Summer is the time for outdoor concerts, but even when 20,000 people gather in a muddy field, they take it for granted that they’ll be able to clearly hear the music…and complain if they don’t. We’re so used to electronically amplified sound the only time we give it any thought is when it doesn’t work properly–but electronic amplification didn’t exist until just a few short decades ago. Before that, the highest-tech form of amplification was the megaphone, a cone open at both ends.
Sound is vibrating air impacting on our eardrums. When you speak, you vibrate a small amount of air in your throats and release it through your mouth. When you speak into a megaphone, you vibrate a small amount of air at the narrow end that in turn vibrates a much larger amount of air at the wide end. More vibrating air means more sound.
Many musical instruments amplify sound in the same way: the bell of a brass instrument is just a type of megaphone. Others, like guitars, use a soundboard; the small vibrations of the strings set up matching vibrations in the soundboard, which is able to vibrate a much larger amount of air than the strings themselves.
Today, however, we want our music far louder than such passive mechanisms permit. If a concert didn’t shake our fillings loose, it was a failure. For sound of such volume, you need electronic amplification.
There are three steps to electronic amplification. First, sound must be converted to a varying electrical current; second, that varying current must be amplified–that is, made more powerful –and third, that amplified current must be converted back into sound.
The first step involves a microphone, or a variation such as a guitar pick-up. There are various types of microphone; the most common is the dynamic microphone, in which a coil of light wire is hung between the poles of a magnet. Sound causes this wire to vibrate–and wire moving in an magnetic field generates an electrical current.
This weak current must then be amplified. Electronic amplification was born when American engineer Lee De Forest invented the triode in the early 1900s. The triode was a variation on the diode. A diode, a glass tube out of which the air has been evacuated, contains two electrodes. One electrode, the “cathode,” is heated, which causes it to release electrons, which flow to the other electrode, the “anode.” In a triode, a metal grid is placed between the cathode and anode. A tiny variation in the voltage of the grid has a much larger effect on the flow of current from cathode to anode. That means a small varying current fed into the grid of the triode can produce matching variations in a much more powerful current. Presto: amplification.
In 1948, Walter H. Brattain, John Bardeen and William Shockley at Bell Laboratories invented a much simpler device called a transistor that could accomplish the same thing. Smaller, cheaper and more efficient, transistors have essentially replaced vacuum tubes in electronic equipment of all kinds.
For the aforementioned filling-loosening sound, you need very powerful amplifiers, and you need more than one. The first amplifier, or “pre-amplifier,” boosts the tiny current produced in the microphone or guitar pickup several thousand times. The boosted current is then fed into another amplifier, and so on, until the current is as strong as it needs to be.
Of course, simply producing a powerful electrical current doesn’t do you much good unless you’re aiming to electrocute your audience. The final step of amplification is turning that current back into sound, which happens in the speaker.
The most common type is the dynamic speaker. Like the dynamic microphone, it contains a coil inside or surrounding the magnetic field produced by a magnet. The amplified current, fed into the coil, produces a varying magnetic field, alternately attracted and repelled by the field of the magnet. This causes the coil to vibrate, and those vibrations in turn vibrate a large diaphragm. The result: sound that matches the original vibrations that entered the microphone, but is much, much louder.
Transistors have also given birth to small, powerful computers, which can modify the electrical signal during the amplification process to achieve special effects, or clean up hisses, pops and crackles–which is why we’ve become so accustomed to good sound at concerts.
But please, please, please, whoever is in charge of these things, remember that “good” does not necessarily equate with “loud.”
Personally, I prefer to emerge from concerts with my fillings intact.
