{"id":32435447,"date":"2026-11-12T10:31:06","date_gmt":"2026-11-12T09:31:06","guid":{"rendered":"https:\/\/migflug.com\/afterburner\/acoustic-liners-jet-engine-nacelle-helmholtz-resonator-honeycomb\/"},"modified":"2026-11-12T10:31:26","modified_gmt":"2026-11-12T09:31:26","slug":"acoustic-liners-jet-engine-nacelle-helmholtz-resonator-honeycomb","status":"publish","type":"post","link":"https:\/\/migflug.com\/afterburner\/de\/acoustic-liners-jet-engine-nacelle-helmholtz-resonator-honeycomb\/","title":{"rendered":"Acoustic Liners: The Honeycomb That Silences Jet Engine Fans"},"content":{"rendered":"<style>.et_pb_title_container h1.entry-title { padding-top: 40px !important; }<\/style>\n<!-- mfsh:top -->\n\n<style>\n.mfsh-trigger{display:inline-flex;align-items:center;gap:10px;height:44px;padding:0 18px;margin:0 0 26px;border:2px solid 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That smooth grey surface is not smooth at all. Up close it is peppered with thousands of tiny holes, as regular as the perforations on a sheet of stamps.<\/p>\n\n<p>Behind every hole sits a small sealed chamber, part of a honeycomb that lines the inside of the nacelle. Together they form one of the quietest pieces of engineering on an aircraft: an acoustic liner, a field of tuned resonators whose only job is to swallow the sound of the fan before it escapes.<\/p>\n\n\n<div style=\"background:#f4f6f9;border:1px solid #e2e7ee;padding:18px 22px;margin:22px 0 26px\"><p style=\"margin:0 0 8px;font-weight:700;color:#0d1117\">Quick Facts<\/p><ul style=\"margin:0;padding-left:18px\"><li style=\"margin:4px 0\"><strong>What it is:<\/strong> A sandwich panel lining the nacelle inlet and bypass duct: perforated face sheet, honeycomb core, solid back sheet<\/li><li style=\"margin:4px 0\"><strong>How it works:<\/strong> Each hole and cell acts as a Helmholtz resonator; airflow through the holes turns sound energy into heat<\/li><li style=\"margin:4px 0\"><strong>Main target:<\/strong> Fan noise: tones at the blade-passage frequency plus broadband noise<\/li><li style=\"margin:4px 0\"><strong>Tuning:<\/strong> Cell depth sets the frequency; a single-layer cell absorbs best near a quarter-wavelength depth<\/li><li style=\"margin:4px 0\"><strong>Typical cell:<\/strong> About 0.375 in (9.5 mm) across, according to NASA<\/li><li style=\"margin:4px 0\"><strong>Zero-splice:<\/strong> One-piece liner with no seams; used on the Airbus A380<\/li><li style=\"margin:4px 0\"><strong>QTD2 result (2005):<\/strong> Seamless inlet liner gave 78% more effective treated area and up to 15 dB lower fan tones in the forward cabin, per Boeing<\/li><\/ul><\/div>\n\n\n<h2 style=\"padding-top:22px\">Why the fan needs a muffler<\/h2>\n\n<p>A modern high-bypass turbofan is quiet behind the engine for the same reason it is efficient: it moves a huge mass of air slowly rather than a small mass quickly. Slower exhaust means less jet mixing noise, which is the low rumble that <a href=\"https:\/\/migflug.com\/afterburner\/chevron-nozzles-nasa-sawtooth-jet-engine-noise-787\/\">chevron nozzles<\/a> attack at the back of the engine.<\/p>\n\n<p>That leaves the fan as the loudest source, particularly on approach, when the engines are throttled back. NASA Langley researchers describe its signature as tones at the blade-passage frequency, the rate at which fan blades sweep past any given point, plus broadband noise from turbulent flow. As bypass ratios have grown, they note, broadband fan noise has become steadily more dominant.<\/p>\n\n<p>The fan noise travels in two directions: forward out of the intake, and backward down the bypass duct. So liners are fitted in both places, on the inlet walls and on the walls of the aft bypass duct. Researchers have been investigating them since the first half of the 1950s.