The cancellation dip shifts to higher frequencies and weakens as measurement moves off axis.

Evidence

The ray diagram in Figure 13.10 illustrates that the direct and reflected path length difference is maximum at 0° (on axis), trending to zero at 90° off axis. Therefore, the frequency at which the first destructive acoustical interference occurs rises from the 200 Hz dip seen in Figure 13.8(b) to ever increasing frequencies as the measurement point moves away from the forward axis. The declining off-axis radiation from the loudspeaker is a major factor in the progressive reduction in the cancellation dip. In the sound power spatial summation this results in a shallow depression in acoustic output, compared to the in-wall installation where there is no interference. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1487

Categorization

• XREF: Connects to boundary-interference concepts from earlier chapters on comb filtering and dipole-style cancellation dips.