How Much Power Is Enough?

Determining amplifier-power requirements for your home theater system.

Power output is often the biggest selling point for receivers and standalone amplifiers. Bells and whistles aside, you can often spend a lot less money for an amplifier or receiver that has a lot less power. While there are several factors that influence an amplifier's sound quality, we're not going to go into many of them in this article. We're going to focus on power. Ideally, an amplifier should be rated with low distortion, measured over the entire audible frequency range and with all channels driven. You should always listen to an amplifier before you purchase it. Whether you should test the 60-watt model or the 150-watt version depends on many factors, including your listening environment, the speakers you'll be using with it, and your listening habits.

For starters, it's important to understand that the perception of loudness is logarithmic. All things being equal, when you double the amount of power that you feed to your speakers, you get a 3-decibel increase in the sound pressure level at the output: 3 dB is considered to be a noticeable difference in volume. A 10-dB increase is twice as loud. Therefore, the difference between 1 and 2 watts is equal to the difference between 10 and 20 watts—likewise for 100 and 200 watts. So, the next time a salesman tries to tell you that you should step up from a 50-watt receiver to a 75-watt receiver, keep in mind that the change in amplifier power will barely create a noticeable increase in volume.

With that in mind, you want to determine how loud you like to listen to movies and music. In reality, most people don't listen to music, or even movies, all that loud. Figure 1 is a listing of various sounds and their relative sound pressure level (SPL). Typical listening levels range between 80 and 90 dB. Figure 2 is OSHA's standards for preventing hearing damage. No matter how loud you like your music, listening to it at a constant 115-dB level will permanently damage your hearing within 15 minutes. Most rock concerts are well into this range.

If you want to create a home theater system that can play at theater-reference levels, you should follow the home THX program's advice and try to achieve a peak of 105 dB. This is also the peak for the five main channels of a Dolby Digital soundtrack. The LFE track can play louder: 115 dB. Again, these are peak, not constant, levels. THX's substantial research with actual program material showed that the most demanding channels were, in descending order, the subwoofer, the center channel, the left and right channels (equally), and the surround channels.

You can start to figure out how much power you need by looking at your speakers' impedance and sensitivity ratings. First, most amplifiers are rated into an 8-ohm load. While most speakers are also rated with a nominal 8-ohm impedance, this rating is more of a mean, or an average. Speakers will often dip well below 8 ohms, particularly at low frequencies. THX-certified speakers are often rated at only 4 ohms. Halving the impedance draws twice as much current, and the amplifier ideally should deliver twice as many watts to keep up. For the sake of this article, we're assuming that you're using 8-ohm speakers and that the amplifiers you're comparing have adequate current capabilities.

Speaker sensitivity is a more-telling number. Sensitivity is often rated with 1 watt (or, more precisely by some manufacturers and in our testing lab, as 2.83 volts) of power and measured at a distance of 1 meter (about 3.3 feet). An average speaker may be rated at 88 dB SPL. Obviously, if you don't listen to music very loud, you won't need much more than a few watts to power a small room with more than enough sound (see figure 3). Music and movie soundtracks are extremely dynamic, though. Even at low listening levels, peaks can quickly require significant power to prevent distortion or clipping. A less-sensitive speaker may not be able to achieve a home theater system's output demands before you reach the speaker's limits.

It would seem, though, that—with an average speaker—you could get away with a mere 64 watts to achieve the theater-reference level for the main channels. This doesn't seem like all that much but also doesn't take into account room acoustics or the speaker's distance to the listener. For starters, if you're twice the distance from the speaker (2 meters or about 6.6 feet), sound will drop off by as much as 6 dB. Since the sound will reinforce itself by reflecting off of nearby walls, the floor, and the ceiling, you may only lose 4 dB or so. This still requires twice the power. The actual amount of attenuation is difficult to predict and is based partly on your room's absorption characteristics. If your room has thick carpet, heavy drapes, and big, plush sofas, there will be more absorption, so your system will require more power.

In some cases, like with subwoofers, you can use the room to your advantage. My Boston Acoustics THX subwoofers have a sensitivity of about 90 dB. (Note: Most powered subwoofers probably don't list the speaker sensitivity, since they have their own amplifier. They should list the speaker/amp combination's output, however.) When I factor in my distance from the subs and the desired 115-dB output level, I should need somewhere in the vicinity of 4,000 watts of power. Even if I could buy an amplifier with that much power, the sub's power handling is only about 150 watts, which means that the driver would likely launch itself into my neighbor's backyard before it would reach the desired output level (assuming my wife, cats, or neighbor would even allow that).

There are a few options, though. Of course, I could buy a more-sensitive subwoofer. A sub with a sensitivity of 93 dB would cut my power requirement in half. Moving the sub closer to a room boundary (putting it on the floor) augments the output by 3 dB, which in turn requires less power. Proximity to two boundaries, the floor and the wall, should garner a 6-dB increase in output. Corner placement (three boundaries) should increase the level by about 9 dB. These increases are theoretical; in practice, the increases are generally somewhat less. Still, this would save substantial power, but it may come at a sacrifice to frequency response, depending on room modes (a topic for a different article). Last but not least, I could add a second subwoofer, which increases the level another 6 dB. The difference in cost between a 500-watt amp and a 150-watt one may be greater than just adding a second sub, assuming that the former is even possible.

Again, you could just follow the THX camp's advice. THX certifies amplifiers at a given voltage with all channels driven for a specific period of time. Using voltage, instead of wattage, ensures that the amplifier will deliver adequate current at different impedance levels. The specification also assumes that the amp is driving THX speakers (a 4-ohm load) in a 3,000-cubic-foot room and takes into account the actual program-material demands mentioned above. The spec for standalone amplifiers is 28.3 volts or 100 watts into 8 ohms. Integrated products, such as receivers, must provide at least 26.1 volts (85 watts) for the front three channels and 20 volts (50 watts) into each surround channel. Then again, if you don't have THX-certified speakers or your room isn't 3,000 cubic feet, your demands may be different. Keep in mind that these specifications are for reference volume levels, something that only the sound engineers themselves actually listen to.

Determining amplifier-power requirements isn't an exact science, nor is it total voodoo. It is, however, somewhat of a juggling act between the desired output, your speakers' capabilities, and the room the speakers will play in. There are other things to consider when you choose an amplifier, as well, including sound quality and appearance. Still, if you keep in mind some of the abovementioned items, you'll at least narrow down your options to the ones that will fit your needs.