Dynaudio AIR 15 Reference Manual page 117

Appendix: Acoustics
Dip frequencies
Cancellation occurs for a comb filter at all the frequen-
cies where the two signals are in opposite phase. This
occurs when the time delay comprises duration of 0.5,
1.5, 2.5 etc. periods. At 1 kHz, the period is 1 millisec-
ond. Half of the period is 0.5 milliseconds.
If a time delay of precisely 0.5 millisecond occurs, this
will result in cancellation not only at 1 kHz, but also at
2 kHz, 3 kHz, 4 kHz etc.
Dynaudio Professional AIR reference manual – 2014-09-28 114