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Room Acoustic Basics

Before considering bass traps in detail, a review of acoustics is in order. This will develop a sense of perspective and scale. The behavior of sound waves and objects depends on the size of the wavelength, in comparison to the size of the object. Simply put, long wavelengths go around small things and small wavelengths get reflected by big things.

The wavelength of a sound is mathematically related to its frequency or tone. The higher the frequency, the shorter the wavelength. Our range of hearing officially spans ten octaves from 20 Hz to 20 kHz and we can perceive or feel sound even below 20 Hz. (1 kHz = 1,000 Hz of cycles per second.) An octave is the doubling of frequency: 20 Hz, 40 Hz, 80 Hz, and so on. For audio playback in small rooms, bass is considered to be the first four octaves (20 Hz to 320 Hz); mids comprise the next two (640 Hz to 5.12 kHz); and the highs occupy the last four octaves. Sounds of the piano keyboard are familiar to most of us; middle-C is a frequency of 256 Hz. The bass range on a piano occupies more than half of the piano keyboard, and about forty percent of the full auditory spectrum.

Bass wavelengths are similar in size to the room in which they exist. It’s easy to calculate the size of a wavelength from the formula: wavelength n_\ = speed of sound (c) /frequency (f). By comparing sound wavelengths to the size of a house, the size of bass wavelengths are evident.

Trapping Bass In Your Project Studio illustration of sound wavelengthsTrapping Bass In Your Project Studio illustration of keyboard and low-end rolloff

The shortest “bass” note–A440–has a wavelength of about 2.5 feet. The longest wavelength is 56 feet, and it belongs to 20 Hz. Full range speakers generally produce sound extending down through most of the lower end of the piano keyboard. Subwoofers produce sound specifically in the last octave of the piano’s keyboard and the one just below it, the first audible octave.

Speaker Directivity

Trapping Bass In Your Project Studio directionality of speakers. Speakers possess frequency-dependent directional qualities. For both mids and highs they produce adequate sound levels only in the forward direction towards the listener. Lower frequencies from the same speakers, however, radiate equally in all directions. This directionality means that mids and highs are efficiently beamed towards the listener, and little acoustic energy is wasted on illuminating the rest of the room.

The lows easily require six or more times the acoustic/electric power than the mids and highs to achieve the same sound level at the listener’s position. Speaker efficiency is one reason for power gulping; the other is directionality. Because bass waves are bigger than the speaker, they travel with equal strength in all directions. The speaker is an “omni” pattern sound source. Often much of the bass wavefront has bounced off of the walls, floor and ceiling of the room before it even reaches the listener.

Sound is an airborne ripple or wave whose speed (c) is about 1,128 ft/second. Consider the piston of a loudspeaker that is vibrating to and fro at 100 Hz. In the exact amount of time it takes for the speaker cone to make one cycle, or complete a round trip (1/100 second), the sound wavefront it generated will have moved away from the speaker (1/100 x 1128) some 11.28 feet. For a continuous tone, this becomes a repeating event. As you move away from the speaker, every 11.28 feet would be the same acoustic condition.

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