Overview of Acoustic Zoning and Features of Small Rooms
The calculation of the point source or the unobstructed spherical propagation in the free field is much simpler than the acoustic calculation inside the room. We generally think that the frequency range of the human ear is 20-20 kHz. In such a wide frequency band, the transmission patterns of different frequency bands in the room are different. For low frequency regions, room reverberation plays a major role in room acoustics, while at high frequencies, linear propagation will have a major impact on room acoustics. Such a wide frequency band, different frequency mixing, and different manifestations will plague us to understand the acoustic characteristics of the room, and the calculation becomes quite complicated. From this we should segment the full frequency domain and calculate it separately to understand the acoustic properties of the room.
When we are facing a small room, we can easily divide the room response into four parts:
Part 1: Free Zone (Z Zone)
Part II: Typical Area (A Area)
Part III: Mixed Zone (Z Zone)
Part IV: Linear Region (C Zone)
In a small room with four partitions: the free zone does not have any modal coupling effect, while the typical zone is more suitable for calculation using the point source sound propagation mode, and the linear zone is more suitable for linear sound source propagation mode calculation. The mixing zone needs to be calculated in a composite manner.
For example, in a space of 6.7 meters long, 5.5 meters wide and 4.3 meters high. We can derive the lowest frequency demarcation point of the space (the demarcation point between the X zone and the A zone) based on the length. The demarcation point is calculated as: the speed of sound (normal air propagation speed is 344 m / s), divided by the distance of twice the longest side of the room, namely: f = 344 / 2L. In this example the demarcation point frequency is approximately 26 Hz. The dividing point frequency of the A zone and the B zone is calculated as f=1905√(T/V), where T represents the reverberation time of the room and V represents the room volume. Assuming that the room reverberation time is 0.5 seconds, we can conclude that the demarcation point between zone A and zone B is about 107 Hz. In general, the boundary line frequency between zone B and zone C is about 4f. In this example we can get about 428Hz. In this way, we divide the small room into 4 parts: 0-26Hz in the X zone, 26-107Hz in the A zone, 107-428Hz in the B zone, and 408-20000Hz in the C zone.
As mentioned above, we need to calculate different areas using different calculation methods. Except for the X zone, the delineation of the zone depends entirely on the length of the longest side of the room and the speed of sound. For the boundary line (f and 4f) of Zone A, Zone B, and Zone C, we need to emphasize that the boundary point is only partitioned through a mathematical model area, which is convenient for us to calculate later, rather than at both ends of the boundary line, the sound Performance will be completely different. The change between them is a continuous transition.
The number of modal resonances increases with the transition from zone A to zone C, but is absent in zone X. This does not mean that the frequency in the X zone (344/2L and below) does not exist in the room, but rather points out that the frequency in this zone does not have a large impact on the room frequency response.
The effect of room size on acoustic quality can be summarized by the following figure:
Before explaining this illustration, we need to make it clear that the room reverberation time used is about 0.5 seconds. The effect of the reverberation time here is limited to a description of the "damping" and the sound absorption coefficient of the room, and the value of the long side 344/2L must be known.
As can be seen from the figure, the lower frequency limit is related to the length of the long side. In a room with a smaller volume, the lower the lower frequency limit and the lower the low frequency response. In a very small, language-only sound reinforcement, this problem may not have serious consequences, because generally only about 10% of the language information will be below 200Hz, of course, this is only Part of the problem.
Area A (typical area) decreases as the volume of the room increases, which means that the smaller the audible frequency, the more the frequency will be transmitted according to the characteristics of the point source diffusion, and there are more The modal resonance is generated. This also means that there will be more steady-state response intervals, resulting in more severe acoustic staining and irregular room response on more bands. Zone B (mixed property zone) also increases as space decreases. Of course, the diffraction and diffusion characteristics dominated in the B region are quite different from those in the A region, and these will also become sound reinforcement problems. The C region (linear region) exhibits a high linear diffusion characteristic, which decreases with the increase of the A region and the B region. When the room acoustic problem is generated in the C region, we can solve the corresponding problem by using a simple acoustic design. .
In general, the system design and commissioning should focus on pre-judging the problems that will arise in Zone A and Zone B. Because the acoustic problems of these two areas may be more difficult to solve. While the X zone is uncontrollable due to physical characteristics and is a frequency band that does not contain the main information, the acoustic propagation physical properties of the C zone will help us to solve the acoustic problems generated in this frequency band relatively easily.
Original: Handbook for Sound Engineers-The New Audio Cyclopedia (2nd edition), Chapter 3-Acoustics of Small Rooms
Compile: Hu Nan
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