Next time you walk past an airliner engine on the stand, look past the fan blades at the wall of the intake. 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.

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.

Why the fan needs a muffler

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 chevron nozzles attack at the back of the engine.

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.

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.

A bottle, thousands of times over

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’s natural frequency, the air in the neck moves violently, and some of that energy is lost to friction.

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.

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.

Tuning the honeycomb

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’s holes shift the exact tuning, which is why engineers measure it rather than trust the formula.

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.

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.

The EU-funded SmartAnswer research network on fan-proximity acoustic treatments for turbofan engines.

The problem with seams

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.

The cure is the zero-splice liner: one continuous acoustic barrel from the intake lip to the fan. Airbus engineers Alain Porte, André Robert and Hervé 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.

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.

Holes, carbon and weight

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’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.

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’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.

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 FAA noise stages. 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.

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

Header photo: Roger Schultz / Wikimedia Commons, CC BY 2.0