有源噪声控制外文

Designing practical active noise control systems

M. Cuesta, T. Bravo, P. Cobo, C. Ranz, and M. Siguero

Instituto de Acústica, CSIC. Serrano 144. 28006 Madrid. SPAIN

Abstract: This paper deals with the practical aspects of the design of two

active noise control (ANC) systems to attenuate low frequency noise. In the

first instance, a hybrid passive-active control system to reduce the noise

radiated by a small generator has been designed. A SISO ANC system, in a

side branch of the exhaust tube, attenuates some of the harmonics of the

lowfrequency exhaust noise as much as 27 dB. In the second instance, a

MIMO ANC system is designed to alleviate the booming inside a van. The

booming is reduced more than 20 dB at the front seats.

1.Introduction

Active noise control (ANC) techniques rely on the destructive interference between the existing, or primary, noise and the anti-phase, or secondary, noise set up by an electronic controller(Nelson and Elliott, 1992; Hansen and Snyder, 1997; Kuo and Morgan, 1996). The simpler adaptive ANC system processes a reference signal to generate the control signal that drives the secondary source. An error signal monitors the control performance. The adaptive filter must extrapolate the acoustic field from the reference sensor to the error sensor (primary path) and compensate for the transfer function between the control source and the error sensor (secondary path) (Cobo, 1997). Since ANC works best on low frequency noises, the active approach complements the traditional passive control methods.

The aim of this paper is to illustrate the design of two ANC systems. In the first instance, a hybrid passive-active control system is designed to reduce the noise radiated by a small generator. Passive control is afforded by a close-fitting enclosure lined with absorbing material. The low frequency noise radiated through the exhaust is controlled by a SISO ANC system implemented in a side-branch configuration. In the second instance, a MIMO ANC system is designed to alleviate the booming inside a van. The ANC system uses the reference signal provided by an accelerometer on the left engine mount to drive six loudspeakers that radiate the out-of-phase booming. The signals picked up at six microphones adapt the controller in the least mean square

way.

2.Passive-active control of noise radiated by a small generator

The noise source is a small combustion Honda EG1900 engine that radiates periodic noise related to its rotational velocity. It is enclosed in a steel box designed to have the highest Insertion Loss. The interior walls of the enclosure are lined with absorbing material and a three-layers damping material has been sandwiched between the generator mounts and the floor.

A MOTRAN HTL6 loudspeaker, which supports temperatures as high as 174 oC, is used as secondary source. An electret FONESTAR 2214 microphone in the exhaust picks up the error signal. To reduce the high noise levels downstream the exhaust pipe, the microphone is inserted into a brass tube, closed at the end in contact with the gas flow. To avoid heating problems, the microphone is isolated from the brass tube by a Teflon ring. The designed active control system is feedforward, so that a good quality reference signal, coherent with the primary noise, is required. The optimal reference is afforded by two accelerometers over the air filter case of the generator, close one another. Two accelerometers filtered in overlapping frequency bands are used to provide redundant reference information.

A configurable ANC system, implemented on a TI TMS320C40 DSP, which incorporates both FX-LMS and FU-LMS algorithms, was used to optimise the SISO control system. An IIR filter yielded the best results. Sampling rate and antialiasing frequencies were 2000 and 500 Hz, respectively. References 1 and 2 were filtered in (40-150 Hz) and (100-300 Hz), respectively. The error signal was filtered in (40-300 Hz).

Fig. 1. (a) Coherence between the error signal and references 1 (dashed, blue) and 2 (solid, red). (b) Relative level of the error signal with the ANC system off (light, blue) and on (thick,

red)

Fig. 1a shows the coherence between the references and error signals. Fig. 1b exhibits the averaged log-magnitude spectrum of the exhaust noise with the ANC system on and off. Most of the harmonics in the frequency band (50-300) Hz were attenuated. The fundamental frequency, N = 32.5Hz, is out of the response frequency band of the loudspeaker. The maximum attenuation, 27 dB, occurs at the frequency 2N = 65 Hz.

3.Active control of the booming inside a van

For a typical four-cylinder powered vehicle, the most significant contribution to the interior noise is the 2nd order harmonic of the engine rotational speed. Under usual driving conditions, when the operating engine speed runs from 1500 to 6000 rpm, the 2nd order excitation varies from 50 Hz to 200 Hz. Internal acoustic modes are excited in this frequency range resulting in increased sound, known as “booming”.

A 6I/6O ANC system has been designed to control the booming inside the van over the whole frequency range under hard driving conditions.

In the 6I/6O ANC configuration, a reference signal feeds a digital controller,

which drives six control signals to minimise the pressure levels at six microphones.

The vertical vibration signal picked up by an accelerometer at the left engine mount has been used in this system as reference signal (Bravo et al., 1999a). Six electret microphones were used, all of them placed on the interior side of the seat headrests (Bravo et al., 1999b). Four loudspeakers were located at the upper corners and two under the middle and back seats.

