【观点】Acoustical Treatment Primer: Diffusion 谈谈声学处理中的“扩散”
【影音中国 hdavchina.com】
Gerry Lemay
HAA家庭声学联盟创始人与主席
My discussion of acoustical treatments for home theater and media rooms now continues with a discussion of the most exotic of the treatments:diffusion. We’ll discuss the concept and how it can be implemented in a home listening room. In the previous article we discussed acoustical absorption.
我对家庭影院和多媒体室声学处理的讨论现在进行到了其中最特别的一环：扩散。在这篇文章中，我们将讨论扩散的概念和它在家庭聆听室的应用。我们已经在之前的文章中研究过声学处理中的“吸声”。
Here’s our definitions for treatment types:
这是我们对声学处理类型的定义：
Absorption: Absorbing sound impacting the panel. It reduces or removes the energy of a reflection.
吸声：吸收冲击面板的声音。这会减少或消除反射声的能量。
Diffusion: Diffusive panels scatter sound. While they are not generally designed to reduce the energy in sound they effectively reduce the energy in a reflection by dividing it into many reflections to various directions.
扩散：扩散板打散声音。虽然它们通常不是为了减少声音中的能量而设计的，但它们通过将一个反射声拆分成许多不同方向的反射声，有效地减少了反射声中的能量。
Reflection: Yes, reflection is an important element in room design.Many reflections are beneficial and should be preserved.
反射：是的，反射是房间设计的一个重要元素。许多反射声是有益的，应该被保留。
Bass traps: This category includes everything from very thick panels to diaphragmatic absorbers. The commonality is that they are most effective absorbing low frequencies.
低音陷阱：这一类型包含从非常厚的吸声面板到薄板共振吸声体等的吸声材料。它们的共同之处在于能最有效地吸收低频。
Hybrid Treatments: These types combine the functions of the above types. They can be very useful when some frequencies should be absorbed while others reflected or scattered.
混合处理：这些类型结合了上述类型的功能。当某些频率应该被吸收而另一些应该被反射或散射时，它们非常有用。
A Diffusive Space
一个扩散的空间
Diffusion is a word that seems to define itself, however the utility of such treatments seems poorly defined by manufacturers and designers. I’ve seen diffusers used almost everywhere in a room without much explanation as to why. Allow me to reference one of the qualities of a large room we discussed in my article on absorption. Large rooms (concert halls, auditoriums, etc.)are referred to as having a diffusive sound field; as in a free space or field.Free, meaning open without narrow boundaries or obstructions. Ultimately,it is the diffusiveness of large rooms that are one of their great sonic benefits. It therefore seems logical that we’d like to make our small rooms more diffusive.
扩散这个词似乎定义了它自己，但制造商和设计者好像对这个声学处理类型的实际作用认识不清。我曾看到房间里几乎每一个地方都使用了扩散体，却没有解释为什么。请允许我引用我们在关于吸声的文章中讨论过的大房间的特点。大房间(包括音乐厅、观众席等等）被认为是属于扩散声场，就像在一个自由空间或声场中。自由，意味着开放，没有狭窄的边界或障碍物。基本上，大房间的扩散性是其最大的声学优势之一。因此，我们想让我们的小房间更具扩散性似乎是合乎逻辑的。
大房间(包括音乐厅、观众席等等）被认为是属于扩散声场，就像在一个自由空间或声场中
In a large room, the diffusiveness is a natural occurrence of having a wide-open area for sound waves to travel. As they travel the long distances between walls there is a homogenization of the various reflections; that is, they begin to become less different as they originate at many angles from the listeners. That’s why we can measure the reverberation time (RT60) of a large room and find nearly the same reading everywhere. Diffusers are used in large rooms to perfect the diffusive condition of a space, especially in rooms where there are unusual surfaces or if the shape of the room is not completely open. Diffusion in a small space like a home theater is more local. I mean that the early reflections tend to dominate the sound quality and the origin of these reflections is highly localized.Moving furniture or a speaker can greatly change the origin point of a reflection or even block it. The problem becomes less about absorbing those reflections and more of filling in the gaps between them or better put, spreading sonic energy more evenly.
