【影音中國(guó) hdavchina.com】
Gerry Lemay HAA
家庭聲學(xué)聯(lián)盟創(chuàng)始人與主席
Absorption Coefficient (AC)
吸聲系數(shù)
In the early days, most used1” (25mm) thick panels. It was cheap and plentiful. Soon it was pointed out that using 2” (50mm) produced better results and then 3” (75mm) and even thicker layers were used. The science behind this is based on understanding the acoustical absorption coefficient (AC). It is defined as the ratio of energy absorbed by a material to the energy incident upon its surface.So, a coefficient of 1.0 means all the sound hitting the panel front surface is absorbed. One inch treatments did a good job of absorbing high frequencies but very little at lower frequencies. That’s because the AC became lower and lower as the frequency became lower. These 1” panels absorbed the lively high frequencies but virtually passed unaffected the lower ones. This means that a 1” panel placed at a mirror point was doing half the job (technically less than half the job).Diagram 2 shows a comparison of the various thicknesses of Owens Corning 703 fiberglass panels. Noticehow 1” panels are operating at well below the desired AC of 1.0 at the all-important range below 1000 Hz.
在早期，大多數(shù)人使用1英寸(25毫米)的厚面板。它既便宜又?jǐn)?shù)量眾多。但很快有人指出，使用2英寸 (50mm)的面板能產(chǎn)生更好的效果，接著是使用3英寸(75mm)，甚至更厚。這背后的科學(xué)是基于對(duì)吸聲系數(shù)(AC)的理解。它是材料吸收的能量和發(fā)射到表面的能量之間的比值。因此，系數(shù)為1意味著所有撞擊在面板表面的聲音都被吸收了。1英寸的面板在吸收高頻方面做得很好，但對(duì)較低頻率的聲音影響甚微。因?yàn)楫?dāng)頻率越低時(shí)，吸聲系數(shù)會(huì)越低。這些1英寸的面板吸收了高頻的聲音，但實(shí)際上沒(méi)有處理較低頻率的聲音。這意味著一個(gè)放置在鏡像點(diǎn)上的1英寸面板只完成了一半的工作(技術(shù)上來(lái)說(shuō)還不到一半)。右顯示了Owens Corning 703纖維板各種厚度的比較。請(qǐng)注意，在1000Hz以下的重要頻率范圍內(nèi)，1英寸面板的吸聲系數(shù)遠(yuǎn)遠(yuǎn)低于所需的吸聲系數(shù)1。
It’s interesting to note that the chart seems to show the thicker panels absorbing more than 100% of the sound. This because the measurement procedure only counts the front surface area in the calculation of the AC. The sides of the panel are not considered in the calculation of the surface area yet they do absorb sound. It’s also important to point out that the measurement of AC includes the amount of random incidence sound absorbed by the panel. This is good for calculating how many panels are required for a concert hall or church but it tells us much less about how the panel handles sound coming from a specific direction; a specular reflection.
有趣的是，根據(jù)圖表顯示，較厚的面板吸收了超過(guò)100%的聲音。這是因?yàn)槲曄禂?shù)的計(jì)算只考慮了面板前表面的測(cè)量，面板的側(cè)面沒(méi)有考慮在內(nèi)，但它們確實(shí)吸收了聲音。另外需要指出的是，吸聲系數(shù)測(cè)量也包括面板吸收的隨機(jī)入射聲量。這對(duì)于計(jì)算音樂(lè)廳或教堂需要多少個(gè)面板是很有用的，但關(guān)于面板如何處理來(lái)自特定方向聲音的信息就少得多了。
各種厚度的Owens Corning 703纖維板吸聲系數(shù)比較
吸收第一次側(cè)面的反射聲可以通過(guò)增加相直達(dá)聲的總量，來(lái)提高聚焦感
New Data 新數(shù)據(jù)
Dr. Floyd Toole in his book“ S o u n d R e p r o d u c t i o n | Loudspeakers and Rooms” (Chapter 21.3.2) discusses the acoustical implications of typical fiberglass panels. He provides the information in part to support his caveat that a good speaker’s first lateral reflections should not necessarily be absorbed; a topic we’ll discuss. The data reveals that these absorbers do not uniformly absorb all frequencies despite the apparent uniformity typical AC plots show. He also points out that regarding specular first reflections from the speaker to the listener, the absorption of typical fibrous panels depends on the angle the sound wave is incident to the panel. Some noteworthy conclusions are:
Floyd Toole博士在他的書(shū)《聲音重播｜揚(yáng)聲器和房間》 （第21.3.2章）中討論了典型的纖維玻璃面板的聲學(xué)影響。他提供了信息來(lái)支持他的理論，即一個(gè)好音箱的第一次側(cè)面反射不一定要被吸收；這是我們將要討論的話題。數(shù)據(jù)表明，盡管這些典型的吸聲體有著明顯一致的吸聲系數(shù)，它們也不能均勻地吸收所有頻率。他也指出，對(duì)于音箱到聆聽(tīng)者的鏡面第一次反射，典型纖維面板的吸聲取決于聲波進(jìn)入面板的角度。一些值得注意的結(jié)論如下：
AC graphs are insufficient because they are based on random incidence measurements and not specular ones. They have different absorption levels for various frequencies when the incident sound is at an angle versus at random angles. They also lack the resolution to show the true impact on frequency response by the reflection. Since the reflected sound has a clear impact on the measured listening position frequency response, this is important to know. Finally, AC measurements do not include data on higher frequencies (up to 20K Hz) which show a lower absorption due to reflection of specular reflections at these higher frequencies.
