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Path Identification for Active Noise Control

Periodic Reporting for period 1 - PIANO (Path Identification for Active Noise Control)

Okres sprawozdawczy: 2020-07-01 do 2021-12-31

One of Clean Sky’s goals is to reduce noise from aircrafts by 50%. The PIANO project has taken up the challenge, in order to improve the noise levels in the passenger cabin of one of Clean Sky’s demonstrators: the Next Generation Civil TiltRotor.
The lack of acoustic comfort in the passenger cabin of tilt-rotors is a demanding challenge in this kind of aircrafts. Quiet environments are being more and more demanded. The global increasing noise pollution in everyday life results in a reduction of the quality of life. Noise is a stressor that affects our circulation largely unconsciously via the autonomic nervous system. Improving acoustic comfort is one of the passengers’ main requests.
The EU-funded PIANO project aims to make progress on this direction, by enabling future low noise tilt-rotor aircrafts with an improved interior acoustic comfort.
Classical noise countermeasures as insulation material were ineffective for the kind of noise produced by a TiltRotor and wearing headphones is not an option for comfort and safety reasons.
In contrast, PIANO is working on specific and innovative solutions for interior noise reduction, with advanced active noise cancelling (ANC) algorithms, as well as making progress on complex vibro-acoustical analysis, through advanced transfer path analysis (ATPA). These innovations will represent a clear advance to meet society and market’s needs.
Therefore, objectives of PIANO are the following:
• Improve tilt-rotor vibro-acoustic characterization for a more accurate assessment of noise abatement measures.
• Improve the accuracy of numerical modelling of aircrafts through experimental techniques.
• Enable aircrafts with enhanced interior acoustic comfort without weight penalization and at a reasonable cost.
Subsystem identification in the TiltRotor from a structural numerical model provided by the Topic Manager (TM) has been the first main task. The model needed to be heavily manipulated for vibro-acoustical purposes, hence it has been refined, substructured and boundary superelements have been created as additional stiffness and mass elements.
The clustering-based subsystem identification algorithm proposed is based on the exponentiation of the Direct transfer (TD) matrix. TD matrices were calculated from the numerical model through eigenvalue decomposition. This data has been clustered by means of distance indicators and dendogram plots.
A sensitivity analysis on clustering results has been done, assessing the influence of the power order of the TD matrix and the maximum number of clusters allowed in the grouping.
In the light of results obtained, it has been concluded that the initial proposal based on exponentiation of the TD matrix has shown to be flawed for the current system, with a complete loss of the matrix structure as the power increases due to a strong coupling between all the elements defining the model. As a result, SEA requirement of weakly connected subsystems is not satisfied. However, this circumstance is often not the case in experimental scenarios. In the context of this investigation the exponentiation process can be best categorized as a method for identifying the strength of connection between already identified subsystems.
ATPA test plan has been designed: static test was already performed at the TM facilities, by instrumenting with accelerometers cabin interior panels and structural linking points of the tilt-rotor wings to the fuselage.
In parallel, local ANC algorithms have been adapted and evaluated for synthetic signals and real noise signals of a similar aircraft, provided by the TM. In this sense, it was experimentally tested a multichannel virtual sensing ANC scheme using
- three parallel adaptive notch filters and a linear extrapolation technique, in a test rig for tonal signals and a beat signal as primary sound disturbance.
- conventional FxLMS and a linear extrapolation technique, in a test rig on flight signals as primary sound disturbance
The preliminary results were promising and will be integrated in the electronics of the ANC system.
Finally, specific electronics have been developed to make a processing with an analogical filtering to obtain an efficient, robust and low cost local ANC processing. The starting idea was to consider that the noise spectrum can vary according to different conditions: speed of the aircraft, internal equipment of the cabin, different layout of the cabin (number of seats, pilot/passenger cabin separation, acoustic treatment of the wall with absorbing materials, etc.). Results are promising.
Work is also being done to find the best way to integrate the loudspeaker and transducers in the TM seat.
ATPA methodology is applied in the field of aerospace for vibro-acoustic characterization for the first time, which is useful to extract relevant information about noise transmission paths, topology of the system and quantifying air-borne and structure-borne noise inside the TiltRotor cabin. This is a key point for acoustic characterization of complex systems and unfeasible with classical TPA methods, but essential in the design of suitable and optimized countermeasures for noise reduction. It will allow extracting the SEA coupling factors from experimental data and a mathematical deduction, a step forward to get a reliable estimation of these parameters and building more reliable models.
A novel methodology for automatically identifying subsystems was developed and implemented for the first time to a real case in the field of aerospace, which has shown to be valid to quantify the degree of coupling among subsystems and could be vital for:
- Assessing whether a subsystem may be studied independently from the rest of the system (numerically or experimentally in laboratory).
- Assessing whether SEA hypothesis of weak coupling between subsystems is satisfied or not.
- Helping in making a selection of subsystems to apply experimental methods

The innovative methodologies in the transfer path analysis field constitute a powerful tool in engineering processes, leading to a continuous improvement of internal workflows in EU companies, from acoustic optimization in aircraft design phase to efficient troubleshooting, and may significantly contribute to the reinforcement of the EU industrial leadership.
The local ANC strategy will address tonal and broadband noise, allowing enhancing the extent, robustness and frequency range of the quiet zone around the passenger’s head with the aim of improving acoustic comfort for passengers and providing an impact in societal and market needs attainment, as quiet environments are more and more demanded.
Global strategy will be focused on cancellation of low-frequency cavity modes, especially at areas with poor LANC results, enhancing the extent of the quiet zones. Results of ATPA, intended to be exploited in the global strategy (hence will improve the knowledge of the noise propagation in between the primary noise sources and the head of the seat passenger) are helping for the first time in the optimization of the number of actuators needed.
As a result of this combined strategy, the use of heavy materials to treat low-frequencies inside the cabin may be avoided or at least limited, causing a weight reduction inside the cabin. Saving of material leads to a direct positive impact, including protection of environment and energy supply saving.
Capture of aircraft model with interface nodes between superelements in orange
Testing of local ANC algorithms in DSL laboratory mockup
Representation of TD matrix and clustering on the cabin fuselage