Dynamic Loudspeaker Principle

A current-carrying wire in a magnetic field experiences a magnetic force perpendicular to the wire. 载有电流的导线在磁场中会受到与导线垂直的磁场力。

An audio signal source such as a microphone or recording produces an electrical "image" of the sound. That is, it produces an electrical signal that has the same frequency and harmonic content, and a size that reflects the relative intensity of the sound as it changes. The job of the amplifier is to take that electrical image and make it larger -- large enough in power to drive the coils of a loudspeaker. Having a "high fidelity" amplifier means that you make it larger without changing any of its properties. Any changes would be perceived as distortions of the sound since the human ear is amazingly sensitive to such changes. Once the amplifier has made the electrical image large enough, it applies it to the voice coils of the loudspeaker, making them vibrate with a pattern that follows the variations of the original signal. The voice coil is attached to and drives the cone of the loudspeaker, which in turn drives the air. This action on the air produces sound that more-or-less reproduces the sound pressure variations of the original signal.

Loudspeaker Basics

The loudspeakers are almost always the limiting element on the fidelity of a reproduced sound in either home or theater. The other stages in sound reproduction are mostly electronic, and the electronic components are highly developed. The loudspeaker involves electromechanical processes where the amplified audio signal must move a cone or other mechanical device to produce sound like the original sound wave. This process involves many difficulties, and usually is the most imperfect of the steps in sound reproduction. Choose your speakers carefully. Some basic ideas about speaker enclosures might help with perspective. 扬声器通常是再现声音保真度的限制因素，无论是家庭还是剧院。声音再现的其他阶段大多是电子的，电子元件已经高度发展。扬声器涉及电磁过程，其中放大后的音频信号必须推动一个锥形或其它机械装置以产生类似原始声波的声音。这个过程涉及许多困难，通常也是声音再现步骤中最不完美的部分。请慎重选择扬声器。一些关于扬声器箱体的基本概念可能会有所帮助。 Once you have chosen a good loudspeaker from a reputable manufacturer and paid a good price for it, you might presume that you would get good sound reproduction from it. But you won't --- not without a good enclosure. The enclosure is an essential part of sound production because of the following problems with a direct radiating loudspeaker: 一旦你从一家信誉良好的制造商处选购了一个好的扬声器，并且以合理的价格购得它，你可能会认为它能产生良好的声音再现。但事实并非如此——没有一个好的箱体，你无法做到这一点。箱体是声音生产中不可或缺的一部分，因为直接辐射的扬声器存在以下问题：

Click image for more details. 点击图像以获取更多细节。

Types of Enclosures

The production of a good high-fidelity loudspeaker requires that the speakers be enclosed because of a number of basic properties of loudspeakers. Just putting a single dynamic loudspeaker in a closed box will improve its sound quality dramatically. Modern loudspeaker enclosures typically involve multiple loudspeakers with a crossover network to provide a more nearly uniform frequency response across the audio frequency range. Other techniques such as those used in bass reflex enclosures may be used to extend the useful bass range of the loudspeakers.

The nature of the enclosure can affect the efficiency and the directionality of a loudspeaker. The use of horn loudspeakers can provide higher efficiency and more directionality, but in extremes can reduce the fidelity of the sound. Line array enclosures can provide some directionality.

The term "infinite baffle" is often encountered in discussions of loudspeaker installations. It visualizes a loudspeaker mounted in an infinite plane with unlimited volume behind it, but in practical use may refer to a loudspeaker mounted in the surface of a flat wall with considerable volume of air behind it. Because of the elastic properties of the loudpeaker suspension, it will still exhibit its natural free-cone resonance, but will be free of the diffraction effects observed with a small box speaker, and essentially free of the effects of the compression of the air behind the loudspeaker cone. '无限腔体'这一术语在扬声器安装的讨论中经常出现。它形象地表示一个扬声器安装在无限大的平面上，其后有无限大的空间，但在实际使用中，可能指的是一个扬声器安装在平墙表面，其后有相当数量的空气空间。由于扬声器悬架的弹性特性，它仍会表现出其自由锥体共振，但不会出现小箱体扬声器所观察到的衍射效应，基本上也免受扬声器锥体后方空气压缩的影响。

