1、 AEROSPACE INFORMATION REPORT Acoustical Considerations for Aircraft Environmental Control System Design Issued 1989-07 Reaffirmed 2004-06 AIR1826 TABLE OF CONTENTS 1. SCOPE 3 2. PURPOSE . 3 3. GENERAL 3 3.1 Background 3 3.2 Definitions and Fundamentals . 3 3.2.1 Sound. 3 3.2.2 Noise 4 3.2.3 Frequen
2、cy Spectrum . 4 3.2.4 Octave Bands 4 3.2.5 Levels and Decibels. 5 3.2.6 Decibel Calculations 6 3.2.7 Comparative Levels of Common Sounds 9 3.3 Design Approach 9 4. NOISE SUPPRESSION METHODS10 4.1 Location and Orientation10 4.2 Absorptive Materials .12 4.3 Transmission Loss 14 4.4 Mufflers.15 4.4.1 A
3、bsorptive Muffler Design 17 4.5 Vibration Isolation .20 4.6 Damping.22 4.7 Wrappings23 4.8 Enclosures .23 SAE Technical Standards Board Rules provide that: “This report is published by SAE to advance the state of technical and engineering sciences. The use of this report is entirelyvoluntary, and it
4、s applicability and suitability for any particular use, including any patent infringement arising therefrom, is the sole responsibility of the user.”SAE reviews each technical report at least every five years at which time it may be reaffirmed, revised, or cancelled. SAE invites your written comment
5、s and suggestions.Copyright 2004 SAE InternationalAll rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE.T
6、O PLACE A DOCUMENT ORDER: Tel: 877-606-7323 (inside USA and Canada)Tel: 724-776-4970 (outside USA)Fax: 724-776-0790Email: custsvcsae.orgSAE WEB ADDRESS: http:/www.sae.orgCopyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without li
7、cense from IHS-,-,-SAE AIR1826 - 2 - TABLE OF CONTENTS (Continued) 5. APPLICATION OF NOISE SUPPRESSION METHODS.24 5.1 Cooling Units 24 5.1.1 Air Cycle Machines (ACM) .24 5.1.2 Vapor Cycle Machines (VCM) 25 5.1.3 Ram Air Inlets.25 5.2 Bleed Air High Pressure Pneumatic Equipment26 5.2.1 Air Turbine Mo
8、tors (ATM) .26 5.2.2 Emergency Power Units 26 5.2.3 Control Valves 27 5.2.4 Bleed Air Discharges .27 5.2.5 Jet Pumps 27 5.3 Air Distribution Systems.29 5.3.1 Ducting.29 5.3.2 Plenums.33 5.3.3 Outlets33 5.3.4 Grilles .33 5.3.5 Gaspers36 5.4 Air Exhaust Systems36 5.4.1 Flow Regulating Valves36 5.4.2 E
9、xhaust Fans .40 6. REFERENCES41 Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SAE AIR1826 - 3 - 1. SCOPE: This Aerospace Information Report (AIR) is limited in scope to the general consideration of
10、 environmental control system noise and its effect on occupant comfort. Additional information on the control of environmental control system noise may be found in the bibliography and in the documents referenced throughout the text. This document does not contain sufficient direction and detail to
11、accomplish effective and complete acoustic designs. 2. PURPOSE: The purpose of this AIR is to provide aid for the reduction of environmental control system noise levels and to minimize their effect on passengers and crew members through engineering design. The reader should be aware that the materia
12、l included in this document is for general guidance purposes only. 3. GENERAL: 3.1 Background: Noise levels in aircraft passenger cabins and flight stations have primarily been caused, in the past, by the engines and the external airflow. The introduction of high by-pass ratio engines of considerabl
13、y lower noise level, plus improved treatments for aerodynamic noise have resulted in significantly quieter interiors. Against these lower background levels, however, the noise from the environmental control system (ECS) is likely to be more noticeable and may be the dominant contributor to the overa
14、ll interior noise level. It is important, therefore, for the designer to appreciate the principles governing the generation of ECS noise and to understand the available means for controlling and reducing this noise. 3.2 Definitions and Fundamentals: 3.2.1 Sound: Pressure alterations or particle disp
15、lacements propagated in an elastic medium produce sound. In air, sound consists of propagated changes in pressure that alternate above and below the ambient pressure. These changes occur when vibrating objects accelerate the air particles next to them. The speed of sound in a particular medium is de
16、fined as the product of frequency and wavelength: c = f where: c is the speed of sound f is the frequency is the wavelength. The speed of sound in air, which varies with temperature, is given by the formula: c = 20.05 T1/2 m/s (c = 49.03 R1/2 ft/s) (Eq. 1) Copyright SAE International Provided by IHS
17、 under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SAE AIR1826 - 4 - 3.2.1 (Continued): where the temperature T is expressed in Kelvins. At 20 C (68 F) c = 343 m/s (1125 ft/s). The wavelength of sound in air as a function of frequency at 20 C ca
18、n be read from Figure 1. FIGURE 1 - Wavelength in Air Versus Frequency at 20 C (68 F) 3.2.2 Noise: Noise is any undesired sound. (If ambiguity exists as to the nature of the noise, a phrase such as “acoustic noise“ or “electric noise“ should be used.) 3.2.3 Frequency Spectrum: The quality of sound i
