A sounder is disclosed which has walling (36, 38) defining a first air
column
(26) leading to a first sound aperture (34) through which the first
column opens to atmosphere, and walling (40, 44) defining a second air column
(60) leading to a second aperture (64) through which the second column
opens to atmosphere. A diaphragm (16) is between the air columns and means
are provided for causing said diaphragm to vibrate.
