Most sound practitioners know that low-frequency wavelengths are much longer than high-frequency wavelengths. But because we can’t see them, to what level do we really understand them?
This is an important subject because understanding the nature of wavelengths can aid in optimally setting up and operating the various types of sound systems that most of us come in contact with.
Let’s look at the physical differential between low frequencies and high frequencies. They are quite radical, in the sense that we do not often encounter such a degree of variance in other fields.
A 20 Hz wavelength is about 60 feet long, or 720 inches. A 20 kHz wavelength is 0.055 feet long, or 0.66 inches. That’s an enormous differential, a ratio of 1091:1, or three orders of magnitude.
What does the length of a wave really mean? In two words, a lot. Sound travels at the relatively low speed of approximately 760 miles per hour in air, compared to light, which travels at approximately 671 million miles per hour.
A long, low-frequency wavelength requires some time to propagate, which means that it must first develop in the atmosphere before the sonic energy can be perceived as a note or tone. A 20 Hz wavelength takes 1/20th of a second to propagate, which is equal to 50 milliseconds.
By contrast, a short, high-frequency wavelength takes very little time to propagate and become audible, and can do so in small spaces, whereas a low-frequency wavelength needs adequate space in which to develop. This is why studio control rooms and other critical listening environments, particularly those that are on the smaller side, will often use bass traps to even out the bass response. Bass traps are acoustic energy absorbers designed to dampen low-frequency energy in order to provide a flatter, more even, low-frequency room response by reducing LF resonances.
Low-frequency (above) and high-frequency waves.
When low frequencies propagate into an echoic room, which describes all rooms that have reflective surfaces, they generate standing waves. Standing waves are pressure nodes created when a sound wave reflected from a wall collides with the direct sound from the loudspeaker. At some frequencies the reflections will reinforce the direct sound, creating an increase in level, while at other frequencies the reflections cancel the direct sound, thereby lowering the level.
One or more bass traps, often located in the corners of the room for maximum effectiveness, will absorb the LF energy rather than let it reflect outward. Non-parallel walls and an angled ceiling can also help reduce standing waves. Incidentally, one reason that early trapezoidal loudspeaker enclosures were developed was to reduce internal cancellations. Within limits, the trapezoidal shape does exhibit certain advantages.