主题：发烧天书写的音箱测量，看看跟我们的有什么不同

电阻抗

（翻译的是李怡南先生，不是我）

如果推动音箱所用的功放具有恒压源特性，则音箱的阻抗代表它将从功放吸取多少电流。由于阻抗不是纯电阻性的，所以其相位角不为0度，而且阻抗值将随频率而变化。  

测量阻抗的方法有两种，第一种方法是用音箱和-个标谁电阻组成分压器，然后测量其脉冲响应，并将所获得的时域数据用快速傅里叶变换转换成频域数据。第二种方法足用正弦扫频信号或一系列离散的点频正弦信号驱动待测的音箱，保持电流为恒定值，分别测量电压幅度和相位。在实际测量中往往不使用恒流源，而足在一个恒压源上串联高阻值电阻来代替它。征测量阻抗值较高的音箱时，这将导致-定的误差。

由于第一种方法是用宽频带噪声信号驱动待测的音箱，而采用第二种方法时，同一时刻只一个频率的信号作用于音箱。一些工程师猜测这两种测试方法得出的结果将会有所不同。但我用两种方法进行测量的结果表明：两者之间的差别极小。在实际工作中，我选用了Audio   Precision  System  One来驱动音箱，信号的频率范围从10Hz至50Hz，分成240个频点。输出电压为6V，通过一个串联的600欧姆电阻接到音箱上。完整的阻抗曲线图应该足三维的，三轴座标分别为阻抗、相位和频率。但为了看得更清楚，实际应用时是把三维图形投影在平面上，绘制成两个两维图形，一张是阻抗--频率的关系曲线，另一张是相位--频率关系曲线。把两张两维图形迭合在一起，就形成了图2所示的两条曲线。

此主题相关图片如下：

图2

音箱阻抗的典型值是多少?

 
此主题相关图片如下：

图3

图3是从1991年1月至1997年6月间测量过的330种音箱的统计结果。平均值是8.6欧姆，中心值为9．25欧姆，只有极少数音箱的阻抗小于4欧姆或大于15欧姆。德国DIN标谁规定：在不同的频率下，音箱的阻抗变化串不应大于正负20％(中国的国家标准也有类似规定--编译者按)。然而，我发现上述这些音箱的阻抗变化率远大于此，平均偏差高达3．7欧姆，相对于平均值8.6欧姆而言，变化率为43％。由于当前的晶体管功放内阻非常小。所以这种超过标推的阻抗变化率并不会对声音造成不良影响。但是，如果所用的是电子管功放的话，由于其内阻较大(可达数欧姆)，过大的阻抗变化率会对频率均衡造成明显的不良影响。

图4

图4是上述330种音箱最小阻抗的统计结果。可以看出：最小阻抗的离散度比标称值小得多，大多数在2．76至4．26欧姆的范围内，平均值及中心值为4．3欧姆。实际上，如果不考虑某些怪异的设计(例如有一款音箱在30kflz以上呈现出短路状态，另有两款早期的样品在100kHz以上的阻抗会跌落到0．5欧姆以下)，只有6种音箱的最小阻抗低于2欧姆。最小阻抗的最大值足8．33欧姆，这是一款1977年产的BB C LS3／5A，此时的频率为171Hz，相位角为-3．1。还可以看到一种饶有趣味的民族差异：英国工程师设计的音箱，最小阻抗往往出现在高音区，大约在10kHz附近,而美国工程师则倾向于使其出现在低中音区。

阻抗测量是一种非常重要的诊断工具，只要看一看阻抗曲线就可以在很大程度上判断出这款音箱的表现。例如我们可以根据阻抗曲线的形状判断出音箱是几路分频的，而不必真的看到达款音箱。还可以判断出它的结构：密闭箱的阻抗曲线在低音区有一个峰；倒相式或传输线式音箱在低音区有两个峰；某些号角式音箱则会出现一系列等间距的峰。根据阻抗曲线低频段的形状还可以估算出音箱的低频下限及Q值。倒相式音箱低频段的两个蜂之间的“鞍部”通常就是低频响应下降6dB的地方，也就是听觉响度下降一半的地方。

