能動騒音制御: Wiping Up Unwanted Sound

騒音は辞書などで調べると，「日常生活の心の平静をかき乱す、不愉快で不必要に大きな音。」と定義されています。したがって、図書館内での話し声や隣の部屋から聞こえてくる音楽、また消防車・救急車のサイレンなども、人によっては騒音と言えるでしょう。とすれば、騒音は私たちにとって非常に身近な存在なのです。

ところで、あなたは騒音を小さくするにはどうしたらよいと考えますか？耳栓をする。壁で音を遮る。吸音材を買ってくる。どれも効果的ではあるでしょう。しかし、これだけでは騒音を完全に除去することはできません。私たちは、これらとは全く別の方法で騒音を小さくする研究をしています。それは、能動騒音制御（アクティブノイズコントロール：ANC）です。

          Active noise control (ANC) (also known as noise cancellation, active noise reduction (ANR) or antinoise) is  the process of reducing existing noise by the introduction of additional noise by means of one or more secondary (or control) noise sources. The introduced noise may achieve the required noise reduction by way of any one or combination of four different physical mechanisms. It works on the principle of destructive interference between sound fields generated by the original (primary) source and the secondary sources, which can be controlled. In the simplest form of ANC, a control system drives a speaker to produce a sound field that is the exact mirror-image of the offending sound (the ”disturbance”). The speaker thus cancels the disturbance by means of destructive interference, and the net result is no sound at all: 

          Why should be an active control? The best answer is,  conventional methods of suppressing acoustic noise using sound absorbers do not work well at low frequencies (wavelength >> thickness of a typical absorber, 100 Hz  about 3.4 m  (using velocity of acoustic waves in air = 333 m/s). The traditional approach of reducing noise (”sound proofing”) is to use passive methods like insulation and silencers. A typical example are the ear muffs of headphones. Good attenuation is achieved when using materials with special characteristics. This form of noise reduction works best for higher frequencies, basically acting like a low pass filter. In some cases, the low frequencies are noticed even more. However, when used for lower frequencies, passive solutions tend to get bulky and heavy as the size and mass of passive treatments usually depends on the acoustic wavelength, making them bigger and more massive for lower frequencies.    

Basic ANC Components

               In control systems jargon, an ANC system consists of the following four major parts: Plant (the physical system to be controlled. A typical example is a headphone and the air inside it), Sensors  (the vibration sensors, microphones, or other devices that sense the primary disturbance and monitor how well the control system is performing by measuring the remaining error), Actuators  (the devices that physically do the work of altering the plant response, usually electromechanical devices such as speakers or vibration generators), and Controller (signal processor that controls the actuators. It bases its commandson the sensor signals and on some knowledge of the plant’s response to the actuators).    

        The variety of behavior characteristics of sound waves in different physical surroundings allow categorization of ANC systems into three different group: 

•  Global free space cancellation, the total annihilation of a sound field in three dimensions. Requires the cancellation source to be placed within close proximity of the acoustic disturbance, as a general guideline within 0.1 wavelengths of the disturbance source in order to obtain 20 dB global reduction in sound intensity at any given frequency.
• Cavity and duct cancellation, deals with noise cancellation in confined spaces,such as a room or a ventilation duct. In a confined space, reflections from the walls create modal responses, which are generally present whenever the wavelength of the acoustic wave approaches or decreases below the dimensions of the cavity. The number of acoustic modes grows rapidly with the increase of frequency of the sound wave.
• Zone-of-silence cancellation, provides a localized cancellation of sound field intensity in a very small region of the overall sound field. A typical cancellation zone will only be about a tenth of a wavelength in diameter

          Under the assumption that the system is linear and time invariant (LTI), there are two different basic approaches to ANC, which shall be discussed in this section.  

• Feedforward Control - These systems depend on a direct measurement of the primary disturbance and an a priori knowledge of the plant transfer function. The approach is known as feedforward compensation because the compensator is not contained within a feedback loop. The advantage of this approach is increased stability since the compensator has no influence on the signal at it’s input.

• Feedback Control — This approach is constantly monitoring the system’s output in order to minimize the remaining error. The advantage is that it can compensate dynamic variations in the plant transfer function due to aging, changes in temperature or humidity, etc.