A speaker is said to play "flat" between two frequencies at a given level of output if the output varies no more than 3dB above (+3dB) or 3dB below (-3dB) the given level. The range is described, for example, as 65Hz - 18KHz +/-3dB. Below the bottom frequency and above the top frequency of that range, the speaker's output will continue to drop off to a point where it is too low to be useful / heard.
Re. the DefTech speakers, it would appear that 79Hz is where the speaker's output drops by -3dB (79Hz @ -3dB). That is the bottom end of the range between which it will play "flat" at a given output level. By 26Hz, the speaker may still be generating some minute amount of output, but since the level has likely dropped by 20dB (26Hz @ -20dB) or 30dB (26Hz @ -30dB) or more, it's pretty much useless.
Looks great for marketing purposes, but it's pretty dishonest "specsmanship".
-- Anecdote time!
My first sub - a DefTech ProSub 200TL - was rated down to 18Hz, with no listed variance (no +/-n
dB, where n
ideally equals a small number). It sounded pretty good.
Then I had the chance to demo an S10.3 I had bought for a friend. It was rated down to 21Hz @ -3dB. I heard/felt deep bass notes I'd never experienced before with the "18Hz" DefTech sub. How could this be?
So I contacted DefTech and was told that the sub was actually rated to 26Hz @ -3dB. That explained a lot!
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