In principle, different types of broadcast speakers should be used depending on the environment. For example, in a room with a ceiling ceiling, an embedded, back coverless ceiling speaker should be used. This type of speaker has a simple structure, is relatively inexpensive, and is easy to construct. The main disadvantage is that there is no back cover and it is easy to be bitten by insects and rats. In a room with only a suspended ceiling and no ceiling (such as an open-air shopping mall), a ceiling-mounted tubular speaker or a ceiling speaker with a rear cover should be used. Since the ceiling is equivalent to an infinite baffle, the use of a speaker without a back cover in the presence of a ceiling does not cause an acoustic short circuit. When there is no ceiling, the situation is very different. If you still use a ceilingless speaker without a back cover, the effect will be very poor. In this case, hoisting speakers should be used in principle. However, if the investment is too large, a ceiling speaker with a rear cover can also be used. The rear cover with the rear cover ceiling speaker not only has the general mechanical protection effect, but also plays a role in preventing the acoustic short circuit to a certain extent.
In ceiling-free rooms (such as underground parking lots), wall-mounted speakers or indoor sound columns should be used.
Outdoors, an outdoor sound column or horn should be used. This type of sound column and horn not only has a rainproof function, but also a large volume. Since the outdoor environment is empty and there is no reverberation effect, it is necessary to select a variety with a loud volume.
In gardens and grasslands, grass speakers should be used. These speakers are rainproof, sleek, and the volume and sound quality are more elegant.
In the halls with exquisite decoration and high ceilings, it is advisable to use hoisting speakers with elegant shapes and harmonious tones. In the case of high fire protection requirements, fire-resistant speakers should be used. This type of speaker is fully sealed and its outlet is mated to the flame-retardant sleeve.
Configuration of Broadcast Speakers Broadcast speakers are in principle arranged in a broadcast service area on a uniform and decentralized basis. The degree of dispersion should be such that the signal-to-noise ratio in the service area is not less than 15 dB.
Generally, the noise floor of a high-grade office corridor is about 48 to 52 dB, the noise floor of a super mall is about 58 to 63 dB, and the noise floor of a busy section is about 70 to 75 dB. Considering the accident, the scene may be very confusing, so for the need 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 is possible to roughly consider the noise floor as 65 to 70 dB (except in special cases). According to this calculation, the sound pressure level of the broadcast coverage area should be above 80 to 85 dB.
Since broadcast speakers are typically distributed, the sound pressure level of the broadcast coverage area can be approximated as a contribution from 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, 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. At a frequency of 90 dB/8 W, the sound pressure level at 8 m from the speaker is about 81 dB. The above calculations do not consider the contribution of the early reflection sound group. Indoors, the contribution of early reflections and adjacent speakers can increase the sound pressure level by about 2 to 3 dB.
According to the above approximate calculation, in the venue where the ceiling is not higher than 3m, the ceiling speakers can be uniformly arranged at a distance of 5 to 8m. If only background music is considered without considering emergency broadcast, the distance can be increased to 8-12 m. In addition, the fire accident design and installation specification (hereinafter referred to as “the specificationâ€) applicable to mainland China has the following rigid rules: “The number of speakers in public places such as walkways, halls, restaurants, etc. should be guaranteed from any part of this floor. The walking distance of a recent speaker is no more than 15m.
Speakers should be provided at the intersection of the walkway and at the corner. The last speaker at the end of the walkway is no more than 8m 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, a sound column composed of a plurality of speakers should be used in principle. The signal power level fed to a group of speakers (eg, a sound column) is doubled (provided the group is acceptable), and the sound pressure level can be increased by 3 dB. Please note the meaning of "double". It is doubled from 1 to 2; it is doubled from 2 to 4. In addition, for every 1 time increase in distance, the sound pressure level will drop by 6dB. According to the above rules, it is not difficult to estimate the arrangement distance of the outdoor sound column. For example, taking the CE-704 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 more than twice that of a single ceiling speaker. In fact, this distance can be larger. Because the sensitivity of the column is higher than that of a single ceiling speaker (about 3 to 6 dB higher), and for every 6 dB increase, the distance can be doubled. In other words, the coverage distance of 540 columns can reach more than 20m. However, the radiation angle of the sound column is relatively narrow, and is effective only about 60 to 90 degrees (horizontal angle) in front of it.
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