<\/p>\n\n\n<figure class=\"wp-block-image size-large\" style=\"margin:0 0 24px;width:100%\"><img decoding=\"async\" class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" width=\"1024\" src=\"https:\/\/migflug.com\/afterburner\/wp-content\/uploads\/sites\/4\/2026\/11\/nasa-fan-noise-test-facility-1969-quiet-engine.jpg\" alt=\"A large engine test rig mounted on a concrete stand outside a building in the snow at NASA Lewis Research Center, 1969\" style=\"width:100%;height:auto;max-width:100%;display:block\"><figcaption style=\"font-size:13px;color:#777;text-align:center;margin-top:6px;font-style:italic\">NASA&rsquo;s Fan Noise Test Facility at the Lewis Research Center in January 1969, built to gather far-field noise data for the NASA and General Electric Quiet Engine Program. Photo: <a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Fan_Noise_Test_Facility_(GRC-1969-C-00189).jpg\" target=\"_blank\" rel=\"noopener\">NASA Glenn Research Center \/ Wikimedia Commons, public domain<\/a><\/figcaption><\/figure>\n\n\n<h2 style=\"padding-top:22px\">A bottle, thousands of times over<\/h2>\n\n<p>Blow across the neck of an empty bottle and it hums at one note. That is a Helmholtz resonator: a slug of air in the neck bouncing on the spring of air trapped in the bottle. At the bottle&rsquo;s natural frequency, the air in the neck moves violently, and some of that energy is lost to friction.<\/p>\n\n<p>An acoustic liner is the same idea, miniaturised and repeated. The liner is a sandwich: a perforated face sheet on the airflow side, a honeycomb core behind it, and a solid back sheet. Each tiny hole acts as a neck, and each honeycomb cell is the bottle. NASA notes that nearly every liner in a modern commercial nacelle uses this perforate-over-honeycomb layout.<\/p>\n\n\n<div style=\"background:#f8f9fa;border-left:4px solid #1565c0;padding:20px 22px;margin:18px 0 24px;font-size:16px;line-height:1.7\"><div><em>&ldquo;Nearly all liners employed in modern commercial aircraft nacelles are perforate-over-honeycomb structures.&rdquo;<\/em><div style=\"margin-top:10px;font-size:14px;color:#555\"><strong>Martha C. Brown, Chelsea Solano, Jordan R. Kreitzman and Michael G. Jones<\/strong> &mdash; NASA Langley Research Center, AIAA AVIATION Forum paper, 2023<\/div><\/div><\/div>\n\n\n<p>When sound waves of the right frequency hit the face sheet, air pumps in and out of the holes. The friction of that flow through the small perforations, the acoustic resistance, turns sound energy into a tiny amount of heat. The liner does not block the noise like a wall; it absorbs it, one cell at a time.<\/p>\n\n\n<figure class=\"wp-block-image size-large\" style=\"margin:0 0 24px;width:100%\"><img decoding=\"async\" class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" width=\"1024\" src=\"https:\/\/migflug.com\/afterburner\/wp-content\/uploads\/sites\/4\/2026\/11\/cf6-fan-nacelle-perforated-acoustic-liner.jpg\" alt=\"Close-up of the micro-perforated acoustic liner on the inlet wall in front of the fan of a General Electric CF6 engine\" style=\"width:100%;height:auto;max-width:100%;display:block\"><figcaption style=\"font-size:13px;color:#777;text-align:center;margin-top:6px;font-style:italic\">The intake wall in front of the fan of a General Electric CF6 on an Airbus A300. The rows of tiny holes are the face sheet of the acoustic liner. Photo: <a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Fan_%26_nacelle_Airbus_A300%27s_CF-6.JPG\" target=\"_blank\" rel=\"noopener\">Duch.seb \/ Wikimedia Commons, CC BY-SA 4.0<\/a><\/figcaption><\/figure>\n\n\n<h2 style=\"padding-top:22px\">Tuning the honeycomb<\/h2>\n\n<p>The magic number is the depth of the cell. For a simple single-layer liner, the cavity behaves much like a pipe closed at one end, and it absorbs best near the frequency at which its depth is roughly a quarter of a wavelength. At 2,000 Hz, where sound has a wavelength of about 17 centimetres, that means a cell only a little over 4 centimetres deep. The face sheet&rsquo;s holes shift the exact tuning, which is why engineers measure it rather than trust the formula.<\/p>\n\n<p>The width of the cell matters too, but in a different way. NASA says traditional honeycomb cells are about 0.375 inches (9.5 mm) across, small enough that sound inside each cell only travels straight in and out. That makes every cell an independent resonator and also keeps the core stiff enough to be part of the structure.<\/p>\n\n<p>A single depth gives a single best frequency. To widen the band, designers stack two honeycomb layers with a porous septum between them, giving two resonators in series. NASA Langley has gone further, testing liners with many different chamber depths side by side, some made by additive manufacturing. Its researchers found that each chamber depth contributes its own resonance, so a mix of depths can spread absorption across a broad range of frequencies.