This paper examines sound level meter errors when measuring direct fly-over aircraft noise in presence of background noise. Since the meter is unable to separate aircraft noise from background noise, an array of 12 microphones is used to measure direct aircraft noise avoiding the effect of the ground reflections and urban background noise. Using this technique, aircraft noise can be separated from the background noise providing a reference value of the aircraft sound pressure level for the sound level meter measurement. Measurement is carried out using both a sound level meter and the array in an urban environment with high background noise level near Barcelona Airport. Finally, the experimental measurement errors of the sound meter are within 1 dB of the theoretical values.

Measurement of aircraft noise in a high background noise environment using a microphone array

The present study addresses the reduction of the sound transmission through an aircraft fuselage test section by means of a combined active noise and vibration control system, so as to reduce the noise level inside the passengers’ cabin. Experiments prove that control loudspeakers and error microphones in the trim cavity yield higher reductions in the radiated sound power than control shakers and error accelerometers on the trim panel. This conclusion remains valid when the skin and the trim panel are mechanically connected by means of four vibration isolators, and when the cavity is filled with thermal insulation blankets. It also appears that, in case of an active cavity noise control system, there exists an almost linear relation between the reduction of the acoustic potential energy in the cavity and the reduction of the radiated sound power. The optimised control system performs much better than a control system with sensors and actuators in arbitrarily chosen positions. * Research Assistant of the Fund for Scientific Research Flanders (Belgium) (F.W.O.)

Experimental study of the active sound transmission reduction through a double panel test section

Aircraft interior noise is governed by the sound characteristics of the fuselage sidewall. Active trim panel attachments provide a promising technology for the low-frequency control of interior noise. Placed between the main fuselage and the cabin trim panels they are used to control the dynamic structural response and consequently the sound radiation of the trim panels. The control concept has been developed by means of a vibro-acoustic simulation model representing a generic aircraft section. A prototype demonstrator was experimentally tested under laboratory conditions proving that substantial reductions in sound transmission for various tonal and broadband perturbations can be obtained with such an active system.

Active Trim Panel Attachments for Control of Sound Transmission through Aircraft Structures

Over the years, the interior noise levels have been improved, and nowadays a high acoustical comfort is of major importance for the commercial success of propeller aircraft. The use and development of high vibro-acoustic performance devices and their integration on the primary structure are, therefore, among the most critical topics in the greening of the next generation air transportation. The purpose of this research was properly focused on the analysis of a dynamic control system appointed to increase the noise attenuation transmitted in the propeller aircraft fuselage. Passive and active dynamic devices have been studied and characterized in this work. Simulations and tests performed on the first stage demonstrator structure highlighted how the passive tuned vibration absorber increases the comfort controlling the structural behaviour of the fuselage at the first blade pass frequency. Further improvements of fuselage comfort can be reached using active devices. A conceptual design of a hybrid active device able to act on the first three blade pass frequencies has been outlined. The characterization of the whole active noise control system has been described and its effect on noise reduction in fuselage of propeller aircraft has been evaluated.

Progress in Studying Passive and Active Devices for Fuselage Noise Reduction for Next Generation Turboprop

Optimization of piezoelectric actuator grouping for aircraft cabin noise control

This paper deals with the experimental validation of an innovative system for the aircraft acoustic signature identification which has been developed by the vibration and acoustics laboratory of the Italian Aerospace Research Center. The system is composed of an algorithm for the acoustic signature identification and a dedicated neural network classifier, trained with a set of experimental aircraft noise data. The algorithm test and validation has been performed for different airplanes during takeoff and landing maneuvers. The experimental activity of ground noise measurements has been carried out at the Naples airport of Capodichino. More than 200 aircraft noise events of five aircraft types (Airbus A320, Boeing B737, McDonnel Douglas MD80, Fokker F100, Aerospatiale/Alenia ATR72), during both takeoff and landing maneuver, have been measured. This paper demonstrates the feasibility of developing a suitable artificial neural network to establish if a time signal, elaborated through a wavelet process, is or is not similar to others, having been recognized as originated from a defined type of aircraft. The artificial neural network was trained by the use of a subset of experimental data and then validated through a comparison with another subset of data from the same experimental campaign. The developed software demonstrated to give more than satisfactory results for each of the acquired spectra, with the maximum error always being under (10)%.

Experimental Training and Validation of a System for Aircraft Acoustic Signature Identification

The work presented in this paper is part of the EU research program ASANCA II and is relative to the main experimental results of an extensive work performed on a fuselage mock-up of the ATR 42/72 regional transport aircraft. An active control of the cabin noise was realized using piezoceramic patches as actuators bonded on the fbselage frames and skin, and microphones as sensors distributed inside the mock-up at seated passenger head height. The external primary field was generated by using two loudspeaker arrays located at the two sides of the mock-up and reproducing, both in amplitude and phase, the flight acoustic pressure field generated by two six-bladed propellers of the ATR 42-500 aircraft. Both primary and secondary field have been measured by means of an advanced acquisition system capable of simultaneously recording 256 channels. The above acquired data have then been used for the sensors actuators optimization that has been done by means of two different optimization algorithms having as cost function the reduction of noise at seated passenger head level and conditioned by the maximum allowable voltage that the amplifiers were capable of supplying to the piezoceramics. The same data have been used to predict the noise reduction with the different system configurations. Several ANVC tests have been performed and the main results are reported. The comparison with the predicted numerical performances is presented and this is showing a very good agreement with the measured reductions. During the tests up to three propeller tones have been simultaneously controlled, coinciding with the Blade Passing Frequency and its first two harmonics, with average noise reductions (over 48 internal microphones) of up to 10 dB for the most significant test condition.

Active noise control on atr fuselage mock-up by piezoceramic actuators

The evaluation and assessment of vibro-acoustic comfort inside civil aircraft requires the knowledge of two different classes of information: the physical feature of both noise and vibration fields and the way the passenger reacts to those stimuli. This paper describes design and performance analysis of a ground-based mock-up for 3D reconstruction of the internal vibro-acoustic field of a helicopter cabin (GINS: Ground-based Internal Noise Simulator); the facility was assembled at the CIRA premise of Capua (IT) as part of the EU-funded project FRIENDCOPTER1 and was aimed at reproducing the spatial distribution of the interior noise of an AW109. The internal acoustic environment was simulated through implementation of the auralisation technique; the reproduced vibro-acoustic field was experimentally measured during in-flight tests on a AW109 helicopter at the Agusta premise of Cascina Costa (IT). The ground-based simulator (GINS) was assembled by integrating the reproducing technology and the CIRA semi-anechoic chamber; a listening test was carried out in order to verify how reliable the noise field 3D spatial reconstruction appeared from a subjective point of view. In details, the listening tests were aimed at evaluating how clearly a good theoretical noise control technique could be identified in the passengers' reactions; those reactions were collected and evaluated through suitable questionnaires that were developed by the psycho-acoustic experts of the University of Oldenburg (D) for the EU-funded project IDEA PACI. The questionnaires analysis showed that the GINS can provide realistic internal acoustic reproductions and that the relevant features of the sound quality are easily perceived by the listening people; this implies that such a simulator can be effectively employed for comfort studies.

Vincenzo Quaranta

Papers

Experimental Training and Validation of a System for Aircraft Acoustic Signature Identification

Active noise control on atr fuselage mock-up by piezoceramic actuators

3D internal noise simulation for vibro-acoustic comfort assessment: an application proposal for AW-109 helicopter

Experimental Training and Validation of a System for Aircraft Acoustic Signature Identification