Sometimes we scratch our heads looking for ideas for this column. At other times, topics stare us right in the face that we fail to recognize! Ray helps run the sound system at our church, a job which is getting increasingly complicated with a growing choir, an orchestra, soloists, special music, visiting groups, and recording sessions.He and the others who service the music ministry are constantlyadjusting and fine-tuning the audio system to best fit the needs of the church. And within that complex job lies the topic for this month’s column.

While there are many important parts to the system, it would make no sense to have all of that equipment without loudspeakers to project the music to the congregation—and loudspeakers are used in many other ways that you may not have thought about. Most people take loudspeakers for granted, but there is more to them than meets the ear!

The World Before the Loudspeaker

Lots of archaeological sites once served as theaters for people to watch plays, listen to concerts, or attend other public events. The earliest sites were constructed in a shape called an “amphitheater.” An amphitheater is an open-air venue where the audience sits on an arrangement of seats that curve slightly less than a semicircle, facing a stage area where the event of interest takes place. Some ancient amphitheaters could seat several thousand patrons, each wanting to see and hear what was taking place.

The problem was that there were no sound systems or loudspeakers to project the sound to the audience. In a testament to ingenuity, amphitheater designers knew that sound would be a problem, so they devised a way to mechanically control the projection of sound toward the audience. How did they do it? Some of you may have experienced a similar situation if you have ever had the chance to use a “whispering wall.” A whispering wall, or a “whispering gallery” as it is sometimes called, is a curved stone wall that seems to amplify your whispered voice so another person can hear it. The trick is that when you whisper, the solid wall projects your voice around the curve to a second person who has his ear near the wall. The wall is not really amplifying (or increasing the intensity of sound)—it’s just controlling the direction of your voice, often while suppressing any outside noises. It helps you hear more clearly.

Early sound engineers designed curved surfaces behind the amphitheater stage that served to channel the sound toward the seating area. Instead of transmitting the sound along a wall, the curved surface was used to focus the sound toward the audience. Pretty clever! If you recall from our article in the Nov/Dec issue of Home School Enrichment, a piano’s soundboard works in a similar manner: it projects the relatively quiet sound of the string vibrations out to the audience.

The early 1600s ushered in Shakespearean plays performed in the Globe Theater. No one knows the exact details of the Globe Theater’s construction, but it was thought to hold over 1,000 people. Like the ancient amphitheaters, it was an open-air configuration with no elaborate sound systems. The performers were instructed to “bellow” out their lines in low voices so they could be heard by the audience. Low sounds project much better than high-pitched sounds. That is why you can hear the rumbling of thunder a long distance from a storm.

A new way to amplify and project sound was needed, but the technology to do so did not exist. People invented many variations of a device we know as a megaphone. Modern megaphones are electrically powered systems, but early megaphones were simply hollow, cone-shaped horns used to project a person’s voice in a specific direction, much like the megaphones that cheerleaders use at football games. Although simple, these work reasonably well. These horns took on many variations, the names of which can give us clues as to what they were used for: the speaking-trumpet, the fireman’s horn, the bullhorn, the blowhorn, and the loud hailer.

Hidden in the development of the megaphone was a subtle hint of physics that would play a role in later developments. The apparent amplification was due to what, in the engineering world, is called “impedance matching.” Impedance matching can get complicated, but the simple explanation is that it provides for a way to transfer the maximum power from a sound source (like a person’s vocal chords) to the air. If you have a television connected to an antenna or a cable, somewhere in the system is a device that impedance matches the antenna or cable to your television so it will receive the maximum possible signal.

The Invention of the Loudspeaker

The loudspeaker, as we know it today, was not invented for the theater, for sports arenas, or for concert halls. It was invented specifically for converting an electrical signal to sound when Alexander Graham Bell invented the telephone. Bell found that he could convert spoken sound into an electrical signal, send it over an electrical wire, and then convert it back into the original spoken sound.

The part he spoke into is what we call a “microphone,” and the part he listened to is what we call the “loudspeaker.” Without both parts, the invention of the telephone would not have been very useful. Actually, a microphone and a loudspeaker are very similar in design. They just operate opposite of one another!

Loudspeaker Technology

The technical name for the modern loudspeaker is “electroacoustic transducer.” That’s a mouthful, but with a little energy, it makes complete sense. “Electro” implies electricity, or in this case, an electrical signal which is a form of electrical energy. “Acoustic” means sound, or in our case, sound energy. “Transducer” means to convert one form of energy to another form of energy. Putting all of the pieces together, then, a loudspeaker is a device that converts electrical energy to sound energy!

