Inertial actuators positioned along the boundary of a structural opening can achieve both vibration control and sound attenuation; however, finding their optimal placement requires taking into account how your system responds to noise excitation.
Engineered materials with exceptional broadband energy dissipation performance have long been sought across a range of engineering disciplines, as they often facilitate effective vibroacoustic sound attenuation and trapping of spectrally broadband vibration and wave energy.
Noise Attenuation
Noise pollution is an increasingly serious global challenge with serious societal impacts. Industrial equipment and household devices have compounded this issue through vibration transfer into higher frequency modes that are more perceptible by humans. Therefore, lightweight solutions to dampen vibration and sound energy from spreading are in great demand; specifically locally resonant vibro-acoustic metamaterial panels provide impressive stop band performance, although they often exhibit a notable insulation dip at lower frequencies that limits their applicability.
Researchers have recently developed a novel class of hyperdamping inclusions to address this limitation, by carefully engineering their design parameters, they have achieved remarkable broadband vibration and sound energy trapping properties with negligible volume changes. Finite element model investigations help guide selection of softening/buckling behaviors which result in outstanding metamaterial vibration attenuation performance.
By exploiting hyperdamping behaviour, it is possible to design noise control systems using structural openings in enclosures as key points of activation of Active Structural Acoustic Control (ASAC), an emerging technique which has the power to significantly improve passive noise barriers as well as decrease vibration and noise levels.
As part of systems with structural openings, it is necessary to comprehend how dynamic interactions between openings and inertial actuators impact control performance. To this end, a novel method for estimating acoustic power transmission through openings was proposed and integrated into an analytical model to optimize actuator placement – thus creating Dual-Actuator-Type Active Noise Control (DATANC), outperforming MISO-ANC and SISO-ANC configurations between 100-200Hz frequency range.
In order to further optimize its acoustic performance, the DATANC system incorporates a hyperdamping metamaterial. Acoustic tests prove that this metamaterial absorbs significantly more vibration energy at lower frequencies than resonant metamaterials; experiments showing this are performed show this. Furthermore, its performance remains comparable in terms of wide frequency range performance despite only being slightly larger – only 2% larger on average!
Sound Attenuation
Sound attenuation is an essential aspect of vibration control for large and high-performance structures, helping protect hearing loss by limiting transmission of noise between rooms, floors, walls and other structural elements. Acoustic leakage prevention also benefits from sound attenuation techniques used in such buildings.
Vibroacoustic sound attenuation can be achieved using insulation materials, construction methods and other techniques to strategically reduce noise transmission through structures. As opposed to total soundproofing which blocks all acoustic waves from entering or leaving an environment, sound attenuation focuses on blocking certain frequencies while attenuating others.
One popular approach to sound attenuation uses porous material sandwiched between external structures and perforated lining, with its hole size and perforation pattern being varied to absorb sound at different frequencies – particularly useful when dealing with low frequency sounds that could otherwise cause unwanted resonance within structures.
Vibroacoustic sound attenuators combined with acoustic panels are another effective means of sound attenuation, creating a shallow air pocket between them and the wall, which resonates into resonance effects that can be reduced by filling this pocket with porous materials such as foam. This type of tuning can then be toned down through secondary porous materials to alter its pitch sharpness.
Passive vibroacoustic sound attenuators work by increasing mechanical impedance of flexible structures and coupling to and dissipating acoustic modes in cavities, providing a cost-effective alternative to active vibration absorbers that require cabling, power amplifiers, signal conditioning hardware and central control schemes.
Ideal conditions for installing a vibroacoustic sound attenuator include placing it in structures where large structural vibration modes contribute significantly to noise transmission, such as spacecraft fairing structures. Figure 5 depicts this pressure response between structures with and without vibroacoustic sound attenuators installed.
Energy Attenuation
Attenuation occurs when energy loses intensity as it travels through a medium. This loss occurs as a result of interactions between its source and atoms in the medium, such as absorption and scattering. Beer-Lambert law describes attenuation’s dependence upon length of path energy takes through medium; similar attenuation occurs with seismic waves which become smaller further from earthquake center.
Vibroacoustic devices offer two means to reduce sound attenuation: structural damping and acoustic dissipation. Simply, this device works by vibrating a dielectric elastomer membrane within a rigid duct to generate vibrations that scatter high frequencies while simultaneously absorbing lower ones, providing sound attenuation across a much wider frequency spectrum than tuned vibration absorbers and similar devices.
Noise coming down from roof top equipment into an occupied space from ducts is often an irritation and distraction for building occupants, especially if located above offices or other noise sensitive areas. Compounding this issue are other roof-mounted appliances like condenser fans, compressors or fresh air intake and exhaust systems located nearby that further increase its volume.
Vibro-Acoustics provides noise control curbs designed to mitigate these problems. Their vibration isolators reduce transmission of structure-borne noise into occupied space below, with options of 1″, 2″ or 3″ deflection to meet individual project acoustic attenuation requirements.
Curb designs also take wind and seismic loading requirements into account, which is important when installing equipment in areas prone to high winds, tornadoes, hurricanes or tornadoes. Curbs may be designed according to local wind and seismic criteria set by licensed engineers for approval and installation.
Medical x-rays offer another application of the energy attenuation principle: diagnosing and treating patients without cutting them open. To make the most of this powerful tool, however, it’s crucial that one understands and effectively attenuates interactions that take place between x-ray radiation and bone structures and soft tissues of a patient.
Noise Control
Vibro-acoustic sound attenuators provide noise control systems with superior acoustic performance due to their spatial positioning and excitation of its actuators, relative positioning of these actuators to each other and vibrational modes within structures also impact acoustic coupling between them. As shown in Fig 7a, loudspeaker-based configuration (MATHRM DATANC_(2,3)) exhibits limited control performance within 100-200Hz frequency range due to resonance at around 225Hz due to proximity of loudspeaker to enclosure opening which reduces driving structural modes effectively.
However, inertial actuator-based (mathrm ‘DATANC_(1,3)) configuration displays acceptable MSE performance over this frequency range. Additionally, inertial actuators are better placed to excite structural modes with lower vibration frequencies and cancel noise from these modes more effectively than with traditional solutions.
An additional way to improve acoustic performance is to add mass to the area surrounding structures with dominant vibration modes, shifting their participation by increasing efficiency in energy transfer from these modes to other parts of the structure. This can significantly decrease modal participation by vibration-based noise control systems across all frequency bands – especially those intended to decrease sound radiation at lower frequencies.
Addition of mass near a structural opening can reduce MSE by as much as 10dB when compared with SISO-ANC configuration without lid. While this improvement may seem minor, it underscores the significance of properly positioning actuators within vibro-acoustic noise control systems with transparent lids to achieve maximum noise suppression.
This study shows how adaptive active noise control with optimal actuator positioning and gain settings can dramatically enhance acoustic performance in systems featuring structural openings. This method can also be applied to vibrational control problems, providing a practical balance between performance and computational cost.
