On 27 March 2026 the ICAO Council adopted Chapter 16, a subsonic noise standard that will require new large aircraft types to come in at least 23 EPNdB below Chapter 3 limits, 6 EPNdB tighter than today’s Chapter 14 (ICCT Policy Update, May 2025; ICAO, Reduction of Noise at Source). EPNdB, effective perceived noise in decibels, is the certification unit that weights loudness, tones and duration. Much of that new margin will have to come from the engine, and inside the engine from the acoustic liner, the perforated honeycomb wall that lines the inlet and the bypass duct. History shows how large the lever can be: a 1971 flight evaluation of treated Boeing 707 nacelles cut landing-approach noise by 15.5 EPNdB (NASA CR-1711, 1971), while a 2022 European study estimated just 0.7 to 0.9 EPNdB at aircraft level for the next generation of liner concepts (ARTEM Final Achievements 2022). We aggregated data from ICAO, EASA type-certificate noise data sheets, NASA Langley and Glenn technical reports, the FAA CLEEN program, the FAA ASCENT center of excellence, and the EU ARTEM project. For the community side of the same problem, see our roundup on aircraft noise exposure around airports; this page stays on the engine.
TL;DR
- Chapter 16, adopted 27 March 2026, applies to new aircraft type designs from 2029 (ICAO, Reduction of Noise at Source 2026).
- Larger new types must sit at least 23 EPNdB and smaller ones at least 19 EPNdB below Chapter 3 limits under Chapter 16 (ICCT Policy Update 2025).
- Jet aircraft show cumulative margins of 8 to 15 EPNdB against Chapter 14 (EASA European Aviation Environmental Report 2025).
- The heaviest Airbus A350-1041 sits a combined 25.6 EPNdB under its three per-point limits, our sum from the data sheet (EASA TCDSN EASA.A.151, Issue 32, 2026).
- A 737-9 at 88,314 kg sits a combined 22.7 EPNdB under its limits, our sum from the data sheet (EASA TCDSN EASA.IM.A.120.6, Issue 14, 2026).
- Treated nacelles cut 707 approach noise by 15.5 EPNdB but take-off noise by only 3.5 EPNdB (NASA CR-1711, 1971).
- A seamless inlet liner raised the effective treated area by 78% on a 777-300ER test aircraft (Boeing QTD2 release 2005).
- Next-generation liner concepts were worth an estimated 0.7 to 0.9 EPNdB on a 2025 large long-range aircraft (ARTEM Final Achievements 2022).
- A slot-based perforate cut the drag penalty of round-hole liner face sheets by 50% against a 60% goal (NASA TM-20220014642, 2023).
- The FAA CLEEN Phase IV draft budgets about 190 million dollars for 2025-2029 (FAA Office of Environment and Energy, November 2024).
- CLEEN’s noise goal is a 25 dB cumulative reduction relative to Stage 5 (FAA CLEEN Phase III announcement 2021).
1. Certification Stringency: From Chapter 3 to Chapter 16
Noise certification works on three measurement points: lateral (beside the runway during take-off), flyover (under the take-off path) and approach (under the landing path). Each point has a limit that grows with the aircraft’s maximum take-off mass, and the “cumulative margin” is the sum of how far the aircraft sits below the limit at all three. Every new chapter of ICAO Annex 16 raises the required cumulative margin, so each chapter is a statement about how much quieter the next generation of engines and nacelles must be.
The new chapter is the first tightening since Chapter 14 was adopted in 2014, and it is asymmetric. Larger new types must find 6 more EPNdB of cumulative margin beyond Chapter 14, smaller ones only 2 (ICCT Policy Update, May 2025). The same analysis frames that as a continuation of a trend of about 0.6 EPNdB per year of cumulative reduction for large aircraft. The applicability is for new type designs, so the aircraft rolling off lines today are not affected; the engineering pressure falls on programs that are still on paper.
