thread: SBIR
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Old 15th January 2012
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Originally Posted by Rod Gervais View Post
For what it's worth - I have always assumed that the effect was caused by
the summing of the signals at the speaker, and although I have been curious about the possibility of room pressure affecting cone travel - I really do not have a strong feeling that this is the case with SBIR......
Nearby boundaries can affect loudspeaker (electrical) impedance, because boundaries are acoustical impedances which load loudspeaker driver, in similar way as loudspeaker box. Basically, an ordinary loudspeaker driver has bidirectional behavior (like electrical transformer), so we can get information about acoustical impedance (of box, room boundaries, etc) through measured electrical impedance.
Also, in any normal use of loudspeakers, non-resonant boundary interferences (SBIR), can't significantly affect loudspeaker driver cone movement (and stop it), because sound pressure level decrease with increasing distance from the source, so we can easily get 50dB of SPL drop between SPL of direct and reflected wave at cone position. So resulted measured SPL will be clean "nearfield loudspeaker SPL measurement" which is widely known and used because its immunity from boundary interferences.
So we can measure SPL at 5mm from cone of bass driver, and we may be pretty sure that nothing from room boundary effects can't affect our measurements significantly.

At the end, it's possibly easier for someone to try this, but try to keep away your measurement microphones from high SPL, because at 5mm from dust cap, loudspeaker driver can be pretty LOUD.

Originally Posted by Ethan Winer View Post
.........This is also why the SBIR peak and null frequencies are related to the spacing between the rear of the speaker cabinet and the boundary, rather than from the cone in the front of the box. The rear of the box is actually the sound source location.
Rear of the box can't be sound source location because ludspeaker cabinet (box) is much heavier than driver cone (5-25kg vs. 15-150g for ordinary two way loudspaker), so radiation (air movement) which originate from the box may be easily neglected, even if box surface is greater than loudspeaker cone surface.

To be clear, radiation from loudspeaker box walls exists, but in this particular case it is absolutely negligible (if we talk about room boundary interferences).