For FR specification, is -6dB and +/-3dB the same thing?

[KEW]

I've always assumed it was, but we all know where assuming can get you!

Frequency response: 62Hz–20kHz (-6dB)

vs

Frequency response: 62Hz–20kHz (+/-3dB)

 

[j_garcia]

They are sort of saying the same thing, both in different semi-deceptive ways lol. -6dB is at least more "honest" in that it is telling you there is almost no sound at that point, but it is really just to make those numbers look better than they are since most don't know what 6dB equates too. Along the same lines, +/- 3 is a way to make it look better than it is, when the swing is really 6dB and likely does actually mean those are the -6dB points. +/- means it remains within that range; aka you should not have swings of more than that between the specified range, 3dB is quite audible, while knowing the -6dB points doesn't tell you if the rest of the range may have larger dips/peaks.

 

[Dennis Murphy]

I've always assumed it was, but we all know where assuming can get you!


vs

That's not how I've always interpreted it. The +/- refers to deviations from some reference level that is the central tendency of the overall response. Suppose a speaker is dead flat from 80 Hz to 15 kHz, but the very top end trends up by 3 dB by the time it hits 20 khz (as many ribbons might do), and trends down by 3 dB when it hits 62 hZ. That would be +/- 3 dB from the reference point, which is the average response. If that same speaker were also flat to 20 kHz, but was 6 dB down at 62 Hz, that would be the - 6 dB spec.

 

[AcuDefTechGuy]

It doesn't seem the same to me.

Frequency response: 62Hz–20kHz (-6dB)...

I think it should read "62Hz-20kHz (-6dB, +0dB)" if the FR never goes above the "reference 0dB level".

I would rather have 62Hz-20kHz +/- 3dB (-3dB, +3dB).

 

[j_garcia]

That's not how I've always interpreted it. The +/- refers to deviations from some reference level that is the central tendency of the overall response. Suppose a speaker is dead flat from 80 Hz to 15 kHz, but the very top end trends up by 3 dB by the time it hits 20 khz (as many ribbons might do), and trends down by 3 dB when it hits 62 hZ. That would be +/- 3 dB from the reference point, which is the average response. If that same speaker were also flat to 20 kHz, but was 6 dB down at 62 Hz, that would be the - 6 dB spec.

What I was trying to say, but I couldn't exactly explain it. +/- means it will not be more than 3dB above or below in the specified range, but it is still a 6dB spread. It may not mean something detrimental, but you need to see the curve.

What would a company benefit from by giving the -6dB numbers other than making them seem better to consumers who don't know what -6dB refers to?

 

[lovinthehd]

I'd never equated the two specs. I've rarely seen the -6dB spec, and that's mostly been for the lower end response alone rather than the full range.

 

[Swerd]

I agree with everyone above. I thought an illustration could help.

Consider this listening window frequency response curve for the KEF LS50.
http://www.soundstage.com/index.php?option=com_content&view=article&id=941:nrc-measurements-kef-ls50-loudspeakers&catid=77:loudspeaker-measurements&Itemid=153

The Canadian NRC says this speaker's sensitivity is 84.25dB (averaged 300Hz-3kHz, 2.83V/1m). Looking at the bass roll-off, at what frequency is the response at ~81 dB (down by 3 dB), and at ~78 dB (down by 6 dB)?

By eyeball, its down by 3 at roughly 70 Hz, and down by 6 at roughly 60 Hz.

If the marketing department wants to use lower frequency in it's specs, it says the frequency response is 60-20,000 Hz (-6 dB), and otherwise it says 70-20,000 Hz (±3 dB). The FR curve is the same.

 

[KEW]

The Canadian NRC says this speaker's sensitivity is 84.25dB (averaged 300Hz-3kHz, 2.83V/1m). Looking at the bass roll-off, at what frequency is the response at ~81 dB (down by 3 dB), and at ~78 dB (down by 6 dB)?

Thanks, the idea of the 84.25dB reference level was what I didn't get!

I was looking at it as the 0 level of a +/-3dB specification was arbitrarily assigned (or maybe I should say optimally assigned by the manufacturer) as long as the response did not fall out of the +/-3dB range.