Now you're plugged in and switched on, but during soundcheck you find out you're not quite loud enough to carry the room. A member of another band generously offers the use of his speaker cabinet. That's great, but there are three questions you should ask before you hook it up: (1) What's the impedance of your cabinet, and what's the impedance of his? (2) What will the total combined impedance be? (3) Is that a safe load for your amplifier?
When an additional speaker is hooked up to a system, it's generally connected in parallel to the other speaker(s);see Fig. 2. This is the case when you use the extension speaker jack(s) on your amp, or the in/out jacks (if provided) on the input panel of your cabinet. As you add speakers in parallel, the total impedance the amplifier "sees" becomes less. Also, other than plugging in an additional speaker in an existing jack, no other wiring is necessary. In a series arrangement, on the other hand, the existing wiring must be broken and the new speaker inserted; in this case, the total impedance becomes greater. Let's look at parallel impedance, since it's more common.
To figure out the total impedance of two or more cabinets of equal value hooked up in parallel, divide the impedance of one cabinet by the number of cabinets:
impedance of one cabinet / number of cabinets = total impedance
Let's say the enclosure you own is 8 Ohms and the cabinet you borrowed is 8 Ohms. The formula is 8 / 2 = 4, so the total impedance will be 4 Ohms. Likewise, if you had four 8 Ohm enclosures, the total impedance would be 2 Ohms ( 8 / 4 = 2). Now, it may be just your luck that the second cabinet is 4 Ohms and yours is 8 Ohms. No problem - just think of the 4 Ohm cabinet as two 8 Ohm cabinets (we know this is true from the first example), so you now have, in effect, three 8 Ohm enclosures. The formula is 8 / 3 = 2.67.
The owner's manual that came with your amplifier should state the lowest (or minimum) impedance your amp is designed to drive. This may also be indicated next to the speaker output jacks. Since the total impedance of the cabinets you want to use is 4 Ohms, your amp must have a minimum load rating of 4 Ohms or less. This being the case, you turn off your amplifier, hook up the second cabinet, and then turn your amp back on. To your disbelief, the sound is not louder—in fact, it's tinny with no body, and turning up the gain makes it sound even worse.
Most likely, the two cabinets are out of phase. This means that while the speaker cones of one cabinet are moving out, the cones of the second cabinet are moving in. The net result is little or no sound at all. To check for this situation, get out that 9-volt battery again. Turn off the amp and unplug the speaker cable from the amp, leaving the other end still connected to the cabinet. As shown in Fig. 1, touch the plus (+) side of the battery to the tip of the phone plug and the minus (-) side of the battery to the sleeve of the phone plug. When you do this, the cone(s) in the cabinet should move out. When the battery is disconnected, the cone(s) will go back to their original position. Now repeat the procedure with the second cabinet; chances are, the cone(s) will move in the opposite direction—in. If so, the speakers are wired out of phase.