吸聲系數(shù)的圖表是有不足的，因?yàn)樗鼈兪腔陔S機(jī)入射聲的測(cè)量，而不包括鏡面反射的部分。當(dāng)入射聲是某一個(gè)角度而不是隨機(jī)角度時(shí)，它們對(duì)不同頻率有不同的缺乏具體的數(shù)字來(lái)顯示反射對(duì)頻率響應(yīng)的真實(shí)影響。因?yàn)榉瓷渎晫?duì)測(cè)量的聆聽(tīng)位置頻率響應(yīng)有明顯的影響，所以了解這一點(diǎn)很重要。最后，吸聲系數(shù)的測(cè)量不包括高于20kHz的頻率，它們因?yàn)殓R面反射而表現(xiàn)出更低的吸聲系數(shù)。
Panels of at least 3” (75mm) or thicker are required to adequately absorb frequencies above thetransition frequency. The transition frequency being the point above which specular reflections dominate our design and calibration strategies;usually between 300 Hz and 500 Hz in the typical small room.
至少3英寸(75mm)或更厚的面板需要吸收轉(zhuǎn)換頻率以上的頻率。轉(zhuǎn)換頻率是鏡面反射主導(dǎo)我們?cè)O(shè)計(jì)和校準(zhǔn)策略的點(diǎn)，在典型的小房間中，通常指300-500Hz之間的頻率。
The fabric used to cover the treatment has little effect on lower frequencies but can reduce the absorption at higher frequencies by making the panel reflective.This points out one of the valuesof using a fabric which has similar properties to speaker grill cloth which is designed to be acoustically transparent (at least it tries to be transparent). It also points to the popularity of the ubiquitous Guilford of Maine FR701 fabric, which, while still reflective at higher frequencies provides a more “grill cloth-like”texture.
聲學(xué)材料表面的纖維布對(duì)低頻幾乎沒(méi)有影響，但會(huì)因?yàn)槁暦瓷涞脑驕p少聲學(xué)材料對(duì)高頻的吸收。這就指出了應(yīng)用在音箱表面的纖維布被設(shè)計(jì)成透聲(至少它試著做具有聲音穿透的能力)的價(jià)值之一。它也指出Guilford of Maine FR701纖維布普遍受歡迎的程度，盡管在更高頻率下仍能反射，但它提供了一種更“具聲音穿透性”的材質(zhì)。
Basic Observations基本的觀察結(jié)果
So, what is a DIY Enthusiast to do? Glue 4” (100mm) panels on the wall? Use no panels? Everyone knows that the sound is better with panels… right?
那么，DIY愛(ài)好者要做什么呢？把4英寸(100毫米)的面板粘在墻上？不使用面板？每個(gè)人都知道用面板的聲音更好，對(duì)吧？
使用厚的吸聲體可以減少頻響的失真，但對(duì)側(cè)墻反射聲的重點(diǎn)保留，可改善聲場(chǎng)的寬度，前后場(chǎng)間的聲音轉(zhuǎn)向及空間感
Let’s look at some basicobservations:
讓我們看一些基本的觀察結(jié)果：
Absorbing the first lateral reflections does indeed improve focus (stereo imaging) and detail by increasing the amount of direct sound with relation to the reflected or indirect sound. If that’s your preference use a thick absorber to reduce frequency response distortion.
