Introduction to public broadcasting system, fire emergency broadcasting, and background music system

1. What is a public address system?

The broadcast sound system involves a wide range, from factories, schools, hotels, stations, docks, squares to venues, theaters, gymnasiums, residential quarters, etc. are all closely related to it.

In civil engineering design, broadcasting systems can be divided into the following categories:

Public broadcasting system for public areas (squares, stations, docks, shopping malls, restaurants, corridors, classrooms, etc.) and parking lots.

This system is mainly used for voice broadcasting, so clarity is of primary importance. Moreover, such systems often broadcast background music,
In the event of a disaster or emergency, it can be converted to an emergency broadcast.

Broadcast sound system for hotel rooms.

This kind of system includes guest room audio broadcasting and emergency broadcasting, which is often delivered by the bedside table in the guest room. The guest room broadcasting contains multiple freely selectable bands. During emergency broadcasting, the guest room broadcasting is automatically interrupted and automatically switched to emergency broadcasting

The hall sound reinforcement system represented by the auditorium, theater and gymnasium.

This is a highly professional sound reinforcement system. It not only needs to consider electro-acoustic technology, but also involves architectural acoustics. Both must be balanced, and must not be discarded. This type of broadcast system often has comprehensive and multi-purpose requirements. It is not only used for venue language sound reinforcement, but also commonly used in cultural performances. For large-scale live performance sound systems, the electric power is less Tens of thousands, and more than hundreds of thousands of watts, so high-power speakers and power amplifiers are required. There are certain requirements in system configuration and equipment selection. At the same time, attention should be paid to the load of power lines.

Broadcast audio system for conference rooms, lecture halls, etc.
This type of system is also generally set up as a dual-use system for background music and emergency broadcasting provided by public broadcasting. However, due to its particularity, a conference broadcasting system is often set up separately in the conference room and lecture hall. For more demanding or international conference halls, special audiovisual systems such as simultaneous interpretation systems, conference voting systems, and large-screen projection TVs need to be designed separately.

From the introduction above, we can see that for various buildings, hotels and other civil buildings, the broadcast sound system can be basically divided into three types: First, the public address system (Public Address System for short), this is a cable broadcast system, It includes background music and emergency broadcast functions, usually combined together, usually playing background music or other programs, when there is an emergency such as a fire, it is converted to an alarm broadcast. The microphone for broadcasting in this system is generally not in the same room as the speaker for broadcasting to the public, so the problem of silent feedback, and the fixed pressure transmission method is its typical system; the second is the hall sound reinforcement system, this system Use professional audio equipment and require a high-powered speaker system and power amplifier. Since the microphone and the speaker for sound reinforcement are in the same hall, there are problems with acoustic feedback and howling, and because of the short distance, the system is generally Adopt low-impedance direct transmission; the third is a dedicated conference system. Although it is also a sound reinforcement system, it has special requirements, just like a sound interpretation system.

2. The composition of the broadcast sound system

No matter what kind of broadcast sound system, you can draw a basic block diagram shown below. It can be basically divided into four parts: program equipment, signal amplification and processing equipment, transmission line and speaker system. Program source equipment: Program sources are usually provided for radio broadcasting, CD players and recording decks, in addition to microphones, electronic musical instruments, etc.

Signal amplification and processing equipment: including mixers, preamplifiers, power amplifiers, various controllers and audio processing equipment. The primary task of this part of the equipment is signal amplification, followed by signal selection. The functions and positions of the mixer and preamplifier are similar (of course, the mixer's functions and performance indicators are higher), their basic function is to complete the signal selection and preamplification, in addition to frank volume and sound effects for various Adjustment and control. Sometimes, in order to better perform frequency equalization and timbre beautification, a graphic equalizer is additionally separately invested. This part is the "control center" of the entire broadcast audio system. The power amplifier amplifies the signal sent from the pre-amplifier or mixer, and then pushes the speaker to play sound through the transmission line. Although the transmission line is simple, it has different requirements with different systems and transmission methods. For auditoriums, theaters, etc., because the distance between the power amplifier and the speaker is not far away, a low-impedance, high-current direct feeding method is generally used. The transmission line requires a dedicated speaker line. The loss caused by the transmission line is often a high-voltage transmission method. Because the transmission current is small, the requirements on the transmission line are not high.

Loudspeaker system: The loudspeaker system requires the entire system to be matched, and at the same time, the choice of its location should also be practical. Auditoriums, theaters, dance halls have high requirements for timbre and sound quality, and speakers generally use high-power speakers; and public broadcasting systems, because of their low requirements for timbre, generally use 3W-6W ceiling speakers.

3. Broadcast system design

The background music is abbreviated as BGM, which is the abbreviation of Back Ground Music. Its main function is to cover up the noise and create a relaxed and harmonious atmosphere. If the listener does not listen intently, he cannot distinguish the location of the sound source. A kind of music that can create a relaxed and pleasant atmosphere.

Therefore, there are two effects of background music, one is to cover up the environmental noise psychologically, and the other is to create an atmosphere suitable for the indoor environment. It is widely used in hotels, hotels, restaurants, shopping malls, hospitals, office buildings and so on. The music should be lyrical or relaxed. Strongly intense music is not suitable.

The background music is not stereo, but mono music. This is because the stereo needs to be able to distinguish the position of the sound source and have a sense of depth, while the background music is not aware of where the sound comes from without listening intently. The location of the sound source is felt for people, so that the sound source is required to be concealed, and the volume should be lighter, so as not to affect the principle of the two people speaking opposite each other.

The public broadcasting mentioned here refers to the sound broadcasting with cable transmission, which is usually used in public venues, buildings, and communities, and is used for background music broadcasting, paging broadcasting, and forcibly inserted disaster broadcasting. The design of this type of public address project is usually carried out in the following order:

1. The selection and configuration of broadcast speakers should be considered first

2. Selection of broadcasting amplifier

3. Broadcast partition

4. Construction of broadcasting system

4. Selection and configuration of broadcast speakers

1. Selection of broadcasting 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. Such as CEH-31T, CEH-33T, CEH-38T series. This type of speaker has a simple structure, is relatively cheap, and is 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 dome speakers or ceiling speakers with rear covers (such as the CE-709T series). 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. The former such as CE-16W, CE-18, series; the latter such as CE-210, CE-220 series. Outdoors, outdoor sound column or horn should be used. The former is like the CE-502 and CE-503 series; the latter is like the CEE-63 series. 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. Such as CE-AG1, CE-S81 series. 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. Such as CEH-38T series. As for the choice of brand and grade, it is naturally related to investment. The CEOPA series is mainly based on popular grades to upper-middle grades (there are also high-end ones, which are few).

2. Configuration of broadcast speakers

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 ~ 52 dB, the background noise of super malls is about 58 ~ 63 dB, and the background noise of busy roads is about 70 ~ 75 dB. 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, you may consider the noise floor as 65 ~ 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 of the speaker coverage area The sound pressure level SPL of the speaker coverage area is related to the sensitivity level LM of the speaker, the electrical power P fed to the speaker, the distance r between the listening point and the speaker, etc .:

SPL = LM +10 lg P? 20 lg r dB (1)
The sensitivity level of the ceiling speaker is between 88 ~ 93dB; the rated power is 3 ~ 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 the venues where the ceiling is not higher than 3m, the ceiling speakers can be arranged approximately 5-8m away from each other. 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 "Code") 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 speaker is not