[ZT]阻尼因子基础

Damping Factor 阻尼因子

by Ben Blish （作者本·博尼西）
April 28th, 2001

Damping Factor is a term audio enthusiasts (and their rabidcousins, the audiophiles) run into time and time again. However,few people actually understand precisely what the term means, whatthe consequences of a particular damping factor rating might be forthe ultimate desired result (sound reproduction!) or what issuesaffect the application or relevance of damping factor. The ideaisn't rocket science, but it's not something you can toss off in afew seconds, either - and so mostly, it stays a mystery. Let's fixthat! It'll be a little longwinded, because there are several waysyou can look at the issue, but it's interesting if you've got abasic understanding of electric current and voltage, which manypeople do.  

阻尼因子是一个音频爱好者和发烧友反复追捧的术语。然而，只有很少人真正能够理解这个术语的准确含义，了解某一特定的阻尼因子量度如何能对最终声音重放的结果产生影响，或者有那些因素和系统或阻尼因子有关。这个概念没有航天科技那么深奥，但也不是几秒钟内就可以解释清楚的事情。所以在很多情况下，它还是一种魔术。让我们来解决这个问题！这个过程会比较繁琐或罗嗦，因为我们可以从几个途径来看待这个问题，但如果你能理解一些电学基础，比如电流电压，概念的话，其实这也是个很有趣的问题。

The Technical Basis 技术基础

One of the things about amplifiers that affects how they can drivea speaker system is the amplifier's output impedance. The lower itis, the better the amplifier can present needed current to thespeaker system, assuming that in other ways (such as frequencyresponse) the amplifier performs well.
输出阻抗是能够影响放大器如何推动扬声器系统工作的诸多因素之一。假如其他因素对放大器的工作状态没有影响的话，输出阻抗越低，放大器就能够更好地为扬声器系统提供所需的电流。

The better the amplifier can provide varying amounts of currentwithout changing the applied voltage to the speaker system, themore accurately the speaker will reproduce what the amplifier istrying to tell it to do.
如果放大器能在不改变其所施加在扬声器系统的电压的情况下，更好地向扬声器提供不同流量的电流，扬声器就能更准确地按照放大器的指示发出声音。

The reason that a speaker might require more - or less - currentgiven a particular applied voltage is that the impedance of thespeaker system changes depending on what direction the speakerdrivers are moving when the voltage is applied, how fast they aremoving that way, and where they actually are in the first place. Ithelps to visualize that the intent of placing a voltage on aspeaker system is to put the speaker cone(s) in (a) particularposition(s). How difficult that is to do depends on the factorsjust mentioned (and others, some subtle.)
然而实际上，如果需要扬声器准确工作，在给定的电压下，扬声器可能需要（比理论情况下）更大（或更小）的电流。这是因为扬声器系统的阻抗是不断变化的。其数值取决于当电压施加时，扬声器的振动部件在向哪个方向移动、移动的速度以及它的确切起始位置。这个描述能帮助我们理解向扬声器系统施加某一电压的企图等同于让纸盆回归到某一特定的位置。而这样做的困难程度就依赖于我们刚提到的因素们的（大小）（也有其他因素会有影响，但相比之下会小一些。）。

Another way to look at this is that as the speaker moves,reproducing the music, its actual impedance changes. At one moment,it might appear to be 6 ohms, and a moment later, it might appearto be 40 ohms. Despite this variation, if the amplifier appliesfive volts to the speaker, it is saying, in effect, "move thedriver(s) to this specific physical position, right now, nowhereelse and as accurately as possible." That's because the appliedvoltage waveform is, for all intents and purposes, an exact graphof the desired physical motion of the speaker.
从另一个方向来看这个问题那就是当扬声器运动时，扬声器才能重放音乐。而扬声器运动的时候，其实际阻抗就会发生变化。在某一时刻，它可能是6欧，而在另一时刻，就变成40欧。如果没有这种变异不存在的话，当放大器施加5伏电压给扬声器，实际上就等同于放大器命令扬声器“把纸盆移动到指定位置，现在，不能是其他地方，而且尽可能准确。”这是因为施加在扬声器上的电压波形，在意图上以及目的上，就是一个期望扬声器所做物理运动的图形。