Did you know that you can actually feel sound energy? Have you ever been in a vehicle when a car near you had a sound system blasting? You can feel the vibrations of the sound-wave energy as it strikes your car. Unfortunately, people who blast their stereos like that don’t realize that the sound energy vibrating your car is also vibrating their eardrums. Years of exposure to that level of sound energy can permanently damage a person’s hearing.

If you listen to music, you know that different instruments generate different pitches. A tuba generates low pitches, while a fife generates high pitches. Other instruments fall in between. No single loudspeaker is good at reproducing sound at all pitches. The physics behind creating the sound impose some limitations on what the speakers are capable of doing. That is why a sound system uses several different kinds of speakers to provide the best results.

A Matter of Hearing

At this point, it would be a good idea to talk a little bit about how God designed our hearing. This will be a brief overview, and readers should explore it in more detail if interested.

Hearing is the ability to perceive sounds. Sound waves are actually vibrations in the air (or some other medium like water) that travel to the ear. The outside of your ear serves to channel the sound into the auditory canal, which takes it to the tympanic membrane. The tympanic membrane, which most of us know as the “eardrum,” vibrates. Other mechanisms inside the ear convert the vibrations to electrical signals that travel to the brain and produce the sensation we call “sound.”

Our hearing has some limitations. The lowest vibrations we can hear are about 20 Hertz, which means 20 vibrations every second. These vibrations could be represented by the very low roll of thunder. The highest vibrations a person can typically hear are about 20,000 Hertz. This would be a high, squealing noise, kind of like chalk squeaking on a chalkboard, only even higher.

Most of what we hear is in between these two limits. As a result, loudspeakers are designed to project frequencies across this range. Low frequencies require very large loudspeakers, and high frequencies require very small loudspeakers. Here are some of the more common loudspeaker designs:

     •Full-Range Driversattempt to provide the broadest range of sound with a single speaker. These speakers range from about 3” in diameter to about 8” in diameter. Some have modifications to help reach into the higher frequencies, but they remain limited and do not do very well when it comes to bass notes.

     •Subwoofersare specialized speakers meant to reproduce the lowest sounds. The construction of subwoofers is simple, but very important. Because they project only the lowest sounds, they are built in rigid cases. The lowest notes will vibrate pieces loose if the subwoofers are not well designed and assembled.

     •Woofers,as one might expect, are used for low pitches, but they handle pitches higher than do subwoofers. The frequency response of woofers makes them able to handle some very low notes, as well as some mid-range notes.

     •Mid-Range Driversreproduce middle frequencies.

     •Tweetersare high-frequency speakers used to reproduce the highest notes in a sound system.

Where Are Loudspeakers Used?

Loudspeakers are used in many other places besides sound systems.

Telephones, cell phones, and similar devices all have loudspeakers inside. Computer systems often have loudspeakers connected either internally or externally. Kitchen appliances, like microwaves and ovens, have loudspeakers that tell you when cooking is completed or a temperature has been reached. Hearing aids have tiny loudspeakers inside. Airline, train, subway, and bus stations have loudspeakers throughout the facility so people can hear announcements.

Try this some time: for one day, pay attention to the sounds you hear around your home, and make a list of everything that has a loudspeaker in it. You will be surprised by how many you find, and maybe even more surprised by where you might find them. One hidden location for a loudspeaker, which you hopefully never hear, is inside of a smoke detector!

Want to Learn More?

If you are interested in learning more about sound, check out Music, Physics, and Engineering by Harry F. Olson. The book is nearly 50 years old, and while technology has changed, the underlying physics have not. Our oldest son modified a box for a bass speaker using what he learned from this book. The modification was simple and made a big difference in the quality of the sound from his music system. 

Ray and Gale Lawsonhave been homeschooling their three children since 1995. Ray holds a BS in Electrical Engineering from the Virginia Military Institute and is also a student, pursuing a Masters in Nuclear Engineering at the University of South Carolina. He works for Savannah River Nuclear Solutions, LLC. Gale holds a BS in Mechanical Engineering from the University of South Carolina and is full-time mom and teacher. They are members of Breezy Hill Baptist Church in Graniteville, SC. Questions, comments, and suggestions are always welcomed and can be e-mailed to them at vmi1981@bellsouth.net (Ray) or galenkids@bellsouth.net (Gale).

This article was originally published in the Jan/Feb 2011 issue of HomeSchoolEnrichment Magazine. To learn more, and to request a FREE sample copy, visit www.HomeSchoolEnrichment.com