Most people think that more watts means more volume. An amplifier with 100 WPC is twice as loud as 50 WPC right? Not exactly.
Let's explore some answers to the above questions.
Sound levels are measured in Decibels (db) In essence, our ears detect differences in volume level in a non-linear fashion. Our ears become less sensitive to sound as it increases. Decibels are a logarithmic scale of relative loudness. A difference of approximately 1 db a minimum perceptible change in volume, 3 db is a moderate change in volume, and about 10 db is an approximate perceived doubling of volume.
To give you an idea of how this relates to real-world situations the following examples are listed:
0 db is the threshold of hearing
Whisper: 15-25 dB
Background noise: about 35 dB
Normal home or office background: 40-60 dB
Normal speaking voice: 65-70 dB
Orchestral climax: 105 dB
Live Rock music: 120 dB+
Pain Threshold: 130 dB
Jet aircraft: 140-180 dB
In order for one amplifier to reproduce 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 watts per channel needs to be 1,000 watts per channel to be twice as loud. In other words, the relationship between volume and wattage output is logarithmic rather than linear.
In addition, the quality of the amplifier is as (or more) important than just the wattage output. 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 with a low distortion level that a much more powerful amplifier with high distortion levels.
For example, an amplifier that has a distortion rating of 10% at full power output is unlistenable (perhaps even at lower output levels as well), however, an amplifier that outputs a .01% distortion level at full power output level would be imperceptible at all listening levels. Distortion specifications are expressed by the term THD (Total Harmonic Distortion).
Also, another factor in amplifier quality is Signal-To-Noise Ratio (S/N), which is 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. A S/N ratio of 70db is much more desirable that a S/N ratio of 50db.
An additional factor is this equation is the ability of a receiver or amplifier to output its full power continuously. In other words, just because your receiver/amplifier may be listed as being able to output 100WPC, doesn't mean it can do so for any significant length of time. Always make sure that, when you check for Specifications, that the WPC output is measured in RMS terms. This means that the listed power output is sustained output at a specific volume level.
How Stated Power Ratings Can Be Deceiving
In addition to the continuous power rating, another factor affecting real amplifier power output, especially with surround sound receivers, is whether the manufacturer is basing their wattage output specification on one or two channels driven or with all channels driven. In addition, was the measurement made using a 1KHz test tone, or with 20Hz to 20KHz test tones?
In other words, when you see an amplifier wattage rating of 100 watts-per-channel at 1 KHz with one or two channels driven, the real-world wattage output when all 5 or 7 channels are operating across all frequencies will be lower, possibly as much as 30 or 40% lower. Of course, not all channels actually require the same power at the same time as variations in audio content affect the requirements for each channel at any given time. However, it is important to note that the wattage output, as stated in the manufacturer's specifications, may not be available across all channels at the same time.
Last (for the purposes of this discussion), but not least (by any means), is the ability of your receiver/amplifier to output power at a significantly higher level for short periods to accommodate musical peaks or extreme sound effects in films. This specification 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.
Once again, Dynamic Headroom is measured in Decibels. If a receiver/amplifier has the ability to double is power output for a brief period to accommodate the conditions described above, it would have a Dynamic Headroom of 3db.
So, when shopping for an receiver/amplifier, be wary of wattage output specifications and also take stock of other factors such as Total Harmonic Distortion (THD), Signal-To-Noise Ratio (S/N), and Dynamic Headroom.
In conclusion, your amplifier or receiver, although the centerpiece your audio or home theater system, other components such as Loudspeakers, Input devices (CD, Turntable, Cassette, DVD, etc...) are also links in the chain. However, you can have the best components available, but if your receiver or amplifier isn't up to the task, your listening experience will definitely suffer.
This has been a very brief overview of some of the factors to take into consideration when buying a Receiver/Amplifier. Don't base your buying decisions from a single specification, such as a Watts-Per-Channel rating. A single spec, taken out of context with other factors, does not give you an accurate picture of the receiver/amplifiers true capabilities. There are many other factors to take into consideration, and never buy any receiver/amplifier without giving a good listen for yourself. Make sure the dealer allows a 30-day satisfaction period or other accommodation for return/exchange if you are not happy with the performance of the unit.
For an additional perspective on this subject, check out two articles from About.com Stereos: What is the Relationship between Loudness and Amplifier Power? and Amplifier Power and Speaker Efficiency.