October 29, 2015

Studying awakening reactions – interview with noise effects researcher Uwe Müller

NORAH study is the largest European-level study on the effects of air, road and rail traffic noise. The results have recently been released by the Environment and Community Centre (Umwelt- und Nachbarschaftshaus), a subsidiary of the German state of Hesse and part of the Forum Airport and Region (Forum Flughafen und Region). Researchers from various disciplines have been conducting investigations in the area around Frankfurt Airport since 2011 as part of this study. One of these scientists is Uwe Müller from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR). He managed the NORAH study. He is working as Project Manager in the field of noise effects research at the DLR Institute of Aerospace Medicine. In this interview, he discusses how this research area emerged at DLR, how a new method simplifies his work and how noise effects research questions and projects will keep him and his colleagues in the department busy in the coming years.

Interview by Falk Dambowsky

Dr Müller, you are a trained physicist and are currently working at the DLR Institute of Aerospace Medicine. How does a physicist come to do research on sleep?

It was not a straight road. Until I finished my PhD in 2000, I focused on atmospheric spectroscopy. After that, I was open to something new and looked around for interesting topics and work areas. A vacancy at DLR sounded particularly exciting – at the time, a physicist was needed to cover the field of acoustics in the newly created field of noise impact research. I got the job and, since then, have been working in the Department of Flight Physiology in a very interdisciplinary team in which medical doctors, psychologists, a statistician, an electronics technician, a medical technical assistant, a biologist and, indeed, physicists collaborate successfully. This helps our highly complex research work immensely.

When did DLR begin with noise effects research?

The foundations were laid before I started to work for DLR. In 1998, the Flight Physiology Department decided to initiate noise effects research at DLR. The first major field study on the impact of aircraft noise on the sleep of residents living in the area around airports was conducted at Cologne/Bonn airport during 2001 and 2002. I was already involved in this study. Sixty-four candidates were examined for nine nights using polysomnography to investigate their awakening reactions and sleep quality.

What is polysomnography?

With this method, the researchers attach numerous measuring electrodes to the subjects before they go to bed. In our study, during the night we recorded the subjects’ brain waves, eye movements, heart rate and breathing rhythm. In the morning, we returned to each of the test subjects and removed the measuring gear again. This, of course, required a tremendous effort. Overall, it took 15 months to carry out the Cologne/Bonn study at two parallel measuring points – but it was worth the effort. We still use the dose-response curves determined at the time – for aircraft noise and the awakening reactions – for our scientific work. Worldwide, there are only a few groups that conduct noise effects research for measuring sleep quality at this high standard.

How did it continue?

In 2004, we also started to investigate the impact of rail and road noise on sleep using our sleep laboratory. At the same time, I was also involved in an EU project in the area of sound engineering. The issue here was to investigate which aircraft noise components are particularly troublesome and how these could be eliminated.

What were the results?

It was very exciting. Back then, we found out that at the same sound level, freight train noise leads to a significantly higher awakening probability than aircraft noise. After initial laboratory studies, this result was further confirmed by an extensive railway noise field study carried out in the Rhine Valley between 2008 and 2009. In the field of sound engineering, we found that both the tonal components of aircraft noise as well as the 'buzz saw' noise generated by the engine when the machine is started were perceived as particularly annoying. For the investigated types of aircraft we were then able to 'rank' the most disturbing frequencies so that the developers could be given a guide on how to optimise the overall sound more effectively.

In 2006, DLR developed a protection concept for Leipzig/Halle Airport. What made it special?

At present, when aircraft noise protection zones are designated, only the mean physical values for the aircraft noise are indicated in the current laws for nocturnal aircraft noise. Moreover, only certain limitations on the maximum levels that should not be exceeded are currently considered. This means that a few very loud flights have the same mean value as that of a series of quieter flights. But the possible awakening reactions of the local residents may be completely different in these cases. That is why, for the first time, we developed a protection concept for the Leipzig/Halle Airport region that incorporated the physiological noise limits, which relate to awakening reactions. The concept was developed taking into account the awakening reactions based on the data collected from the Cologne/Bonn study. These data are now also being used to determine the Frankfurt Night Index and the Aircraft Noise Index in Zurich. A next and very important step, in our opinion, would be to incorporate these physiological limits in laws regarding aircraft noise protection.

In 2013, the only legally valid method available to study the sleep impact of people living near airports was polysomnography. What new methods are available now and what are their benefits?

Together with the University of Pennsylvania in the United States, we have developed the vegetative-motoric method, which measures the nocturnal heart rate accelerations and body movements of residents near to an airport. Instead of many measuring electrodes, only two, easy-to-apply electrodes need to be attached, as is the case for performing an ECG (electrocardiogram). The volunteers can do this on their own in the evening, before going to bed, allowing us to easily examine much larger groups of subjects with the same budget. We began developing the method and we started to look at data collected from the polysomnography studies in our sleep laboratory. In this way we researched how heart rate and body movement changed during a noise-induced awakening. We were then able to optimise this method significantly based on the data acquired in the NORAH study during 2011 and 2012. In 2013, the methodology was first used in Frankfurt and in 2014 in Philadelphia.

How is sleep history displayed using the new method?

With the vegetative-motoric method, we no longer measure the quality of sleep in the true sense and also do not analyse awakening reactions. Nevertheless, we have been able to demonstrate that recording heart-rate accelerations and body movements provides a more sensitive measurement curve, because the noise dose and the resulting physiological effects can thus be correlated. This is possible because the measured heart-rate accelerations are an accurate measure of the body’s behaviour to aircraft noise events even below the wake-up threshold.

You have been busy with the NORAH noise impact study for the last few years. Which DLR projects are keeping you busy at the moment?

Right now, the department is beginning preparations for a field study on the effects of nocturnal aircraft noise on children's sleep. This study will run until 2018 at Cologne/Bonn airport. As the physiological reactions of children are different from that of adults, we could not apply the simplified methodology; we needed to go back to using polysomnography. This meant that we had to undertake a pilot phase to see whether children would participate well in a study where we had to completely wire them up every night for a period of four nights. This did indeed work out, and we will therefore begin our direct investigations next year.

What are your plans beyond this; what are your goals?

In addition to the research efforts mentioned above, we are carrying out a field study on the effect of road traffic noise on sleep, which will run until 2017. We are repeatedly asked to provide exposure-response curves for noise protection concepts. So far, we have been able to determine this for aircraft noise and railway noise, and now we want to devote ourselves to road noise. In other external projects we are carrying out research on the effects of infrasound, a long-wave sound inaudible to humans and the combined effects of noise and vibration of freight trains on sleep. Overall, the noise effects research we conduct at DLR is aimed at understanding the causes of sleep disturbances and the effects of traffic noise in greater detail. It is important for us to provide a continuously growing knowledge base through our projects that will help to better protect people that are affected by traffic noise.

For further information and results on the recently published NORAH noise impact study, please visit NORAH study.

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Falk Dambowsky

Head of Media Relations, Editor
German Aerospace Center (DLR)
Corporate Communications
Linder Höhe, 51147 Cologne
Tel: +49 2203 601-3959

Dr. rer. nat. Uwe Müller

German Aerospace Center (DLR)
DLR Institute of Aerospace Medicine
Linder Höhe, 51147 Cologne