Home Theater & Entertainment Audio 421 421 people found this article helpful Understanding Amplifier Power Output Specifications Don't base an amplifier's quality just on its wattage output by Robert Silva Writer Robert Silva has written about audio, video, and home theater topics since 1998. Robert has written for Dishinfo.com, and made appearances on the YouTube series Home Theater Geeks. our editorial process Facebook Twitter LinkedIn Robert Silva Updated on September 11, 2020 Audio Speakers Stereos & Receivers Tweet Share Email The watts-per-channel (WPC) rating always stands out in advertisements and product descriptions for amplifiers, stereos, and home theater receivers. There is a perception that more watts is better, with more watts equalling more volume. But that's not necessarily true. D&M Holdings / Sound United Stated Power Ratings Can Be Deceiving When it comes to real amplifier power output, especially with surround sound receivers, you can't take a manufacturer's amplifier power rating statements at face value. You need to look closer at what they are basing their statements on. For example, for home theater receivers that have a 5.1 or 7.1 channel configuration, is the stated wattage output specification determined when the amplifier is driving just one or two channels at a time? Or is the specification determined when all channels are driven simultaneously? In addition, was the measurement made using a 1 kHz test tone, or with 20Hz to 20KHz test tones? Zeroing in on Stated Power Ratings When you see an amplifier wattage rating of 100 watts-per-channel at 1 kHz (which is considered the standard mid-frequency reference) with one channel driven, the real-world wattage output when all 5 or 7 channels are operating at the same time across all frequencies will be lower, possibly as much as 30 to 40 percent lower. It's better to base the measurement with two channels driven, and, instead of using a 1kHz tone, using 20Hz to 20kHz tones. They represent the widest frequency range that a human can possibly hear. However, that still doesn't take into account the amplifier's power output capability when all channels are driven. In a home theater receiver, not all channels require the same power at the same time. Variations in audio content affect the requirements for each channel at any given time. For example, a movie soundtrack will have sections where only the front channels may be required to output significant power, while the surround channels may output less power for ambient sounds. On the other hand, the surround channels may need to output a lot of power for explosions or crashes, but the front channels may be de-emphasized at the same time. Based on those conditions, a power specification rating phrased in context is more practical to real-world conditions. One example would be 80 watts-per-channel, measured from 20Hz to 20kHz, 2-channels driven, 8 ohms, .09% THD. What all that jargon means is that the amplifier, stereo, or home theater receiver has the ability to output 80-WPC using test tones over the entire range of human hearing, when two channels are operating with standard 8-ohm speakers. This is more than enough for an average size living room. Also included in our example is the notation that the resulting distortion (referred to as "THD" or Total Harmonic Distortion) is only .09%. This represents a very clean sound output. Continuous Power Another factor to consider is whether a receiver or amplifier can output its full power continuously. Just because a receiver or amplifier is listed as being able to output 100 WPC, doesn't mean it can do so for any significant length of time. When checking amplifier specifications, see if the WPC output is measured in RMS or FTC terms, and not terms such as Peak or Maximum Power. Decibels Sound levels are measured in Decibels (dB). Our ears detect differences in volume level in a non-linear fashion. Ears become less sensitive to sound as it increases. Decibels are a logarithmic scale of relative loudness. A difference of approximately 1 dB is the minimum perceptible change in volume, 3 dB is a moderate change in volume, and about 10 dB is an approximate perceived doubling of volume. Here is how this relates to the real world: 0 dB: the threshold of human hearing15-25 dB: whisper35 dB: Background noise40-60 dB: normal home or office background65-70 dB: normal speaking voice105 dB: orchestral climax120 dB+: live rock music130 dB: pain threshold140-180 dB: jet aircraft For one amplifier to produce sound twice as loud as another in decibels, you need 10 times more wattage output. An amplifier rated at 100 WPC is capable of twice the volume level of a 10 WPC amp. An amplifier rated at 100 WPC needs to be 1,000 WPC to be twice as loud. This follows the logarithmic scale mentioned above. Distortion The quality of the amplifier isn't just reflected in wattage output and how loud it gets. An amplifier that exhibits excessive noise or distortion at loud volume levels can be unlistenable. You are better off with an amplifier of about 50 WPC that has a low distortion level than a more powerful amplifier with high distortion levels. Distortion specifications are expressed by the term THD (Total Harmonic Distortion). However, when comparing distortion ratings between amplifiers or home theater receivers, things can get "cloudy." On its spec sheet, "amplifier or receiver A" might have a stated distortion rating of .01% at 100 watts of output, while "amplifier or receiver B" might have a listed distortion rating of 1% at 150 watts of output. You might assume that "amplifier/receiver A" might be the better receiver, but you have to consider that the distortion ratings of the two receivers were not stated for the same power output. It may be that both receivers have the same (or comparable) distortion ratings when both are running at 100 watts output, or when "A" was driven to an output 150 watts it might have the same (or worse) distortion rating as "B". On the other hand, if an amplifier has a distortion rating of 1% at 100 watts and another has a distortion rating of only .01% at 100 watts, then it is more obvious that the amplifier or receiver with the .01% distortion rating is the better unit with regards to that specification. As a final example, an amplifier or receiver that has a stated distortion rating of 10% at 100 watts, would be unlistenable at that power output level. It may be more listenable with less distortion at a lower power output level, but if you run into a amplifier or receiver that lists a 10% distortion level (or any distortion level higher than 1%) for its stated power output, try to get some clarification from the manufacturer before buying. Signal-To-Noise Ratio (S/N) Another factor that affects amplifier quality is Signal-To-Noise Ratio (S/N). This is a ratio of sound to background noise. The larger the ratio, the more the desirable sounds (music, voice, effects) are separated from acoustical effects and background noise. In amplifier specifications, S/N ratios are expressed in decibels. An S/N ratio of 70db is much more desirable than an S/N ratio of 50db. Dynamic Headroom Last (for the purposes of this discussion) is the ability of a receiver or amplifier to output power at a significantly higher level for short periods to accommodate musical peaks or extreme sound effects in films. This is very important in home theater applications, where extreme changes in volume and loudness occur during the course of a film. This specification is expressed as Dynamic Headroom. Dynamic Headroom is measured in decibels. If a receiver/amplifier has the ability to double its power output for a brief period to accommodate the conditions described above, it would have a Dynamic Headroom of 3db. The Bottom Line When shopping for a receiver or amplifier, be wary of wattage output specifications and take stock of other factors such as Total Harmonic Distortion (THD), Signal-To-Noise Ratio (S/N), and Dynamic Headroom. Also pay attention to the efficiency and sensitivity of the speakers you are using. An amplifier or receiver may be the centerpiece of your audio or home theater system, but other components such as CD players, turntables, and Blu-ray Disc players may also be links in the chain. You may have the best components available, but if your receiver or amplifier isn't up to the task, your listening experience will suffer. Although each specification contributes to the ultimate performance capability of the receiver or amplifier, it is important to emphasize that a single spec, taken out of context with other factors does not give you an accurate picture of how your home theater system will perform. Also, although it is important to understand the terminology that is thrown at you by the Ad or salesperson, don't let the numbers overwhelm you. Base your buying decision on what you hear with your own ears, in your own room.