还可以利用阻抗曲线找出是否存在共振，以及共振的类型。如果测量时选择适当的频率间隔，曲线上对应于共振频率的地方就会出现小毛刺。例如图2中200Hz和450Hz处的毛刺是箱体共振造成的，27kHz处的毛刺则是由于高音单元的共振。

 
此主题相关图片如下：

图5

图5是另一款音箱的阻抗相位曲线。设计者决定在箱体中不使用任何阻尼材料，它认为这样可以使声音更好。也许这样作能使声音变得较为“振奋”，但却在整个中频段造成了空腔共振和箱板共振，在阻抗曲线上可以明显地看到达些共振所形成的毛刺。我在试

听这款音箱之前，就可以根据曲线作出预言：这款音箱的中音部分必定存在严重的声染色。而音响评论家Tom Noton在未看到这条曲线的情况下曾试听了这款音箱，并指出该音箱确实存在这种问题。

最重要的是你可以根据阻抗曲线来判断一款音箱是否易于被功放推动。音箱的阻抗不是纯电阻性的，随着相位角的不同，它的电流有可能超前或滞后于电压。在最坏的情况下--相位角等于90度时，信号电压为0时将要求功放提供最大的电流。因此，只是简单地标明一款音箱的阻抗是8欧姆是不够的，甚至会造成误导。阻抗的相位角是随频率而变化的，从功放的角度来看，音箱的实际阻抗会比标称值低得多。然而，Peter Baxadall在音响工程学会1987年度的学术报告会上曾经指出：最大相位角绝不会与最低阻抗同时出现。阻抗和相位角都是同一三维现象的二维投影，因此二者是存在着数学关联的。

在过去的二十年中，发烧圈中对这个问题曾出现过许多推测。Dolby公司的Eric Benjarming曾于1992年详细地探讨过这个问题。他特别研究了B类功率放大器输出级的理论功耗，功耗的大小将依赖于负载阻抗的大小和相位角。他发现：音箱从功放吸取的最大电流几乎从来不会超过根据最小阻抗值预测出的电流。随着阻抗和驱动信号的不同，由计算得出的功放输出级的功耗将为预测值的120％至270％。因此，在选择功放时，输出功率需要留出一定的余量。余量的大小与所驱动的音箱的阻抗特性有关，有些音箱只需要很小的余量，有些音箱则需要较大的余量。这个问题过去并末引起特别的重视，这是因为在典型的房间里用典型的音量听音乐的时候，所需的功率一般不会超过数瓦，所以这个问题对于民用功放来说并不显得十分严重。

音箱的阻抗特性将对其音质产生重大的影响。例如图6和图7是我所测量过的音箱中阻抗最低的两款，它们的阻抗曲线颇具代表牲，特别应该注意它们的相位角。图6的音箱在2KHz以下的阻抗保持在5欧姆以上，但在12k以上时下跌到1欧姆。在3．3kHz处形成了一个由可能使功放损坏的组合--阻抗为2．4欧姆，相位角为-70。相比之下，图7的音箱虽然虽然几乎在整个频段的阻抗都低于3欧姆，但相位角并不很大。单凭这些曲线并不能预澜出哪一款音箱更难推。Stereophile正在计划进行一个新的项目：计算出我所测量过的音箱的“Benjaming峰值推动难度”，并建立起一个可供器材评论采用的索引。

[后记：到目前为止，我们所讨论的那是音箱的频域特设，从下一期开始，我们将讨论音箱的时域特牲。]

 
此主题相关图片如下：

图6

此主题相关图片如下：

图7

有人翻译啦？原文似乎是：

Measuring Loudspeakers, Part One

John Atkinson, November, 1998

This series of articles was initially written (in slightly different form), as a paper presented at the 103rd Audio Engineering Society Convention, New York, September 1997. The preprint, "Loudspeakers: What Measurements Can Tell Us—And What They Can't Tell Us!," AES Preprint 4608, is available from the AES, 60 East 42nd Street, Room 2520, New York, NY 10165-0075. The AES internet site, offers a secure transaction page for credit-card orders.

