If you have purchased an audio amplifier, it makes sense to spend a few minutes to figure out some key parameters of loudspeakers in order to purchase speakers which will work best with your amplifier.
One of the most important speaker parameters is the speaker impedance. The impedance is basically a ratio that determines how much voltage the amplifier needs to apply across the speaker in order to generate a certain amount of wattage. In other words, the higher the speaker impedance, the higher the voltage swing of the amplifier has to be in order to generate a certain speaker wattage. The speaker wattage will determine the loudness of the speaker.
Many small amplifiers work with an external power supply. If you look at the power supply label you will notice a voltage rating. The voltage rating determines how much voltage the power supply will deliver to the amplifier. If the power supply is rated for 12 V for example then you will need some fairly low-impedance speakers in order to take advantage of the amplifier. At 12 V, the amplifier will have a hard time driving some high-impedance speakers.
Many power amplifiers have a built-in power supply. That means that you will not easily be able to determine the internal operating voltage of the amplifier. The only way to determine the voltage swing that the amplifier can deliver at its output is to do a measurement or to read the specifications. I personally prefer taking a quick measurement. To do this, you will need an oscilloscope. Then apply a fixed sine wave signal to the amplifier input. The easiest way to generate such a signal is to record a simple tone or download a sine wave from several websites. Then transfer this way form want to an MP3 player. Plug the output of the MP3 player into the input of the amplifier.
Then take the probe of the oscilloscope and measure both output terminals. Most amplifier will work in four-bridge configuration. This means that both terminals are actively driven. In that case, the amplifier can generate more power given a specific supply voltage versus having a single-in that configuration. With a single-and the configuration, one of the speaker terminals will be decoupled against ground by using a decoupling capacitors and only one speaker terminal is driven.