Absorbing the first reflections also reduces the apparent soundstage width and spaciousness of the sound field. If you prefer a wider, and more three-dimensional sound stage, don’t absorb first lateral reflections. This where Dr Toole’s assertion that these reflections should not necessarily be absorbed originates. A good speaker (one with excellent off-axis response) will create reflections that enhance envelopment and spaciousness.Since the reflections are less frequency response distorted (due to absence of panels) the listening position frequency response will be more like the speakers engineered response.
吸收第一次側(cè)面的反射聲確實(shí)可以通過(guò)增加相對(duì)于反射或間接聲的直達(dá)聲的總量，來(lái)提高聚焦感(立體聲像)和細(xì)節(jié)。如果你偏愛(ài)使用厚的吸聲體來(lái)減少頻響的失真，那么需要注意對(duì)第一次反射聲的吸收也會(huì)明顯減少聲場(chǎng)寬度和空間感。如果你更喜歡更寬的三維聲場(chǎng)，不要吸收第一次側(cè)面反射聲。Floyd Toole博士主張這些反射不一定要被吸收的緣由如下。一個(gè)好的音箱(有著出色的離軸響應(yīng))所帶來(lái)的反射聲可增強(qiáng)包圍感和空間感。因?yàn)榉瓷渎暤念l率響應(yīng)失真更小(由于沒(méi)有吸聲板)，所以聆聽(tīng)位置的頻率響應(yīng)將更像音箱設(shè)計(jì)時(shí)的頻率響應(yīng)。
If you are a “near-field” listener (sitting very close to the speakers),panels will have less effect because the primary sonic source you are hearing is the direct sound of the speaker which minimizes the reflections effect on the sound at the listening position. This scenario, is not uncommon for many hard-core two-channel audiophiles.
如果你是一個(gè)近場(chǎng)的聆聽(tīng)者(坐在非常靠近音箱的位置)。面板的影響更小，因?yàn)槟阏诼?tīng)的主要音源是音箱的直達(dá)聲，它最小化了聆聽(tīng)位置聲音
的反射影響。這種方案在老派的兩聲道發(fā)燒友中并不少見(jiàn)。
If you have a theater and more than one seat (especially if you have more than one row), it’s not practical to hog the one good seat (the sweet spot for near-field listening). You’ll need to have the seats deeper in the room to provide a satisfactory experience to all. Reflections begin to take on a much larger role. See my article on the sweet triangle in this blog. Over-absorbing reflections can also reduce the width of the sweet triangle. In the case of a surround sound system, over absorbing can create a scenario where speakers do not create a cohesive surround field.
如果你有一個(gè)影院，而且座位不止一個(gè)(特別是如果你有多于一排的座位)。只選擇一個(gè)好位置(近場(chǎng)聆聽(tīng)的甜區(qū))是不實(shí)際的。你需要在房間靠后的位置擁有多個(gè)座椅位置，來(lái)提供一個(gè)讓所有人滿意的體驗(yàn)。反射聲開(kāi)始扮演更重要的角色。在HAA網(wǎng)站中可以看到我關(guān)于甜區(qū)三角形的文章。過(guò)分吸收反射聲也會(huì)減少甜區(qū)三角形的寬度。在一個(gè)環(huán)繞聲系統(tǒng)的案例中，過(guò)分吸聲會(huì)導(dǎo)致音箱不能產(chǎn)生一個(gè)有凝聚力的聲場(chǎng)。
Unfortunately, you see the use of absorbing panels is not a cut-and-dried situation. The proper use depends on the speaker you own,your listening habits, and, of course,your room.
