This paper describes the development of adaptive acoustic impedance control (AAC) technologies to achieve a larger fan noise reduction, by adaptively adjusting reactance and resistance of the acoustic liner impedance. For the actual proof of the AAC technology III performance, the advanced fan noise absorption control duct liner II was made on trial basis, with the simple control system and the plain device. And, then, the duct liner II was examined for the AAC technology I, II, and III models, using the high speed fan test facility. The test results made clear that the duct liner II of the AAC technology III model could achieve the fan noise reduction higher than O.A. SPL 10 dB (A) at the maximum fan speed 6000 rpm, containing the reduction of fundamental BPF tone of 18 dB and 2nd BPF tone of 10 dB in response to the fan peed change from 3000 to 6000 rpm.
Recently, due to the strong demands by communities in the vicinity of airports, the new aircraft noise certification for the subsonic aircraft, ICAO Chapter 4, took effect in 2006 with the aim to reduce the aircraft noise.
The ICAO Chapter 4 becomes substantially stringent for the development of the new subsonic and supersonic transports. In order to overcome the stringent regulation and to solve the recent aircraft noise issues at night flight, is strongly required the large reduction of the intensive fan noise emitted from the multistage fan in the supersonic transport engines, over the wide engine operational range.
In order to provide the adequate fan noise reduction, the fan noise suppressors need to be satisfied with the following demands (I)
As one of the fan noise suppressors satisfied with these demands, the application of the active noise control (ANC) technology by cancelling the fan tones with the secondary noise sources is considered, which is developed by the other researchers and the authors in [
Comparison of property and method between AAC technology and ANC technology.
AAC technology | ANC technology | |
(1) Technology name | Adaptive acoustic impedance control technology | Active noise control technology |
(2) Noise reduction method | Adaptively controlled sound absorption | Actively controlled sound cancellation |
(3) Reduced noise contents | Both tones and broadband noise | Tones or broadband noise |
(4) Control target | Maximum noise reduction in overall sound pressure level | Maximum tone reduction in sound pressure level |
(5) Extension to low frequency noise region reduction | Usage of acoustic resonance and absorption in acoustic resonant chamber space | Usage of low frequency sound generated by speakers |
(6) Control method and procedure | (i) Large peak frequency shift and peak level increase in acoustic absorption spectra of duct liner for tones and broadband reduction, by adjusting reactance and resistance parts of acoustic impedance of liner with mobile reflective plate apparatus and tunable perforated plate apparatus | (i) Determination of acoustic spinning modes of tones with microphone array measuring systems and acoustic modal analyzer. |
(7) Feedback control system of technology | Simple and plain feedback control system | Complicated feedback control for multi-channel system |
(8) Available range of technology | Low-high intensive sound (Max. 170 dB) | Low-medium intensive sound (Max. 120 dB) |
Low-high temperature (Max. 900 K) | Low temperature (Max. 350 K) |
Fan 1st BPF tone reduction by ANC technology.
Furthermore, the acoustic liners with the tunable wall impedance control are researched [
Therefore, at JAXA (Japan Aerospace Exploration Agency), adaptive acoustic impedance control (AAC) technology I was contrived and developed. The technology aims at absorbing both the fan tones and the broadband noise largely and reliably, by controlling the acoustic impedance of the acoustic duct liners with the mobile reflective plates. For largely decreasing the low frequency fan tones in the AAC technology I, the resonance of their tones with the acoustic resonant chamber, where the Honeycomb structure panel is removed, is made full use of, in addition to the one-fourth wave length resonance with the resonant chamber depth. Because the Honeycomb structure panel is removed, the chamber space is enlarged in the transverse directions parallel to the duct liner surface plate. When the transverse dimensions of the resonant chamber are further larger than the chamber depth dimension, the fundamental resonant frequency of the resonant chamber becomes lower than the lowest frequency of the one-fourth wave length. Therefore, in the adaptive duct liners, the shallower resonant chamber depth than the ordinarily used chamber depth can be used. In addition, the acoustic absorbent materials stuck on the resonant chamber surface can reinforce the low frequency noise reduction [
In the succession of the AAC technology I, the AAC technology III was contrived and developed using both the mobile reflective plate apparatus (AAC technology I model) and the tunable perforated plate apparatus (AAC technology II model) set in the acoustic duct liners, in order to adaptively adjust both the reactance and the resistance of the acoustic liner impedance. For the actual proof of the AAC technology III performance, the advanced fan noise absorption control duct liner II was made on trial basis, with the simple control system and the plain device. The duct liner II was examined for each of the AAC technology I, II, and III models, using the high speed fan test facility.