在一个大房间中，扩散性是为声波传播提供开放空间的自然现象。当声波在墙与墙之间长距离传播时，各种各样的反射是均匀的，也就是说，聆听者聆听多个位置发出的声音实际区别并不大。这就是我们为什么能测量一个大房间的混响时间，并在每一个地方找到基本一致的读数的原因。大房间使用扩散体来改善空间的扩散条件，尤其是在有着不寻常表面或是形状不完全开放的房间中。在像家庭影院这样的小空间中，扩散更具有定位性。我的意思是早期的反射往往支配着音质，而这些反射的来源是高度定位的。移动家具或音箱可以极大地改变反射声的起点，甚至阻塞反射声。因此问题变成不在于吸收这些反射声，而在于填补它们之间的空隙，更恰当地说，是更均匀地传播声音的能量。
When we add diffusion, we are,in fact, filling in the gaps where direct reflections or speakers are absent. We can add absorption to tame some early reflections to improve the focus of the sound field. This can provide some benefit; however, the early reflections are also the genesis of later reflections.Early reflections eventually become later reflections with origins at other locations. In effect, by over-absorbing we are reducing the diffusiveness of the space by killing the late reflections as we kill the early ones. That’s why we are careful when adding absorption in limited quantities where it can be helpful. Good locations for absorption are the side walls (in limited quantities),behind floor-standing speakers, andoften in the center of the back of the room. Often we’ll put a distributed pattern of absorption or hybrid treatments on the side walls to manage them yet not kill them. We need much of the energy from the early reflections to survive and become later reflections to enhance a sense of diffusion. We call the uniformity of the sound field “envelopment”. Obviously, in a home theater the surround speakers play a role in the envelopment of the system as well but that’s for another discussion.
当我们加入扩散时，我们实际上是在填补直达反射声或音箱缺失的空白。我们可以增加吸声体来遏制早期反射，以提高声场的聚焦。这可以带来一些好处。但是，早期反射也是后期反射的起源。早期反射会成为起点在其它位置的后期反射。实际上，因为过度吸收，我们在消除早期反射的同时，也消除了后期反射，减少了空间的扩散。这是我们增加有限数量吸声体时要小心的原因。吸声体摆放的好位置通常是在侧墙(有限数量)、前置落地音箱的后面和房间后墙的中间部分。一般来说，我们会在侧墙放置分散式吸声板或混合式的声学处理材料来管理反射声而不是消灭它们。我们需要早期反射留下的大量能量，来生成后期反射，并增强扩散的感觉。我们将声场的均匀性称为“包围感”。显而易见地，在家庭影院中，环绕音箱也在营造系统包围感中扮演重要角色，但那是另一个讨论的话题了。
扩散体的基本原理很简单，不均匀的表面会将反射分解成许多较小的反射，以不同的角度发生偏移
吸声体摆放的好位置通常是在侧墙(有限数量)、前置落地音箱的后面和房间后墙的中间部分
Scattering Sound
散射的声音
The basic mechanics of a diffuser are straightforward; an uneven surface that breaks up a reflection into many smaller reflections deflected at various angles. There are many diffusers on the market. Most are simply elaborate surfaces designed to please the designer. Others are very scientifically designed surfaces which provide a very uniform scattering of the sound.The qualification of the scattering characteristics of a surface is called the scattering coefficient (SC). It is defined as one minus the ratio between the specularly reflected acoustic energy and the total reflected acoustic energy.In other words, it shows how much energy is scattered from a reflection due to the uneven surface. The higher the number the more energy is scattered. Unfortunately, this doesn’t help us much. We can assume that the higher the scattering coefficient is, the more it is diffused. The real utility in the SC is with acousticians plugging in numbers to a computer model to predict how a large space will behave acoustically. As with the acoustic absorption coefficient, we don’t really have any computer models that do a good job in a small room. In addition,this tells us little about the uniformity of the scattering. For that we need to look at the diffusion coefficient.