But because the speaker's load impedance will vary widely dependingon what it was doing when the five volts was applied, the amount ofcurrent that the speaker will draw with the applied five volts willvary widely as well. The amplifier needs to present the appliedvoltage correctly (five volts in this example) no matter what thespeaker impedance is. If it can do that, the speaker's conemovements will produce the most accurate reproduction of theapplied voltage changes - in other words, the least distortion willoccur because the driver(s) will move to the desired position withthe least amount of error in time and space.
但是由于扬声器的负载阻抗会发生大幅度的变化，而且这种变化依赖于当5伏电压施加的时候，扬声器的运动状态，那么当电压施加是扬声器所吸收的电流也会发生大幅度的变化。而扬声器需要向扬声器施加准确的电压，无论当时扬声器的阻抗实际是多大。如果能做到这样的话，扬声器纸盆的运动就能最准确地反映出所施加的电压的变化。换句话说，就是失真会最小，因为在这种情况下，扬声器的运动部件会在最小的时间和空间误差下，运动到期望的位置。

Summing up, since music is comprised of a constantly changing setof applied voltages, except when silence is being reproduced, thespeaker is always moving in some complex pattern of strokes in andout, and that in turn means that when the amplifier changes theapplied voltage, the amount of current consumed by the speakersystem in complying with that applied voltage will vary over a widerange. So the amplifier must, to the best of its ability, supplythat current and not change the applied voltage as a result. If thevoltage changes in the process, the speaker will not reach theoriginally intended excursion, and distortion is the result.
总而言之，由于音乐就是一段段不断变化的施加到扬声器的电压的组合，停顿的时候除外，扬声器也一直在按照一些复杂的模式往复地运动，那么当放大器施加的电压发生变化的时候，相应于所被施加的某一电压，扬声器所消耗的电流会在一个很大的范围内变动。（意思是即使放大器施加的都是5伏，但喇叭有时候要消耗1A的电流，有时要200mA的电流）。所以放大器必须能，在准确施加电压的前提下，尽可能地提供这些电流，而不能因此就没有施加正确的电压。如果在这一过程中所施加的电压不同于期望电压，那么扬声器就不能运动到原始期望的偏移，那么失真就此产生了。

Deriving Damping Factors 追溯阻尼因子

Technically, the damping factor is the specified load impedance (inuse, that'd be the speaker system) divided by the amplifier'soutput impedance.
技术上，阻尼因子就是指定的负载阻抗（实践中，也就是扬声器系统）除以放大器的输出阻抗。

To explain this, we're going to avoid the idea of impedance (whichhas resistive, capacitive and inductive components, some of whichvary in effect by frequency, thus varying the actual loading) anduse pure resistance to describe the concept. We'll come back toimpedance later.
为了便于解释，我们准备暂时避免使用阻抗这一概念，因为它有电阻性、电容性和电感性成分，而其中一部分会随着频率的变化而变化，从而改变了实际的负载。我们首先用纯电阻性来阐述这一概念，稍迟再回到阻抗上。

For a perfectly resistive 8 ohm load impedance, an amplifier with aperfectly resistive output impedance of 4 ohms (that's pretty high,by the way) presents a damping factor of eight over four, or two: 8/ 4 = 2
对于一个完全电阻性8欧姆负载阻抗，一个有4欧姆完全电阻性输出阻抗（顺便提一下，这是很高的）的放大器意味着其阻尼因子等于八除以4：8/4=2.

Another one: For a perfectly resistive 8 ohm load impedance, anamplifier with an perfectly resistive output impedance of .1 ohmspresents a damping factor of eight over point one, or eighty: 8 /.1 = 80
另一个：对于一个完全电阻性8欧姆负载阻抗，一台具有0.1欧姆完全电阻性输出阻抗的放大器意味着其阻尼因子等于八除以0.1，或8/0.1=80.

