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.
Decibels
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 is the minimum perceptible change in volume, 3 db is a moderate change in volume, and about 10 db is an approximate 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.
Distortion
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).
Signal-To-Noise Ratio (S/N)
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.
Continuous Power
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 measure in RMS terms. This means that the listed power output is sustained output at a specific volume level.
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