Ear plugs

Sounding off on noise

It's the bane of modern life, or a sign of a vibrant society. But while excessive noise is definitely dangerous, engineers are learning to mitigate, modify and redesign it.

For many Europeans who live near major airports or under flight-paths, last year's Icelandic ash clouds had a welcome silver lining: a few days of peace and quiet. It was a reminder, if any were needed, of just how noisy modern life has become, due in large part to the work of engineers and scientists.

Fortunately, though, engineers and scientists are also working just as hard – if not harder – to fix that. They need to as well, because excessive noise is now blamed not only for hearing loss but for stress and hypertension problems, for making workplaces more dangerous by masking hazards and warning signals, and even for causing social and sexual problems.

And while noise in the public space has evolved and grown, noise in the industrial space has changed too. Gone are many of the noisiest old machines, but in their place have come new sources of noise, and new regulations for those tasked with managing it.

One of the biggest dangers is if workers – especially younger workers – don't take the risk seriously, says Dr Odd Pettersen, research director at Norwegian technology group SINTEF's ICT Acoustics division.

'It's like smoking – you don't get the cancer for another 50 years,' he says. 'Showing that to young people is the problem. We made a demonstration of hearing loss for Statoil, we took a video of a comedy sketch and put on more and more sound loss. After a while it's impossible to hear the jokes. We also let [the audience] put in earplugs and see the effect of hearing loss that way.'

Pettersen is part of the team that took active earplug technology, originally developed for the military, and applied it to earplugs for workers on offshore platforms (see news, E&T Vol 10 #13). He says that a key feature of this is supporting and reinforcing the wearer's behaviour. For instance, the earplug itself measures how well it is attenuating noise, so if you fit it wrongly, you are signalled to re-fit it.

And he says the whole field is a complex interplay of different environmental and engineering factors, with unintended and unexpected consequences: 'For example, the oil and gas industry had insulated for noise, but then they had corrosion problems so they had to remove the insulation, and now noise loads are a big problem again.'

New technologies also change the 'soundscape', says Maria Franco-Jorge, principal engineer in NVH (noise, vibration and harshness) at motor industry consultancy MIRA. She says that while car manufacturers have experience in soundproofing, the arrival of new electric and hybrid powertrains is exposing the limits of that expertise.

She explains that while electric motors are quieter of themselves, 'if you have different motors and different shaft weights, you get new heavy things rotating', which produces a different NVH profile. 'There's a lot of research now concerning putting motors into wheels, which will add mass to the wheels and could have a major effect.

'One other cause of noise in electric and hybrid vehicles is the weight of the batteries. Hybrid vehicles are also noisier on broadband noise because they have taken sound-proofing out' to reduce weight and compensate for the batteries, she adds.

Safety first

Then there is the chicken-and-egg problem of road noise, she says. 'The tyre manufacturers say they cannot make quieter tyres without compromising safety and durability. They blame road engineers for not making quieter tarmac, but the road engineers say quiet tarmac is not so good for drainage, and that we need quieter tyres!'

She adds that even when engineers manage to reduce noise levels in one area, it simply shifts the perceptual focus elsewhere. For example, the driver no longer gets the aural feedback they expect from the engine, or starts to hear other niggling little noises – both interior and exterior – that the engine used to mask. The problems are not insuperable, she says, but they can require considerable experience and effort.

Fortunately, just as powerful DSPs (digital signal processors) have made active noise cancellation a practical proposition (see box), greater computing power is also helping in the area of acoustic design.

'We are doing a lot more noise mapping,' says Adam Lawrence, an acoustics, noise and vibration associate with engineering consultancy Atkins. 'There's been a lot of technology change in the software – you can now do noise-mapping both inside and outside buildings, for example to compare different building finishes.'

SINTEF's Dr Pettersen agrees: 'The capacity of PCs to do modelling has grown, and it is now possible to integrate noise modelling with other modelling, where it needed huge computers a few years ago. Now you can do noise modelling of HVAC, say, as you are modelling the physical system – you can listen to the acoustics as you work on the design.

'So far though it is quite specialised, and every industry has its own models. I think it is currently possible to integrate external effects, but it is hard to do. But we will eventually see it will be integrated – we will see Matlab and the others steadily combining different packages.

'For example, we have a vision that a traffic planner should have noise in their planning tool, and be able to listen to the noise effects when they change the roads. Another example is an architect designing a concert hall – acoustics is a separate task, but maybe we could make an expert system and let the architect at least see a first idea of the noise effects.'

That, in turn, reminds us that noise is subjective, so there can be such a thing as good noise – although of course the definition of 'good' is a personal one.

'I like to see the big picture,' says Pettersen. 'I hope at least we see increased consciousness of noise in the public, and a greater awareness of the quality of life and of silence. It could be of value to have quiet areas in cities where people can appreciate the value of silence.'

As anyone who has experienced the absence of sound in an anechoic chamber will know, though, true silence is an eerie and unnatural thing. Adam Lawrence suggests that engineers instead concentrate on quality rather than quantity, creating a soundscape that is more pleasurable and less stressful.

'We know a lot about the adverse effects of noise, but not so much about the other end of the scale,' he says. 'Common sense says the sound of a fountain is better than road noise, and that you can find a noise environment that makes people relax or feel calm, but there has not been much research yet into why and how it works. If you can find health benefits, however, you can include that when you're designing.

'So if you've got a road, you can either change the surface, put up a barrier, or slow the traffic down – and for most noise causes it's those same options. We hope to get software doing auralisation in the outdoors environment to show the difference and hear which is subjectively 'better'. It's the same in industry – if you have a motor or pump creating more noise than you want, which solution sounds best?'

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