The test results made clear that the duct liner II is available as the excellent fan noise suppressor for the current and future aircraft engines. And it is applied for reducing the wide noise fields of the turbomachinery noise and the factory noise. The duct liner II could achieve the fan noise reduction higher than the O.A. SPL 10 dB (A) at the max fan speed of 6000 rpm, containing the reduction of fundamental BPF tone of 18 dB and of 2nd BPF tone of 10 dB in the fan speed range from 3000 to 6000 rpm. It can provide with noise suppressors having the small weight, the small installation thrust loss, and the high noise reduction for aircraft engines. The AENC technology is composed of the adaptive acoustic impedance control (AAC) technology and the active jet noise control (AJC) technology.
Figure
Concept of active engine noise control technology and pictures of adative noise absorption control duct liners.
This paper describes each of the development of AAC technologies I, II, and III and the main results of the actual proof test of the AAC technology I, II, and III models with the acoustic duct liner II.
Table
Three models of AAC technology I, II, and III.
AAC technology | AAC technology | AAC technology | |
(1) Usage of apparatus | Mobile reflective plate apparatus | Tunable perforated plate apparatus | Both mobile reflective plate and tunable plate apparatuses |
(2) Mainly controllable acoustic impedance part of liner | Reactance part of acoustic impedance of liner | Resistance part of acoustic impedance of liner | Both reactance and resistance parts of acoustic impedance of liner |
Figure
Concept of adaptive acoustic impedance control technology I (mobile reflective plate apparatus—reactance part control of acoustic impedance).
The active control system comprises sensing sensors, a multistepping motor control device, stepping motor equipments installed on the panel casing wall, and a mechanics jointed to the motor equipment. The mechanics is consisted of pin joints, rods, arms, and gears for moving the reflective plates in three modes of parallel shift, forward shift and backward shift. The motor control device consisted of a personal computer, a bus bridge unit, stepping motor control boards, and stepping motor drivers connecting to the stepping motor equipments.
We try to explain the control system and control algorithm when in advance the order of moving modes of the mobile reflective plate is decided and the fan rotates at constant speed. The new position of mobile reflective plate is searched in the region covered by limiters, obeying the LMS (least mean square) algorithm, and the signal of displacement sensor sensing the movement of mobile reflective plate is sent to computer. The position of mobile reflective plate is changed so that the minimum value of overall SPL can be caught by the sensing sensor of a microphone set in the anechoic chamber room or a miniature pressure transducer set on the duct casing. Then, the moving mode and position of the mobile reflective plate and the minimum overall SPL are stored in the computer. The computer sends the pulse to stepping motor drive control, and the mobile reflective plate is moved to the position of the minimum O.A. SPL by the stepping motor equipments.
When the movement of mobile reflective plate is stopped by the limiters and the next moving mode is set, the same feedback loop is circulated. From the stored overall SPL, the minimum overall SPL is determined as well as the mode and the position of mobile reflective plate. Finally, the mode and the position of the mobile reflective plate return to ones where the overall SPL becomes the minimum value. When other fan rotational speed is fixed, the same operational technique is conducted in order to determine the mode and the position of the mobile reflective plate and the minimum overall SPL at the fan speed.
