1. Selection of broadcast speakers
In principle, different types of broadcast speakers should be selected depending on the environment. For example, in a room with a ceiling, it is advisable to use a built-in ceiling speaker without a rear cover. This type of speaker is simple in structure, relatively cheap in price, and easy to construct. The main disadvantage is that there is no rear cover, and it is easy to be bitten by insects and rodents. In rooms with only a framed ceiling and no ceiling (such as open-frame shopping malls), it is advisable to use ceiling-mounted tube speakers or ceiling speakers with rear covers. Since the ceiling is equivalent to an infinite baffle, the use of speakers without a back cover under the condition of the ceiling will not cause an acoustic short circuit. The situation is very different when there is no ceiling. If you still use a ceiling speaker without a rear cover, the effect will be very poor. In principle, ceiling speakers should be used. However, if the investment is too large, a ceiling speaker with a rear cover can also be used. The rear cover of the ceiling loudspeaker not only has general mechanical protection, but also plays a role in preventing acoustic short circuits to a certain extent.
In rooms without suspended ceilings (such as underground parking lots), wall-mounted speakers or indoor sound columns should be used.
Outdoors, outdoor sound column or horn should be used. This type of sound column and horn not only has a rainproof function, but also has a large volume. Since the outdoor environment is open and there is no reverberation effect, it is necessary to select a variety with a larger volume.
In gardens and lawns, lawn speakers should be used. This type of speaker is rainproof, beautiful in shape, and the volume and sound quality are more particular.
In the halls with exquisite decoration and high ceilings, ceiling-mounted speakers with elegant shapes and harmonious colors should be selected. In the occasions with high fire protection requirements, fire-resistant speakers should be used. This type of speaker is fully sealed, and its outlet can be matched with a flame retardant sleeve.
2. Broadcast speaker configuration
Broadcast speakers are arranged in the broadcast service area on the principle of uniformity and decentralization. The degree of dispersion should ensure that the signal-to-noise ratio in the service area is not less than 15dB.
Generally, the background noise of corridors of high-grade office buildings is about 48-52dB, the background noise of super malls is about 58-63dB, and the background noise of busy roads is about 70-75dB. Considering that the scene may be very confusing when an accident occurs, so for the purpose of emergency broadcasting, even if the broadcasting service area is an office building, the noise floor should not be estimated too low. For this reason, as a general consideration, in addition to the bustling and lively places, it may be considered that the noise floor is generally 65 to 70dB (except for special cases). According to this calculation, the sound pressure level in the broadcast coverage area should be above 80 ~ 85dB.
In view of the fact that broadcast speakers are usually distributed, the sound pressure level of the broadcast coverage area can be considered approximately as the contribution of a single broadcast speaker. According to the relevant electroacoustic theory, the sound pressure level SPL of the speaker coverage area has the following relationship with the sensitivity level LM of the speaker, the electric power P fed to the speaker, the distance r between the listening point and the speaker, etc .:
SPL = LM ＋ 10lgP-20lgrdB (1)
The sensitivity level of the ceiling speaker is between 88 and 93dB; the rated power is 3 to 10W. Calculated with 90dB / 8W, the sound pressure level at 8m away from the speaker is about 81dB. The above calculation does not consider the contribution of the early reflected sound group. Indoors, the contribution of early reflected sound groups and adjacent speakers can increase the sound pressure level by about 2 to 3 dB.
According to the above approximate calculations, in venues where the ceiling is not higher than 3m, the ceiling speakers can generally be evenly spaced from each other by 5 to 8m. If only background music is considered and emergency broadcasting is not considered, the distance can be increased to 8-12m. In addition, the fire accident broadcast design and installation specifications applicable to mainland China (hereinafter referred to as "the specifications") have the following hard rules: "The number of speakers in public places such as aisles, halls, restaurants, etc., should be able to ensure that from any part of this floor to The walking distance of the nearest loudspeaker should not exceed 15m. Loudspeakers should be installed at the intersections and corners of the walkway. The last loudspeaker at the end of the walkway should be no more than 8m away from the wall.
There is basically no early reflection sound group in outdoor places, and the effective coverage of a single broadcast speaker can only take the lower limit calculated above. Since the distance corresponding to the lower limit is very short, in principle, a column composed of multiple speakers should be used. The sound pressure level can be increased by 3dB every time the electrical power of the signal fed to the speaker group (such as the sound column) is doubled (provided that the group can accept it). Please pay attention to the meaning of "double". Doubled from 1 to 2; doubled from 2 to 4. In addition, every time the distance is doubled, the sound pressure level will drop by 6dB. According to the above rules, it is not difficult to calculate the placement distance of the outdoor sound column. For example, taking the CS-540 outdoor column as an example, its rated power is 40W, which is more than 4 times that of a single ceiling speaker. Therefore, its effective coverage distance is greater than twice that of a single ceiling speaker. In fact, this distance can be larger. Because the sensitivity of the sound column is higher than that of a single ceiling speaker (about 3 to 6dB higher), and each additional 6dB, the distance can be doubled. In other words, the coverage distance of the 540 sound column can reach more than 20m. However, the radiation angle of the sound column is relatively narrow, and it is only effective within about 60 to 90 degrees (horizontal angle) in front of it. The specific calculation can still use formula (1).
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