不幸的是，你看到吸聲板的使用并不是一成不變的。正確的使用取決于你的音箱，你的聆聽(tīng)習(xí)慣，當(dāng)然還有你的房間。
過(guò)分吸收反射聲會(huì)減少甜區(qū)三角形的寬度
混響聲會(huì)對(duì)直達(dá)聲產(chǎn)生影響
The Room
房間
One topic which seems to pop up in this kind of discussion is that of the RT60 of the room. The reverberation time, at a particular frequency, is defined as the timen taken for sound to decay by 60 dB;this is often abbreviated to RT60.The significance of RT60 is that the reverberation time is proportional to the reverberant sound level in the room. Basically, its how we compare the reverberant versus the direct sound levels. It was impractical to measure the reverberant sound level directly in the early days (still is) so a series of empirical values of RT were collected over time representing the desired RT60 for different size spaces and different uses. Larger performance spaces have long RT60’s, school class room auditoriums tend have lower targets.In an HAA calibration we primarily use our RT measurement to tell if we are over-treating the room. An especially small room can becomeover treated with just a handful of absorptive treatments. The truth is that for most home listening rooms,calculating RT60 is not a useful tool. We use RT measurements as a warning sign to stop putting absorptive panels on the wall (or to remove some in a finished install).This is true for a variety of reasons:
在這種討論中出現(xiàn)的一個(gè)話題是房間的RT60。一個(gè)特定頻率的混響時(shí)間被定義為聲音衰減60dB所需的時(shí)間，通?？s寫(xiě)為RT60。因?yàn)榛祉憰r(shí)間與房間內(nèi)的混響聲壓成正比。RT60的意義在于，我們基本上會(huì)用它來(lái)比較混響聲和直達(dá)聲的聲壓。早期(包括現(xiàn)在)直接測(cè)量混響聲聲壓是不切實(shí)際的。
所以，隨著時(shí)間的推移，我們收集了一系列RT的實(shí)驗(yàn)值，這些值代表了不同大小空間和不同用途所需的RT60。較大的視聽(tīng)空間有較長(zhǎng)的RT60，學(xué)校教室、禮堂的目標(biāo)值往往較低。在HAA的調(diào)校中，我們主要使用RT測(cè)量來(lái)判斷是否過(guò)度處理了房間。而一個(gè)特別小的房間可能只做了一小部分吸聲處理就會(huì)變成過(guò)度處理。真相是對(duì)于大部分房間來(lái)說(shuō)，計(jì)算RT60并不是一個(gè)有用的工具。我們只是使用RT測(cè)量作為警告信號(hào)，來(lái)決定是否不在墻上安裝吸聲板(或在完成安裝后移除一些)。這么做的原因有很多：
The fundamental basis for acoustical calculation in large room acoustics depends on a “free space”model. This why random incidence measurements of AC for treatments are so useful for commercial acoustics. The reverberant sound field in a large space is largely uniform and diffusive; randomized.Most home theaters are anything but a free space (no pun intended). Early reflections are considerably louder in a small room because of the short distance they travel to our ears. The sound field is not uniform,highlighting our interest in specular reflections over calculating the aggregate effect of all reflections asin a concert all or auditorium. The fundamental requirement for an accurate RT60 is a largely uniform reverberant sound field. That’s why in the HAA we use the term RT (reverberation time) to let everyone know we know the RT60 is not a true RT60 even though we use the same procedure to measure it.
大房間聲學(xué)計(jì)算的基本依據(jù)是“自由聲場(chǎng)”模型。這是吸聲系數(shù)的隨機(jī)入射聲測(cè)量在商業(yè)聲學(xué)處理中如此有用的原因。大空間內(nèi)的混響聲場(chǎng)基本上是均勻、隨機(jī)分散的。但大多數(shù)家庭影院絕不是一個(gè)自由的空間(沒(méi)有雙關(guān)的意思)。在小房間里，早期的反射聲音要大得多，因?yàn)樗鼈儌鞯轿覀兌淅锏木嚯x很短。聲場(chǎng)不是均勻的，增強(qiáng)了我們對(duì)鏡面反射的興趣，而不是像在音樂(lè)廳或禮堂那樣計(jì)算所有反射的綜合效應(yīng)。得到精確的RT60的基本要求是一個(gè)均勻的混響聲場(chǎng)。這就是為什么在HAA中，我們使用RT(混響時(shí)間)這個(gè)術(shù)語(yǔ)來(lái)讓所有人明白，其實(shí)我們知道RT60不是一個(gè)真正的RT60，盡管我們使用了相同的程序來(lái)測(cè)量它。
Our rooms are small. The recommended RT times for small rooms, as given by a variety of organizations, is between 200 ms and 500 ms (500 milliseconds = ? second). Most residential rooms already fit that specification. There is little need to reduce the RT unless the room is exceptionally large or lacks the typical absorptive elements found in a home (furniture, drapes, carpeting, moose heads, etc.).
我們的房間很小。各種各樣的組織都給出了小房間的推薦混響時(shí)間，介于200ms到500ms(500ms=?s)之間。大部分住宅的房間都適應(yīng)這個(gè)標(biāo)準(zhǔn)。除非房間特別大，或者家中缺少常見(jiàn)的典型吸收元素(家具、窗簾、地毯、麋鹿頭等)，否則不需要減少混響時(shí)間。
Large room acousticians use RT60 as a calculation tool in the design phase of a project. Using formulas like Sabines, Fitroy’s, Eyring, and others, they can accurately calculation how much surface area needs to be treated to hit a target RT60. These formulas break down as the space becomes smaller. They are not a useful tool to use in a small room. In the HAA HT3 workshop we demonstrate how placing treatments in various places has a greater effect on the RT than changing the number of panels in the room. Using a formula to calculate how many panels you need usually over-treats a room and it doesn’t tellyou where to put the panels.
