# What exactly does the +/- 2dB spec mean?

Discussion in 'Speakers & Subwoofers' started by Matt_Smi, Oct 16, 2004.

1. ### Matt_Smi Second Unit

Joined:
Apr 11, 2004
Messages:
327
0
Trophy Points:
0
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.

2. ### ChrisWiggles Producer

Joined:
Aug 19, 2002
Messages:
4,791
1
Trophy Points:
0
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.

3. ### Wayne A. Pflughaupt Producer

Joined:
Aug 5, 1999
Messages:
6,123
40
Trophy Points:
6,610
Location:
Katy, TX
Real Name:
Wayne
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

4. ### Lev-S Second Unit

Joined:
Jun 29, 2003
Messages:
324
0
Trophy Points:
0
This is Total Harmonic Distortion (THD), right?

5. ### ChrisWiggles Producer

Joined:
Aug 19, 2002
Messages:
4,791
1
Trophy Points:
0
no.

6. ### ChrisWiggles Producer

Joined:
Aug 19, 2002
Messages:
4,791
1
Trophy Points:
0
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.

7. ### Craig Chase Gear Guru Supporter

Joined:
Oct 21, 2002
Messages:
1,658
66
Trophy Points:
1,610
Location:
Pennsylvania
Real Name:
Craig
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...

Joined:
Sep 22, 2002
Messages:
2,031