The AAC technology I makes an large peak frequency shift and a big peak spectra increase of the acoustic absorption spectra of duct liner for large fan noise reduction, by adjusting the reactance of acoustic impedance of the duct liner with the control of reflective plate movement in response to fan speed change. Through these experiments, the acoustic impedance of the duct liner is neither measured nor analyzed. However, in AAC Technology I, the acoustic absorption act is considered to largely depend on the acoustic resonance of duct liner and, therefore, the control of the reflective plate movement in the acoustic resonant chamber is considered to largely affect the reactance of the acoustic impedance of the duct liner.
The acoustic absorption mechanism of AAC technology I is considered to be composed of the resonance of incident sound with the acoustic resonant chamber, of noise absorption by acoustic absorbent materials stuck on the acoustic chamber wall, and of noise absorption by acoustic absorbent materials stuck on the mobile reflective plate. For largely decreasing low frequency fan tones by the AAC technology I, the resonance of their tones with the acoustic resonant chamber is made full use of, in addition to one-fourth wave length resonance with the resonant chamber depth. When transverse dimensions of the resonant chamber are further larger than the chamber depth dimension, the fundamental frequency of resonant modes of the resonant chamber becomes lower than the lowest frequency of one-fourth wave length resonance. Therefore, for the adaptive duct liners, the shallower resonant chamber depth than the ordinarily used chamber depth can be applied. As an example, to obtain the one-fourth wave length resonance with the chamber depth (frequency 270 Hz), the chamber depth is necessary to be 315 mm, while, the chamber depth of adaptive duct liner is sufficient to be about 100 mm [
When fan noise changes in accordance with fan speed change, the acoustic impedance of the duct liner is adaptively controlled using the AAC technology I. The AAC technology I makes a large peak frequency shift and a big peak spectra increase of the acoustic absorption spectra of the duct liner for large fan noise reduction, by adjusting the reactance of acoustic impedance of the duct liner with the control of the reflective plate movement, in response to fan speed change. Figure
Cut view and pictures of acoustic absorption chamber, mobile reflective plate, and mechanics controlling mobile reflective plat impedance.
Figures
Concept of adaptive acoustic impedance control technology II (tunable perforated plate apparatus—resistance part control of acoustic impedance).
Tunable perforated plate apparatus of adaptive fan noise absorption control duct liner II.
Through the experiments, the acoustic impedance of the duct liners is neither measured nor analyzed. However, in the AAC technology II, sound absorption is considered to depend on the porosity variation of the perforated plate apparatus and, therefore, the tunable perforated plate is considered to largely affect the acoustic resistance of the duct liners.
Figure
Concept of adaptive acoustic impedance control technology III (both mobile reflective plate apparatus and tunable perforated plate—two parts control of acoustic impedance of liner).
The adaptive fan noise absorption control duct liner II was made on trial base, and the AAC technology I, II, and III models of the duct liner II were examined using a high speed fan test facility. In Figure
Adaptive fan noise absorption control duct liner II.
Figure
General view and picture of high speed fan test facility.
The far-field fan noise is measured at 10 deg. intervals between 0 and 90 deg. from the fan axis in the anechoic room, by a microphone array set on the circle of 1.5 m radius from the fan inlet center. Also, the in-duct fan noise is measured with flush-mounted microphones installed upstream and downstream of the adaptive duct liner II. The microphone signals are amplified by the acoustic measuring Amps and analyzed with a FFT digital analyzer.
The noise reduction and the response performance of the adaptive duct liner II for the AAC technology I, II, and III models were examined using the high speed fan test facility. The AAC technology model of the adaptive duct liner II was exchanged, and the experiments were conducted for the AAC technology I model (mobile reflective plate apparatus), the AAC technology II model (tunable perforated plate apparatus), and the AAC technology III model (both the mobile reflective plate apparatus and the tunable perforated plate apparatus). And the position and the moving mode of the mobile reflective plates and the porosity of the tunable perforated plates were selected, at an interval of 1000 rpm from 3000 to 6000 rpm. At each fan speed, when the overall SPL of far-field fan noise measured at the position of 60 deg. reaches to the lowest value, the chosen moving shift mode and the position of the mobile reflective plates, the porosity of the tunable perforated plates, and the measured fan noise data were recorded in the personal computer.