<\/p>\n\n\n<div style=\"position:relative;padding-bottom:56.25%;height:0;overflow:hidden;margin:24px 0 8px\"><iframe class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" src=\"https:\/\/www.youtube.com\/embed\/zD9YJMTKUN8\" style=\"position:absolute;top:0;left:0;width:100%;height:100%;border:0\" allowfullscreen><\/iframe><\/div>\n\n<p style=\"font-size:13px;color:#777;text-align:center;font-style:italic;margin:0 0 24px\">The EU-funded SmartAnswer research network on fan-proximity acoustic treatments for turbofan engines.<\/p>\n\n<h2 style=\"padding-top:22px\">The problem with seams<\/h2>\n\n<p>Here is the catch. An intake is a barrel, and for decades the liner inside it was built from curved panels joined along longitudinal seams. Those joints, called splices, are not acoustically treated. Worse, they break up the regular pattern of the liner, so sound waves hitting them scatter into new patterns that the liner is less good at absorbing.<\/p>\n\n<p>The cure is the zero-splice liner: one continuous acoustic barrel from the intake lip to the fan. Airbus engineers Alain Porte, Andr&eacute; Robert and Herv&eacute; Batard developed a one-piece nacelle lining for the A380 and were recognised for it by the European Patent Office, which credits the design with helping to make the A380 the quietest long-range airliner of its time.<\/p>\n\n\n<div style=\"background:#f8f9fa;border-left:4px solid #1565c0;padding:20px 22px;margin:18px 0 24px;font-size:16px;line-height:1.7\"><div><em>&ldquo;An intake liner is usually manufactured in two or three curved segments which are assembled to longitudinal splices. But with a zero splice, one-piece design, you get a much better noise attenuation, as you are able to apply an uninterrupted acoustic treatment to the liner, which has the potential to reduce noise.&rdquo;<\/em><div style=\"margin-top:10px;font-size:14px;color:#555\"><strong>Colin Elliott<\/strong> &mdash; Vice-President of Engineering and Product Development, Bombardier Belfast, February 2015<\/div><\/div><\/div>\n\n\n<p>Boeing and its partners proved the point in flight in 2005. On the Quiet Technology Demonstrator 2, a 777-300ER with GE90-115B engines, GE fitted a one-piece inlet liner running from the lip to the fan blades and treated the inlet lip itself instead of leaving it as bare metal. Boeing reported that the seamless construction increased the effective treated area by 78 per cent without making the nacelle bigger, and cut fan tones heard in the front of the cabin by up to 15 decibels.<\/p>\n\n\n<figure class=\"wp-block-image size-large\" style=\"margin:0 0 24px;width:100%\"><img decoding=\"async\" class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" width=\"1024\" src=\"https:\/\/migflug.com\/afterburner\/wp-content\/uploads\/sites\/4\/2026\/11\/acoustic-liner-perforated-facesheet-honeycomb-1979.jpg\" alt=\"A curved perforated metal face sheet lying next to a piece of honeycomb core on a table\" style=\"width:100%;height:auto;max-width:100%;display:block\"><figcaption style=\"font-size:13px;color:#777;text-align:center;margin-top:6px;font-style:italic\">The two main ingredients of an acoustic liner, photographed in an acoustic inlet liner fabrication shop in October 1979: a perforated face sheet and a piece of honeycomb core. Photo: <a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:ACOUSTIC_INLET_LINER_IN_THE_FABRICATION_SHOP_-_NARA_-_17473641.jpg\" target=\"_blank\" rel=\"noopener\">U.S. National Archives \/ Wikimedia Commons, public domain<\/a><\/figcaption><\/figure>\n\n\n<h2 style=\"padding-top:22px\">Holes, carbon and weight<\/h2>\n\n<p>Making a one-piece liner is harder than it sounds. In 2015 Bombardier in Belfast built a one-piece carbon-fibre intake liner for Rolls-Royce&rsquo;s ALPS composite-fan demonstrator, under the European Clean Sky SAGE programme. The University of Limerick researchers who worked on it explained that a carbon composite face sheet brought its own challenges, starting with the holes.