# Subwoofer -3db level?



## leenorm1 (May 7, 2012)

I guys my understanding of the -3db point is that if a speaker plays at 100db average then the -3 db point will be at whatever frequency the spl reading is 97db. Could somebody please clarify. Thanks in advance lee


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## wgmontgomery (Jun 9, 2011)

I believe you are referring to a speaker's frequency response. The -3dB point usually refers to the frequency in the bass that is down 3 dB (aka f3) from a specific point like 1 kHz. It is also used for high frequencies.

You will also see specs like 40Hz-25kHz +/- 3 dB which means that from 40Hz to 25kHz the frequency varies anywhere from 3 dB down to 3 dB above "flat."

This is a rather simplistic explanation, but I hope that it helps.


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## leenorm1 (May 7, 2012)

Thanks for a quick response. Your information is valued. I was referring to subwoofers which say for example-3db at 28hz. So from what I understand from your comment is that if the specs say -3db at 28 hz then this means -3db from a specified level not the max output? If this is right then the information the manufacturers supply isn't very good as it bears no relation to the subs max output.


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## wgmontgomery (Jun 9, 2011)

It usually refers to the lowest "usable" frequency, but with subwoofers (all speakers actually) the in-room response will usually be lower. A -3dB point of 25Hz may give you ~20Hz _in-room_. The maximum output is usually listed as something like "max SPL" and _may_ specify a frequency.


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## wgmontgomery (Jun 9, 2011)

"So from what I understand from your comment is that if the specs say -3db at 28 hz then this means -3db from a specified frequency not the max output?"

Correct


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## leenorm1 (May 7, 2012)

Thanks. So if I found two subs that when used together achieve 105db at 25 hz at the -3db point that would be good?


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## wgmontgomery (Jun 9, 2011)

Multiple subs are great for added SPL and-_more importantly_-reducing room modes. You will not only get "more" bass, you will also get cleaner bass.


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## phreak (Aug 16, 2010)

leenorm1 said:


> Thanks. So if I found two subs that when used together achieve 105db at 25 hz at the -3db point that would be good?


Probably. If for music only then this would be perfect for pretty much everything other than the lowest notes on a pipe organ. With movies, this would still exceed most people's expectations, but without a frequency response graph or knowing more about the slope of the low frequency drop off, it is hard to say whether or not you would feel the stomach churning low end that some movies have for earthquakes/cannon shots/depth charges and other sub killing effects. But don't let me discourage you. 99% of the general population have probably never heard 105dB with a -3 of 25 hz. It will sound good.


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## tesseract (Aug 9, 2010)

Many manufacturers measure the sub in a corner, indoors. This inflates the -3 dB rating. Look for outdoor ground plane measurements.


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## leenorm1 (May 7, 2012)

Thanks for the info. The subs i am talking about are thx certified so I assume the measurements are more accurate?


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## wgmontgomery (Jun 9, 2011)

leenorm1 said:


> Thanks for the info. The subs i am talking about are thx certified so I assume the measurements are more accurate?


As far as I know THX does not mandate how a company measures its speakers. The -3dB point could be in an anechoic chamber or "room loaded;" unless the specs state something like "in-room" response you really just don't know.


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