A configurable ANC system that implements the FX-LMS algorithm in a TI TMS320C40 DSP was used. Both the system identification and the adaptive filters were FIR in order to reduce the computational effort. Fig. 2 (a,b) shows the 2nd order tracking curves at the front seats, for a fast run-up of the engine at 3rd gear, with the ANC system off (solid, blue) and on (dashed, red). The tests were carried out in a rolling bank within a semi-anechoic room. The booming is evident around 3600 rpm. The ANC system reduces the 2nd order SPL more than 20 d

B at some frequencies. This reduction in the 2nd order harmonic provides an improvement in overall Aweighted sound pressure level. Fig. 2 (c, d) shows this A-weighted sound pressure level at the front seats (error microphones M1 and M2), under the same driving conditions (3rd gear, fast acceleration). The booming is again obvious in the overall levels around 3600 rpm. The AN

C system is able to cancel out the booming.

Fig. 2. 2nd order tracking in the front-left (a) and frontright (b) error microphones with the ANC system off (solid, blue) and on (dashed, red). Overall A-levels at front-left (c) and front-right (d) microphones with the ANC system off (solid, blue) and on (dashed, red)

Acknowledgments

This work was supported by the CICYT, through grant AMB97-1175-C03-01. We are grateful to Nissan European Technology Centre-Espa?a (NETC-E) for supplying the van.

References

Bravo, T., Cobo, P., and Martin, J.M. (1999a). “Active noise control of harmonic sound in a van:

A case history”, in Proceedingsof European Automotive Congress, edited by C. Grasas,

European Automobile Engineers Cooperation (Barcelona, Spain).

Bravo, T., Cobo, P., and Martin, J.M. (1999b). “A multichannel active system for control of the booming inside a van”, J. Acoust. Soc. Am. 105, 1244.

Cobo, P. (1997). Control Activo del Ruido: Principios y Aplicaciones, Colección Textos Universitarios, No 26 (CSIC, Madrid).

Cuesta, M. and Cobo, P. (1999). “A hybrid passi ve-active system for control of the noise radiated by a small generator”, J. Acoust. Soc. Am. 105, 1156.

Cuesta, M. and Cobo, P. (2000). “Active control of the exhaust noise radiated by an enclosed generator”, Applied Acoustics, accepted for publication.

Nelson, P.A. and Elliott, S.J. (1992). Active control of sound, (Academic Press, London). Hansen, C.H. and Snyder, S.D. (1997). Active control of sound and vibration, (E&FN Spon, London).

Kuo, S.M. and Morgan, D.R. (1996). Active noise control systems. Algorithms and DSP implementations, (Wiley, New York).

有源噪声控制系统设计实践

M.Cuesta,T.Bravo,P.Cobo,C.Ranz,and M.Siguero

西班牙马德里赛拉诺声学研究所，CSIC.144.28006

摘要：本文论及了两例应用有源噪声控制系统来削弱低频噪声的实例。在第一个例子中，我们设计了一个用可以产生辐射的小发生器来削弱噪声的无源—有源混合控制系统。将这个单输入单输出噪声控制系统布置在单侧分支的排气管道上，可以削弱至少27dB的部分谐波低频排气噪声。在第二个例子中，我们设计了一个多输入多输出有源噪声控制系统，它可以有效缓和面包车上的轰鸣声，在前排座椅位置，轰鸣声可以被削弱超过20dB。

1.引言

有源噪声控制（ANC）技术依靠现存的初级噪声和由电子控制器产生的反相位次级噪声间的相互干涉来实现（Nelson and Elliott，1992年；Hansen and Snyder，1997年；Kuo and Morgan，1996年）。简单的自适应有源噪声控制系统通过处理参考信号产生控制信号来驱动次级声源，用误差信号来反映系统控制性能。自适应滤波器需要推算从声场参考传感器和误差传感器（初级反馈）到补偿声源和误差传感器（次级反馈）间的传递函数。由于有源噪声控制系统适用于低频噪声的控制，所以，有源控制方法只能作为传统无源控制的补充。

本文的目的是要说明两个ANC系统的设计。在第一个例子中，设计了一个无源-有源混合控制系统，它用一个小型发声装置来减少噪音辐射。无源控制采用紧贴外壳的内衬吸音材料。通过排气管道的低频辐射噪音是由一个单输入单输出的ANC系统在侧分支管道中实施控制。在第二个例子中，设计了一个多输入多输出ANC系统，来削弱一辆面包车内的轰鸣声。ANC系统是用一个悬置在左侧发动机上传感器所提供的参考信号来驱动6个可以产生外部辐射噪音的扬声器。从6个麦克风上拾取的信号反馈在控制器上。

2.应用辐射发生器的无源—有源噪声控制

我们采用的噪声源是一个小型本田EG1900发动机，它发出的周期性噪声与其旋转速度有关。它被包围在一个具有最高插入损耗的钢框内，内墙外壳内衬吸音材料，并在发生装置与地板之间夹了三个层阻尼材料。

系统中的MOTRAN HTL6扬声器，可以支持高达174oC的温度，用来作为辅助噪声源。排气管中的驻极体FONESTAR2214用来拾取麦克风产生的误差信号。为了降低排气管下游的较高的噪音水平，麦克风被插入到铜管中，在气流接触的