| Metric | Value | Source |
|---|---|---|
| Years from which Chapters 3, 4 and 14 became applicable | 1977, 2006 and 2018 | EASA European Aviation Environmental Report 2025 |
| Required cumulative margin below Chapter 3 limits: Chapter 4 vs Chapter 14 | At least 10 EPNdB vs at least 17 EPNdB | ICCT Policy Update (2025) |
| Chapter 14 applicability for new type designs | From 31 December 2017; from 31 December 2020 for aircraft under 55 tonnes | ICAO, Reduction of Noise at Source (2026) |
| Chapter 16 adoption by the ICAO Council | 27 March 2026 | ICAO, Reduction of Noise at Source (2026) |
| Chapter 16 required cumulative margin below Chapter 3 limits | At least 23 EPNdB for larger aircraft; at least 19 EPNdB for smaller aircraft | ICCT Policy Update (2025) |
| Chapter 16 applicability | New aircraft type designs from 2029 | ICAO, Reduction of Noise at Source (2026) |
| Expected Chapter 16 benefit | More than one million people removed from DNL 55 dB areas between 2032 and 2048 | ICAO, Reduction of Noise at Source (2026) |
Context: the European Union is transposing the package. EASA published NPA 2026-02 on 3 July 2026, implementing Amendment 15 to Annex 16 Volume I, with the ICAO amendments applicable from 1 January 2027. The ICCT update also notes that certification itself can take 4 to 5 years, which is why 2029 is closer than it looks for any team choosing a liner architecture today.
2. Certification Margins: How Much Headroom In-Production Jets Have
Margin is the currency the liner engineer is paid in. EASA’s latest environmental report says jet aircraft continue to show cumulative margins of 8 to 15 EPNdB against the Chapter 14 limit, while margins for turboprops are lower (EASA, Aircraft environmental standards, EAER 2025). The same page adds that the average margin of new regional, single-aisle and twin-aisle deliveries has flattened out, because deliveries are now almost entirely types certified in the 2010s. Put simply, the easy step from the previous generation has already been banked.
To make the margin concrete, we read two EASA type-certificate data sheets for noise (TCDSN) and added up the three per-point margins for selected records. The arithmetic is ours and is shown in each row; the levels and limits are as printed by EASA in the A350 data sheet (EASA.A.151, Issue 32, 25 June 2026) and the 737 MAX data sheet (EASA.IM.A.120.6, Issue 14, 19 February 2026).
| Metric | Value | Source |
|---|---|---|
| Jet aircraft cumulative margin against the Chapter 14 limit | 8 to 15 EPNdB | EASA European Aviation Environmental Report 2025 |
| State-of-the-art designs: margin by which they already comply with Chapter 14 | 10 to 15 EPNdB | ICCT Policy Update (2025) |
| A350-1041, Trent XWB-97, 322,000 kg: level vs limit at lateral, flyover, approach | 94.8 vs 102.2; 90.0 vs 100.0; 96.8 vs 105.0 EPNdB | EASA TCDSN EASA.A.151, Issue 32 (2026) |
| A350-1041 at 322,000 kg: sum of the three margins (7.4 + 10.0 + 8.2) | 25.6 EPNdB | Our arithmetic from EASA TCDSN EASA.A.151 |
| A350-941, Trent XWB-75, 210,000 kg: sum of the three margins (9.6 + 17.6 + 7.6) | 34.8 EPNdB | Our arithmetic from EASA TCDSN EASA.A.151 |
| 737-8, LEAP-1B25, 82,644 kg: sum of the three margins (9.8 + 7.6 + 6.7) | 24.1 EPNdB | Our arithmetic from EASA TCDSN EASA.IM.A.120.6, Issue 14 |
| 737-9, LEAP-1B27, 88,314 kg: sum of the three margins (9.8 + 6.2 + 6.7) | 22.7 EPNdB | Our arithmetic from EASA TCDSN EASA.IM.A.120.6, Issue 14 |
Outlier note: the lightest A350-941 record has 17.6 EPNdB of margin at flyover alone, more than the 17 EPNdB cumulative total Chapter 14 asks for across all three points, while its approach margin is only 7.6. Approach was the tightest of the three points in two of the four records above, and the widest in none of them. That matters for liners, because section 3 shows that inlet treatment historically paid off far more on approach than on take-off. As an illustration only (Chapter 16 does not apply to in-production types), the 737-9 record would sit 0.3 EPNdB short of the 23 EPNdB line set for larger new types, and the heaviest A350-1041 would clear it by 2.6.