<\/p>\n\n\n<div style=\"background:#f8f9fa;border-left:4px solid #d32f2f;padding:20px 22px;margin:18px 0 24px;font-size:16px;line-height:1.7\"><div><em>&ldquo;The manufacture of acoustic liners from carbon fibre composites material presents a host of new challenges. New technologies for the drilling of thousands of small holes had to be developed.&rdquo;<\/em><div style=\"margin-top:10px;font-size:14px;color:#555\"><strong>Dr Trevor Young<\/strong> &mdash; Materials and Surface Science Institute, University of Limerick, March 2015<\/div><\/div><\/div>\n\n\n<p>Every square metre of liner is also a square metre of nacelle that has to be carried around the world. NASA researchers describe the designer&rsquo;s dilemma as two conflicting criteria: maximise liner treatment for noise, and minimise nacelle size and weight. That is why so much current research goes into thinner liners, larger lighter cells and broadband designs that do more with the same wall.<\/p>\n\n<p>Chevrons and liners are two halves of the same noise strategy. Chevrons tame the jet behind the engine; liners eat the fan noise inside it. Both exist because a few decibels decide whether an aircraft meets the certification rules explained in our post on <a href=\"https:\/\/migflug.com\/afterburner\/faa-noise-stages-epndb-heathrow-quota-count-night-flights\/\">FAA noise stages<\/a>. The next time an engine whines past you on approach, remember that most of the noise it might have made is being turned into heat inside a honeycomb you will never see.<\/p>\n\n\n\n<p><em>Sources: NASA Langley Research Center (AIAA papers, NASA Technical Reports Server), Boeing, GE Aerospace, European Patent Office, Bombardier via Airframer, Composites Today, University of Limerick<\/em><\/p>\n<!-- mfsh:bottom -->\n\n<button type=\"button\" class=\"mfsh-trigger mfsh-bottom\" data-mfsh-placement=\"bottom\" aria-haspopup=\"dialog\"><svg><use href=\"#mfsh-i-share\"\/><\/svg>Share this story<\/button>\n\n<!-- \/mfsh:bottom -->\n\n<p class=\"mf-photo-credits\" data-no-translation style=\"font-size:13px;color:#777;font-style:italic;margin:6px 0 18px\">Header photo: <a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:A380_ENGINE.jpg\" target=\"_blank\" rel=\"noopener\">Roger Schultz \/ Wikimedia Commons, CC BY 2.0<\/a><\/p>\n\n\n<style>.mfq{margin:34px 0 8px}.mfq h2{font:26px\/1.3 \"Gilroy semiBold\",Helvetica,Arial,sans-serif;color:#0d1117;padding-top:22px;margin:0 0 6px}.mfq .qa details{border-top:1px solid #5C91FF;margin:0;padding:0}.mfq .qa details:last-of-type{border-bottom:1px solid #e2e7ee}.mfq .qa summary{cursor:pointer;list-style:none;display:flex;justify-content:space-between;align-items:center;gap:16px;padding:22px 26px;font:18px\/1.7 \"Gilroy semiBold\",Helvetica,Arial,sans-serif;color:#0d1117}.mfq .qa summary::-webkit-details-marker{display:none}.mfq .qa summary::after{content:\"+\";font:24px Helvetica,sans-serif;color:#3568e0;flex:0 0 auto}.mfq .qa details[open] summary::after{content:\"\\2013\"}.mfq .qa details:hover summary{color:#3568e0}.mfq .qa .a{font:16px\/1.7 \"Gilroy regular\",Helvetica,Arial,sans-serif;color:#454e5e;padding:0 26px 24px}.mfq .qa .a a{color:#3568e0}@media(max-width:680px){.mfq .qa summary{padding:18px 14px;font-size:17px}.mfq .qa .a{padding:0 14px 20px}}<\/style>\n\n\n<section class=\"mfq\"><h2>Frequently Asked Questions<\/h2><div class=\"qa\"><details open><summary>What is an acoustic liner in a jet engine?<\/summary><div class=\"a\">An acoustic liner is a sandwich panel lining the inside of a jet engine nacelle, in the inlet and the bypass duct. It has a perforated face sheet, a honeycomb core and a solid back sheet. Each hole and cell acts as a small Helmholtz resonator that absorbs fan noise and turns it into heat.<\/div><\/details><details><summary>How does a Helmholtz resonator reduce engine noise?<\/summary><div class=\"a\">In an acoustic liner, each perforation acts like the neck of a bottle and each honeycomb cell like the bottle itself. At the tuned frequency, air pumps rapidly in and out of the holes, and friction in that flow dissipates sound energy as heat. Thousands of these resonators together absorb fan tones and broadband noise.<\/div><\/details><details><summary>How are acoustic liners tuned?