The EM Poll

P.A. 101

Mar 14, 2008 3:24 PM, By Emile Menasche

           

HOW TO CHOOSE THE RIGHT SOUND SYSTEM FOR YOUR NEEDS WITHOUT TEARS

Impedance and Speaker Load

Impedance-the total opposition by an electrical circuit to the flow of a signal or current at a given frequency-is one of the most important specs you need to know when matching a speaker or a group of speakers to a power amp. Why should you care? Because impedance-expressed in ohms, the symbol for which is the Greek letter omega (z)-determines the amount of load placed on your system. In audio terms, mismatched impedance wastes power and can result in poor sound or, worse, damaged gear. The impedance of the speaker system should exactly match the output impedance of the amplifier.

The number of watts (W) an amp puts out depends in part on the amp's output impedance. Change the impedance, and the power rating changes, too. As a rule, the amount of power is inversely proportional to the impedance, so if you halve the impedance, the power rating will double. For instance, a 200W output at 8z would become 400W at 4z. When evaluating an amp's power rating, it's important to know the impedance of the circuit used to measure that power. An amp that puts out 400W at 8z is twice as powerful as an amp that puts out 400W at 4z.

Figure A: Of the three ways to wire speakers (series, parallel, and series/parallel), parallel connections are the most common in sound reinforcement applications.

The speaker connection also impacts the load (see Fig. A). Speakers are part of an electrical circuit, and the laws of physics govern that circuit. Let's say a power amp delivers 100W into an 8z load. If you connect one 8z loudspeaker, you'll be averaging 100W through that circuit. (This isn't literally true, because other factors come into play, but it's close enough for our purposes.) But what if you want to connect a second 8z speaker? There are three ways to connect multiple speakers to an amplifier: series, parallel, and series/parallel. The impedance-and consequently the power output-changes depending on which wiring scheme you use.

Wiring in series. In a series connection, the speakers connect in a linear fashion, from one to the next. The total impedance (Z) is the sum of the impedance of each speaker in the circuit-that is:

So if you have two 8z speakers wired in series, your total impedance will be 16z; two 4z speakers would give you a total impedance of 8z; and so on. One note about series connections: if one speaker fails, the whole circuit stops working.

Wiring in parallel. Parallel connections are much more common in sound reinforcement. In this method, the positive terminals of each speaker connect to each other (from the positive terminal on the power amp) and the negative terminals connect to each other (from the negative terminal on the power amp). Wiring in parallel reduces the impedance, allowing the amp to put out more power. How do you figure out the change in impedance that results from parallel wiring? One way is to use the following formula:

Z1, Z2, and Z3 represent the impedance of each speaker in the circuit. Fortunately, the equation is a lot easier to grasp when you're only dealing with a pair of matching speaker cabinets. In that instance, you can simply divide the nominal impedance by 2. So if you have two 8z cabinets, you divide the rated impedance (8z) by the number of cabinets (2) to get 4z.

Wiring in series/parallel. The third wiring option is series/parallel. Commonly used in multispeaker guitar cabinets, this scheme turns up less often in sound-reinforcement installations. However, it works well as a method to maintain a specific impedance load while adding additional speakers to a system.

The math needed here is a bit more involved. First, you calculate the impedance of each serial circuit. We'll use lowercase letters (a, b, c, and so on) to indicate the individual drivers that are wired in each serial circuit.

Then you divide those totals as you would in a standard parallel connection:

Let's say you have four 8z speakers wired in series/parallel. Each series would offer an impedance of 16z:

We have two 16z loads wired in parallel. The equation is:

With two circuits wired in parallel, we could simply divide 16 by 2 and get with a net impedance of 8z.

Emile Menasche is a guitaris, writer, and songwriter in New York City.