Why do Off axis measurements matter so much

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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.
 
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.

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

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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! =)

From my personal experience it seems to, but it's not obvious what's going on unless you've done the polars first. Even with a very broad gate I can see my power response issues, but they are not easily observed if you haven't done fairly accurate gated polars. I still have never done great measurements, but useful none the less. Room responses can easily hide issues from the unexperienced and more experienced alike(if they don't know their polars). To me they are only useful b/c I know less approximately what's going on with my gated polars. So do the polars first, then remeasure your room and see if it doesn't teach you something. I think it's great that another DIY guy is taking up measuring. Just don't let pride get in the way of your assessment. That seems common and it's not useful. IOW, don't go into this "measurements don't matter camp" if your measurements aren't very good. Really that's just silly pride. You can be happy listening to your (and I'm not directing this at any one person, just at listeners in general) response anomalies, but know it's not a good representation of what's on the recording. You'll never get perfect, but one thing is certain; your polar response matters immensely.

Dan
 
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I'm with you there DAN. It can be very difficult to know what's what. So having good polars should help with understanding the room results.
One thing it's hard to get the measurements to do is sort out direct from reflected. Though if you know what you're doing, it helps. The ear and brain are better at this, but still hear the whole room.
Maybe that's what makes it all so difficult.

Thanks for the plot Ouro - very cool! My CARA room simulation software does something similar to that. Simulated, of course.
 
Thanks Everyone for your opinions,

I think ill have to work through all the different measurement methods and come to my own conclusions.

The advantage of course is that i will hopefully get enough practise and experience to accurately measure and understand the response plots and that my measurement techniques improve.

Dantheman,

Very insightful words of advice. It would probably be very easy to fall into the 'trap' of not being critical enough of my own measurements so i will have to be very objective when it comes to my results.

I don't expect my first speaker build to be ground breaking however, i ham doing a few things which are a little different from the norm.

1. 7' high electrostatic curved tweeter panels,
2. Open baffle mid line array (i know its been done many times.)
3. Torus shaped baffle
4. variable pitch and size vented baffle (baffle size grows with wavelength)
5. fully active computerised crossovers with linear phase
6. distinct convolution of each individual speaker set, 2 sets mids, 2 set tweets etc.
7. each speaker in the line array will (hopefully) have its own ampllifier to allow for power tapering of the array.

I've a lot of work and a lot of measuring to do.
 
This is the first time I've seen that plot style- it's gorgeous, really shows the information effectively.

I think that its pretty, but not as easy for me to read as a flat map done in color - like on my web site. And I don't think +- 180 degress is a good idea because the 90 - 180 data is pretty useless, but this cuts the forward resolution in half. Best just to leave it off. And that curve is normalized to the axial response, which is a very bad idea. See the frquencies with the big response bumps? Those probably don't exist and instead there is a small hole on-axis.

But the concept of actually looking at the polar response in some detail is far too long overdue, so I applaud the attempt.
 
If you are the only one, then fire your speakers at your listening position or whatever sounds best, turn it up and enjoy.

I think that people forget that at least 50% usually much more, of the sound energy that arrives at your listening point is not direct. You simply cannot ignore the non-direct sound aspects of a loudspeaker if you want things to sound right.
 
It's not only the position you seat and being on axis to the driver while you are listening. Dr.Gedlee is absolutely correct about reflected high frequency signal arriving to the listener. Attached is a FR of a full range driver. It appears to be fairly flat and having a good extension to 20 khz. In reality, off axis starts rolling off pretty sharply around 4 khz and the loudspeaker sounds very "dark" in the room.
 

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