大房間的聲學(xué)工作人員在項(xiàng)目設(shè)計(jì)階段使用RT60作為計(jì)算工具。使用的公式像Sabines、Fitroy’s、Eyring等等。它們可以精確計(jì)算要達(dá)到目標(biāo)值RT60，需要對(duì)多大的表面區(qū)域進(jìn)行處理。但這些公式會(huì)隨著房間變小而失去作用，它們?cè)谛》块g中不是有用的工具。在HAA HT3課堂上，我們展示了在不同的地方放置聲學(xué)材料對(duì)RT的影響比改變房間的面板數(shù)量更大。使用公式來(lái)計(jì)算你需要多少面板通常會(huì)過(guò)度處理一個(gè)房間，它也不會(huì)告訴你把面板放在哪里。
相對(duì)面板，大部分高端影院與多媒體室采用掩蓋板來(lái)實(shí)現(xiàn)更加精準(zhǔn)的多反射處理
對(duì)稱的擺位會(huì)加強(qiáng)SBIR 在相同頻率的強(qiáng)度
房間的前半部分側(cè)面使用“分散”擺放的吸音材料保留供反射聲發(fā)揮的空間
Other Factors
其它因素
Ok, so it looks like absorptive acoustical treatment is of limited value in a small room… but it sounds better when we use it!!! The reality is that treating a small room is necessary in a high performance installation but it doesn’t depend on absorbing all or even most reflections.It depends on understanding the complex structure of the sound field and the nature of each reflection point. We use absorption (along with other treatments) to craft a seamless and spacious sound stage while preserving high quality focus (imaging) and detail. You can read my post about focus and envelopment here. Other factors include the utility of using the right kind of absorber to control low frequency effects like SBIR (speaker boundary interference response), compensating for poor speaker off axis response, crafting the sound stage by using a distributed pattern of absorption, increasing the diffusiveness of the sound field, and others.
所以在一個(gè)小房間中，吸收性的聲學(xué)處理價(jià)值是有限的……但當(dāng)我們使用它了，確實(shí)聲音聽(tīng)起來(lái)更好！實(shí)際情況是，在高性能的安裝中，對(duì)小房間的聲學(xué)處理是必要的，但它不依賴于吸收全部或大部分反射聲。它取決于理解聲場(chǎng)的復(fù)雜結(jié)構(gòu)和每個(gè)反射點(diǎn)的性質(zhì)。我們使用吸聲(以及其他聲學(xué)處理類型)來(lái)創(chuàng)造一個(gè)無(wú)縫和寬敞的聲場(chǎng)，同時(shí)保持高質(zhì)量的聚焦(成像)和細(xì)節(jié)。你可以在HAA網(wǎng)站閱讀我關(guān)于聚焦和包圍感的文章。其它因素包括使用合適的吸聲體來(lái)控制低頻效應(yīng)，像SBIR(音箱邊界干擾效應(yīng))、補(bǔ)償不好的揚(yáng)聲器偏離軸響應(yīng)、使用分散式的吸聲模式來(lái)創(chuàng)造聲場(chǎng)、增加聲場(chǎng)的擴(kuò)散以及其它因素。
Perhaps the most tangible thing this discussion points out is that treating reflections with absorption depends on the reflection; its source and its direction. We need a strategy based upon specular reflections referencing the personal preference of the client. Panels in the correct position are good, but panels in the wrong place are bad. Above all, over treating a room with absorption is a definite no-no. A good treatment strategy is possible in a small room but I’m afraid this post is already too long and I need to promise I’ll continue this series in my next blog article. Let me know your questions in the forum and I’ll tailor the articles to your needs.
也許文章指出的最實(shí)在的事情是用吸聲處理反射依賴于反射本身，比如它的來(lái)源和位置。我們需要一個(gè)基于參考客戶個(gè)人喜好的鏡面反射策略。放在正確位置的面板是好的，在不正確位置是不好的。總而言之，用吸聲過(guò)度處理一間房間是明確的禁忌。小房間也可以有好的聲學(xué)處理策略，但我擔(dān)心這篇文章已經(jīng)太長(zhǎng)了，我需要保證在我將在下一篇文章中繼續(xù)這個(gè)系列。讓我知道你的問(wèn)題，我將會(huì)根據(jù)你的需要來(lái)撰寫(xiě)文章。
編譯_杜曉文
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