Picture this: you’re standing in your kitchen fixing a cup of coffee, and you notice a low, throbbing sound of unknown origin. You chalk it up to your caffeine-starved imagination, and proceed to your window overlooking the city. As you make your way through the apartment, the sound disappears…that is, until you reach the window, where the throb returns. As you traverse back and forth, you realize that the sound disappears halfway between the coffee machine and window. Before starting on any conspiracy theory, consider this: you may be affected by a room mode! Read on to understand how these acoustic phenomenon form and how to combat them.
Room modes occur when a pitch’s wavelength is twice the length as the distance between two surfaces in a room. The phenomena can occur in any direction – between walls, a floor and ceiling, or even corners of a room. A wavelength of sound is simple to calculate; it is determined by the speed of sound divided by the frequency (Hz). For example, 56 Hz has a wavelength of 20’; in a room with dimensions of 10’ X 10’, a 56 Hz tone will resonate perfectly between those walls and will increase in sound level depending on where you stand in that room. As the size of the room increases, the fundamental frequency of room modes decreases, eventually getting below the range of human hearing. For the most part, room modes are usually only problematic at very low frequencies.
Modes occur in all rooms but are most pronounced in those with parallel surfaces, which also happens to be how most general purpose rooms are built. So, the question on all our minds is obvious: who cares?! The honest answer is not us…most of the time. Typical space types in run-of-the-mill projects, such as corporate offices or residential buildings, don’t suffer much from modal deficiencies: modes, if even noticed, tend to be broken up by furnishings in the space. For larger, more critical spaces, such as performance halls or churches, the lowest modes fall outside of human hearing range and higher modes are so saturated that none stand out. For the most part, the recording studio industry cares about room modes since they are usually smaller rooms where the frequency is still in the audible range. Professional monitors in a control room can excite modes to produce exceptionally loud or quiet pockets. This can be detrimental to the mixing environment with some listeners having too much low frequency content while someone else just a foot away may wonder where all the bass went.
There are a few easy methods that may be used to prevent room modes. Angling opposing surfaces to be just a few degrees from parallel can prevent a room mode from nestling perfectly between them. If you find room modes occurring in an existing space, incorporating acoustic panels on one of the parallel walls will also help mitigate the problem. For more critical spaces like studios, tuned absorbers – often called bass traps – can help manage prominent modes.
When considering what a room will be used for, it’s important to keep in mind the overlooked and often forgotten room modes, so the occupants are the only ones getting excited.