Megaphone

Overview

A megaphone or loudhailer is a portable voice amplifier built for maximum acoustic output per watt of battery power. Everything in its design serves efficiency: a Compression Driver working into a folded Reentrant Horn converts 10–45 electrical watts into 100–130 dB SPL of directed speech, intelligible at hundreds of meters in open air. The speech band is all it attempts, response from roughly 400 Hz to 5 kHz covers the frequencies that carry consonant intelligibility, and nothing below or above is worth the battery cost.

How it works

The operator squeezes the Trigger Switch in the Trigger Grip, which powers the Amplifier Board only while held. Speech enters the Microphone Capsule behind the rear opening, gets amplified by the Amplifier IC, a Class-AB or Class-D chip running directly off the battery rail, and drives the compression driver. An AGC Limiter compresses sudden shouts so the amplifier stays out of hard clipping, which on a horn sounds harsh and actually reduces intelligibility. The Volume Potentiometer sets level, and a Siren Circuit sweeps a 500 Hz – 2 kHz alert tone when its button is pressed; many models add a whistle tone and a record-and-loop message function via the Mode Buttons.

The compression driver

An ordinary cone speaker couples poorly to air: the diaphragm is heavy and the air is compliant, so most amplifier power becomes heat. The compression driver fixes the mismatch. Its Driver Diaphragm, a phenolic or mylar dome driven by a Copper Winding voice coil in the field of a Ferrite Magnet, radiates into a chamber whose exit area is roughly a tenth of the diaphragm area. The Phase Plug channels this compression chamber to the throat through slots of equal path length, so pressure waves from different parts of the dome arrive in phase. Loading the diaphragm against this stiff, small air volume raises the radiation resistance and lifts electroacoustic efficiency to 10–25 %, against roughly 1 % for a direct-radiating cone.

The horn

The horn is an acoustic transformer. Starting from the small throat behind the Throat Adapter, its cross-section expands exponentially to the mouth of the Horn Bell, stepping the acoustic impedance down gradually from the dense throat to open air. A straight horn long enough to cut off at 400 Hz would be unwieldy, so megaphones fold it: the Reentrant Core routes the sound forward, back, and forward again through concentric ducts, packing the required path length into a body a fraction as deep. The mouth diameter, 150–300 mm, sets the low-frequency limit and the beamwidth, the horn concentrates output into a forward lobe of roughly 30–60 degrees, which is where the range advantage over an omnidirectional speaker comes from.

Microphone and feedback

The microphone sits at the acoustic rear null of the horn, but a megaphone is still an amplifier with its input near its output, so feedback management matters. The Microphone Capsule is noise-cancelling: ports on both faces of the element cancel far-field sound (including horn spill) while close-talking speech, which reaches the front face much louder than the rear, passes through. A Microphone Windscreen suppresses breath pops, and the rubber Microphone Mount keeps body vibration out of the capsule. Handheld-mic variants put the capsule on a coiled cord so police and event users can aim the horn independently of their head.

Power

The Battery Pack is sized for hours of intermittent use, the trigger duty cycle, not continuous draw, dominates. Rechargeable models carry four Li-ion Cell, 18650 cells with a BMS Board for protection and a Charging Jack for the wall adapter; the Battery Holder in cheaper units takes four to eight C or D alkaline cells instead, accessed through the Battery Door. At a typical 10 W average draw a fresh pack sustains four to eight hours of normal use.

Construction

The Body Shell is injection-molded ABS throughout, the Body Molding clamps the horn bell, encloses the electronics, and carries Strap Lugs for the Shoulder Strap. Weather resistance matters more than appearance in this product class: gasketed seams and a sealed driver let the unit work in rain at outdoor events, marine use, and emergency response, where the megaphone remains standard equipment precisely because it has no infrastructure dependencies.