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Passive, tuned acoustic absorbers, such as Helmholtz resonators (HR) and quarter-wave tubes, are commonly used solutions for abating the low-frequency tonal noise in air induction systems. Since absorption at multiple frequencies is required, multiple absorbers tuned to different frequencies are commonly used. Typically, the large size and multiple numbers of these devices under the hood is a packaging challenge. Also, the lack of acoustic damping narrows their effective bandwidth and creates undesirable side lobes. Active noise control could address all of the above-mentioned issues. Most active noise control systems use feedforward adaptive algorithms as their controllers. These complex algorithms need fast, powerful digital signal processors to run. To ensure the convergence of the adaptation algorithm, the rate of adaptation should be made slow.

Large vehicles, such as SUVs and minivans, exhibit body boom phenomena during multiple source excitation events including rough road/impact and power-train induced events. The main cause of the boom is the low-frequency acoustic/vibroacoustic modes of the cavity being excited vi a the high acoustic transfer functions at multiple paths, due to an inherently weak body structure and/or existence of popular features such as tailgates with their corresponding dynamics. Abating the boom noise by modifying the response is the more viable and less costly option than body changes. Active acoustic damping can do such modification, cost-effectively. A number of active, feedback controlled, boom noise damping techniques have been developed at the University of Dayton.

The performance of shunt piezo damping is demonstrated by adding damping to the first mode of a plate with the dimensions of 28 by 38 cm and thickness of 0.8 mm. A small 1 by 2 inch piezoelectric patch with the thickness of 10 mil is bonded to the plate at a location where strain due to the first mode of vibration is high. The peizo is shunted with a resistance-inductance (RL) circuit, tuned to the first resonance frequency of the plate at 38 Hz. The plate is excited at its first natural frequency and the power spectrums of the acceleration at the center of the plate with and without the damping treatment were measured. These measurements showed that the shunt piezo damping treatment tuned to the first mode added an appreciable amount of damping to that mode.

Two active boom noise damping techniques using a Helmholtz resonator-based compensator and a lead compensator called a positive pressure feedback have been developed at the University of Dayton [1]. The two damping techniques are of feedback type and their compensators can be implemented in software or hardware (using inexpensive operational amplifiers). The active damping system would rely on a speaker, a low-cost microphone, two accelerometers, and an electronic circuit (or a micro-controller) to add damping to the offending low-frequency vibroacoustic modes of the cavity. The simplicity of the active boom noise damping system lends itself to be incorporated into a vehicle's sound system. The Helmholtz resonator-based strategy is implemented on a Dodge Durango sport utility vehicle. The control scheme adds appreciable amount of damping to the first cavity mode and the first structurally induced acoustic mode of the cabin.

We propose the use of an active element in conjunction with a passive, reasonably-damped Suspension to make an engine mount capable of satisfyingBoth damping and isolation requirements while avoiding 1) the loss of performance due to detuning of tunable (such as hydraulic) engine mounts and 2) the undesirable on/off switching associated with the decoupler in hydraulic mounts. In the proposed scheme, the damping will be provided by the proper choice of damped elastomeric material and isolation will be achieved by controlling the active element using a novel kalman-estimator based algorithm developed by the authors. To demonstrate the effectiveness of the proposed mounting scheme the distributed parameter models of a typical engine/chassis system was developed. Finite element analysis of the chassis was performed to find its natural frequencies and mode shapes which in turn were used to construct the state space control model.

Because of the recent trend for testing products in multi- directions and to higher and higher frequencies there is a need to develop excitation systems with high natural frequencies, i.e. having a stiff construction with small moving mass(es). In this work, a 3- degree of freedom (DOF) excitation mechanism with closed-chain kinematics is proposed that can satisfactorily fulfill these requirements. Closed loop control allows this device to be used for testing parts under realistic combined loads that include not only vertical but also two axial horizontal loads. The 6-degree of freedom version of such device can, in addition to 3 axial forces, also load a part with 3 moments. The use of this device can be extended to active vibration control applications such as active seats for off-road vehicles.

Due to the limited effectiveness of sound absorbing material at low frequencies the first (first few) acoustic mode(s) of enclosures, the size of automobile cabins, are very difficult to damp, resulting in the boom noise problem. The use of one or a bank of Helmholtz resonators for increasing the absorption in automobile cabins has been studied by different researchers. This approach is effective, but the large size of the resonators (due to the low frequencies they are tuned for) makes their use rather prohibitive. This has led to the use of active noise control, to add damping to the first (or first few) acoustic modes by nearly-collocating a microphone with a volumetric source (speaker) in the acoustic field and employing appropriate feedback compensators. Significant levels of low-frequency, global acoustic damping can be achieved in enclosures such as automobile cabins, with the proposed feedback control schemes.