3. Measured Attenuation: What Liners Have Delivered in Flight and Engine Tests
The measured record spans 55 years and shows a shrinking increment. Treated nacelles on a Boeing 707-320C with JT3D-7 engines cut landing-approach noise by 15.5 EPNdB (16 PNdB), against only 3.5 EPNdB on take-off (NASA CR-1711, 1971). A liner only treats the fan and compressor noise travelling through the inlet and bypass duct, so a gap of 15.5 versus 3.5 EPNdB between approach and take-off says where that noise mattered most on a certification day. Most recent available data for a full aircraft-level liner retrofit: NASA CR-1711, 1971.
Several later gains came from more treated area and fewer seams rather than from new materials. A splice is the joint between liner panels; each one is an untreated strip that scatters sound. Boeing’s Quiet Technology Demonstrator 2 on a 777-300ER reported that a seamless inlet liner raised the effective treated area by 78% without making the nacelle larger (Boeing QTD2 release, November 2005). Insertion loss, used in the engine tests below, is the noise reduction measured by swapping a hard wall for the liner.
| Metric | Value | Source |
|---|---|---|
| Boeing 707 with treated nacelles: noise reduction on landing approach, on take-off, and on take-off with power cutback | 15.5 EPNdB; 3.5 EPNdB; 4.5 EPNdB | NASA CR-1711 (1971) |
| 777-300ER seamless inlet liner: effective treated area vs the previous production liner | +78% | Boeing QTD2 release (2005) |
| 777-300ER QTD2: reduction in fan tones heard at the front of the aircraft | Up to 15 dB | Boeing QTD2 release (2005) |
| One-piece (zero-splice) intake liner vs a traditional two-piece intake, in trials | 3 to 4 dB lower noise | GKN Aerospace release (2008) |
| DART turbofan inlet, peak total insertion loss: advanced multi-layer liner vs conventional single-layer liner | 5.9 dB vs 6.2 dB | Sutliff et al., AIAA/CEAS Aeroacoustics 2019 |
| DART turbofan inlet, peak tonal insertion loss: advanced vs conventional liner | 7.46 dB vs 6.96 dB | Sutliff et al., AIAA/CEAS Aeroacoustics 2019 |
| DART turbofan, forward-quadrant overall sound pressure level reduction (100 Hz to 40 kHz) | 1.5 to 3.2 dB | Sutliff et al., AIAA/CEAS Aeroacoustics 2019 |
| Multi-focal and slanted-septum liners: estimated benefit for a 2025 large long-range aircraft | 0.7 to 0.9 EPNdB | ARTEM Final Achievements (2022) |
Context: the DART rig is a small research turbofan (fan diameter about 14 inches, bypass ratio 7.6), so its absolute numbers do not scale to an airliner; its value is the head-to-head comparison (NASA NTRS 20190025673). A “degree of freedom” here means a resonant layer: single-layer (SDOF) liners have one cell depth, multi-layer (MDOF) liners stack or vary depths to absorb a wider band. The GKN figure is a 2008 supplier trial (GKN Aerospace via Airframer), and the ARTEM figure is an industrial assessment, not a flight test. Most recent available data: Boeing QTD2 (2005) for a seamless-liner flight result, GKN Aerospace (2008) for a zero-splice trial, the DART test (2019) for a published NASA engine comparison of liner types, and ARTEM (2022) for an aircraft-level estimate; the later QTD3 inlet liner flight test is discussed in the Data watch note. The test data that would show how such absorbers are located on a real engine comes from phased arrays, the subject of our roundup on acoustic cameras and noise source localization.
