Patent ID: 8847779

Claim:
An alarm device, comprising: a sealed speaker enclosure assembly comprising a tubular section, a loudspeaker mounted at one end of and facing outward from the tubular section, and a substantially flat plate that encloses an opposite end of the tubular section, such that a sealed enclosure volume is defined within the tubular section, said loudspeaker comprising a diaphragm driven by a coil; and an alarm signal generation circuit that drives the loudspeaker with an alarm signal that comprises a fundamental frequency and multiple harmonics, said multiple harmonics including a set of low-order harmonics at frequencies above the fundamental frequency and a set of higher-order harmonics at frequencies above the frequencies of the low-order harmonics, said fundamental frequency falling in the range of 400 to 700 hertz; said speaker enclosure assembly configured to cause energy in the second set of higher-order harmonics to be transferred downward in frequency to the fundamental frequency and the set of lower-order harmonics, said transfer caused at least partly by an interference effect in which waves emitted from a rear of the loudspeaker into the tubular section are reflected by the substantially flat plate, causing constructive or destructive interference with waves emitted from a front of the loudspeaker.