What's the point of measuring the performance of an audio component if what is owners do is listen to it? Of the 20 or so regularly published English-language consumer magazines on audio, only three routinely publish rigorous loudspeaker measurements: Hi-Fi News & Record Review in the UK; Audio and Stereophile in the US (footnote 1). It would seem that supporting a reviewer's subjective impression of how a loudspeaker sounds with measured data is an idea that is out of fashion these days. But without measuring a loudspeaker's performance, how can a "Golden Ears" reviewer avoid falling into error? I believe that it is the responsibility of any magazine that publishes judgments on audio components to fully describe their performance. And that cannot be done without measurements.

For electronic components, measuring their effect on an input signal is relatively straightforward (though correlating those effects with perceived sound quality is not a job for the fainthearted). But for many decades, getting a true idea of how a loudspeaker performed was only possible using swept sinewaves in large, expensive anechoic chambers. However, the relatively recent advent of low-cost, PC-based measuring equipment capable of assessing acoustic performance in normal rooms has made it possible for more people to measure more loudspeakers than ever before. However, simply placing a microphone in front of a loudspeaker and pressing ENTER on the computer keyboard will still not produce a measurement that is likely to predict how that loudspeaker will sound in a room.

If you examine the vocabulary typically used to describe loudspeaker performance by audiophiles and in magazine reviews [1, 2, 3, 4], it covers the following areas:

• Musical and technical accuracy—how good? How close is the loudspeaker to reproducing an overall sound that will convince its listeners they're in the presence of live sounds?

• Frequency range—bass and treble extension. Does the sound have its full complement at the frequency extremes? Or has, say, the designer discarded ultimate bass extension, perhaps to maximize midrange clarity or overall power handling?

• Frequency balance—how neutral? Does the speaker have a natural timbre? If you play a recording of someone's voice, does it actually sound like that person's voice, or does it sound more like a combination of bass boom and a bat squeak? Is the presentation tilted-up at one or both frequency extremes? Is there a shelf effect present at one or more frequencies?

• Response and balance anomalies—coloration. Do voices sound like someone is speaking through a megaphone or pinching their noses. Is there excessive sibilance on a female voice or too much chest tone on male voice?Can the sounds of similar but different musical instruments be distinguished? Or does the loudspeaker's own character make a violin sound like a viola, an oboe like an English Horn, a Fender Stratocaster like a Gibson Les Paul? With some chronic loudspeaker cabinet problems, it can almost sound as though someone is banging a xylophone along with the sound of the recorded instruments.

• Clarity and transparency. How much detail can you hear? I remember taking part in a blind test in the early '80s where we A/B'd two speakers using a live microphone feed of a bunch of keys being rattled in an anechoic chamber. One speaker made it sound like there was just one key. With another loudspeaker, you could almost hear how many keys there were. Do individual images within the soundstage sound sufficiently separate from each other, or do they seem more like spatial highlights emerging from a background of sonic soup?

• Grain, hardness, and distortion. Does the loudspeaker sound significantly more distorted at high playback levels? Does the loudspeaker make listeners want to run screaming from the room? Do the listeners feel as if their eardrums are being gently caressed with sandpaper?

• Stereo imaging precision. When a dual-mono recording is played, do listeners perceive a narrow sonic object precisely midway between the loudspeakers, or do they just hear a vague, amorphous blob? Do central images stay centrally located at some frequencies but not others?

• Soundstage width and depth. With appropriate recordings, can listeners hear acoustic objects precisely positioned anywhere on a two-dimensional grid defined by the loudspeaker and listener positions?

• Dynamics (micro and macro). Are loud musical passages appropriately louder than the quiet passages (macrodynamics)? Can listeners hear subtle changes in one acoustic object when something else is playing very loudly (microdynamics)? Does it all blur at high playback levels, or does it take being played loudly to make the music "come to life"?