扩散体的基本原理很简单，不均匀的表面会将反射分解成许多较小的反射，以不同的角度发生偏移。市面上有许多扩散材料。大部分表面精心制作的设计是为了取悦设计师。而其它进行了非常科学设计的表面，能提供非常均匀的声音扩散。量化一个表面扩散声音特性的数值称为散射系数(SC)。它的定义是1和镜面反射声能和总反射声能之比之差。另一方面，它展示了不均匀表面的反射体散射了多少能量。不幸的是，这起到的帮助有限。我们只能假设散射系数越高，扩散越多。SC的真正用途其实是让声学家将数字输入计算机模型，以预测大空间将产生的声学问题。但就像吸声系数一样，我们并没有什么计算机模型能在小房间里很好地工作。另外，它关于散射均匀性的信息也很有限。为此我们需要关注扩散系数。
The diffusion coefficient is also a number between 0 and 1. A higher number means the surface scatters the sound more uniformly in many directions. Diffusive materials differ in how they scatter sound. Some scatter sound in a way that favors certain angles and not others. Such a treatment could be considered a poor diffusor since the sound is not scattered uniformly. The concept of a diffusion coefficient was pioneered by Dr Peter D’Antonio of RPG Diffusor Systems Inc,and Dr Trevor Cox of the University of Salford. The key to a quality diffuser is not just how well it scatters sound but also that it does so uniformly in many directions. Many, if not most of the cheap diffusers on the market do not reveal these coefficients.
扩散系数也是从0-1的数字。一个更高的数字意味着表面能向许多方向更均匀地扩散声音。扩散材料的不同之处在于它们如何散射声音。一些材料散射声音更喜欢采用集中特定角度的方式。这样的声学处理会被认为是很糟糕的扩散，因为声音没有均匀地散射。扩散系数的概念是由RPG扩散系统公司的Peter D 'Antonio博士和索尔福德大学的特雷弗·考克斯博士首创的。高质量扩散体的关键不仅在于它能多好地散射声音，还在于它在许多方向上都是均匀的。很多即便不是市场上廉价的扩散体也没有揭示这些系数。
在房间中恰当摆放扩散体可以减少环绕音箱和后环绕音箱的定位感，减少“出口效应”
Placement
放置
So, where do we place our diffusers? I’ll bow to the opinions of various acousticians out there by once again pointing out they seem to appear everywhere in some home theater designs. That’s not a bad thing. After all, we are seeking a more diffusive room. HAA recommends the best location for diffusors is in the rear half of the home theater, particularly between the surround and rear speakers. The audible effect is one of adding spaciousness to the sound.In addition, properly placed diffusorscan reduce the localization of the surround and rear speakers; reduce the “exit door effect”. One location I do not like to used too much diffusion is on the side walls at the LCR reflections (mirror) points. While good diffusers do not reduce sonic energy (at least not much), they can reduce the energy of good reflections that we want. If I use diffusion on the LCR mirror points,I’ll still want plenty of the “good”reflections to be retained; just as I do when adding absorption there.
所以，我们要把扩散体放在哪里呢？我要向各种各样的声学家的意见鞠躬，再次指出它们在一些家庭影院的设计中，似乎出现在每一个地方。这不是一件坏事。毕竟，我们一直在寻找更具扩散性的房间。HAA推荐的扩散体最佳摆放位置是在家庭影院的后半部分，特别是在环绕音箱和后环绕音箱之间。可以明显听到声音空间感的增加。另外，恰当摆放扩散体可以减少环绕音箱和后环绕音箱的定位感，减少“出口效应”。我不喜欢使用太多扩散体的位置是在侧墙的左中右反射(镜像)点。虽然好的扩散体不会减少声音的能量(至少不是太多)，但是它们会减少我们想要的好的反射声的能量。如果我在左中右镜像点的位置使用扩散体，我仍然希望保留大量好的反射声，就像我在那里加入吸声体一样。
编译_杜晓文