相

干

性 声压级（d B ） 最后关闭。为了避免发热，麦克风用聚四氟乙烯环与铜管隔离开。设计，为了获取质量较好的参考信号来发出相干反相噪声，需要设计采用前馈方式的有源控制系统。最佳的参考信号是由两个以上的发生装置经空气滤波后产生的，关闭一个发生器，另外其它发出多频段噪声的发生装置提供附加参考信号。

一个配置好的ANC 控制系统，需要用FX-LMS 和FU-LMS 算法在TI TMS320C40 DSP 上实现，它可以用来优化单输入单输出有源噪声控制系统。加入IIR 滤波器可以取得更好的效果。这样，采样率和抗锯齿频率分别为2000Hz 和500Hz 。分别在参考文献1和2中被过滤（40-150Hz ）和（100-300Hz ），误差信号被过滤（40-300Hz ）。

图1.（a ）误差信号和参考信号间的一致性1（蓝色虚线）及2（红色实线）

（b ）有源噪声控制系统误差信号相对水平，断开（淡蓝色）接通（深红色） 图1（a ）表示了误差信号和参考信号间的一致性。图1（b ）表示了有源噪声控制系统断开与接通时排气噪声的日平均幅度谱，大多数的谐波（50-300Hz ）是衰减的。基本频率在N=32.5Hz 的谐波超出了扬声器的频率响应范围。最大削减值27dB 发生的频率为N=65Hz 处。

3. 面包车轰鸣声有源控制

对于一个典型的四缸动力汽车，

车内噪声最显着的贡献是第二级发动机的转

速谐波噪声。在一般的驾驶条件下，ECU系统控制发动机的转速在1500r/min到6000r/min时，二阶激励范围在50Hz到200Hz。系统的内嵌声学模块很乐意频率在这个范围内，增加的声音则被称为“轰鸣声”。我们设计了一个6输入6输出的有源噪声控制系统来控制整个频率范围内的硬盘驱动条件下面包车内的轰鸣声。

对于这个6输入6输出有源噪声控制系统的配置，参考信号被送入一个数字控制器，即可驱动6个控制信号，以尽量减小在6个麦克风处的声波压力水平。我们以在左侧发动机上悬置的发生器垂直振动信号作为参考信号（布拉沃等人，1999年）在本系统中使用。六个驻极体传声器，它们都放在驾驶室侧座椅的头枕位置（布拉沃等，1999年）。四个扬声器，分别位于上角和两个中间和后排座位下。

在使用了一个可配置的有源噪声控制系统后，我们实现了TITMS320C40 DSP的FX-LMS算法。系统识别模块和自适应滤波器模块都被解除，以减小计算难度。图2（A，B），表示了二阶响应在前排座椅的曲线，引擎在3档的快速运行状态下，ANC系统关闭（实线，蓝色）和（虚线，红色）的曲线。我们在一个混响室中而不是一个半消声室中进行测试。轰鸣噪声在3600r/min左右是显而易见的。有源噪声控制系统可以减小二阶声压级在某些频率超过20分贝的噪声。二阶谐波的减少改善了一个整体A加权声压级。图2（C，D），表示了这个在前排座椅位置（提供误差信号的次级声源M1和M2）相同的驾驶条件下，（3档，加速）的A加权声压级。轰鸣声又很明显，在3600r/min左右的总体水平。有源噪声控制系统能够取削减这种轰鸣声。

图2。二阶跟踪左前（a ）及（b ）与frontright ANC 系统关闭（固体，蓝色）和错误（虚线，红色），麦克风。整体左前（c ）和右前A-级（D ）与ANC 系统关闭（固体，蓝色）和麦

克风（虚线，红色）

4. 致谢

这项工作是由CICYT 支持，通过授予AMB97-1175-C03-01。我们感谢日产欧洲技术中心-，西班牙（NETC - E ）提供的面包车。

参考文献

Bravo, T., Cobo, P., and Martin, J.M. (1999a). “Active noise control of harmonic sound in a

van: A case history”, in Proceedingsof European Automotive Congress, edited by C. Grasas, European Automobile Engineers Cooperation (Barcelona, Spain).

Bravo, T., Cobo, P., and Martin, J.M. (1999b). “A multichannel active system for control of the

booming inside a van”, J. Acoust. Soc. Am. 105, 1244.

Cobo, P. (1997). Control Activo del Ruido: Principios y Aplicaciones, Colección Textos Universitarios, No 26 (CSIC, Madrid).

Cuesta, M. and Cobo, P. (1999). “A hybrid passive -active system for control of the noise radiated

声压级（d B ） 转速（r/min ）

by a small generator”, J. Acoust. Soc. Am. 105, 1156.

Cuesta, M. and Cobo, P. (2000). “Active control of the exhaust noise radiated by an enclosed generator”, Applied Acoustics, accepted for publication.

Nelson, P.A. and Elliott, S.J. (1992). Active control of sound, (Academic Press, London). Hansen, C.H. and Snyder, S.D. (1997). Active control of sound and vibration, (E&FN Spon, London).

Kuo, S.M. and Morgan, D.R. (1996). Active noise control systems. Algorithms and DSP implementations, (Wiley, New York).