From this we can fairly easily see that the damping factor numberitself represents a ratio, one that tells you as it becomes larger,that the source (output) impedance of the amplifier isprogressively lower. Damping factors can be quite high; forinstance, an amplifier with an output impedance of .01 ohms wouldresult in a damping factor of 800 against an 8 ohm load impedance,a very large number as compared to what we usually see poweramplifiers rated. And yes, it is possible to achieve that kind ofoutput impedance in a power amplifier. Not easy or necessarily agood idea, mind you, but possible.
从此，我们可以简单地看到阻尼因子本身也就是一个比率值。如果它逐步变大的话，那么阻抗源（放大器的输出阻抗）也就在逐步变低。阻尼因子可以是一个很高的数值。例如，相对于8欧姆负载阻抗，如果一台放大器的输出阻抗是0.01的话，那么其阻尼因子就是800.和我们经常所见的功率放大器额定值相比，这是一个非常大的数值。当然我要提醒你：我们是可以在功率放大器上做到这样的输出阻抗的。这并不容易，也没有那个必要，但它是可以做到的。

We can also see that because damping factor is a ratio thatrequires the load impedance to compute, it must be stated againstsome known impedance to be meaningful. When someone says anamplifier's damping factor is 87, that's not enough. They need tosay that it is 87 at eight ohms, or 87 at four ohms, or you can'tuse the number as a basis for any meaningful comparison. Eight ohmsis common, but you might run into a rating against four ohms, orsixteen... or ratings for all three. Maybe even one ohm for some ofthese gee-whiz ultra-stable megawatt car audio amplifiers meant tocause your eardrums to meet in the center of your skull.
我们还可以看到因为阻尼因子是一个比率值，而且需要负载阻抗已知才能够进行计算，所以阻尼因子必须和某一已知的阻抗同时具明才有意义。如果有人说某一放大器的阻尼因子是87，那是不够的。它们必须说明它在8欧姆时是87，或者说在4欧姆时是87，否则你不能利用这一数值做出任何有意义的比较。8欧姆确实是最常见的数值，但你可能会碰到一个额定值是4欧姆的系统，或者16欧姆，或者同时具备三种阻抗的系统。也许你会看到一个只有一欧姆阻抗的系统，例如那些刺耳、无比稳定、而且标称上兆瓦功率的车载放大器，那么这个时候，你只能耳膜打鼓了。

That's ok, though. You can easily move everything to a comparabledamping factor number. If an amp was rated as having a 100 dampingfactor against four ohms, then it is 200 against 8 ohms. Why?because the only way you can get a damping factor of 100 at fourohms is with these numbers... 4 / .04 = 100
(we got that .04 by computing damping factor / speaker impedancerating) ...so we know the amp is rated for .04 ohms outputimpedance. Now, we just do this... 8 / .04 = 200 ...and there youhave it. Having worked through the math, you can simply observethat for the common speaker impedance (4, 8 16 ohms) you can double(as rated load impedance goes up) or halve (as rated load impedancegoes down) the damping factor and you'll get the right number aslong as you don't get confused and go the wrong way. When in doubt,do the math.
简而言之，你能简单得把所有的东西都用一个可以通用可以互相比较的阻尼因子来衡量。如果一台放大器额定在4欧姆时阻尼因子100,那么其在8欧姆时阻尼因子就是200.原因就是，计算出100阻尼的唯一公式就是：4/0.04=100
（0.04是从阻尼因子/扬声器额定阻抗计算得出）。这样我们知道该放大器额定输出阻抗为0.04欧姆。现在我们只需要计算8/0.04=200,然后我们就得到了这个答案。通过公式计算，你能简单地观察到针对常用的扬声器额定阻抗，你能加倍或减半阻尼因子。只要你没有犯糊涂，或者用错了方法。如果你还有点担心的话，就做做数学题吧。

