Flying can be stressful. Before even boarding you must pass the luggage weigh-in, navigate security and snake your way through the duty-free assault course. But, for many, the flight itself is a real source of stress – particularly when travelling long-haul.
Not least of the inflight annoyances is noise. The hum in the cabin – a mix of the plane’s workings, crew announcements, airflow and fellow passengers’ chatter – can be overwhelming.
But a lab in Hamburg, Germany has been working to reduce the problem, with efforts including the construction of a replica plane cabin. At the ZAL Centre for Applied Aviation Research sits an 8.5 metre-long A320 fuselage demonstrator.
Minus wings, nose or tail, it has the unusual addition of 444 loudspeakers – each one is controlled by an algorithm to test how sound enters and moves around the cabin.
Unlike testing in a real plane cabin, researchers do not have to account for weather changes and are able to change parts to test different acoustics.
Since opening in 2016, ZAL has been researching engine noise. A new loudspeaker array (added in 2017) will enable its researchers to reproduce "the turbulent boundary layer noise" – the main source of broadband (or background) noise on a flight. This noise is produced by the air flow on the fuselage (or cabin) during flight.
We spoke to one of the lab’s acoustic experts, Dr Kay Kochan, head of cabin and systems, about ZAL's work and his tips for improving your experience on flights. Here’s what we learned.
Flights have become a lot quieter
“If you go 50 years back, the aircraft noise was about 90 dba [A-weighted decibels, an expression of the relative loudness of sounds as perceived by the human ear] now it’s decreased to 74dba in the aircraft cabin,” explains Kochan
The evolution of plane engines has helped. Most recently, geared turbofan engines have brought about quieter engines. In these engines, a gearbox is fitted between the fan at the front and the compressor behind it – the gearbox allows the fan to be slowed down, which makes it quieter.
But, as ZAL's research suggests, the noise you hear on a plane doesn’t just come from engines or the passengers and crew. Researchers found that: “high speed turbulent flow over an aircraft fuselage is responsible for a substantial component of the interior noise, and is probably the most important source of cabin noise for jet powered passenger aircraft in steady cruise.”
On modern aircraft, 70 to 80 per cent of the noise comes from this airflow.
White noise makes flights bearable
What makes for good on board acoustics depends – unsurprisingly – on the plane and the number of passengers, says Kochan.
“In large passenger aircraft with more than 150 passengers, the interior noise should be without tonal [or sudden] noise components, which may be caused by hydraulic pumps or cooling fans.
“These tonal noise components are perceived as very disturbing by most passengers. On the other hand, the broadband [or ambient] noise level should be not too high and not too low. This broadband noise is sometimes very helpful to mask the conversations (or snoring) of other passengers. This guarantees a bit of privacy in the aircraft cabin.”
Together, broadband noises from the engine and airflow act as a kind of white noise making the sudden or tonal noises within the cabin more bearable.
So, what's the ideal level of background noise? Kochan says: "If we consider that human speech has a noise level of about 60 dB(A) in 1m distance to the speaker, I assume that 63 – 65 db(A) cabin noise should be enough for masking the noise from other passengers.
"On the other hand, if we assume a 2dB decrease in the cabin interior noise level every 10 years, I would expect a target noise level of 70 dB(A) for next generation of commercial aircraft in 2030. From my perspective there will be a lot of work even for the next generation of acoustic engineers."
Choose your seat wisely for a quieter flight
The front of the plane is the least noisy over the course of the flight, explains Kochan.
“If you cannot afford a business class ticket then I would recommend sitting in the centre of the aircraft [in front of] the wing, for example. The engines are mounted below the wing so you have some kind of covering from the wing”.
A preference for an aisle or window seat divides plane passengers like Marmite. However, according to Kochan, if you’re flying long-haul and noise is your biggest bugbear you should really favour the aisle seat. Research found that the noise experienced in window seats was four decibels higher than the middle and aisle seats.
Meanwhile, at the back of the aircraft, there is a lot of low frequency noise, says Kochan.
Tiny factors can influence noise levels
Kochan says: “We compared the effect of different types of seats, so a fabric seat or a leather seat. Fabric seats produce a cabin that is a little bit quieter than seats with leather covering.”
Even passengers in winter clothing can muffle noise to a measurable level.
But a product that Kochan’s lab has been developing could make a real change – meaning relatively low-noise flights without adding bulk to the plane.
The product in question is a meta-material – a kind of thermal-acoustic insulation. Kochan explains: “A meta-material is a material where you have a resonant structure inside. You have tiny vibration absorbers, for example, which are distributed all over the structure.”
The next step is to “industrialise” the material, so that it can eventually be used in commercial plane cabins.
Quieter inflight announcements could be on their way
According to researchers at Istanbul University, during a flight instantaneous noises (the type which is more likely to disturb), mainly came from the activities of the crew. It suggested that noise levels from in-flight announcements could be reduced by 12 db(A) by replacing live announcements with taped ones.
Improving in-flight announcements is another area of research for ZAL. Kochan says: “If you’ve [been on a flight] you know that the announcements of the flight attendant are sometimes very difficult to understand.
"For this reason, we are working on new methods to optimize the loudspeaker placement in the cabin and to tune the volume automatically.”