4. The Price of Treatment: Weight, Drag and Fuel
A liner is never free. Every square metre of treatment adds mass, its perforated face sheet adds skin-friction drag, and its depth competes with structure for the space inside a nacelle. The 1971 Boeing 707 study estimated that its production liner package would add 3,140 lb to operating empty weight, and measured fuel mileage fell by 0.7 to 2.1 percent at cruise (the same NASA report). It is still the most recent published full-aircraft penalty accounting we could find; modern manufacturers do not publish equivalent figures.
The current research agenda attacks exactly these penalties. NASA Langley’s low-drag liner work measured drag in the Grazing Flow Impedance Tube, a rig where air flows along the liner surface rather than into it (NASA TM-20220014642, 2023). The FAA ASCENT Project 079 optimization shows the space side of the trade: printed lattice cores reached their broadband optimum at roughly a third of the depth of a classic honeycomb.
| Metric | Value | Source |
|---|---|---|
| Boeing 707 liner package: estimated operating empty weight increase, and measured fuel mileage loss at Mach 0.80 to 0.83 | 3,140 lb; 0.7 to 2.1 percent | NASA CR-1711 (1971) |
| Boeing 707 liner retrofit: cost per airplane, direct operating cost increase (international; domestic), and capacity payload range reduction | 1 million dollars; 9.2 and 9.6 percent; about 200 nautical miles | NASA CR-1711 (1971) |
| Slot-based perforate: test speeds and tone frequencies in the Grazing Flow Impedance Tube | Mach 0.3 and 0.5; 400 to 3,000 Hz at 140 and 150 dB | NASA TM-20220014642 (2023) |
| Slot-based perforate: reduction in the drag penalty of conventional round-hole perforates, vs the program goal | 50% achieved vs 60% goal | NASA TM-20220014642 (2023) |
| Broadband-optimized liner depth: honeycomb single-layer vs Diamond lattice core | 36.0 mm vs 11.6 mm | ASCENT Project 079 Annual Report (2025) |
| Maximum liner depth allowed in the ASCENT 079 design study | 1.5 in (38.1 mm) | ASCENT Project 079 Annual Report (2025) |
Context: the 707 cost figures are 1971 dollars and were for a retrofit, not a clean-sheet design, so they overstate what treatment costs when it is designed in from the start. NASA notes that further drag reductions may be possible but would need lower measurement uncertainty to be proven statistically.
5. Next-Generation Liner Research: Multi-Layer, Printed and Lower-Frequency Designs
The research direction is driven by engine architecture. NASA’s DART paper states the problem plainly: higher bypass ratios and shorter nacelles need fan noise reduction over a wider frequency range, with less wall area to put it on. A design study for the 22-inch ADP fan rig predicted that an inlet double-layer liner would be 83% more effective per unit area than a 1992-technology liner, and a segmented aft double-layer liner 86% more efficient, against a program goal of 25% (NASA NTRS 20220015374). Those are predictions from a model-scale design report, not flight results. Most recent available data for a published per-unit-area liner benchmark: NASA NTRS 20220015374, posted 2022 and measured against 1992-technology liners.
The 2024-2026 NASA Langley output is about measurement and modeling discipline, the unglamorous step before a new liner can be certified with confidence. One recent paper found that the reference microphone in a standard impedance tube can misread the level at the sample surface by up to 35 dB when spacers are used (NASA NTRS 20260005691, 2026). Another showed that clustering face-sheet holes improves lower-frequency absorption, part of the wider band that shorter nacelles need to cover (Kreitzman and Jones, AIAA Journal 2025).
