Megaphones: Heard above the Noise

The megaphone is one of the oldest acoustical devices. People learned early that horn-shaped or cone-shaped tubes could be used to increase the audibility of the human voice. However, it took much longer for humanity to discover exactly how megaphones amplify sound.

A Quick History

Many early civilizations used “speaking horns.” Some Native American leaders used birch bark horns to communicate with large groups. In ancient Greece, actors wore masks with cone-like openings around the mouth which amplified their voices. Many inventors and scientists developed types of megaphones, the most famous being Thomas Edison. Edison created a massive megaphone that is reported to have made whispers audible at a thousand feet and normal speaking audible at over a mile away. Unfortunately, Edison’s megaphone, while functional, was too large to be of any practical use.

Today, megaphones are used in crowd communication. Cheerleaders, lifeguards, and movie directors use them to direct masses of people. In this article, however, we will be using megaphones not to direct masses of people, but to discuss two important acoustics principles: directivity and acoustic impedance.

Directivity

In acoustics, directivity is a measure of how much sound energy goes in what direction. In an ideal, free-field situation, a simple point source creates sound that radiates equally in all directions at all frequencies. In the real world, most sources do not radiate equally in all directions. For example, sound from the human voice radiates much more strongly from the front of the head (where the mouth is) than from the back of the head.

While this may sound like common sense (and in many ways, it is), megaphones take advantage of this principle. The cone shape of a megaphone acts like a funnel to channel sound energy in the direction it is pointed. Rather than spreading sound energy across a large swath of space, megaphones concentrate sound energy into a narrow region, boosting the sound’s level to the hearer’s benefit.

While directivity helps explain how megaphones work, it is not the full explanation (although many internet articles refuse to venture farther.) The other half of the explanation relies upon the principle of impedance.

Acoustic Impedance

All matter has an associated acoustic impedance—a measure of how much it resists transferring acoustic power. Factors like mass, stiffness, size, and shape affect acoustic impedance. Acoustic power is lost when sound tries to move between two media with different acoustic impedances. This is effectively what happens when sound leaves the small cavity of your mouth and enters the expansive space of open air. As a result of this impedance mismatch between your mouth and the open air (due to the differences in size and shape), some of the sound energy is reflected back into your mouth.

In other words, despite all your effort to yell louder, some of the sound energy produced by your vocal cords doesn’t ever leave your mouth.

Megaphones make the transition from the geometry of your mouth to the geometry of the air more gradual, which allows more sound power to transfer to the air. You can think of a megaphone as a sort of bridge for sound that helps match the impedances of the two spaces. This impedance matching works best at frequencies with wavelengths similar to the length of the megaphone itself. Thus, many megaphones are a couple feet long.

Megaphones tend to amplify the higher frequencies of the human voice more than the lower frequencies due to resonance. This helps with speech intelligibility for consonants—the speech sounds that include the most high-frequency content.

The principle of acoustic impedance matching shows up in many places, including why humans have bones in the middle ear and why gel is applied to skin during ultrasonic medical imaging. So the next time you visit the beach, attend a sporting event, or walk on set for your next movie role, impress your friends with your extensive knowledge of the acoustics of megaphones. Doing so, you might just be heard above all the noise.