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Speaker Off Axis: Dispersion Specifications and Off Axis Response Plots
by ct_acoustic_frontierJuly 06, 2022
This will be a quick article that explains how to interpret manufacturer published speaker dispersion specifications and in particular how these relate to what you might see on an off axis response plot.
The Importance of Speaker Dispersion
To properly position speakers in a home theater, you must understand speaker dispersion. An important goal in any home theater is to provide even SPL coverage of all seats. To do this you need to understand the effect of listening angle on SPL.
You must also understand speaker off axis response to properly design the acoustic treatment for a room. A wide dispersion speaker that puts a lot of energy into the early and late reflected field will need a very different acoustic design than a narrow dispersion speaker.
Speaker Off Axis Response Plots
The graphic above shows the frequency response for a wide dispersion cone/dome speaker. We've taken measurements at angles from 0 degrees (on axis) to 90 degrees (mic sideways on to the baffle). You can see that the SPL at different frequencies (the frequency response) changes shape as you measure at different off axis angles.
It's pretty easy to read these charts for the purpose of examining speaker dispersion. You simply calculate the difference between the plots at two off axis angles. Many acoustic measurement tools have arithmetic functions that allow you to normalize data. You can then see how similar the on axis sound is to the off axis sound, and how much it is down at certain frequencies.
For example the plot below shows the magnitude of the difference between the 45 degree off axis plot and the on axis plot. The on axis plot has been normalized to 0dB.
In this particular case you can see that the speaker has wide dispersion, and that the response on axis and 45 degrees off axis keep within a 3dB window through 6kHz or so.
Understanding Dispersion Specifications
Many pro-audio manufacturers publish dispersion specifications. These are commonly quoted as xx degrees. For example: "90 degrees".
Correctly, these dispersion figures should be qualified by both the frequency and dB difference from the on axis SPL. For example: "90 degrees -3dB @ 1kHz". That would mean that at 1kHz the speaker covers an angle of 90 degrees with a nominal difference from the on axis SPL of 3dB. For what it's worth, the accepted pro-audio "standard" is to quote dispersion figures at -6dB and 1kHz.
One important thing to understand is what the angular coverage refers to. This is a source of potential confusion for those reading and interpreting specifications.
A dispersion figure of 90 degrees means 45 degrees either side of "on axis". Likewise a dispersion figure of 80 degrees would mean 40 degrees either side of "on axis". So if you saw a published specification of 90 degrees with no frequency or dB qualifier, you'd assume this means that if you measured the speaker at 45 degrees off axis you'd see that at 1kHz the response is -6dB down.
At Acoustic Frontiers we don't think this is a very useful standard for home theater design or acoustic design for two reasons. We think -3dB at 4kHz is a better specification for our purposes:
Firstly, -6dB is a very significant perceptual change in SPL. We prefer -3dB, which is around a 20% perceived loudness difference.
Secondly, 1kHz is not a high enough frequency to cover the main energy output of popular musical instruments or the key frequencies that contribute to localization. 4kHz is a better frequency for this purpose.
The other important way you'll see dispersion specifications like "90x45" or "80x60". There are two dispersion figures here, the first is accepted to be the horizontal / lateral dispersion and the second the vertical. So we'd interpret "90x60" to mean that the speaker is -6dB down at 45 degrees laterally off axis at 1kHz and also -6dB down at 30 degrees vertically off axis at 1kHz.
Much confusion still exists about what a room correction product does, what problems it can (and cannot) solve and therefore its 'place' in a modern high quality sound reproduction system. Part of the challenge of understanding room correction is that it requires a reasonable level of understanding of sound quality, acoustic science, acoustic measurement and psychoacoustics (how humans perceive sound). The majority of the articles I have read online or in print magazines do not cover the fundamentals in enough depth to allow the curious and committed reader a chance to understand room correction on anything more than a cursory level. By the end of this article I hope that you will have learnt enough to judge for yourself what room correction can and cannot do and how best to apply it in the context of a world class music or home theater system.