| Metric | Value | Source |
|---|---|---|
| 22-inch ADP fan rig design study: predicted gain per unit area vs 1992 baseline, inlet single-layer liner | 34% more effective | NASA NTRS 20220015374 (posted 2022) |
| 22-inch ADP fan rig design study: inlet double-layer liner and aft segmented double-layer liner vs 1992 baseline | 83% and 86% more effective | NASA NTRS 20220015374 (posted 2022) |
| NASA Langley perforate face-sheet model validation dataset | 73 normal-incidence samples and 9 grazing-flow samples, at Mach 0.0, 0.3 and 0.5 | NASA NTRS 20240006417 (2024) |
| Additively manufactured samples in the hole-clustering study, and accurate prediction range | 19 samples; accurate up to 3,000 Hz | Kreitzman and Jones, AIAA Journal (2025) |
| Possible level error between the impedance-tube reference microphone and the sample surface | Up to 35 dB | NASA NTRS 20260005691 (2026) |
| ASCENT 079 lattice liner optimization: design samples per liner type, and frequency range | 500 samples; 300 to 3,400 Hz | ASCENT Project 079 Annual Report (2025) |
| ASCENT 079 printed face sheet: designed vs measured percent open area | 5.94% vs 4.26% | ASCENT Project 079 Annual Report (2025) |
Context: the open-area shortfall in the last row is the practical warning for printed liners. A face sheet that prints with 28% less open area than designed, a figure we derived from (5.94 minus 4.26) divided by 5.94, will not have the acoustic resistance the optimizer assumed. Face-sheet perforation is the same lever studied in architectural micro-perforated panels; our roundup on micro-perforated absorbers covers that design space. NASA Langley’s transmission line impedance model, used at the center for five decades, remains the baseline these new geometries are measured against (NASA NTRS 20250004554).
6. Research Programs, Funding and Noise Goals
The US money behind quieter nacelles flows mostly through public-private programs. The FAA’s CLEEN program has a draft Phase IV budget of about 190 million dollars for 2025-2029, the largest of its four phases, with industry matching FAA funds 1:1 (FAA Office of Environment and Energy overview, November 2024). Nacelle acoustics is explicitly on the Phase III list: Boeing’s Advanced Nacelle Next Generation Inlet, Collins’ Large Cell Exhaust Acoustic Technology, Safran’s Acoustic Air Inlet Lip Skin and GE’s Advanced Acoustics.
The goals have stayed ambitious while the delivered increments stay small. CLEEN’s 25 dB cumulative target relative to Stage 5 sits against project-level liner gains measured in fractions of an EPNdB (section 3). NASA’s Environmentally Responsible Aviation project set an even steeper target of 42 dB cumulative below Stage 4, but that number assumed a hybrid wing body airframe that shields engine noise, not a tube-and-wing aircraft with better liners (Thomas, Burley and Olson, NASA Langley, AIAA 2010-3913). Most recent available data for that system-level assessment: NASA Langley, AIAA 2010-3913 (2010).
| Metric | Value | Source |
|---|---|---|
| CLEEN Phase I, II and III: FAA budget and timeframe | About 125 million dollars (2010-2015); about 100 million (2016-2020); about 125 million (2021-2026) | FAA Office of Environment and Energy overview (November 2024) |
| CLEEN Phase IV (draft): FAA budget, timeframe and target entry into service | About 190 million dollars; 2025-2029; about 2035 | FAA Office of Environment and Energy overview (November 2024) |
| CLEEN noise goal | 25 dB cumulative reduction relative to the FAA Stage 5 standard | FAA CLEEN Phase III announcement (September 2021) |
| CLEEN Phase III awards, and total with industry match | More than 100 million dollars; at least 200 million dollars over five years | FAA CLEEN Phase III announcement (September 2021) |
| Projected effect of CLEEN Phases I and II on the DNL 65 dB contour area by 2050 | 10% reduction vs technology evolution without CLEEN | FAA overview citing Georgia Tech, ASCENT Project 37 (2024) |
| ASCENT Project 079 (printed noise liners): FAA funding, and total with partner match (1.2 + 0.11 + 0.45 + 0.40 + 0.24 million) | 1.2 million dollars; 2.4 million dollars | ASCENT Project 079 Annual Report (2025) |
| EU ARTEM noise project: total cost, EU contribution and partners | 7.9 million euros; 7.5 million euros; 24 partners | ARTEM Final Achievements (2022) |
| Hybrid wing body study: extra system-level reduction from a crown pylon acoustic liner | 0.8 dB | NASA Langley, AIAA 2010-3913 |
Context: the FAA’s 2021 announcement says Phase III technologies could enter commercial service by 2031 (FAA newsroom, September 2021), so liners funded today line up with the first Chapter 16 type certifications rather than with today’s fleet. ICAO’s independent experts set medium-term (2027) and long-term (2037) noise goals in 2019 (ICAO Doc 10127), and EASA notes the 2027 goal is approaching and should be reviewed. The ARTEM final report is published by DLR.
