Why do Off axis measurements matter so much

[oublie]

Having read as much as i can there are a few things i just can't find the answer to.

I'm about to start doing some measurement (post xmas) of my prototype open baffle line arrray and was wondering the following:-

What is the importance of the off axis measurements considering the fact that i'm the only listener 90% of the time?

What's the best way to set up your measurement gear for 15 degrees, 30, 45 etc off axis?

To my mind the best place for me to measure is head height at the listening position and also 1 metre from the speakers - is this also true of line arrays?

should i do all measurements with 2.83v on the output of the amp?

thanks.

[TheSeekerr]

Originally Posted by oublie

What is the importance of the off axis measurements considering the fact that i'm the only listener 90% of the time?

If you always listen to music seated in the same spot, no importance at all.

Originally Posted by oublie

What's the best way to set up your measurement gear for 15 degrees, 30, 45 etc off axis?

Leave it where it is and simply turn the speaker?

[oublie]

Originally Posted by TheSeekerr

If you always listen to music seated in the same spot, no importance at all.



Leave it where it is and simply turn the speaker?

Thanks TheSeekerr,

No off access measurement for me then

[John Sheerin]

Originally Posted by TheSeekerr

If you always listen to music seated in the same spot, no importance at all.

Not true. You hear the off axis sound reflect from the walls of your room. If the reflected sound has a different frequency response than the direct on axis sound, you will hear that. Read Toole's book for more details.

[oublie]

Originally Posted by John Sheerin

Not true. You hear the off axis sound reflect from the walls of your room. If the reflected sound has a different frequency response than the direct on axis sound, you will hear that. Read Toole's book for more details.

So how would i go about correlating the off axis with on axis response or would i already be measuring the off axis response due to reflections when measuring on axis?

[godfrey]

measure out-doors in a wide open space?

[John Sheerin]

You can correlate the off axis response (sound power) to on axis response if you know the directivity of the speaker. However, you probably don't, or at least you don't know what it is in your finished cabinet.

Typically you would measure in an anechoic chamber or outdoors or use gated measurements to measure the off axis response at various angles. Then you could use that information while designing your crossover.

If you're correctly using gated measurements indoors, then you're not measuring the reflections, but you're also probably not getting very high resolution at very low frequencies.

[Ouroboros]

Measuring outdoors on a windless day is perfectly practical. Many audio measurement programs such as Clio or Arta allow you to take a MLSSA or noise impulse measurement at different off-axis angles and merge the results to produce polar plots showing the polar response at different frequencies. If you really want to show off, you can even produce 3-D waterfall plots showing the delayed resonances at different off-axis angles. (Not that I've tried to do this last bit though!)

[Pano]

Off axis is important for the reasons that John mentions, but what if the measurements are done at the listening position - and not too tightly gated? Wouldn't that take into account most of the power response?


Ouroboros - I'd like to see one of those 3-D, off-axis waterfall plots. That would be fancy! =)

[Ouroboros]

This is one showing amplitude/frequency/angle, but you can also get 3D plots of amplitiude/time/angle I believe.