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# What exactly does the +/- 2dB spec mean? (1 Viewer)

#### Matt_Smi

##### Second Unit
I have been wondering this for a little while, I am using my STF-2 as an example it is rated at +/- 2dB: 25 Hz. I know that the lower the dB rating the better, so I am guessing that it means the sub can play to 25 Hz without dropping more than 2 decibels, or without getting 2 decibels louder. But I am really not sure, someone please explain, thanks.

#### ChrisWiggles

Senior HTF Member
So it should be flat within 2db to that freq, pretty much, in a proper space. Obviously in an untreated room, the room will push this all over the map.

#### Wayne A. Pflughaupt

Moderator
Senior HTF Member
Matt,

It means that the sub’s frequency response curve over its full operating range (probably 25Hz-100Hz?) varies from flat by no more than + or – 2dB.

Regards,
Wayne A. Pflughaupt

#### Lev-S

##### Second Unit
This is Total Harmonic Distortion (THD), right?

#### ChrisWiggles

Senior HTF Member
no.

#### ChrisWiggles

Senior HTF Member
This is frequency response. It is telling you, that it is flat across such a band of frequencies, within this limit.

THD is the distortion created by an amplifier, etc, that adds harmonics to the fundamental frequency. Totally different.

#### Craig Chase

##### Gear Guru
No - Total harmonic distortion is the sum of the harmonics of the fundamental.

For example, If the Fundamental is 20 Hz, and the subwoofer is generating 100 dB, you would add 40, 60, 80, 100 ... etc... signals to get the THD. We will stop at 100 Hz for simplicity.

If, in this example, each of the 40-60-80-100 Hz levels had a 60 dB signal while the 20 Hz signal was 100 dB, you would have four 1% distortion levels (-40 dB is a 1 % distortion level) ... or a THD of 4 %.

In the real world, it is never that simple... but this gives an idea...

#### Edward J M

Senior HTF Member
When measured outside at 2 meters, the STF-2 does indeed meet rated spec, measuring 25-100 Hz +/- 2 dB.

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