Summary: Commercial Aircraft Engine Nacelle Acoustic Liner Attenuation by the Numbers
| Metric | Value | Source |
|---|---|---|
| Chapter 16 adoption by the ICAO Council | 27 March 2026 | ICAO, Reduction of Noise at Source (2026) |
| Chapter 16 applicability | New aircraft type designs from 2029 | ICAO, Reduction of Noise at Source (2026) |
| Required cumulative margin below Chapter 3 limits: Chapter 14 vs Chapter 16 (larger; smaller aircraft) | At least 17 EPNdB vs at least 23 EPNdB; at least 19 EPNdB | ICCT Policy Update (2025) |
| CLEEN Phase III awards, and total with industry match | More than 100 million dollars; at least 200 million dollars over five years | FAA CLEEN Phase III announcement (2021) |
| Jet aircraft cumulative margin against Chapter 14 | 8 to 15 EPNdB | EASA European Aviation Environmental Report 2025 |
| A350-1041 at 322,000 kg: sum of the three per-point margins | 25.6 EPNdB | Our arithmetic from EASA TCDSN EASA.A.151 (2026) |
| 737-9 at 88,314 kg: sum of the three per-point margins | 22.7 EPNdB | Our arithmetic from EASA TCDSN EASA.IM.A.120.6 (2026) |
| Boeing 707 treated nacelles: approach vs take-off noise reduction | 15.5 EPNdB vs 3.5 EPNdB | NASA CR-1711 (1971) |
| Inlet double-layer liner: predicted effectiveness per unit area vs 1992 baseline | 83% higher | NASA NTRS 20220015374 (posted 2022) |
| 777-300ER seamless inlet liner: effective treated area gain | +78% | Boeing QTD2 release (2005) |
| Zero-splice intake vs two-piece intake | 3 to 4 dB lower noise in trials | GKN Aerospace release (2008) |
| DART inlet: peak total insertion loss, advanced vs conventional liner | 5.9 dB vs 6.2 dB | Sutliff et al., AIAA/CEAS 2019 |
| Next-generation liner concepts on a 2025 large long-range aircraft | 0.7 to 0.9 EPNdB | ARTEM Final Achievements (2022) |
| Slot-based perforate drag penalty reduction vs goal | 50% vs 60% | NASA TM-20220014642 (2023) |
| Broadband-optimized depth: honeycomb vs Diamond lattice core | 36.0 mm vs 11.6 mm | ASCENT Project 079 Annual Report (2025) |
| Impedance-tube reference microphone level error | Up to 35 dB | NASA NTRS 20260005691 (2026) |
| CLEEN Phase IV draft FAA budget | About 190 million dollars (2025-2029) | FAA Office of Environment and Energy overview (2024) |
| CLEEN noise goal | 25 dB cumulative relative to Stage 5 | FAA CLEEN Phase III announcement (2021) |
Methodology and Sources
We read every source listed below during research for this update and took each figure from the document itself. Where a row says “our arithmetic”, the calculation is shown in the row and uses only numbers printed by the source.
