Quote:
Originally posted by eaadams
on a speaker what does it mean "-3dB point = 80hz?"
In adition to that could someone point me to a site that explains frequency responce as well as those speaker charts like in Sound and Vission.
Thanks |
Evan,
I've looked at this thread a few times now, and the main problem I keep coming across is where to start. Obviously one tangent on the problem has been taken up, but it is easy to get overly confused with what is going on if some basics are not clearly understood. I'll pick out a couple issues to take note of and then hopefully you can ask some further questions based on those answers and how they relate to what you are looking over in the magazines.
First and foremost, be cautious when you are reading measurements in magazines and especially those published by manufacturers (yes, I work for a loudspeaker manufacturer

). While many different graphs may all have similar horizontal and vertical markings with different "curves" plotted, rarely are two measurements directly comparable.
I'm sure someone else here can dig out a link showing where different instruments' frequency ranges fall which will give you a basis for what type of sounds we are talking about when we look at different frequency ranges on a response curve.
These response curves typically have a vertical scale in dB or decibels. In this context, a decibel is a unit measure of sound pressure level, often abrieviated as SPL. SPL is measured using a microphone and associated electronics. Realize that the decibel is a logrithmic measurement, which means that unlike the power from our amplifiers, 2x the power to a speaker does not mean twice the decibel reading. In terms of power into a loudspeaker, every doubling of power equates to approximately a 3dB increase in sound pressure level, while every halving equates to approximately a 3dB decrease in sound pressure level. The -3dB roll-off points for the low end response of a speaker comes from this as it is considered the "half power point."
Of course what we need to remember is that a loudspeaker's response doesn't just turn off below this half power point, but rather it rolls off at some rate, with some speakers rolling off faster than others. Likewise output from a speaker can most certainly be useable even when it is more than 3dB below the arbitrary reference level. The above said, another interesting slight of spec shall we say is the casual use of a +/-3dB specification. The "+" in the spec allows for a total of a 6dB swing within the frequency range noted. This allows a manufacturer to specify the low end cut-off or response as where ever the response drops 6dB below the peak of the response curve while still giving entirely accurate specifications.
Beyond the notation games, without understanding the conditions of the tests, and the smoothing (or averaging) applied, it becomes much more difficult to compare curves from different sources. Most manufacturer's response curves have 1/3rd octave smoothing, which hides many blips in the response, where 1/6th to 1/12th octave resolution is more appropriate to identify problems in a speaker, but of course don't look as pretty or smooth. Also be sure to check the vertical scale of the curve, as a 10dB/ division curve will look a lot smoother than the same curve plotted at a 3dB/ division vertical scale. Referring specifically to the Sound & Vision curves, it is important to understand what their different graphs are really depicting. The average response over 30deg is more useful to show the power response of the speaker yet may hide dips or peaks in the response as you move from on to off axis.
Realize that no one curve, measurement or specification will give you a complete picture of what a speaker will sound like or how it behaves. In fact it is precisely the combination of many measurements and the interpretation of their relative significance which is key.
Hope this helps more than confuses,