Only in the past ten years has the negative impact of traffic noise on our health become truly apparent, tells Maarten Hornikx, Professor of Building Acoustics at TU/e's Department of Built Environment. “And by that I mean the consequences of prolonged exposure to background noise. Rather than leading to hearing damage, this elevates a person's stress level. This in turn causes sleep problems, exacerbates cardiovascular diseases and reduces cognitive performance.”

The consequences of relatively new sources of noise nuisance, such as the hum of heat pumps and the rhythmic whistling of wind turbine blades as they sweep round, on the other hand, are as yet unclear, says Hornikx. “Existing regulations generally assume an average noise level, measured over an entire day. While this norm is unlikely to be exceeded, it isn't a very good indicator of the nuisance people experience. This is determined to some extent by the meaning people attach to the noise. It appears, for example, that local residents are less bothered by wind turbines when they themselves are benefitting from the energy being produced.”

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Often it is relatively low tones that cause nuisance. “These carry farther due to their long wave length,” Hornikx explains. “Which also makes it tricky to locate their source. By way of comparison, think of seismic stations measuring earthquakes; in order to locate the epicenter, you need a number of stations spread out over a wide area.” And similarly, people who are bothered by low-frequency sound can find it very difficult to pinpoint the culprit, says Hornikx. “Sometimes they hear the noise indoors while outdoors the drone is drowned out by ambient sound.”

In theory it is even possible for a sound wave to travel almost unnoticed by us until it enters a home and is amplified there - the home acts as a sound box for that particular frequency. “The sound waves corresponding to the lowest tones we can hear, about 20 Hertz, have a wave length that fits roughly in an average lounge.”

When wind turbines are causing the nuisance, identifying the source is not the problem. There is, however, little that can be done, according to Hornikx. "You can't very well insulate a home against low frequencies; you simply can't keep them out. This is one of the challenges in my specialist field." In the case of electrical heat pumps, whose wholesale introduction is supposed to end our use of gas-fired central heating boilers in the coming years, noise nuisance usually goes hand in hand with faulty installation. “Of course they produce a certain amount of noise, but not in excess of the statutory norm. But if the pump is attached in such a way that the wall starts to vibrate with it, it can become a nuisance for an entire apartment building or row of houses. This is not something the manufacturer can do anything about, but the engineer who installed it probably can.”

Attenuation

Ines Lopez Arteaga is both Dean of the TU/e Bachelor College and Chair of Acoustic and Noise Control at the Department of Mechanical Engineering. She agrees that the way in which heat pumps are attached to a building makes a huge difference. “What you really need to avoid is structure-borne sound, where the building starts acting as a loudspeaker, as if were.”

By way of illustration she reaches for a small music box. “Look, if I hold this in the air, you can hear the music playing quite softly, but if I place it on my desk, the music sounds much louder. This is because the desktop starts to vibrate at the same frequencies.” Similarly with heat pumps, the real nuisance is caused by structure-borne noise, she says. “A music box sounds very innocent, but I know a colleague who sleeps poorly due to the noise of a boiler in the basement of his apartment block; and he lives on the fifth floor.”

Lopez Arteaga takes a little packet of paper tissues and places it under the music box. “Do you hear the difference? You can also add this type of sound insulation to heat pumps, simply by placing vibration dampers, rubber cylinders called silent blocks, between the pump's casing and the roof or wall of the building. If you do that really well, the pump itself will also vibrate less and so it will produce less noise anyway.”

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The source of the noise emitted by heat pumps - at least the types that draw their heat from the air - is the fan that sucks in the air, she explains. “The fundamental frequency - the lowest tone - is equal to the rotation frequency (r.p.m.) of the axis multiplied by the number of blades on the fan - usually five or seven. At 1000 r.p.m. this gives you about 100 Hertz. Added to this, you have the harmonics, whose frequency is a multiple of the fundamental frequency.”

It is mainly these identifiable frequencies - or tones - that cause noise nuisance. While the air movement will also cause sound, “this is a noisy, broad spectrum sound that is less intrusive. This, incidentally, is the reason why the profile on car tires is random. Otherwise they would produce annoying tones.”

Looking to the future, Lopez Arteaga thinks that for devices like heat pumps smart forms of sound damping close to the source could help. “Attenuation close to the source, in this case the blades, is in principle the most effective way of preventing noise nuisance. This may involve using metal plates punched with very small holes: micro-perforated plates.” In indoor spaces the same technique, in the form of perforated fabric, is already improving acoustics, she says. “But for machines, of course, you need a robust solution and so metal is an obvious choice.”

Perception

Armin Kohlrausch, Emeritus Professor of Auditory and Multisensory Perception (Department of IE&IS) emphasizes that the context in which sounds occur also plays an important role. “People have a whole string of associations with certain sounds, like those caused by wind turbines or road traffic or aircraft. This means the impact of these sounds bears little comparison with the impact of artificial stimuli in a laboratory environment. This makes noise nuisance research a complex business.”

Sound over which people can exercise some control - coming from a device they can switch off, say - is often perceived as being less annoying, Kohlrausch says. If nothing can be done about a sound, you can still try to mask it with other sounds. This can be done in one of two ways, he says. By drowning out the annoying sound or by distracting attention from it.

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