The overall fan noise, 1st fan blade passing frequency (BPF) tone, 2nd BPF tone, and 3rd BPF tone reduced by the adaptive duct liner II of the AAC technology I model are shown in Figure
Variation of overall fan noise, 1st BPF, 2nd BPF, and 3rd BPF tones reduction by adaptive duct liner II of AAC technology I. Fan rotational speed 4000 rpm; acoustic measuring point 60 deg.; mobile reflective plate; parallel shift, acoustic absorbent material; poal + acousticel.
Figure
Variation of fan noise spectra reduction by adaptive duct liner II of AAC technology I model with mobile reflective plate position change. Fan rotational speed 4000 rom; acoustic measuring point 60 deg.; mobile reflective plate; parallel shift, acoustic absorbent material; poal + acousticel.
The comparison of the effect of the mobile reflective plate shift modes between the case in the backward shift of mobile reflective plate and the case in the parallel shift of mobile reflective plate is shown in Figure
Effect of mobile reflective plate shift mode on fan noise spectra reduction by adaptive duct liner II of AAC technology I model. Fan speed 4000 rpm; measuring point 60 deg.; mobile reflective plate parallel shift and backward shift, acoustic absorbent material stuck on reflective plate; poal + acousticel.
Figure
Fan noise spectra reduced by adaptive duct liner II of AAC technology I with and without mobile reflective plates. Fan speed 6000 rpm; measuring point 60 deg; with and without mobile reflective plates; acoustic absorbent material; poal + acousticel.
The variation of fan noise overall SPL reduction is shown in Figure
Variation of overall fan noise reduction by adaptive duct liner II of AAC technology II model with duct liner porosity change. Fan rotational speed 4000 rpm; acoustic measuring point 60 deg.; fixed perforated plate hole dia. = 1.0; pitch = 2.0 acoustic absorbent material stuck on plate; poal + acousticel.
The fan noise overall SPL reduction is shown in Figure
Effect of tunable perforated plate hole diameter on overall fan noise reduction by adaptive duct liner II of AAC technology II model. Fan rotational speed 4000 rpm; acoustic measuring point 60 deg.; fixed perforated plate hole dia. = 1.0; pitch = 2.0; acoustic absorbent material stuck on plate; poal + acousticel.
Figure
Fan noise spectra emitted from duct liner II of AAC technology III model and emitted from hard wall duct. Fan rotational speed 6000 rpm; acoustic measuring point 60 deg.
When the mobile reflective plate is fixed at 25 mm position and the porosity of the tunable perforated plate is changed, the variation of the fan overall SPL noise reduction is shown in Figure
Variation of overall fan noise reduction by AAC technology III model with tunable perforated plate position change. Fan rotational speed 4000 rpm; acoustic measuring point 45 deg.; mobile reflective plate position fixed at 25 mm; parallel shift.
In the case when the porosity of the tunable perforated plates is fixed and the mobile reflective plates are moved in the duct liner II and in the case when the porosity of the tunable perforated plates is fixed and the mobile reflective plates are removed from the duct liner II shown as the linear line of 10 dB, the comparison of the fan overall SPL noise reduction is illustrated in Figure
Variation of overall fan noise reduction by duct liner II of AAC technology III model.
Figures
The effect of the acoustic absorbent materials stuck on duct liner II against the fan noise spectra is shown in Figure
Effect of acoustic absorbent material on fan noise spectra emitted from duct liner II of AAC technology III model. Fan rotational speed 3000 rpm; acoustic measuring point 60 deg.; with mobile reflective plate; parallel shift; 25 mm position fixed; without tunable perforated plate.
The comparison of the fan noise absorption performance for three-duct liner II of the AAC technology I model, the AAC technology II model, and the AAC technology III model is illustrated in Figure
Comparison of overall fan noise reduction by three-duct liner II of AAC technology I model, AAC technology II model and AAC technology III model.
