Sound Amplification Systems

Simplified Model: Sound Reinforcement

简化模型：声音强化

Assume*:

假设:*

Inverse square law dropoff of sound
声音的反平方定律衰减
Omnidirectional loudspeakers and microphones
全向扬声器和麦克风

The loudspeaker provides more sound to the listener than would otherwise have been received, but it also produces sound at the location of the microphone. This feedback to the microphone limits the amount of amplification which can be used. Control of the feedback generally is the determining factor for the potential acoustic gain that can be achieved by a sound reinforcement system.

扬声器比麦克风原本接收的声能更多，但它也会在麦克风位置产生声音。这种反馈限制了可以使用的放大程度。对反馈的控制通常是决定声学增益潜力的关键因素。

The microphone creates an electrical image of the sound which is amplified and used to drive a loudspeaker.

麦克风将声音转换为电信号，该信号被放大并用于驱动扬声器。

*These idealizations are never met in any real room, but they provide a framework for building a model. The assumptions will be relaxed later.

这些理想化在任何实际房间中都无法实现，但它们为构建模型提供了框架。这些假设将在后面被放宽。

Numerical Example: Need for Amplification

数值示例：放大器的必要性

By the inverse square law, a doubling of distance will drop the sound intensity to 1/4, corresponding to a drop of 6 decibels. Note that in the table at right, the distance is doubled in each successive step.

Simplified sound
system model

简化声音系统模型

根据反平方定律，距离加倍会使声音强度降至1/4，对应降6分贝。注意右边的表格中，每一步的距离都是加倍的。

Level dB 电平 dB	Distance ft 距离英尺
80	2
74	4
68	8
62	16
56	32
50	64
44	128

The following simplifying assumptions allow us to calculate the sound levels and assess the need for amplification:
1. The sound drops off according to the Inverse square law.
2. The microphones and loudspeakers are omnidirectional.

以下简化假设允许我们计算声压级并评估放大需求：1. 声压按照反平方定律衰减。2. 话筒和扬声器是全向的。

Numerical Example: Need for Amplification

数值示例：放大器的必要性

Level dB 电平 dB	Distance ft 距离英尺
80	2
74	4
68	8
62	16
56	32
50	64
44	128

Starting with 80 dB at two feet and using the fact that every doubling of distance will drop the level by 6 dB, we learn that a listener at 128 ft would receive a sound intensity of only 44 dB!

从两英尺处的80分贝开始，利用每增加一倍距离声强会降低6分贝的事实，我们得知在128英尺处的听众只接收到了44分贝的声强。

Maximum Amplification Condition

最大放大条件

When the feedback to the microphone is equal to the original sound input level, the gain can no longer be increased without an uncontrolled increase in output, usually in the form of a loud ring at one frequency. Practically, the sound should be kept about 3 dB below that point. If 80 dB is the input level to the microphone, then 80 dB feedback return is an upper bound. Using the inverse square law, the resulting levels are as shown.

当反馈到麦克风的信号等于原始声音输入电平时，增益将不能再增加，否则通常会以某一频率产生刺耳的啸叫。实际上，声音应保持在该点下方约3 dB。如果麦克风的输入电平为80 dB，那么80 dB的反馈返回就是上限。利用反平方定律，所得的电平如图所示。