- ICAO, Technology Standards: Reduction of Noise at Source (2026): icao.int
- ICCT, The International Civil Aviation Organization’s CAEP/13 Aircraft Noise Standards, Policy Update by Dan Rutherford, May 2025: theicct.org
- EASA, NPA 2026-02, Implementation of the latest CAEP amendments to ICAO Annex 16 (July 2026): easa.europa.eu
- EASA, European Aviation Environmental Report 2025, Aircraft environmental standards: easa.europa.eu
- EASA, TCDSN EASA.A.151 Airbus A350, Issue 32 (25 June 2026): easa.europa.eu
- EASA, TCDSN EASA.IM.A.120.6 Boeing 737 MAX, Issue 14 (19 February 2026): easa.europa.eu
- NASA CR-1711, Study and development of turbofan nacelle modifications to minimize fan-compressor noise radiation, Volume 1 (1971): ntrs.nasa.gov
- Boeing, Boeing and Industry Partners Prove Quiet Technologies for Commercial Jets (QTD2), November 14, 2005: boeing.mediaroom.com
- GKN Aerospace, zero-splice intake liner release (2008), via Airframer: airframer.com
- Sutliff, Nark, Jones and Schiller, Design and Acoustic Efficacy of a Broadband Liner for the Inlet of the DGEN Aero-Propulsion Research Turbofan, AIAA/CEAS Aeroacoustics Conference 2019: ntrs.nasa.gov
- ARTEM, Project Overview and Final Achievements (DLR, 2022): dlr.de
- Howerton and Jones, Low-Drag Acoustic Liner Development, NASA TM-20220014642 (2023): ntrs.nasa.gov
- Low, Premo and Yu, 22-inch ADP Fan Rig Liners Design Report, NASA contractor report (posted 2022): ntrs.nasa.gov
- Jones, Kreitzman and Nark, Examination of Perforate Facesheet Impedance Prediction Models (2024): ntrs.nasa.gov
- Kreitzman and Jones, Assessment of Acoustic Liner Hole Clustering in a Normal-Incidence Impedance Tube, AIAA Journal (2025): ntrs.nasa.gov
- Kreitzman, Consideration of Distance from Sample Surface to Reference Microphone in a Normal-Incidence Impedance Tube (2026): ntrs.nasa.gov
- Jones, Nark and Kreitzman, A Transmission Line Impedance Prediction Model for Acoustic Liners (2025): ntrs.nasa.gov
- FAA ASCENT, Project 079 Novel Noise Liner Development Enabled by Advanced Manufacturing, 2025 Annual Report: s3.wp.wsu.edu
- FAA Office of Environment and Energy, Overview and Update for the CLEEN Consortium, November 20, 2024: faa.gov
- FAA, FAA Awards 100M dollars to Develop Next Generation of Sustainable Aircraft Technology, September 10, 2021: faa.gov
- Thomas, Burley and Olson, Hybrid Wing Body Aircraft System Noise Assessment With Propulsion Airframe Aeroacoustic Experiments, AIAA 2010-3913: ntrs.nasa.gov
- Data watch: (1) ICAO describes Chapter 16 as 6 dB and 2 dB more stringent, while ICCT gives the same steps as EPNdB of cumulative margin; the official Annex 16 Amendment 15 text, and EASA’s final rule following NPA 2026-02, will settle the exact wording and the mass split between larger and smaller aircraft. (2) EASA says jets show 8 to 15 EPNdB of margin to Chapter 14; ICCT says 10 to 15 EPNdB for state-of-the-art designs. (3) EASA lists Chapter 14 as applicable from 2018, ICAO from 31 December 2017; both refer to the same rule. (4) Our TCDSN margin sums assume the per-point limits printed in the data sheets are the Chapter 3 per-point limits that the cumulative requirement is counted against; check each record’s certification basis before reusing them. (5) The aircraft-level weight, fuel and cost penalties come from a 1971 report, and the 22-inch ADP liner predictions were benchmarked against 1992 technology although the report was posted to NTRS in 2022; no manufacturer publishes current equivalents. (6) The FAA lists CLEEN Phase III as 2021-2026 in its 2024 overview, while its 2021 announcement described a five-year period; Phase IV figures are marked draft. (7) Boeing and NASA Langley have published phased-array results from the QTD3 flight test of a NASA-designed advanced inlet liner on a 737 MAX-7 (NTRS 20200002461), but the record we read reports no dB values, so it is not used here.
Last updated: October 18, 2026. We update this roundup quarterly, and the next refresh is expected when ICAO publishes the Annex 16 Volume I Amendment 15 text with the Chapter 16 limit equations and EASA adopts its final rule following NPA 2026-02.