The sound reinforcement system of the school Lecture hall is no longer an orderly combination of single sound equipment, but a sound closed-loop system integrating Architectural acoustics (sound field) and information electronic technology (audio electronic equipment). It also has increasingly high requirements for the environment, requiring the auditorium to have sufficient sound pressure and a uniform sound field distribution. Correspondingly, the sound reinforcement system no longer only provides simple amplification functions, but will present different acoustic effects and usage functions according to different usage requirements. Generally speaking, the sound reinforcement system is mainly composed of microphone, loudspeaker, Mixing console, sound field processor, audio processor, automatic mixing console, digital power amplifier and other equipment.
2、 Design requirements for sound reinforcement systems
The school's Lecture hall is mainly responsible for speech based sound reinforcement of academic reports, conferences, etc., which requires that the sound reinforcement system can restore the language signal truly and clearly. Firstly, the system should ensure good language clarity and sufficient and uniform sound pressure, with a language clarity frequency band of 300-6000Hz. In addition, the Lecture hall is also a place for performing songs, dances, operas, music and other artistic performances. The sound reinforcement system is required to restore and reproduce various sound sources. Therefore, this places higher demands on the clarity and richness of the signal.
3、 Design standards for sound reinforcement systems
The design standard for volume is related to the rated power of the sound reinforcement system, and sufficient volume is the basic objective requirement of the sound reinforcement system. According to the industry standard GB50371-2006 "Code for Design of Hall Sound Reinforcement Systems", the requirements for multifunctional sound reinforcement standards are that the loud pressure of halls mainly focused on language is ≥ 90dB, the first dance hall is ≥ 103dB, and the second dance hall is ≥ 110dB. The volume level is determined based on the signal-to-noise ratio and system operating characteristics.
The design standards for sound quality mainly depend on the nonlinear distortion and transmission frequency characteristics of the sound reinforcement system. The national standard stipulates that the transmission frequency characteristic index of a hall is 0dB with an average sound pressure of 100-6300Hz. The allowable range within this frequency band is -4~+5dB, and the allowable range for 50-100Hz and 6300-12500Hz is -8~+4dB. The frequency transmission characteristics are a key indicator of sound quality, and nonlinear distortion is also one of the factors affecting the sound quality effect. In addition, the clarity of music and language is also an important criterion for measuring sound quality. There are many methods for measuring clarity, and in practical operations, evaluation usually relies on subjective sensations on site.
The design standard for sound field requires that the unevenness of a hall should not exceed 6dB at 1000Hz, 8dB at 4000Hz, and 10dB at 100Hz. Therefore, the school Lecture hall should construct the sound field according to the standard, and the sound field should be as uniform as possible.
4、 Selection and application of microphones
The microphone is the 'entrance' of the entire system. The microphone converts the collected sound wave signal into a weak voltage signal for output, which requires the back-end audio processing equipment to amplify and improve the signal. Therefore, if there is noise input in the microphone, it is difficult for the back-end. The selection and use of the microphone will have a significant impact on the quality and effectiveness of sound reinforcement. In addition, there is a contradiction in acoustic feedback indoors, so there are high requirements for the sensitivity, frequency response range, and directionality of microphones (microphones).
Sensitivity refers to the ratio of the open circuit output voltage U of a microphone to the sound pressure P. In fact, the higher the sensitivity, the better. Moderate sensitivity is sufficient, and high sensitivity may not necessarily be pursued. The average sound transmission gain of 125-6300Hz shall not be less than -8dB.
For indoor sound reinforcement, the level of sound that the sound reinforcement system can achieve is constrained by sound feedback (feedback is the process of sound being output from the speaker through the amplifier speaker and then transmitted back to the speaker. When the feedback reaches a certain level multiple times, howling will occur). Therefore, in the same sound reinforcement system, if the sensitivity of the microphone is high, the output voltage will increase, and the sound emitted by the speaker will also increase accordingly. In order to prevent feedback howling, the volume has to be turned down to achieve good sound reinforcement effect.
The frequency response range refers to the variation in output voltage measured by a microphone at different frequencies under a constant sound pressure. Generally speaking, the flatter the frequency response curve, the better. For conference microphones, their frequency response range is between 250 and 4000Hz. When designing, the spectral range should be placed within a higher frequency band to improve language clarity. Because the frequency spectrum of human language generally ranges from 100 to 4000Hz, the high-frequency band has a significant impact on clarity.
The directionality of a microphone refers to the ratio of sensitivity in a certain direction to high sensitivity at a certain frequency. For language reinforcement such as academic reports and conferences, in order to effectively prevent and suppress the sound feedback caused by howling, directional microphones should be used. The directionality is closely related to the frequency of the received acoustic signal: the lower the frequency, the worse the directionality; The higher the frequency, the stronger the directionality.
The output impedance is the modulus value of the internal resistance measured at the output end of the microphone using a frequency of 1kHz sound signal, which is divided into two types: high impedance and low impedance. The recommended values for use in China are 200 Ω, 600 Ω, and 2000 Ω. The specific applications of microphones are as follows.
1. Pickup method. There are two main ways to pick up a microphone: when performing language reinforcement, the microphone's sound axis should be facing the speaker; When amplifying a concert, three-dimensional sound pickup is generally used.
2. Relative position. The larger the distance, the less sound feedback can be reduced. Therefore, when placing the microphone, it should be placed relatively far away, and the height should be reduced as much as possible to avoid facing the speaker directly.
3. The distance between the microphone and the speaker. Generally speaking, the distance between the microphone used for speech and the mouth shape should be 10-20cm. If you are too close to the microphone, the low-frequency sound will worsen due to the influence of speech airflow, resulting in mixed and unclear sound, stuffy sound, and directly affecting the clarity of the language. Therefore, you should stay slightly away from the microphone to avoid speech airflow directed towards the microphone; It should not be too far, otherwise the sound signal will weaken and affect the pickup sensitivity. In addition, a suitable distance can effectively suppress the entry of environmental noise. Generally speaking, the near-field sensitivity is higher than the far-field sensitivity, and environmental noise comes from the far-field. If the speaker's mouth is 10-20cm away from the microphone, the sound emitted belongs to the near-field, and the low-frequency sensitivity of the speaker is higher than that of environmental noise, thus relatively suppressing noise sensitivity.