• Pace'n'rhythm. First defined by Stereophile contributor and UK reviewer Martin Colloms [5]. Some loudspeakers clearly make the music sound like it's going slowly, while others make it sound like it's going faster, even though the recording's playing time and the music's tempo obviously cannot be affected by anything that a loudspeaker does.

Subjective loudspeaker performance is thus a multidimensional phenomenon. However, to make objective measurements that are both meaningful and practicable involves a subjective choice about what parameter to plot against one, or at most two, other parameters. All other parameters have then to be held constant. If you plot, say, a loudspeaker's sound-pressure level against frequency for a given input voltage, the result is the typical amplitude or "frequency" response. But this measured response will only be valid on the chosen axis in an anechoic chamber at the chosen sound-pressure level at one instant of time. How typical will it be of what the loudspeaker does with music in a real room played at widely differing spls? It is important, therefore, to keep in the back of your mind that to make "objective" measurements involves subjective choices!

A list of measurements that I typically perform in connection with the loudspeaker reviews published in Stereophile includes:

• Voltage sensitivity on the chosen axis.

• Electrical impedance (magnitude and phase).

• Impulse and step responses.

• Amplitude and phase response on the chosen axis in the farfield.

• Nearfield amplitude response (at low frequencies).

• Polar behavior—Dispersion in horizontal and vertical planes.

• Power and other in-room responses.

• Nonlinear distortions of various kinds.

• Delayed acoustic resonances.

• Cabinet vibrational behavior.

It should be obvious that not one of the parameters in this second list appears to bear any direct correlation with one of the subjective attributes in the first list. If, for example, an engineer needs to measure a loudspeaker's perceived "transparency," there isn't any single two- or three-dimensional graph that can be plotted to show "objective" performance parameters that correlate with the subjective attribute. Everything a loudspeaker does affects it to some degree or other.

Of course, there are some performance parameters that correlate significantly—perceived bass extension with measured low-frequency extension, for example—but it is important to remember that there will always be other aspects of measured performance that also contribute. Anyone who looks at published measurements should never assume that one measurement—a frequency response, or an impedance curve, or a dispersion pattern—fully or even partially describes the sound that they will hear. It's only the totality of all possible measurements looked at simultaneously that will give the reader any idea of what's going on. What you hear always depends on more than one measurement. Ergo, no one measurement can tell the whole story.

And given half a chance, all measurements will tell lies. It's very easy to assume that if you get a piece of test gear, turn it on, and hook it up to the device-under-test, that the resultant graph is meaningful. It's never safe to assume that a) the graph is correctly plotted, or b) that you are actually measuring what you think you're measuring. You still need another source of data, much as in pre-calculator days someone using a slide-rule needed to know approximately what the answer would be before they did a calculation. When you measure a loudspeaker's complex impedance, for example, it is helpful to look at the waveform of the signal present at the speaker terminals with an oscilloscope and to listen to the speaker's acoustic output. The test set might still produce a nice-looking graph, even if the speaker isn't making a sound!

是的，不过我弄丢了前面的一部分

好文章，阐述的很详细。音箱的阻抗对音质的影响，简单的说，也就是音箱和功放的匹配问题。我们这个论坛 虽然是扬声器设计论坛，但我觉得孤立的来谈扬声器设计似乎总有那么点缺憾。最终扬声器产品还是需要扩大机来推动，需要人的耳朵来鉴赏。当我们孤芳自赏，对别人不屑一顾的时候，是否应该偶尔反思一下：在Audio的大海洋里，我们的知识面是否足够宽广？

赞同上边的说法，系统是最关键的 。

音箱的阻抗对音质的影响，不是简单的音箱和功放的匹配问题。因为在电类学科中"匹配"是仅涉及功率传输的概念.

好文章。

翻译的文章跟原文比，节约了不少的语句，先看看原文吧，这样好理解作者的原意。

另外，这个JA主编毕竟不是设计系统的，所以大部分的东西他是以一个局外的观测者来检查他手上的数据的。