|IMPEDANCE IN AUDIO TECHNOLOGY(cont.)
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.
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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