So, here we are, thinking we've got a good handle on this. Well...not exactly. It turns out that electrically speaking, there is nodistinguishing between the amplifier's output resistance and theseries resistance (not impedance, just the resistance) of thespeaker system. So an actual amplifier output impedance in usemight be a low number (like .1 or .01) added to perhaps 6 ohms (asan average guess at the DC resistance of a particular 8-ohmspeaker.) And that affects the control ability of the amplifier.Think about it: If the speaker has a basic resistance of 6 ohms,and the amplifier can apply 10 volts to the speaker, then ohm's lawtells us that the most current the amp will ever be called on todeliver, period, end of story, is 10/6=1.666... amperes; if that'snot enough to put the speaker drivers into the right position,they're not going to get there. No matter what the output impedanceof the amplifier is!
好吧，此时此刻，你是不是觉得我们已经可以很好地运用这个术语了？其实还没有。因为这样的话就变成，用电子学术语来描述，放大器的输出阻抗和扬声器系统的串联电阻（不是阻抗，仅仅是电阻（个人觉得用电阻性内阻更确切））没有任何区别。这样一台真实放大器的实际运作中的输出阻抗可能只是个很小的数值（比如0.1或0.01欧），而这个数字添加到大概6欧姆上（6欧姆是针对额定8欧姆的扬声器的直流电阻的平均估计值）。这样会影响放大器的控制力。想想看：如果扬声器的基础电阻是6欧姆，而且放大器施加10V电压到扬声器上，那么欧姆定律告诉我们在整个过程中放大器能为扬声器提供的电流只是10/6=1.666 安。如果这个电流不足以推动扬声器的运动部件到正确的位置，它们就永远不会到那。这和放大器的输出阻抗究竟是多少无关。

But doesn't this dilute the meaning, and so the actual usefulness,of the damping factor rating? Yes. It does. But it doesn't removeit. If the amplifier has a very low damping factor, that says ithas a high output impedance, and that will reduce the amount ofcurrent it can deliver even further, and therefore also reduce itsability to control the speaker.
那么这是不是意味着阻尼因子的额定值就没有任何实际用途呢？是的。但这也不能表明我们可以完全不需要这一指标。如果放大器有一个非常低的阻尼因子，那么意味着它具有很高的输出阻抗。这样该放大器可以输出的电流强度会比较有限，因此它控制扬声器系统的能力也就非常有限。

There is another implication lurking here; speakers with lowerresistances will support higher current drive. That means that anamplifier can control them better. However, most amplifiersgenerate more distortion when they produce higher current, so youdon't always win this battle!
这还意味着，低电阻的扬声器能支持更高的电流。这就意味着放大器可以更好地控制它们。遗憾的是，大部分放大器在产出更大电流的时候会同时产生更大的失真，所以这种方法也不是百试百灵的。

And then there's the real world...实际运用中的情况

As most of us are aware, speakers are not pure resistive loads.They are anything but, in fact. Sometimes they present a capacitiveload, sometimes they present an inductive load, sometimes simplyresistive, sometimes a combination of these. Also, the actualresistance of a particular speaker system is almost never the sameas the rated impedance. That rating is a general description of thespeaker system impedance, and the reason that is so is simplybecause you just can't make a typical voice-coil based speakersystem act like a resistor. After all, speakers are based on theidea of an inductor, a reactive coil of wire wrapped around amagnet - so this isn't anything surprising. Plus, everything elseaffects the impedance: the speaker wire, the connections,contributions from crossover circuits inside the speaker,corrosion, fuses and circuit breakers, loading by changing a portedcabinet when you plug or unplug a port insert, condition of thespeaker assembly (like the foam surrounds of a woofer, or thecondition of the ferromagnetic fluid of some tweeters), the cabinetseals, etc. Or in other words, just about everything you can thinkof or see when you look at or into a speaker.
大部分人都知道，扬声器不是纯粹的电阻性负载。它们什么都有可能是，事实上，有时候它们表现出电容性负载，有时它们表现出电感性负载，有时候它们表现出电阻性负载，有时候它们根本就是三种的混合体。而且，事实上，扬声器系统的实际电阻从来没有等同于它的额定阻抗。额定值只是对扬声器系统阻抗的一个一般性描述，原因在于不可能制造一个类似于电阻那样工作的动圈扬声器系统。毕竟，所有的扬声器都是基于电感的理念，是一个藏绕在磁体上的电抗线圈。这样理解这种现象就不会有任何令人惊奇的地方了。另外，还有其他东西会影响到扬声器的阻抗：喇叭线，接线柱，内部分频线路，部件的腐蚀情况，保险丝或断路器，箱线头子和接线柱之间产生的负载，扬声器组装时的生产环境，箱体的密封情况，等等。换句话说，就是任何你可能看到或想到的与扬声器有关的东西。

