Active Control of Sound, Applications of
- 109 Downloads
The active control of sound is realized with active noise reduction systems. These systems are made up of actuators, sensors and digital signal processing. Unlike classical passive sound control the active control of sound is especially effective at low frequencies. Active noise reduction systems are able to reduce unwanted sound with a minimum of added mass and volume which makes them useful for lightweight applications for example in the automotive or aerospace industry.
Active noise control
Active noise vibration control
Active structural acoustic control
Active vibration control
Carbon fiber reinforced plastics
Counter-rotating open rotor
Finite impulse response filter
Frequency response function
Infinite impulse response filter
Printed circuit board
Radiation modal expansion
Structural health monitoring
Scanning laser Doppler vibrometer
Sound pressure level
Signal processing unit
Turbulent boundary layer
Due to the fact that the methods to achieve active control of sound are highly application dependent, it is reasonable to describe the most prominent approaches using dedicated applications in mind.
Sound control is an important issue for many lightweight structures because they are prone to structural vibration and sound radiation. Especially at frequencies below 500 Hz, passive sound control is generally not compatible with lightweight construction. This is due to the fact that low-frequency sound transmission loss (TL) is determined by mass. According to the so-called mass law, an increase in TL by 6 dB (factor of four) requires a doubling of mass per unit area. If certain frequencies are to be addressed, tuned vibration absorbers or Helmholtz resonators might be an option. These systems are usually not able to track frequency changes, and their mass and volume also scale with the desired amount of disturbance rejection. Alternatively, active sound control methods can be applied to achieve high performance with a minimum of added mass and volume. These methods use actuators, sensors, and control to reduce the sound emission of structural parts. Generally, active sound control can be subdivided into active noise control (ANC), active noise vibration control (ANVC), and active structural acoustic control (ASAC). In addition, also active vibration control (AVC) is often used to attenuate unwanted noise even if this principle is not especially tailored to acoustics. AVC is limited to cases, where the occurring vibrations directly induce the unwanted noise, and is most effective if the correlation of the vibration patterns and the radiated sound is high. In this case, elaborate models of the sound radiation process can be avoided leading to simpler and sometimes more robust active systems. As AVC, ASAC systems are built from structures with embedded actuators and sensors. In contrast to AVC, the objective function however is based on acoustic measures. In the case of ANVC and ANC, the sensor information is usually obtained from sound pressure-based devices like microphones, and the latter principle uses loudspeakers as actuators for the active control. Microphones and loudspeakers can be positioned independently of the walls and sound-emitting parts to optimize the performance of the active system.
Whereas ANC and ANVC generally lead to local sound control at the error sensor positions, ASAC aims at a reduction of the radiated sound power which is a global metric. This difference is caused by and has implications on the error sensor scheme. Generally, a prerequisite for global control is the observability of the global system behavior which can either be achieved by filtering local sensor data through an observer (a global system model) or by a global physical sensor scheme. A performance metric for ASAC is obtained from the so-called radiation modes of a structure. These modes couple independently into the sound power, and a reduction of their modal amplitude is directly linked to a sound power reduction. This is a key difference to the suppression of structural modes, which may even lead to an increase in radiated sound power. Since radiation modes are frequency dependent, they must be modeled by a dynamic system. The radiation modal expansion (RME) technique provides a trade-off between accuracy and efficiency. Special filtering of sensor signals facilitates the incorporation of psychoacoustic metrics to achieve higher sound quality or a comfortable acoustic environment. Psychoacoustic metrics are, however, not easily included in real-time control applications. A typical simple example is the postprocessing of microphone data with an A-weighting filter to account for human sound perception. Regarding the actuator schemes, ANC uses loudspeakers, and ANVC and ASAC directly actuate the radiating structure. For ASAC this is a requirement, but for ANVC the actuated and the noise radiating structural parts must not necessarily coincide. In the latter case, the structure acts as a loudspeaker, and ANVC is similar to ANC. Usually an ANVC system acts on the vibration of the radiating structural parts aiming at a reduction of sound pressure or sound power.
Specifies in general the accumulated decrease in intensity of a sound wave as it propagates through a certain type of structure. Measures of TL are expressed in decibel and directly quantify the success in sound attenuation through windows, walls, or other separating constructions.
Describes the situation that a vibrating structure emits sound into a region with no or insignificant reflections. The sound can ideally radiate indefinitely without hitting any obstacles. This can approximately be the case in large rooms with sufficient damping. Sound radiated under free-field condition almost keeps its energy while traversing the media if the damping of the media is neglected. A sound field can be roughly divided into a near field and a far field. The former is in the region of the emitting structure and exhibits interference effects, while the latter is present in the outer region with almost plane waves. Measuring the sound energy on spherical surfaces in the far field, so-called radiation modes can be determined in the case of free-field radiation, which can be used to determine the radiated sound energy of the given vibration pattern of the radiating structure. This can significantly simplify the construction of ASAC systems.
