The present invention relates to an acoustical amplifier for increasing the sound pressure level of an audio signal, and more particularly, to a method and apparatus for amplifying the output signal of a portable receiver without requiring additional electrical power.
Portable receivers, such as pagers and miniature hand-held radios, utilize miniature magnetic transducers as sound output devices. Conventionally, these transducers are located in acoustical cavities within the housing of the receiver, and the audio signals produced by the transducer are conducted to the exterior of the housing by means of a channel leading directly from the output port of the transducer to an output aperture in a wall of the housing.
Typically, the output signal from a portable receiver has a sound pressure level (SPL) in the range of 80-84 decibels. This output level may not be sufficient for easy recognition in a relatively noisy environment. For example, an alerting signal at this output level from a pager beeper may not be adequate to gain the attention of a worker at a construction site.
Portable receivers of the type described previously use a small battery for a power supply. Since such a battery has a limited amount of available power, an increase in the output level of the audio signals produced by the sound transducer through electrical amplification is not a desirable solution to the problem of low level transducer signals. The extra electrical power required to increase the output signal to a satisfactory level may provide an increased drain on the battery power and therefore necessitate more frequent changing of the batteries, which is both bothersome and more expensive.
One proposed solution for amplifying the audio output signal of a portable radio receiver is illustrated in U.S. Pat. No. 3,748,583 issued to Anderson et al. This patent discloses an acoustical horn which is directly coupled to the sound transducer to amplify the output signal thereof. One problem associated with this proposed solution is the increase of the size of the receiver. The acoustical horn requires an extra compartment to be added to the receiver, thereby increasing the size of the receiver considerably. It is desirable to have the portable receiver occupy as little space as necessary, and therefore the solution proposed by this patent is not entirely satisfactory. Furthermore, the acoustical horn requires that additional components be added to a conventional receiver, and therefore inreases manufacturing costs.