There are serious consequences. As the frequency(s) of thesignal(s) applied to the speaker change, and/or the speaker'sphysical and/or electrical condition changes, the speaker's loadimpedance changes too. Sometimes a lot - for instance, you mightsee an impedance result of anywhere from one to as much as 50 ohmsfrom a particular speaker design within it's normal operatingfrequency range. Amazing, eh? Sure it is... But absolutelytrue.
这会带来严重的后果。由于施加到扬声器的信号的频率在变化，以及（或者）扬声器的物理及（或）电学条件在变化，扬声器的负载阻抗也在变化。有时这种变化的幅度会非常大，例如，你可能会观察到一个扬声器在普通的工作频率范围内其阻抗可能是从1欧姆到50欧姆之间的任何一个数值。好玩吧？确实好玩，但这是事实。

"But if damping factor is a number derived from the speakerimpedance, and speaker impedance dances all over the chart, how candamping factor just be a single number?" ...I hear you cry.
“但是如果阻尼因子是从扬声器的阻抗计算出来的数值，而且扬声器阻抗会在整个图表上跳舞的话，怎么有可能阻尼因子只是一个单一的数字？”。。。我听到你的反驳。

That's right. It can't. It's a curve, but its a nasty, wiggly linethat looks more like the trail of a worm on LSD than it does likeany curve worthy of the name. We typically call things like that"scribbles", not curves. That line will vary depending on theparticular speaker you use, the music you play, the volume you playit at, the quality and condition of your connections to thespeakers, everything mentioned above and a lot that isn't - even,believe it or not, factors like wind blowing on the cone of awoofer or the ambient humidity and temperature. So in that way,damping factor is indeed a major simplification of what is actuallygoing on and your instincts are right on.
这是正确的。因子不能是一个单一数值。它是一条曲线，但它是一条没有固定模式的，波动的线，就好像吃了迷幻药的虫子的尾巴一样的曲线，而不是任何一条已经知名的曲线。我们通常叫它“画符”，而不是曲线。这条线还会不断变化，它会收到的影响包括你所使用的扬声器，你正在播放的音乐，你所播放的音量，你的扬声器连接线的质量和条件，几乎是所有的上面提到的事情，还有无数上面没有提到的事情，甚至，信不信由你，包括像吹过低音单元的风，或环境湿度与温度这些因素。这样，阻尼因子事实上是一个主要的简化指标，指示什么实际正在发生的和你的本能所感知到的所有事情。

However, we can still draw some general useful conclusions; becausethe damping factor ultimately is telling us what the outputimpedance of the amplifier is, it is also telling us an importantand easily understood thing: When amplifiers are rated into thesame impedance (both into eight ohms, for instance) or when youconvert the numbers to the same impedance as I've shown you how todo above, you are now looking at a direct comparison of the outputimpedance of those amplifiers.
无论如何，我们还是可以得到一些普遍性的结论。因为阻尼因子最终是要告诉我们放大器的输出阻抗是什么样的，它也告诉了我们一件重要而且容易理解的事情：如果一些放大器的额定阻抗相同的话（比如都是8欧姆），或者说你按照前述的计算得出它们的阻抗是一致的话，你现在就可以看到你可以对这些放大器的输出阻抗做一个直接的比较。

