Go bigger to rock louder? Maybe, says science

Scientific St. Albert
The Gazette is using science to answer questions from St. Albert-area residents about the world around them. Send in your questions to [email protected].

Guitars are loud, as any heavy metal enthusiast will tell you. Sometimes, you want them to be louder so your tunes reach the back of the crowd.

But how do you do that if you don’t have an amplifier? “If a guitar was larger, would it be louder?” wondered Elyse, a Grade 5 student at Sister Alphonse Academy.

Physicists say going bigger can make you louder, but only under some circumstances.

Size (sometimes) matters

Sound is caused by the displacement of molecules, said University of Alberta physicist Roger Moore. When you apply a force to air molecules by plucking a guitar string, you cause them to vibrate back and forth, causing zones of high and low pressure. Those molecules bump into their neighbours before springing back to their original location, transmitting the energy of your pluck as a sound wave.

Loudness is a result of a pressure change in a medium and is measured in decibels (dB), said Thomas Moore, retired professor of physics and musical acoustics at Rollins College, Fla. If we want a louder guitar, we need to make our guitar move more air and cause a bigger pressure change.

Plucking a guitar string causes it to displace air, creating sound, said Andrew Morrison, a professor of the physics who studies the physics of musical instruments at Joliet Junior College in Illinois — not much sound, as the string is small and doesn’t move much air. If the string is attached to a big wooden box with a hole in it and vibrates at the right frequency, the box will resonate/vibrate along with it, pushing more air and making the sound louder.

Making the guitar bigger on its own will actually make it quieter, Roger and Thomas said. The force you apply to the string is now being applied to a bigger object, so it doesn’t move as much. You have to increase the amount of force you put into the string in proportion to the size increase of the guitar to get more loudness.

Your best bet to make your guitar louder is to pluck the string harder, Roger said.

“The energy of vibration comes from you plucking the string,” he said, so the more power in your pluck, the more loudness in your rock.

Roger said you will want to make sure that your body isn’t touching the guitar, as doing so would absorb vibrational energy and muffle its sound. (Some sort of anti-gravity model would be ideal.)

You also have to consider the structure of the guitar, Morrison said. If you make it bigger in every aspect, the box may become too thick to vibrate. Longer strings will produce lower frequency sounds unless you tighten them, and too much tension could snap the strings. Put too much force into your pluck, and the string or guitar could snap.

“You really want to try and find that sweet spot,” Morrison said.

While Thomas, Roger, and Morrison weren’t sure what material would make for the loudest possible guitar, Roger said one made of antimatter would be exceptionally loud. A one-kilogram antimatter guitar would instantly annihilate one kg of air, and guitarist around it, to produce a 43-megaton explosion.

“While definitely louder than a typical guitar, I suspect any melody would be lost, although it would definitely be a performance that the audience would remember for the rest of their lives!” Roger said.

How loud can we get?

Let’s assume we have a magic guitar that works at infinite size. How loud can it get?

The maximum loudness our guitar can reach under normal atmospheric conditions is 194 dB. Sound is a wave that consists of zones of high and low pressure, Roger explained. At 194 dB, your low-pressure zones are so low they are a complete vacuum and can’t get any lower. If you try to go louder, the energy of your wave stops moving through the air and starts pushing the air along with it, distorting the sound in the process. Your sound wave has become a shock wave, the force of which will probably kill/explode anyone who tries to listen to it.

Venus has an atmosphere 93 times denser than Earth’s, so our magic guitar could get many times louder there, Roger said. (The crushing pressure, clouds of acid, and 460 C temperatures might be unpleasant, though.) Research suggests we could get up to 270 dB if we played underwater. Presumably, we could get even louder if we played inside solid steel, but it might be hard to hear due to all the metal in our ears.

If you want to rock louder, a bigger guitar can help you crank up the volume. If you want to rock better, though, you’ll just have to practise.