An overview of Room Modes - Bass Frequency concerns in small rooms

So far in this blog we've discussed horn vs. cone speakers, box vs. architectural speakers, and an overview of speakers in general.

Today I'd like to provide a little background on the physics of sound, which will lead to a future entry on subwoofers/satellites vs. full-range speakers.

In the days when I first started playing with and learning about audio, the only way to get full-range sound (both good low frequencies and good high-frequencies) was to use a good full-range speaker. At its core, a full-range speaker contains one or more cones and/or horns in a single box, and the aggregate device reproduces all sounds from the lowest to the highest.

Cheap full-range speakers had only a single cone (or horn) that had to deliver all sound frequencies at the same time. Better speakers had two cones, a driver or woofer for low frequencies and a tweeter for high-frequencies. There was a crossover circuit that fed low-frequency sound to the driver, and high-frequency sound to the tweeter. The best full-range speakers added a mid-range cone and more crossover circuitry. Some more exotic speakers had multiple tweeters, mid-ranges, and even woofers to deliver more power or punch in those frequency ranges.

Note that all of these sounds are delivered from a single box. This means that all of the sounds come from essentially the same place in the room.

However, research by Floyd Toole and others over the last 40 years or so has essentially demonstrated that, depending on the frequency, sound can have very different characteristics. High-frequency sounds are very directional, while low-frequency sounds are omni-directional - they radiate in all directions.

Low-frequency sounds, also known as bass, have the further characteristic of long wavelengths. The wavelength of a bass frequency ranges from about 56.5 feet (at 20 Hz) to 11.3 feet (at 100 Hz). Relative to a typical listening room in a home (which we consider to be a "small" room when compared to concert halls), these are very long waves, and have very different characteristics than higher frequency sounds that have wavelengths measured in inches or fractions of inches.

When a high-frequency sound is produced in a small room, the directional sound travels in one direction to a boundary (wall, floor, ceiling), and bounces to the next boundary, etc. until its energy is dissipated. This means when the speaker is pointed at the listener, the sound travels directly to the listener's ears, and also bounces off the walls. These secondary "bounces" hit the listener's ears a few milliseconds later than the original direct sound. Third- and fourth-order reflections hit the ear even later. At these frequencies, the human ear is very good at combining and differentiating those reflections. In fact, the earlier reflections actually reinforce the original sounds, making the listener's perception clearer and stronger.

Later reflections, on the other hand, are perceived as echoes. They are so far out in time from the original sound that the ear cannot recombine them into the original sound, but rather perceives them as a separate sound.

Low-frequency sounds act very different. The wavelengths are long relative to the room, and they emanate from the speaker in all directions. Every room will have one specific frequency for which the room length exactly matches the length of the sound wave. It will also have a frequency for which the room width exactly matches the sound wave length, and a third for which the room height matches the wavelength.

Let's consider just one of these for the moment - the frequency at which the wavelength exactly matches the length of the room.

When the sound emanates from the speaker at one end of the room, it travels the length of the room to the opposite wall, and bounces back and forth between the front and back wall. These reflections will reinforce themselves - the peak of the wave will be higher, and the valley of the wave will be lower. This is called a standing wave.

You can hear this effect very easily by putting a subwoofer in a room and sending a pure test tone through it. By varying the tone, it's easy to find a frequency that is too loud in one spot, but too soft in another. Quite literally, you can find a "hotspot" for that frequency, where it's too loud, and then take a step or two in any direction, only to have that sound completely disappear! This frequency is called a "room mode", and it also happens for wavelengths that are 1/2, 1/3, etc. of the room length.

Of course, this is all basic physics, so with a little math we can predict which frequencies will cause a room mode. Since every room has dimensions, it's impossible to completely eliminate room modes. However, by understanding what causes a room mode, we can compensate for them by proper subwoofer placement, multiple subwoofers, parametric equalizers, or other mechanisms of good audio design.

If you want the best possible sound out of your audio equipment, whether it's your home theater, stereo system, or even a mixing room, classroom, or auditorium, call your local experts! Inspired Electronics, Inc. is one of just a handful of companies in Illinois certified for both audio and video calibration, and we can make your room sound better. Call us today at 847-471-4420 to set up an initial consultation.