How to choose broadcast speakers and their configuration

1. Selection of broadcast speakers

In principle, different types and specifications of broadcasting speakers should be selected according to the environment. For example, in a room with a ceiling, it is appropriate to use an embedded ceiling speaker without a back cover. This type of speaker has a simple structure, is relatively cheap, and is easy to install. The main disadvantage is that it has no back cover and is easily bitten by insects and rodents. In a room with only a frame ceiling but no ceiling (such as an open-shelf shopping mall), it is appropriate to use a hanging tube speaker or a ceiling speaker with a back cover. Since the ceiling is equivalent to an infinitely large baffle, the use of a speaker without a back cover under the condition of a ceiling will not cause a short circuit. The situation is very different when there is no ceiling. If a ceiling speaker without a back cover is still used, the effect will be very poor. In principle, a hanging speaker should be used at this time. However, if the investment is too large, a ceiling speaker with a back cover can also be used. The back cover of a ceiling speaker with a back cover not only has a general mechanical protection function, but also plays a role in preventing short circuits to a certain extent.

In indoor spaces without suspended ceilings (such as underground parking lots), it is advisable to use wall-mounted speakers or indoor sound columns.

Outdoor sound columns or horns are suitable. These sound columns and horns are not only rainproof, but also have a higher volume. Since the outdoor environment is open and there is no reverberation effect, it is necessary to choose a type with a higher volume.

In gardens and grasslands, it is advisable to use lawn speakers. This type of speaker is rainproof, beautifully shaped, and has a high volume and sound quality.

In halls with exquisite decoration and high ceilings, it is advisable to use hanging speakers with elegant shapes and harmonious colors. In places with high fire protection requirements, it is advisable to use fire-proof speakers. This type of speaker is fully sealed, and its outlet can be connected to a flame-retardant casing.

2. Configuration of broadcast speakers

In principle, the broadcast loudspeakers should be evenly and dispersedly arranged in the broadcast service area, and 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 in high-end office buildings is about 48-52dB, the background noise of supermarkets is about 58-63dB, and the background noise of busy roads is about 70-75dB. Considering that the scene may be very chaotic when an accident occurs, for the needs of emergency broadcasting, even if the broadcasting service area is an office building, the background noise should not be estimated too low. Therefore, as a general consideration, except for busy and lively places, it is better to roughly regard the background noise as 65-70dB (except for special circumstances). According to this calculation, the sound pressure level in the broadcasting coverage area should be above 80-85dB.

Since broadcasting speakers are usually dispersed, the sound pressure level in the broadcasting coverage area can be approximately considered as the contribution of a single broadcasting speaker. According to relevant electroacoustic theories, the sound pressure level SPL in the speaker coverage area has the following relationship with the speaker's sensitivity level LM, the power P fed to the speaker, and the distance r between the listening point and the speaker:

SPL=LM+10lgP－20lgrdB (1)

The sensitivity level of the ceiling speaker is between 88 and 93 dB; the rated power is 3 to 10 W. Based on the 90 dB/8 W calculation, the sound pressure level at 8 meters away from the speaker is about 81 dB. The above calculation does not take into account the contribution of the early reflection sound group. In the room, the contribution of the early reflection sound group and the adjacent speakers can increase the sound pressure level by about 2 to 3 dB.

According to the above approximate calculation, in venues with ceilings no higher than 3m, ceiling speakers can be evenly arranged at a distance of 5 to 8m from each other. If only background music is considered without emergency broadcasting, the distance can be increased to 8 to 12m. In addition, the Fire Accident Broadcasting Design and Installation Specifications applicable to mainland China (hereinafter referred to as "Specifications") have the following rigid regulations: "In public places such as corridors, halls, and restaurants, the number of speakers should be able to ensure that the walking distance from any part of the floor to the nearest speaker does not exceed 15m. Speakers should be installed at intersections and corners of corridors. The last speaker at the end of the corridor 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 range of a single broadcast speaker can only take the lower limit calculated above. Since the distance corresponding to this lower limit is very short, in principle, a sound column composed of multiple speakers should be used. For every doubling of the signal power fed to the speaker group (such as the sound column) (provided that the group can accept it), the sound pressure level can be increased by 3dB. Please note the meaning of "double". From 1 to 2 is doubled; and from 2 to 4 is doubled. In addition, for every doubling of the distance, the sound pressure level will drop by 6dB. According to the above rules, it is not difficult to deduce the configuration distance of the outdoor sound column. For example, taking the CS-540 outdoor sound 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 2 times that of a single ceiling speaker. In fact, this distance can be even larger. Because the sensitivity of the sound column is higher than that of a single ceiling speaker (about 3 to 6dB higher), and for every 6dB increase, the distance can be doubled again. That is to say, 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 is only effective within about 60 to 90 degrees (horizontal angle) in front of it. The specific calculation can still be done using formula (1).