Use of Multiple Drivers in Loudspeakers

Even with a good enclosure, a single loudspeaker cannot be expected to deliver optimally balanced sound over the full audible sound spectrum. For the production of high frequencies, the driving element should be small and light to be able to respond rapidly to the applied signal. Such high frequency speakers are called "tweeters". On the other hand, a bass speaker should be large to efficiently impedance match to the air. Such speakers, called "woofers", must also be supplied with more power since the signal must drive a larger mass. Another factor is that the ear's response curves discriminate against bass, so that more acoustic power must be supplied in the bass range. It is usually desirable to have a third, mid-range, speaker to achieve a smooth frequency response. The appropriate frequency signals are routed to the speakers by a crossover network.

Horn Loudspeakers

Horn type loudspeakers use a large diaphragm which supplies periodic pressure to a small entry port of a long horn. More compact versions use a folded horn geometry. The large diagphragm system is called a "compression driver" since its large air displacement which feeds into a small port causes a larger pressure variation than ordinary loudspeakers. The long tapered horn increases the sound production efficiency by perhaps a factor of ten compared to an ordinary open cone-type loudspeaker. 号型扬声器使用一个较大的膜片，它将周期性压力施加到一个长号管的小入口孔上。更紧凑的版本使用折叠号型几何结构。由于其较大的空气位移通过小孔进入，该大膜片系统被称为“压缩驱动器”，因为它导致的压力变化比普通扬声器更大。长的锥形号管相比普通开放型锥形扬声器，可将声音产生效率提高十倍左右。

Reference:

Wiki: Compression Driver

Wiki: Horn loudspeaker

Line Array or Column Loudspeakers

One way to achieve a degree of directionality with bass loudspeakers and to achieve a modest increase in potential acoustic gain is to use several identical bass speakers in a "line array" or column geometry. 用几个相同的低音扬声器组成‘线阵’或柱状结构，可以实现低音扬声器的一定方向性，并获得适度的声学增益提升。 It may be surprising that the effect of this geometry is to spread the sound in the horizontal plane and narrow its spread in the vertical plane. This is the geometric effect of sound diffraction, and is analogous to the spreading of light through a narrow slit in the direction perpendicular to the narrow dimension. 这种几何效应可能让人感到意外，它会使声波在水平面上扩散，而在垂直方向上则会使其扩散变窄。这是声波衍射的几何效应，类似于光通过狭窄缝隙在垂直于狭缝方向上的扩散。 The directional patterns below show the spreading of the sound in the horizontal plane on a polar scale indicating the diminishing of the sound in decibels compared to the centerline direction of the loudspeakers. For a column of bass speakers of size 15" or 38 cm, λ=4D corresponds to a frequency of about 225 Hz while the shortest wavelength, λ = 0.25D corresponds to a frequency of 3570 Hz. 下面的定向图显示了声音在水平面上的扩散情况，以极坐标表示，表明与扬声器中心方向相比，声音强度以分贝为单位逐渐减弱。对于尺寸为15英寸或38厘米的列阵低音扬声器，λ=4D对应约225 Hz的频率，而最短的波长λ=0.25D对应3570 Hz的频率。

Example of a line array loudspeaker collection which is ceiling-mounted in an auditorium. It points generally at the audience and spreads the sound perpendicular to the array. 图示为一种安装在礼堂天花板上的线阵列扬声器系统。它大致面向观众，声波垂直于阵列传播。

Directionality of Loudspeakers

While loudspeakers cannot achieve the extremes of directionality of microphones, efforts to control the directionality of sound from loudspeakers can be productive. Horn type loudspeakers are generally more directional than open cone-type loudspeakers. Speakers in a line array can be arranged to spread the sound in the horizontal plane more than the vertical plane to direct the sound energy more at the listeners. Making the loudspeakers more directional can increase the potential acoustic gain that can be achieved with a sound amplification system.

The example of a small loudspeaker shows that high frequencies from loudspeakers are typically more directional than low frequencies. For sound from a single loudspeaker, this means that the bass-treble balance will become more prominent in bass as you move further off-axis. Most sound systems use some kind of cross-over network so that the bass and treble can be controlled separately to achieve a good balance. Often, horns for the bass are used because they are more directional than other bass speakers and can be combined with treble sources of comparable directionality.