Q: I want to buy a new receiver and am confused by the amplifier specifications. How do I know what they mean and what I need?

A: When choosing an amplifier or receiver the first things to consider are the speakers' sensitivity and the room size.

Speakers have a sensitivity rating (also called efficiency) that specifies how many decibels of sound they produce from one watt of power when measured from one meter away. Numbers above 90 are considered very efficient, 87-89 is average, and below 87 is inefficient. The lower the sensitivity, the more power you need. To make a 3-decibel difference you must double the power, so the difference between an 87-decibel and 90-decibel sensitivity rating is actually quite large. A pair of speakers with a 90-decibel sensitivity rating with a 50-watt amplifier will perform the same as a pair of 87-decibel rated speakers with a 100-watt amplifier. Though it seems to be a paradox, bigger speakers are usually more efficient than smaller ones.

Room size is simple. The bigger the room, the more power you will need.

There is also a speaker rating called impedance. Most speakers are 8 ohms, which most any amplifier can handle. If your speakers are rated at 6 ohms or 4 ohms make sure the amplifier is rated to work with them. Many receivers are 8 ohms only.

Next, check the specifications of the amplifier itself. What you want to note are the rated bandwidth, total harmonic distortion (THD) and signal-to-noise ratio. Manufacturers do not always provide the signal-to-noise number.

The human ear can hear between 20 Hz (deep bass) and 20,000 Hz (high treble). The best amplifiers are rated between 20 and 20,000 Hz. Sometimes a manufacturer will rate an amplifier at a single frequency, such as 1,000 Hz. This is misleading and allows the manufacturer to advertise a higher wattage number. A THD rating below .1 percent is inaudible to the human ear. The best solid-state amplifiers are rated between 20 and 20,000 Hz with less than .1 percent THD. Tube amps typically have higher THD but will still sound excellent.

To complicate matters further, most multichannel surround-sound receivers are measured with just one or two channels driven. That's why a separate amplifier rated at 75 watts per channel with all channels driven will be noticeably more powerful than a 100-watt receiver rated with one or two channels driven.

There also is the matter of current. High current is especially important if you have low-impedance speakers rated at 4 ohms. If you have a powered subwoofer you can get away with less power to the main speakers because the subwoofer will reproduce the power-hungry bass sounds and leave all the receiver power for the speakers.

To sum it up, look for a receiver or amplifier rated between 20 and 20,000 Hz, with less than .1 percent THD. A signal-to-noise ratio above 95 decibels is excellent. A receiver with 50 watts per channel should be adequate for small to medium-sized rooms or speakers above 90 decibels sensitivity. If you have a large room or speakers below 87 decibels sensitivity, try to get 100 watts or more.

Next week I will provide some amplifier comparisons and examples of matching amplifiers and receivers to speakers, using some of the best values on the market.

Send questions to Don Lindich at donlindich@gmail.com. Get more recommendations and read past columns at www.soundadviceblog.com.