Audio and Alarms Signal to Noise Ratio Primer

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Signal to Noise Ratio Primer
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Signal To Noise Ratio:
The signal to noise ratio is the difference between the noise floor and the reference level. The reference level is determined by the person making the measurements. For amplifiers, the reference may be, full power, one volt, one watt into a given load or any number of other things. For you to compare two pieces of equipment which were tested by different methods, you must know precisely what reference was used.

Common Misconception:
Many people believe that a piece of electronic equipment (equalizer, amplifier, crossover...) with a slightly higher signal to noise (s/n) ratio is infinitely better than one with a slightly lower s/n ratio. Many people don't realize how much a 30 or 40 dB ratio is. A piece of electronic equipment with a S/N ratio of 80dB may be good enough for all but the best systems.

Example:
If you were looking at 2 amplifiers and one had a S/N ratio of 102dB and the other was rated at 80db. You'd probably think that the amp with the higher ratio was quieter (and better). If both amplifiers were rated at 600 watts and the amp with the higher S/N ratio was rated at full power but the amp with the lower S/N ratio was rated at 1 watt, we could use the formula dB=10*log(value1/value2) to find the difference in decibels between 600 watts and 1 watt (so that we could compare apples to apples).

dB = 10*log(value1/value2)
dB = 10*log(600/1)
dB = 27.78

We can see that the difference in 600 watts and 1 watt is 27.78dB. To compare the 2 amplifiers, we have to subtract 27.78 from 102dB. This gives us a S/N ratio of 74.22dB for the amp with the 102 dB S/N ratio when its reference is changed from full power output to 1 watt. When both amplifiers are measured with the same reference, the amplifier that you thought was noisier (and a total piece of junk) is actually quieter.

Even at a 50dB ratio, there is a significant difference between the 2 levels. With most computer speakers, anything more than a 50dB would be difficult to reproduce.

You should remember:
1.Because a 90 dB s/n ratio is so high already, another 10 dB (for a 100 dB s/n ratio) is not going to be noticable and you should not decide to purchase a piece of audio equipment solely based on that specification. In amplifiers, you wouldn't likely be able to pick out the noisier amp if one amp had a s/n ratio of 60 dB and another had a 90 dB s/n ratio (as long as all else remained equal). This, of course, excludes all of the 13, 14 and 15 year old guys out there because, as we aaalllll know, they know more than anyone (especially their elders). :- ) I can just imagine all the email I'll get from that!

As most of you have noticed, the noise ordinances have become much tougher lately. Most of this is due to idiots, yes IDIOTS, who drive through residential areas with their windows down while their system is playing at full power. To make things worse, the music they listen to has all sorts of foul language that's not suitable for small children, (who may be playing outside). There are even a few people, who are even beyond idiot status, that play their systems at full power through residential areas after 10:00 PM (when many people go to bed). I don't believe that this type of behavior is good for the industry. If the fines get too stiff, people will stop buying large systems. If this happens, more people will get out of car audio (who wants a mediocre system). People get interested in things because they're exciting. A deck and four 6.5" speakers are not going to interest many of the younger car audio enthusiasts. If car audio enthusiasts keep annoying more and more people, the fines will keep getting tougher. All of this will only reduce interest in the equipment that fuels the industry. If you want to listen to your system at full volume, get out on the highway where there's little chance of bothering anyone. When you get to a red light, turn it down. If the only thing attractive about you is your 'system', you have some work to do. Bottom line... Think about what you're doing. Think about other people. It's not the end of the world if you have to turn the volume down for a little while.