We mentioned that speakers are not purely resistive. That means, inelectronic terms, that they are often (almost always, in fact)reactive. And that word in this context can be taken to mean aboutwhat it sounds like; the amplifier attempts to apply a signal tothe speaker, and bingo, the speaker reacts, sucking more currentthan it needed a moment ago, or actually trying to feed currentback into the amplifier. Or, it might resist a change in appliedcurrent, and as a direct consequence, not do what it is being toldto at the moment. Yes, we're talking about behaviors that introducenothing other than distortion, that evil and annoying nemesis ofall of us. Ideally, the speaker should do what it is told, andnothing else. But they never do.
我们已经提到扬声器不是纯粹的电阻性的。用电学的术语来描述就是它们经常是（几乎就是）电抗性的。电抗性这个词在现在的上下文环境中可以意味着：放大器意图施加一个信号给扬声器，而且，“好”，扬声器回应了，同时吸收的电流瞬时提升，或者实际上要馈送一些电流回放大器。有时，它也许会对抗所被施加的电流，不愿改变，直接后果就是它没有即时去做放大器告诉它要做的事情。这些行为没有带来任何好处，唯有失真。这些是针对我们的邪恶的恼人的报应。理想中，扬声器应该做所有告诉它要去做的事，而不是其他事情。但实际上它们从来没有这样做过。

It helps when an amplifier has a high damping factor (in otherwords, a low output impedance) because the degree to which it isaffected, itself, by these annoying speaker behaviors is reducedalong with its output impedance. If the speaker needs more current(its overall impedance drops) then a lower amplifier outputimpedance will allow that current to be drawn out of the amplifierwithout causing much of a change in the applied voltage - and thatmeans less distortion.
这个时候如果放大器的阻尼因子足够高的话它就可以帮助我们了（换句话说，低输出阻抗），因为阻尼因子的量度直接影响到这个问题，放大器的输出阻抗越小，相应的这些恼人的放大器的行为也会减少。如果扬声器需要更大电流（扬声器的整体阻抗降低），那么一个低输出阻抗的放大器可以容许在输出相应电流的同时，所施加的电压变化更小，这也意味着失真更小。

And so...因此...

Like most specifications, damping factor does not tell you anamplifier is "good". It can't; no more than a tuner with a greatimage rejection specification is guaranteed to be a "good tuner".There is a lot more to good amplifier performance than outputimpedance. Frequency response, mentioned previously, is a goodexample; suppose the amplifier had a damping factor of 10,000 at 8ohms, a stunning number (because it implies a very, very low outputimpedance)... but could only reproduce frequencies between 300 Hzand 1 KHz? Your music would sound pretty awful.
像大部分指标那样，阻尼因子并没有告诉你哪个放大器是“好”放大器。它告诉不了你。就好像一台调谐器，哪怕它的镜像抑制指标无比好也不能保证这是个好的调谐器。除了输出阻抗外，还有很多因素才能构成一部好的放大器。之前有提过的频响，就是一个例子。假如一台放大器的阻尼因子是8欧姆时10000，这是个令人窒息的数字（因为它意味着非常非常低的输出阻抗）。。。但如果频响只能300-1kHZ? 你的音乐会听起来非常可怕。


Another is distortion: Suppose the amplifier distorts the music?What good is it then to have the speaker do more (even justslightly more) precisely what the amplifier says? Not much! Sodon't let damping factor go to your head, so to speak - it's one ofmany specs, one where higher numbers are better because they tellyou that the amplifier has a lower output impedance, which is good.Read all the specs, try to learn what they all mean, and thenyou'll be able to get a good picture of amplifier performance. Thebottom line is, a really low damping factor can tell you an ampisn't going to be all that great with a highly reactive speaker(the larger a speaker driver is physically, and the more power itis designed to handle, the more likely it is to be highlyreactive.)
另一个例子是失真：加入放大器重放音乐有失真？还希望扬声器系统更准确的表达放大器传递的信号有什么作用呢？答案当然是不。如是说，不要让你自己被阻尼因子所左右。它只是许多指标中的一个，这个指标越大越好是因为它告诉你放大器有很低的输出阻抗，而这件事是好事情。你需要阅读放大器的全部指标，并试着去了解它们的意义，这样你才会对放大器的表现能力有一个整体的认识。