19、s determined primarily by its frequency spectrum. A plot of sound pressure level or sound power level against frequency is called a frequency spectrum. 3.2.4 Octave Bands: Part of the audible frequency range, approximately 20 to 10 000 Hz is divided into bands, usually one octave wide. The term octa
20、ve means that the frequency of the upper limit of the pass band is twice that of the lower limit. An octave band sound pressure spectrum is obtained with an octave band analyzer containing electronic filters that pass only those components of the measured sound that have frequencies within the limit
21、s of the filter. The first set of octave band limits shown in Table 1 is specified by USA Standard S1.6-1967. The second set has been in general use for some time and is given for comparison. Data from the second set can be converted to the first set by graphical interpolation with sufficient accura
22、cy. Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SAE AIR1826 - 5 - TABLE 1 - Octave Pass Bands Frequency Bands Frequency, Hertz Preferred Mid-frequency 63 125 250 500 1000 2000 4000 8000 Approxima
23、te frequency limits Lower 45 90 180 355 710 1400 2800 5600 Upper 90 180 355 710 1400 2800 5600 11 200 Previously Used Frequency limits Lower 37-1/2 7 5 150 300 600 1200 2400 4800 Upper 75 150 300 600 1200 2400 4800 9600 Approximate geometric 53 106 212 425 850 1700 3400 6900 mid-frequency 3.2.5 Leve
24、ls and Decibels: The magnitude of sound is expressed in levels. The level is a measure of the ratio of sound power, or sound pressure to a reference value, and is expressed in decibels. The relationship between sound power and sound pressure corresponds to that between heat and temperature. The ear
25、and instruments respond to sound pressure, but equipment sound ratings are best expressed in terms of sound power output. Sound power cannot be measured directly; it has to be computed from sound pressure measurements. Acoustical energy is proportional to the square of sound pressure. Sound pressure
26、 level Lp (or SPL) in decibels, referenced to the approximate threshold of hearing i.e. 2 x 10-5 Pascal (N/m2): L px pxp = = 10 2 10 20 2 1010 5210 5log log (Eq. 2) Where: p = sound pressure in Pa Sound power level Lw (or PWL), in decibels re 10-12 watt, is defined as: L Ww = 10 1010 12log (Eq. 3) W
27、here: W = sound power in watts Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SAE AIR1826 - 6 - 3.2.5 (Continued): Whenever the word level is used as a modifier for sound power or pressure, a refere
28、nce power or pressure has to be given and the ratio between the measured quantity and the reference quantity is used to calculate the decibel level. The reference for sound power level must be noted since 10-13 watt is still often used. If the 10-13 watt reference is used, simply subtract 10 dB from
29、 the power level to obtain levels referenced to 10-12 watt. Figure 2 is a graphical representation of the pressure and power level formulas. When the pressure or power level changes, its ratio to the referenced value changes accordingly. For a 3 dB increase, the power ratio is doubled while a 6 dB i
30、ncrease in pressure level doubles the pressure ratio. About the smallest change the ear can detect is 1 or 2 dB. While stationary, 3 to 5 dB changes are detected, however, they are not detected walking from one area to another. A 10 dB noise increase would be perceived as twice as loud. 3.2.6 Decibe
31、l Calculations: The ratio of the sound power entering an element of a system to the sound power continuing beyond the element, expressed in decibels, is called the attenuation of that element: Attenuation in db WWec( ) log= 10 10 (Eq. 4) Where: We = Sound power entering, watts Wc = Sound power conti
32、nuing, watts Figure 2 may be used to convert the ratio of sound power to decibels and vice versa. If an attenuator absorbs 99.9% of the entering sound power, the power ratio is 100/0.1 = 1000, and the attenuation is 30 dB. Since this is relative, no reference is necessary. If referenced decibel leve
33、ls are used, the attenuator equation reduces to a simple arithmetic expression: Attenuation in dB L Lw we c( ) = (Eq. 5) Where: Lwe = sound power level entering (dB) Lwc = sound power level continuing (dB) Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproductio
34、n or networking permitted without license from IHS-,-,-SAE AIR1826 - 7 - Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SAE AIR1826 - 8 - FIGURE 3 - Chart for Combining Levels 3.2.6 (Continued): The
35、 attenuation of a group of elements through which the sound passes in succession is the arithmetic sum of all individual attenuation values in decibels. When the decibel system is used, the levels of two or more sounds cannot be added arithmetically. Figure 3 may be used to combine two sound levels.
