive reactance or opposition (impedance) to
current flow. A capacitor blocks direct current (DC), and stores
a charge, but for alternating current (AC) a capacitor has
high opposition to current flow at low frequencies, and low
opposition at high frequencies. When alternating current encounters
a capacitor, the voltage lags behind the current.
An inductor is a coil of wire that offers high opposition to
current flow at high frequencies and low opposition at low
frequencies. When alternating current encounters an inductor,
the current lags behind the voltage because inductance is a
circuit element that opposes changes in current.
Here is an analogy for impedance in the physical world. You
have loaded a wheel barrow with dirt, and now you must move
the payload. When you pick up on the handles of the wheel barrow,
the weight offers a resistance. However, because the handles
operate with the wheel and the axle to form a kind of inclined
plane (lever), the resistance is less than the actual weight.
In order to get the wheel barrow moving, you must apply even
more force, but the mass (real weight of the dirt) offers inertia
or opposition to the force applied (Inductive reactance). Now
imagine you have moved the payload to its intended location,
and must now stop the wheel barrow's forward motion. But now
the opposition to the deceleration is in the form of momentum
or stored energy in the actual motion of the wheel barrow (Capacitive
reactance).
In the case of our loudspeakers, their opposition to current
flow from the power amplifier is their impedance. Audio electrical
signals are electrical analogues or representations of the
positive and negative fluctuations of air pressure that have
been converted to positive and negative fluctuations of voltage.
This fluctuating electrical signal that represents the vibrations
of air or sound is by its very nature Alternating Current or
AC (i.e., the direction of current flow changes directly with
the number of audio cycles per second being reproduced).
Loudspeakers actually involve three forms of impedance. The
first is the electrical impedance offered to the power amplifier
discussed above. The second is the mechanical impedance of
the loudspeaker, which is taken into account in the design
of the loudspeaker enclosure. Third is the impedance of the
air or the acoustic impedance that the combination loudspeaker/enclosure
encounters.
The air itself, which is the medium through which we transmit
sound in the form of pressure variations, has an impedance
(the medium of transmission offers opposition to the vibrations
of its air molecules). A loudspeaker is a transducer that changes
energy from one form to another. The loudspeaker changes electrical
energy into acoustical energy or sound as we know it.
A basic loudspeaker is quite a bit inefficient in that most
of the energy produced is in the form of heat generated in
the voice coil of the speaker. Loudspeakers intended for use
as direct radiators are anywhere from 0.25% to 4% efficient,
meaning that more than 96% of the energy is lost as heat and
not converted into sound or acoustical energy. Loudspeakers
actually make better space-heaters than they do electrical
to acoustical transducers.
There are ways to somewhat improve upon the efficiency of a
basic loudspeaker, and that is to use a kind of transformer
to couple it with its acoustic environment. Many of you already
know about electrical transformers that can isolate (1:1 ratio),
step up (1: 10), or step down (10:1) electrical signals. The
ratio represents the proportion of the number of turns in the
primary to the number of turns in the secondary. In addition
to isolating and stepping voltages up or down, a transformer
can match impedances: i.e., a very high impedance source can
be coupled to a low impedance load via a step down (high to
low turns ratio) transformer. The source that is coupled to
the primary of the transformer now sees the high turns ratio
as its load impedance, while the secondaries lower turns ratio
sees the device coupled to the secondary of the transformer
as the actual load impedance.
In loudspeaker transducer technology, we use a horn as a transformer.
The horn couples or matches the loudspeaker to the air in a
manner in which the efficiency of the loudspeaker as a system
is increased (i.e., with one watt of power going to the loudspeaker,
the sound pressure on-axis with the horn will be greater, because
all of the acoustic energy radiated from the loudspeaker is
focused by the horn). Since the acoustic signal produced by
the loudspeaker is now restricted within the walls of the horn,
the speaker is said to be loaded by the horn. The horn offers
an acoustical impedance to the loudspeaker and, like a transformer,
the horn changes the impedance that the source amplifier sees.
In this case our amplifier actually sees a somewhat higher
impedance or opposition to current flow than the speaker would
offer if it were directly coupled to the air itself.
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