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Discussion Starter · #1 ·
Ok, I know a bit about electronics, music, sound, acoustics etc. But I am a far cry from calling myself knowledgable.


A fool who doesnt ask questions remains a fool forever... so here goes.


Could someone help me out by explaining (first as if I'm an idiot, and then more technically) why the -3dB rating on a speaker/woofer/sub is so important.


For example, some small speakers (eg 6") claim to go as low as 20Hz, while other bigger speakers (eg 12") state 32Hz @ -3dB. But the 12" obviously plays deep bass much better.
 

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Just the quickie not very technical version.... dB referrers to the decibles of output sound produced by the speaker. When a speaker shows the limit of it's frequency range, a speaker will not produce it's entire frequency range at the same SPL's (sound pressure levels) or level of output. IOW, when you see a speaker rated to 32 Hz @ -3dB.... it means the speaker will play down to 32 Hz, but the 32 Hz sound waves will be 3 dB quieter than the rest of the output accross the frequency range.


The output of any driver will not be a straight line or constant. It will be more like a bell curve. At some point there will be roll of of output where the ability of the driver to produce sound starts to decrease. Since at -3 dB's you most likely will still be able to hear the 32 Hz, they publish the spec. But once you start to get to the point where the driver is down -6 dB's or -12 dB's or more.... you aren't really going to be able to hear what's being played because it's so much quieter than the rest of what's being produced.


As far as a 6" driver going down to 20 Hz, versus a 12" only going down to 32 Hz, I'd say you've got the sizes reversed. The smaller the driver, generally.... the less ability it has to go very deep. Bass sound waves are very long. The longer they are (as they go deeper) require a larger driver to reproduce them. I don't think I've ever seen a 6" driver with the ability to get down to 20 Hz even at -3 dB.
 

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To expound a bit further, it's probably a bit more important on a sub woofer because at some point you stop actually hearing very low frequencies but may still be able to feel them. It's that tactile impact of "feeling" those ultra low sound waves that can give you goose bumps when watching an action movie.


Not the ones that thump you in the chest, but that tiniest of "vibrations" that you subtlely feel right before the big BOOM slams you in the gut. Those are the very, VERY deep sound waves being reproduced by the driver.
 

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Discussion Starter · #4 ·
Thanx Quadriverfalls,


Thats pretty much spot on as to what I understand. So the -3dB in a nutshell is just illustrating the "useful" range of the driver, even though it can produce lower frequencies.


FTR, i didnt have the sizes mixed up, it was just an example, but you got the just of it. I think that some manufacturers put (for example) 30-20,000Hz on their little 6" driver. Technically the speaker is capable of producing all those frequencies, but their "useful" range is only as low as 49Hz (@ -3dB )


But obviously 30Hz looks far more appealing than 49Hz
 

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Quote:
Thats pretty much spot on as to what I understand. So the -3dB in a nutshell is just illustrating the "useful" range of the driver, even though it can produce lower frequencies.

LOL.... yup. I've been accused of being too "long winded" before.
But essentially, the - 3 dB point is generally referred to the point where the driver is starting to "roll off."
 

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Depending upon the application, a lower f3 (-3 dB point) may or may not be desireable. For example, a full-range speaker system designed to have an f3 of 30 Hz probably will be less sensitive than the same system designed for an f3 of 45 Hz. In other words, a higher -3 dB point will probably mean the speaker will play louder and have more headroom. There are many high-sensitivity 18" pro woofers made for sound reinforcement that are 105 dB sensitive and have an f3 of 40 Hz or higher.


This becomes important if the main speaker was designed for maximum headroom and output, and for use with a subwoofer. There's no reason to coax 20 Hz bass out of a speaker that will always be used with a sub, especially when it will most likely have more output if the drivers chosen for the design roll off higher. Our $7,000 Platinum LCR is always used with subs, and at 150 lbs., it's f3 is 51 Hz. We always recommend that in setup, it's set to "small."


If you're not using a subwoofer and you can live with a few dB less output, a lower -3 dB is desireable.
 

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Quote:
Originally Posted by Paul Scarpelli /forum/post/15551122


Depending upon the application, a lower f3 (-3 dB point) may or may not be desireable. For example, a full-range speaker system designed to have an f3 of 30 Hz probably will be less sensitive than the same system designed for an f3 of 45 Hz. In other words, a higher -3 dB point will probably mean the speaker will play louder and have more headroom. There are many high-sensitivity 18" pro woofers made for sound reinforcement that are 105 dB sensitive and have an f3 of 40 Hz or higher.


This becomes important if the main speaker was designed for maximum headroom and output, and for use with a subwoofer. There's no reason to coax 20 Hz bass out of a speaker that will always be used with a sub, especially when it will most likely have more output if the drivers chosen for the design roll off higher. Our $7,000 Platinum LCR is always used with subs, and at 150 lbs., it's f3 is 51 Hz. We always recommend that in setup, it's set to "small."


If you're not using a subwoofer and you can live with a few dB less output, a lower -3 dB is desireable.


Yep Paul nailed an important point. You can tune a speaker very low but when you start pumping a lot of SPL's through it (full range) that woofer will likely bottom out faster than if the speaker was tuned a little bit higher and not trying to move all that air at a really low hz. And obviously crossing over at 80 hz really takes the load off those 6 or 7" drivers.
 

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And odds are the 20Hz number for the smaller speaker would probably be down around -12db. If that manufacturer had provided a -3db number it would like be higher than the 32Hz of the other speaker.


lies, damn lies and statistics !
 

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Quote:
Originally Posted by rick240 /forum/post/15551236


And odds are the 20Hz number for the smaller speaker would probably be down around -12db. If that manufacturer had provided a -3db number it would like be higher than the 32Hz of the other speaker.


lies, damn lies and statistics !

A sealed box rolls off at 12 dB/octave, and a ported box does 24 dB/octave. If a small speaker starts rolling off at 80 Hz, the sealed version is down 24 dB at 20 Hz, and the ported one has, guess what, NO bass at 20 Hz.
 
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