36、 If more than two levels are to be combined, the two highest levels should be combined, and the resulting value combined with the next highest level. Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-S
37、AE AIR1826 - 9 - 3.2.7 Comparative Levels of Common Sounds: TABLE 2 Sound Pressure Sound Pressure (dB) (Pa) psi Common Sounds 160 2 x 103 3 x 10-1 Medium Jet Engine 140 2 x 102 3 x 10-2 Large Propeller Aircraft-Air Raid Siren-Riveting 120 20 3 x 10-3 Discotheque 100 2 3 x 10-4 Heavy City Traffic Sub
38、way 80 2 x 10-1 3 x 10-5 Busy Office 60 2 x 10-2 3 x 10-6 Normal Speech 40 2 x 10-3 3 x 10-7 Quiet Residential Neighborhood 20 2 x 10-4 3 x 10-8 Whisper 0 2 x 10-5 3 x 10-9 Threshold of Hearing Sound essureLevel L pxpPr ( ) log= 20 2 1010 5 (Eq. 6) Where: p = Sound pressure in Pa 3.3 Design Approach
39、: To reduce the ECS noise in the aircraft interior, two approaches are possible, source reductions and transmission path attenuations. In order to reduce noise at the source, it is necessary to identify the components or parts of the system generating the noise and to determine the mechanism of the
40、sound generation process. Modifications may then be designed as appropriate. When all available steps have been taken to reduce noise at the source, any further improvement must come from the addition of insulation, absorption, or other treatments into the path between source and cabin. This require
41、s that the dominant sound transmission path be determined, i.e., either structure-borne or air-borne. Excessive noise can be generated by active components such as air cycle machines and by the flow of air through the system. Air noise is generated at valves, in the ducts at positions of rapid area
42、or shape change, and at exits from the ducts. Noise radiated from the surface of ducts may be due to unsteadiness in the internal air flow. Vibration of ducts and/or equipment may be transmitted via the airframe structure and radiated as noise into the cabin. Copyright SAE International Provided by
43、IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SAE AIR1826 - 10 - Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SAE AIR1826 - 11 - 3.3
44、(Continued): Information on noise suppression methods is given in Section 4. Section 5 provides information on the application of noise suppression methods. Although the major problems arising from ECS noise concern the passenger cabin and flight station, efforts should be made to protect ground cre
45、ws working around the aircraft. Air exhausting from air cycle machine fans or other overboard bleed flows can generate unacceptably high noise levels. Potential noise problems can be minimized in the design stage of equipment (such as fans) by adopting minimum noise generation principles. Further im
46、provement can come from the treatment of exhaust ducting and by reducing the exhaust velocity of emergent jets to as low a value as possible. A general design rule to be borne in mind is that high frequency sound is more easily attenuated and absorbed than low frequency sound. (See Figure 4 for typi
47、cal four-engine jet interior noise level.) 4. NOISE SUPPRESSION METHODS: 4.1 Location and Orientation: The overall acoustic design for occupied areas in a passenger aircraft should be such that the noise due to the ECS does not contribute to the ambient noise level, i.e., it is not greater than a le
48、vel about 10 dB below the combined engine and aerodynamic noise in each octave band. If the ECS equipment noise contains discrete frequencies or tones, the restriction becomes more severe and is a special problem. The ECS and its components can produce noise in two ways: (1) direct radiation of air-borne sound waves into a space and (2) transmission of vibration or structure-borne noise through the mechanical mountings to large radiating surfaces. Noisy ECS equipment should, whenever practical, be mounted in areas as far away acoustically from occupied compartments as possible, such tha
