Basic Knowledge of Room Acoustics

- Oct 11, 2018-

When the sound waves propagate indoors, they are repeatedly reflected by the closed walls, and a very complicated acoustic phenomenon occurs, which makes the indoor sound field completely different from the outdoor one. Because the geometry of each room is different, the materials used are different, and the acoustic phenomena caused by the sound propagation in each room are completely different. The effect of these phenomena on the listening effect is very large.

First, the acoustic process in the room

When the sound source sounds at a certain point in the indoor space, the listener first hears the direct sound from the sound source, and then hears the early reflections from the walls, ceilings, etc., and then the re-reflection of these reflected sounds, even Multiple reflections. These reflected sounds are absorbed by the wall and eventually decay to zero.

In the actual state, the acoustic characteristics are different due to the different structures and materials of the rooms. In the same size room, their frequency attenuation curves are also different. The undulating characteristic curve indicates that there are acoustic defects such as standing wave reflection surface, resonance and tremor echo in the room.

Second, the reverberation time

The time it takes for the sound wave to stop sounding from the sound source to the time it takes for the sound pressure level to drop to 60 dB is called the reverberation time.

The air has a relatively high absorption of sounds with a relatively high frequency (generally above 2000 Hz), and the higher the frequency, the greater the absorption, especially when the room is relatively large.

Under normal circumstances, the room with too short reverberation time is dry, dull, without warmth and vitality; while the reverberation time is too long, the previous reverberation masks the subsequent direct sound, which will produce a bathroom effect, and the sound is vague. Unclear, the sound quality is poor.

The low frequency reverberation time should be higher than the intermediate frequency, and the sound can be thick, full and flexible. The high frequency (4 ~ 8 kHz) can be slightly lower than the intermediate frequency, the brightness of the sound, the gorgeous color and the fine details can be reflected, and the burden on the tweeter is not too heavy.

Third, the resonance of the room

When pouring water into the water bottle, as the water level increases, the sound of pouring water will go from low to high. Similarly, changing the length of the instrument can change the level of the sound.

Every closed room will be excited by the sound and resonate. Both theory and practice have proved that rectangular rooms with relatively small spaces, especially those with a wall size of positive integers or cubes, are excited by sound, and various resonance points are generated, which are particularly easy to concentrate and superimpose, so that the sound of a certain frequency is obviously enhanced. , forming unnecessary dyeing. When the resonance is strong, the sound is significantly distorted. Music is more likely to excite resonance than language.

Fourth, the transmission frequency characteristics of the room

Each room has its own resonant frequency. Generally, the low frequency band resonance frequency is unevenly distributed, so that some frequencies will be greatly enhanced. Therefore, any sound that propagates in the room must be superimposed on its frequency characteristics. In the actual situation, it is much more complicated. The reflection and absorption of the walls are different. Many furniture, wall decorations and curtains have their own different sound absorption characteristics, and the frequency and ability to absorb sound are different. For these two reasons, each room responds differently to the frequency of each sound.

When the sound source is fixed, the position of the listener is different, and it will be affected by different reflections, and the points are also different. These differences need to be effectively adjusted by the audio device to pick up, record or reproduce the desired sound.

 

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