Acoustic standard and test of professional audio system

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（1） . common acoustic standards and specifications for theatres and stadiums

The standard of sound reinforcement characteristics of halls has been formed since 1985, for example: measurement method of sound reinforcement characteristics of halls [GB 4959-85]; The acoustic characteristics of hall sound reinforcement system (gyj25-86) and so on are the commonly used standards after continuous modification:

1. measurement method of sound amplification characteristics of halls GB 4959-95

2. optimal connection value of equipment interconnection of sound reinforcement system gb/t 14197-93

3. specification for reverberation time measurement of hall GBJ 76-84

4. RASTI gb/t 14476-93 for objective evaluation of the comprehensibility of hall language

5. analog program signal GB 6278-86

6. code for design of hall sound reinforcement system GB 50371-2006

7. code for design of theatre architecture JGJ 57-2000/j67-2001

8. regulations for acoustic design and measurement of gymnasium jgj/t 131-2000

9. code for design of sports buildings JGJ 31-2003 / j265-2003 GB national standard; JGJ - industry standard

In the national standards, mandatory provisions, such as safety related content, must be strictly implemented, and other more provisions are recommended.

When designing the sound reinforcement in the interior and outside, the corresponding national standards or specifications are often followed in the "acoustic characteristics index of sound reinforcement system". The owner of the project also has clear requirements for "acoustic characteristics index of sound amplification system" during the bidding of the project.

There is no written standard for the acoustic characteristics of stadiums. Recent data from FIFA and DFB show that the maximum sound pressure level of stadium audience is about 105dB.

The acoustic characteristics of the main sound amplification system of the newly built or reconstructed stadium in 2008 Beijing Olympic Games are as follows:

Sound pressure level: 95dB for normal use; Maximum sound pressure level (emergency broadcast) 106db.

Transmission frequency characteristics: 100Hz ～ 5KHz for language ± 5dB；

100Hz to 15KHz for music ± 5dB。

Language clarity: fast language transfer index RASTI ≥ 0.5.

（2） . sound quality control and system debugging of sound reinforcement system

1. deepen the understanding of the content of sound field 'time domain' in sound amplification

When it comes to sound field, people will immediately think of some basic parameters of sound amplification acoustic characteristics, such as maximum sound pressure level, transmission frequency characteristics, sound gain and sound field inhomogeneity. These characteristic parameters are important, but not sufficient, to measure the hall sound amplification (or sound effect) in "overall". Here we call it "normal parameter" or "conventional control" for the time being.

In terms of the hall reinforcement, we can regard the sound amplification box working simultaneously in the auditorium as "multi-point sound source". Because of the different geometric positions they are installed in the space, the time they arrive at the audience seat will be different when the sound is amplified, that is, there is time difference. The difference of time brings about the change of sound amplification phase, which forms a typical interference, and has comb filter effect. The existence of interference in the field of sound amplification will affect the clarity of language and music, and will damage the sound quality effect of sound amplification and playback. The comb filter effect will have a clear "squeeze" sense especially in the high range when listening to the replayed program. Here we call the "noise interference" and "comb filter effect" formed in the sound reinforcement as "refinement parameter" or "refinement control".

2. limitations of conventional 1/3 octave real-time spectrum analyzer

The "normal parameters" or "conventional control" of sound amplification system are usually detected by a 1/3 octave real-time spectrum analyzer. But the use of "1/3 octave" to detect "thinning parameters" or "refinement control" because of its low precision, it will cover up or find the existence of phase difference and comb filter effect. After the detection based on the 1/3 octave real-time spectrum analyzer, the 1/3 octave graph equalizer is usually used to adjust or compensate the amplitude frequency response curve, that is, the valley value part of the amplitude frequency response curve is upgraded and the peak part is attenuated, so as to "flatten" the amplitude frequency response as far as possible. But sometimes after such adjustment or compensation, the listening effect is not good. Because such "amplitude frequency equalization" can not reflect and solve the sound quality problems caused by phase difference and comb filter effect, it will make the system debugging into error when adopting such "adjustment or compensation" for the existence of the hidden phase difference and comb filter effect.

3. the superior type of sound amplification system is debugged by sim system tester

Sim (source independent measurement) can be translated into "sound source independent measurement".

Sim is basically FFT real-time spectrum analyzer, which is made up of Bob in the United States

McCarthy was developed in 1984 and has been developed to sim- Ⅲ, the third generation product of today after more than 20 years of continuous improvement. The basic characteristics of sim-Ⅲ system:

(1) . multichannel test points (usually three test points), i.e. output point of the tuning station, output point after system signal processing and the sound field position of the test microphone. When connecting the corresponding expansion interface (accessories), up to 8-64 different position test microphones can be used to connect the test system to test one by one;

(2) . any audio frequency signal such as music, pink noise, sine wave can be used as test signal for analysis, so it can provide the test and analysis work of theater during performance;

(3) The high resolution display is to use 1/24 octave high-precision real-time spectrum analysis, so it can accurately find the comb filter effect that may exist in the sound field of sound amplification.

The sound reinforcement system debugging by sim-Ⅲ is more effective than the usual 1/3 octave real-time spectrum analyzer, which can refine the phase difference, delay and comb filter effect in the sound amplification field. Then, the system can be optimized in terms of equalization, delay and gain, which can make the detection of sound field more comprehensive and accurate, and the interference will be minimized.

4. the importance of subjective listening evaluation

After the adjustment of the sound reinforcement system in the theatre auditorium, the acoustic characteristics of the sound reinforcement system shall be measured according to the national standards. Objective physical measurement is necessary, and subjective sound evaluation is also necessary. It is an important supplement to objective measurement. It is often necessary to modify system debugging according to the audition of different program sources. Objective physical measurement and subjective sound evaluation are the real embodiment of comprehensive assessment of the sound amplification effect (or quality) of the theatre.