Fan rotating speed 4000 rpm
Acoustic absorbent material; poal + acousticel
For overcoming the stringent aircraft noise regulation and solving the recent aircraft noise issues at night flight, the adaptive acoustic impedance control (AAC) technology I, II, and III models were contrived and developed as the fan noise reduction part of the active engine noise control technology for the current and future aircraft propulsion. Following the development of the AAC Technology I, the AAC technology III was contrived and developed, using both the mobile reflective plate apparatus (AAC technology I model) and the tunable perforated plate apparatus (AAC technology II model) set in the acoustic duct liner and by adaptively adjusting reactance and resistance of the acoustic duct liner impedance. For the actual proof of the AAC technology III performance, the advanced fan noise absorption control duct liner II was made on trial basis with the simple control apparatus and the plain device. The duct liner II was examined for each of the AAC technology I, II, and III models, using the high speed fan test facility.
The test results made clear that the duct liner II is available as the excellent fan noise suppressor for the current and future aircraft engines and can be also applied to wide fields of the turbomachinery noise and the factory noise reduction. It can achieve the fan noise reduction higher than O.A. SPL 10 dB (A) at the max fan speed 6000 rpm, containing the reduction of fundamental BPF tone of 18 dB and of 2nd BPF tone of 10 dB in response to the fan speed change from 3000 rpm to 6000 rpm. Also, it was clarified that the tunable perforated plate apparatus (AAC technology II model) is ineffective to the fan noise reduction, in the comparison with the mobile reflective plate apparatus (AAC technology I model).
This paper describes development of the AAC technology I, II, and III and the main results of the actual proof test of the technologies I, II, and III models with the acoustic duct liner II.
Main concluding remarks are as follows. The AAC technology I, II, and III models are the new adaptive noise absorption control technology which uses the simple apparatus and the plain control device assembly. They are different from the recently developed active noise control (ANC) technology which has the complex apparatus and the complicated control device assembly necessary for the acoustic mode detection, the data reduction, and the acoustic cancellation. The fan noise absorption in the low frequency region from 300 Hz to 1000 Hz is largely reinforced by removing the honeycomb panel structure from the duct liner and by enlarging the acoustic resonant chamber and the absorbent chamber in the AAC technology I, II, and III models. In addition, the mobile reflective plate can achieve both the large peak frequency shift and the big peak level increase of the acoustic absorption spectra, in response to fan speed change. The fan overall SPL noise reduction of 14.8 dB (A) including the fan 1st BPF tone noise reduction of 18.0 dB is achieved by the duct liner II of the AAC technology III model. The AAC technology I model can reduce the same fan noise level as the ones obtained by the AAC technology III model. However, the AAC technology II has lower frequency fan noise reduction performance than the AAC technology III model. In the fan noise reduction, the mobile reflective plate apparatus is more effective and is more responsible for the fan noise spectra change than the tunable perforated plate apparatus. The fan noise reduction of 1.5 to 2.0 times of the ones obtained by the conventionally used acoustic duct liners can be achieved by moving the mobile reflective plates and by controlling it in the AAC technology I and III models. The duct liner II has the great possibility of the more fan noise reduction by the use of the more efficient acoustic absorbent materials stuck on the acoustic resonant chamber and on the mobile reflective plates. Therefore, the choice of the adaptive acoustic absorbent materials is important for reinforcement of the fan noise reduction performance. Also, it has the great possibility to be easily extended to the self-controlled smart acoustic-lined duct using the very simple feedback loop to find the lowest overall sound pressure value of fan noise. The adaptive duct liner II has the great possibilities for overcoming the ICAO Chapter 4 for the fan noises of the supersonic and subsonic aircraft transports. The acoustic absorption control silencers and the panel liners developed as the application of the experimental results of these AAC technology I, II, and III models can be used for the improvement noise issues in the wide engineering fields of the traffics, the mechanics, the electronics, the architecture, and the civil.
The authors would like to thank many staffs at JAXA, Sugawara N., Takeda K., Takamori S., and students Endo H. and Nishimoto Y. for helpful discussion and assistant on this experiment.