Imagine you are the speaker and facing front. 30 degrees off axis means that somebody listen to you 30 degree away from your front. On axis means somebody listen to you right in front of you. What does it do for speaker? Actually what does it do for listener. You hear more treble response if you listen to speaker on axis. Hope this helps.
What exactly does 30° off axis mean for a speaker?
Imagine light coming from a bare light bulb; it can be seen equally well from all angles. If the light is a spotlight with a reflector, it beams light only in a narrow pattern. It’s bright if you view it from directly in front, or on axis, but if you view it from the side, off axis, the light is much dimmer.
Sound radiates from its source in all directions, like light from a bare light bulb. For most speakers (those with cone drivers mounted on the side of a box), the sound can radiate in a hemispherical pattern (180°). That’s true only in a perfect world with ideal drivers. In the real world, cone drivers can act as a bare light bulb or a spotlight depending on the sound wavelength they are producing. In general, when the wavelength produced by a driver is larger than the diameter of its cone, the sound is radiated in a wide pattern. As the wavelength produced by a driver approaches the diameter of its cone, the driver starts beaming. At shorter wavelengths than the cone diameter, the driver radiates sound in a narrow pattern in front of the speaker, like light from a spotlight.
For speakers to create a well-dispersed sound image, they must avoid beaming. That is where the skill of the designer becomes important.
When speaker response is measured, it is usually measured on axis, 30° off axis, and 60° off axis to describe this problem of beaming at higher frequencies.
The off axis response is an indicator of how room friendly a speaker is. The sound you hear consists of sound coming directly from the speakers and sound reflected from room surfaces, providing both near reflections and far reflections. A speaker with poor off axis response can sound good on axis if placed well away from room surfaces so that there are no near reflections. A speaker with wide off axis response will sound better in most rooms, since the near reflections will be balanced almost the same as the direct response.
John Atkinson of Stereophile interviewed Paul Barton of PSB a few years ago, and Paul gives some information about the dispersion patterns of speakers.
Most speaker measurements are done on-axis. But, since stereo and 5.1 soundtracks require more than one speaker, the L and R main speakers are placed a few feet apart. Unless you toe-in the speakers to directly face you, the sound will be off-axis. A 30-degree off-axis position is optimal for stereo and 5.1 imaging. Some speaker manufacturers and audio reviewers publish 30-degree off-axis frequency response measurements because that's just how people typically position their speakers at home. And speakers can differ significantly in terms of how they sound off-axis. Some have a wide dispersion and sound similar off axis as on axis, others sound very different. In the days of mono and one speaker facing directly at you, an on-axis response measurement was all you needed.