If a structure emits sound into a cabin, usually the free-field condition does not hold, because significant parts of the radiated sound waves are reflected at the boundary of the cavity and contribute to the resulting sound field. In addition to the structural resonance frequencies, the cavity itself also has acoustic resonances, which must be considered as well. Adequate modeling needs elaborate models based on volumetric finite element models of the cavity or surface-based boundary element models. In general the reflections of the boundary are described by complex impedances leading to inherent complex system models with real and imaginary parts.
Active Noise Control (ANC)
If the noise sources are distributed or the sound-emitting structural parts are otherwise inaccessible to active measures, ANC is the principle of choice to attenuate the noise or change its patterns as desired. It is most effective if the controlled region is local to the microphones in use or if the sound path is restricted. Successful applications contain the attenuation of machine-induced noise passing through industrial chimneys to the environment or the reduction of sound waves travelling through air conditioner ducts. More generally, if good reference signals are available, feedforward-based ANC systems can be very effective. Noise-cancelling headphones are also based on ANC and provide great possibilities to filter certain noise patterns from the human ear perception due to the fact that the target region is well determined and sound waves travel mainly through the auditory canal.
Placement of Actuators and Sensors
The efficiency of an active system is directly connected to the controllability and observability of the used actuators and sensors. Besides the properties of the devices itself, their location naturally has major influence. Poorly chosen positions might fully prohibit the detection or stimulation of a certain structural vibration mode. Therefore carefully chosen positions are an important step in the design of a reasonable adaptive structure.
It is clearly shown that the optimized system with 24 error sensors shows the largest reduction averaged over the 5 addressed CROR frequencies. It is also shown that no amplification is present for the optimized system because only negative values indicate a sound power reduction in this study. Therefore, it can be stated that the AVC feedforward control systems optimized with the optimization tool indeed reduce the radiated sound power.
Active Windshield of a Passenger Car
The effect of active global vibration control on the interior SPL can be seen in Fig. 6. The SPL is monitored at six positions in the front part of the cabin on the driver and co-driver side. A maximum reduction of 16 dB was achieved at the third EF at the driver side. It can be concluded that the active system is able to reduce the interior SPL induced by the windshield. Regarding the acoustic performance in real driving operation, it must be noted that other external and internal disturbance sources will contribute to the interior SPL which must also be controlled by passive or active means. Nevertheless, the windshield is a critical part because it has a large vibrating surface in the vicinity of the (co-)driver and the applicability of passive damping or absorption is limited.
Active Fuselage Panels
Due to the special experimental setup with a closed test section, additional wind noise was present in the wind tunnel. As a consequence, acoustic measurements with a sound intensity probe in front of the panel showed poor results. Therefore, the radiated sound power of the panel is estimated with the well-known approach of the radiation resistance matrix (Fahy and Gardonio, 2007). The panel is subdivided into elementary radiators, and the normal velocity of the center point is needed for the calculation of the sound radiation. For this panel, the grid of 260 scan points is used to determine these radiators. The procedure has been validated at the experimental setup using shaker excitation and acoustic measurement equipment in the inactive wind tunnel.
The TBL-induced disturbances are modeled as process noise d entering the closed loop between controller R and plant G. The control objective is the reduction of the influence of the disturbances onto the velocity outputs p to achieve global control of the vibrations of the entire panel (Fig. 9). The final H∞ controller is synthesized as discrete state-space model in Matlab/SIMULINK with a sampling time of 1 kHz. Afterward, it is implemented in a dSPACE rapid prototyping system.
The results from the presented experiments show that an active control system is able to reduce the transmission of TBL noise below 500 Hz significantly. The usage of the extended plant in combination with H∞ control leads to global vibration reductions and finally to less noise radiation of the panel.
Active Aircraft Sidewall Panels
- Elliott S (2001) Signal processing for active control. Academic Press, San DiegoGoogle Scholar
- Fahy F, Gardonio P (2007) Sound and structural vibration: radiation, transmission and response, 2nd edn. Academic Press, OxfordGoogle Scholar
- Kuo S, Morgan D (1995) Active noise control systems: algorithms and DSP implementations. Wiley, New YorkGoogle Scholar
- Misol M (2014) Aktive Steuerung des Transmissionsverhaltens stochastischer Störquellen durch flächige Leichtbaustrukturen [Active feedforward control of the transmission of stochastic disturbances through lightweight panel structures]. DLR-Forschungsbericht 2014-38, Technische Universität Carolo-Wilhelmina zu Braunschweig, Köln. http://elib.dlr.de/99510/
- Misol M (2019) Active sidewall panels with remote microphone technique for aircraft interior noise reduction. In: 26th international congress on sound and vibrationGoogle Scholar
- Misol M, Algermissen S, Monner HP (2013) Experimental study of an active window for silent and comfortable vehicle cabins. In: Sinapius M, Wiedemann M (eds) Adaptive, tolerant and efficient composite structures, research topics in aerospace, chap 36. Springer, Berlin/Heidelberg, pp 439–447. https://doi.org/10.1007/978-3-642-29190-6 CrossRefGoogle Scholar