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Speaker efficiency vs. power requirements

Recently someone gave me the "math" behind speaker efficiency ratings and power requirements. Although I am not sure if the information below is 100% accurate, it is what I have been told. Can we lay this on the table for discussion and try to resolve this confusing issue once and for all?

0 db is a power ratio of 1. Records and tape have dynamic ranges of 30-40db. To achieve a 30db dynamic range requires a power ratio of 1,000:1 and 40 db requires 10,000:1. So if you assign 1 watt of power to a speaker yielding 90db SPL, you need 1000 watts to deliver a true 30db dynamic range. With digital material we find dynamic ranges of 60-70db requiring power ratios of 1,000,000:1 & 10,000,000:1 respectively. Power amps of 1-10 million watts are not feasible today but the point is, more power offers more dynamic realism. Forget power vs. loudness because that really is not a factor in the overall scheme.
bwhite
rockvirgo
587 posts
 
Power ratio??? Never heard of it. What is it?
It appears the flaw in your reasoning is that speakers rated at 90dB can and often do play at much lower levels. By definition a person can hear any level of signal above the noise floor. Consequently think of starting at the whole system's noise floor - including the noise embedded in the software - not the efficiency rating of the speakers to discern your dynamic range. Also remember that the power ratings of amplifiers do not dictate output. The wattage ratings serve as one measure of an amp's prolonged sustainable capability without failure.
drjjpdc
80 posts
 
I DON'T KNOW WHO YOU WERE TALKING TO BUT BOY ARE THEY CONFUSED. Speaker sensitivity is simply the amount of db's produced as a reference (usually 1 watt at 1 meter). What your amp kicks out are electrical watts (you know the RMS variety) while what's (sorry for the pun) coming out of your speaker are acoustic watts. Even a very powerful 200 watt amp may in fact only have a watt or two coming from a speaker, after the inevitable heat loss from electrical to mechanical. The wattage of your amp must be taken in context with your speaker sensitivity, how far you listen from, what kind of music and how loud? You are also confusing dynamic range with the decibel scale. The decibel scale is a fixed number, 70 db is always 70 db. Dynamic range is just the ratio of a quietest sound to a loudest sound (also called signal to noise ratio). Don't forget that power ratings double with a corresponding 3 db change. Therefore a 200 watt amp has twice the wattage of a 100 watt amp and is only 3 db louder, which is about the limits that you can detect by ear. I have thrown out some basic stuff out there as I am mainly a biology major. I'm sure some physics major or enginneer can add plenty more.
herman
2,273 posts
 
Your math is Ok, but 90 dB is not the starting point for your dynamic range. Dynamic range is the difference between the softest and the loudest, and 90 dB is pretty loud to start with. 120 dB is EXTREMELY LOUD, equivalent to a jet taking off. 140 dB is the threshold of pain. Your million watt amplifier producing 160 dB would destroy your hearing. A more reasonable 60 dB range would be 40 to 100 dB, well within the capabilities of most hi end amplifiers and speakers.
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