(Not) Understanding Frequency Response

[ronnie 1.8]

The tech info on the frequency response of my PSB T65's (mains and surrounds) reads:

on axis @ 0 degrees +/- 1.5dB 42-20,000 Hz
on axis @ 0 degrees +/- 3.0dB 30-23,000 Hz
off axis @ 30 degrees +/- 1.5dB 42-10,000 Hz
Lf cutoff-10dB 25 Hz

Can someone explain these please? The more I learn, I realize the less I know!

Thanks.

 

[no. 5]

imagin a circle around the speaker drawn on the floor, marked off like a compass, 0 degrees would be facing the drivers of the speaker:

on axis @ 0 degrees +/- 1.5dB 42-20,000 Hz

at 0 degrees, any sound between 42Hz and 20kHz that the speaker makes will be within 1.5dB of eachother. in other words, between 42Hz and 20kHz the frequency is close to flat -- if you are facing head-on to the speaker's drivers.

on axis @ 0 degrees +/- 3.0dB 30-23,000 Hz

agan at 0 degrees, 30Hz and 23kHz are the points where the FR begins to roll-off.

off axis @ 30 degrees +/- 1.5dB 42-10,000 Hz

if you are standing at 30 degrees from zero on that circle, the speaker's FR would only be flat from 42Hz up to 10kHz rather than up to the 20kHz that it would have been if you were facing head-on to the drivers (0 degrees).

Lf cutoff-10dB 25 Hz

25Hz is reproduced at half the volume of any frequence sound higher than 25Hz.

 

[Sheep]

Actually, it will vary from -1.5db to +1.5 so each frequency between the 2 points is no more the 3 dB away from the next. Same goes for the other +/- figures.

On axis is basically the speaker dead on facing you. Off axis is marked by degrees. The farther off axis in degrees, the less the speaker is facing you.

25Hz is reproduced at half the volume of any frequence sound higher than 25Hz.

Not quite. 25Hz is -10dB down from the highest (dB wise) frequencies. I believe -3dB is have the volume, not 10dB.

SheepStar

 

[j_garcia]

-10dB cutoff means that is the lowest usable frequency output. -3dB is the point at which a drop of 50% output occurs, which is why that is given as the typical measurement. -6dB would be 100% drop off, so -10dB is a huge drop in output at the specified frequency.

 

[no. 5]

Actually, it will vary from -1.5db to +1.5 so each frequency between the 2 points is no more the 3 dB away from the next. Same goes for the other +/- figures.

oops, missed the +/- sign.

Not quite. 25Hz is -10dB down from the highest (dB wise) frequencies. I believe -3dB is have the volume, not 10dB.

-3dB is half the power, but -6dB to -10dB is psychoacousticly about half the perceived volume.

 

[MDS]

-3dB is half the power, but -6dB to -10dB is psychoacousticly about half the perceived volume.

Correct. The 'perceived loudness' varies with frequency and from individual to individual. As a rule of thumb, most people just say -10 dB is half the perceived loudness or +10 dB is double the perceived loudness.

One thing though, the frequency response measurement is not a measure of how the amplitude of one frequency compares to another. It is a measure of how the amplitude of that frequency as reproduced by the speaker varies from the amplitude of the input signal.

When the spec says 'x Hz - y Hz +/- 3 dB' that means that the output from the speaker is at most - 3 dB lower than the input or + 3 dB higher than the input. It does not tell you at which frequencies it is down exactly +/- 3 dB and that is one reason that speakers with near identical specs can sound different.

 

[no. 5]

When the spec says 'x Hz - y Hz +/- 3 dB' that means that the output from the speaker is at most - 3 dB lower than the input or + 3 dB higher than the input. It does not tell you at which frequencies it is down exactly +/- 3 dB and that is one reason that speakers with near identical specs can sound different.

ooooh, now that's verry interisting, thanks MDS.

 

[ronnie 1.8]

Thanks, guys. With these replies, I believe I can check this off my "I don't quite understand" list!