<\/summary><div class=\"a\">Acoustic liners are tuned mainly by the depth of the honeycomb cells. A simple single-layer cell absorbs best near the frequency at which its depth is about a quarter of a wavelength, so a cell a little over 4 cm deep suits roughly 2,000 Hz. The perforated face sheet also shifts the tuning.<\/div><\/details><details><summary>What is a zero-splice acoustic liner?<\/summary><div class=\"a\">A zero-splice liner is a one-piece acoustic barrel with no seams. Conventional intake liners are built from two or three curved segments joined at splices, which are not acoustically treated and scatter sound. Airbus used a one-piece lining on the A380, and Boeing&rsquo;s QTD2 tests in 2005 showed a seamless inlet liner cutting forward fan tones by up to 15 dB.<\/div><\/details><details><summary>Why do jet engines need both chevrons and acoustic liners?<\/summary><div class=\"a\">Chevrons and acoustic liners attack different noise sources. Chevrons on the nozzle trailing edge reduce low-frequency jet mixing noise behind the engine. Acoustic liners inside the nacelle absorb fan noise, which dominates on approach and has become more important as bypass ratios have grown.<\/div><\/details><\/div><\/section>\n\n\n\n<script type=\"application\/ld+json\">{\"@context\":\"https:\/\/schema.org\",\"@type\":\"FAQPage\",\"mainEntity\":[{\"@type\":\"Question\",\"name\":\"What is an acoustic liner in a jet engine?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"An acoustic liner is a sandwich panel lining the inside of a jet engine nacelle, in the inlet and the bypass duct. It has a perforated face sheet, a honeycomb core and a solid back sheet. Each hole and cell acts as a small Helmholtz resonator that absorbs fan noise and turns it into heat.\"}},{\"@type\":\"Question\",\"name\":\"How does a Helmholtz resonator reduce engine noise?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"In an acoustic liner, each perforation acts like the neck of a bottle and each honeycomb cell like the bottle itself. 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Acoustic liners inside the nacelle absorb fan noise, which dominates on approach and has become more important as bypass ratios have grown.\"}}]}<\/script><div style=\"background:#f0f4ff;border-left:4px solid #5C91FF;padding:16px 20px;margin:32px 0 8px\"><p style=\"margin:0 0 8px;font-weight:600;color:#333\">Related Posts<\/p><p style=\"margin:4px 0\"><a href=\"https:\/\/migflug.com\/afterburner\/chevron-nozzles-nasa-sawtooth-jet-engine-noise-787\/\">NASA Chevron Nozzles: The Sawtooth That Quieted the 787<\/a><\/p><p style=\"margin:4px 0\"><a href=\"https:\/\/migflug.com\/afterburner\/faa-noise-stages-epndb-heathrow-quota-count-night-flights\/\">FAA Noise Stages and Heathrow&rsquo;s Quota Count: Who Flies at Night<\/a><\/p><p style=\"margin:4px 0\"><a href=\"https:\/\/migflug.com\/afterburner\/fighter-jet-noise-how-loud-decibels-f-35-measured-explained\/\">Fighter Jet Noise: How Loud Is an F-35 in Decibels, Measured<\/a><\/p><p style=\"margin:4px 0\"><a href=\"https:\/\/migflug.com\/afterburner\/turbojet-vs-turbofan-vs-turboprop-turboshaft-bypass-ratio-explained\/\">Turbojet vs Turbofan vs Turboprop: How Jet Engines Differ<\/a><\/p><\/div>\n\n\n<!--mfcred1-->","protected":false},"excerpt":{"rendered":"<p>Acoustic liners line every airliner nacelle with thousands of tuned Helmholtz resonators. How the perforated honeycomb works, how it is tuned and why zero-splice liners matter.<\/p>","protected":false},"author":23,"featured_media":32435321,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"editor_notices":[],"footnotes":""},"categories":[665],"tags":[],"class_list":["post-32435447","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-aviation-world"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v28.8 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Acoustic Liners: How Jet Engines Absorb Fan Noise | MiGFlug<\/title>\n<meta name=\"description\" content=\"Acoustic liners explained: the perforated honeycomb Helmholtz resonators inside jet engine nacelles, how they are tuned and why zero-splice liners matter.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link 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