Axiom Home Page Forums General Discussion Technical Questions Help understanding decibles

More proof of my ignorance coming through...

When a receiver says -10dB. what exactly is that ratio? I understand that a jet plane taking off produces 140+ dB within a reasonable distance. How such a common number equated to the performance of a receiver. Second question would be regarding speaker performance. The stats on the M80s says,"Freq Resp +/-3dB (Hz)34-22K". I understand the frequency reference; however, I don't get why we have a plus/minus of 3 for that value. What is its significance?

If anyone would like to point me to some good reading, rather than explain in their own words, please don’t hesitate.

Thanks!

Good question...seems the last few years someone changed things a bit and now the volume is +/- for the volume controls on the recievers......
Whatever happened to 0-10

decibels

In reply to:

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Whatever happened to 0-10

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not to brag, but mine actually has 0-11

"3 dB attenuation means that 0.50 of the input power survives"

In other words, when you see a freq. range with a 3dB +/-, it means it can get to low frequencies before losing half of the total volume, correct? In other words, a speaker rated to go from 57 Hz - 220 MHz +/- 3dB will surely hit 57, but it will be at half output. Indeed, it will probably hit below 57, but even more quietly.

Is this right?

"In other words, when you see a freq. range with a 3dB +/-, it means it can get to low frequencies before losing half of the total volume, correct? In other words, a speaker rated to go from 57 Hz - 220 MHz +/- 3dB will surely hit 57, but it will be at half output. Indeed, it will probably hit below 57, but even more quietly."

You're mixing apples and oranges here. A 3db decrease requires half as much power. Power does not equal loudness, they are two seperate things. As the link above shows, db is a logarithmic function. For example, if it takes 10 watts to create a 100 db noice, then a 103 db noise will require 20 watts, 106 db requires 40 watts, 109 db requires 80 watts, etc.

OK, but that leaves me as confused as ever about the original quesiton of the thread - what does the +/-3dB part of the "XX Hz - XX MHz +/- 3dB" spec tell you?

It tells you what frequency range the speaker will reproduce. If a speaker is +/- 3db from 50Hz to 20kHz, then that speaker has a fairly flat response from 50Hz to 20kHz. This means that the speaker can produce sound within 3db of the original material as long as the original material doesn't go below 50Hz or above 20kHz.

How loud the speaker is depends on it's efficiency and how much power you feed it.

OK, thanks. But is +/- 3dB really all that "flat"? Since dB is on a log scale, isn't 3dB a fairly notable difference, ultimately?

I just re-read your original post and I think you had basically the right idea, just misapplied a little bit. Yes, 3db is a noticable difference. Whether or not it is exactly "twice as loud" is rather sujective (I've seen some sources say 4db is twice as loud). The fact that +3db requires twice as much power is not subjective.

And I'm not an expert on this subject, so if anyone else wants to jump in and explain things better...feel free!