Acoustics expert


Decibels and reverberation time are (almost) obsolete when it comes to determining perfect room acoustics. Interestingly, this statement comes from two experts at the Fraunhofer Institute for Building Physics. In this interview, building physicist Dr.-Ing. Horst Drotleff and psychologist Dr. phil. Andreas Liebl explain why architects and suppliers of acoustic systems should listen more to the users. The experts use psychoacoustic findings to develop innovative acoustic systems.

Dr.-Ing. Horst Drotleff

Group Leader Room Acoustics

start-page-image Dr.-Ing. Horst Drotleff Group Leader Room Acoustics
  • Dr Drotleff, Dr Liebl – you conduct research and development in the field of room acoustics. To broach an exciting aspect of your work right away: you use psychological findings for this. What do acoustics have to do with our psyche?

A. Liebl: A whole lot. Here at the Fraunhofer Institute, we develop solutions which configure acoustics according to how the room is used. This is fundamental for acoustics: you have to first know how the room is used before you can acoustically optimise it in a suitable way – namely, in line with the subjective psychological auditory impressions.

  • That sounds almost unheard of. Why are physical indicators like decibels not enough for you?

H. Drotleff: In the past, these were the predominant indicators for describing room acoustics. This purely technical approach is a relic from our manufacturing culture in the middle of the last century, when work environments were still very noisy. Today, however, every other workplace is an office workplace. And those have very different requirements. That is why we look at things much more from a user's point of view today. What's important for this is the so-called disturbance load.

A. Liebl: We use the term disturbance load to describe, for example, how strongly people are distracted by sound levels containing information. We can quantify that.

H. Drotleff: Another question is: how should rooms be designed so that people will feel comfortable in them? There are different expectations for a restaurant compared to a classroom, for example. Discretion is important in one, speech intelligibility in the other. Consider the foyer of an insurance group: a large hall, lots of wood, lots of natural stone – but the acoustics do not reverberate, they are elegantly discreet. The objective is to meet the right expectations for the rooms.

  • Can expectations actually be measured objectively?

A. Liebl: Of course. With different psychological methods, like surveys or experiments. In this respect we use the same methodology as our colleagues from engineering. In order to obtain a user opinion, we regard people as measuring instruments. Take the term “noise”, for example: a signal with the same measured value is assessed differently by different people. One man's pleasure is another man's pain. In this regard, the physical measurement is of little consequence, whereas personality traits such as noise sensitivity are.

  • Many different people have to work together in open plan offices. Is there something like a happy medium?

A. Liebl: Definitely. There is no arbitrary individuality in the perception of sound. There are patterns which apply to certain groups of people. We define acoustic conditions for specific uses accordingly.

  • For example?

A. Liebl: Let's stay with the open plan offices. Our examinations show that many users often perceive a work environment as noisy – not because the measured sound level is high, but because there is a lot of talking going on. People just have to listen, that's how we've evolved to be. Additionally introduced sound is perceived as quieter by test subjects because speech intelligibility decreases. And that's despite the fact that a microphone would measure a louder sound level in the room.

Dr. phil. Andreas Liebl

Group Leader Psychoacoustics and Cognitive Ergonomics

liebl Dr. phil. Andreas Liebl Group Leader Psychoacoustics and Cognitive Ergonomics
  • So “the quieter, the better” does not apply?

A. Liebl: No. People are still thinking too much in terms of levels and the minimisation principle. That can't be applied across the board. We have to develop concepts and components for different requirements.

  • Modern architecture uses a lot of concrete and glass. Both materials are acoustically difficult. Do architects have to sacrifice their designs for the sake of acoustics?

H. Drotleff: Not at all. This is where the suppliers in particular come in. Functionality and design are only seemingly contradictory. We have to regard both as integral components of possible solutions. Systems such as StoSilent show that the architectural ideal of seamless surfaces is possible. These systems feature excellent absorption and fit in with the overall design. At first glance, you would not think that they provide any sound absorption.

  • Do architects still not pay enough attention to the issue of acoustics?

A. Liebl: I think so. Acoustics should be part of the planning process from the outset. This often happens too late. Making corrections later on is never ideal. I would like to see planners and acoustics experts work more hand in hand.

H. Drotleff: In the end, we are a support for architects – not an obstacle. When an architect plans a building with a lot of concrete and glass, for example, we just have to develop the concrete to be absorbent and design the glass surfaces so that they, too, absorb sound and provide suitable room acoustics.

  • So you are saying that everything is possible from a building physics point of view?

H. Drotleff: A lot is possible. A good acoustics engineer is always someone who can implement the architect's vision.

Contact info

You can learn more about StoSilent in our brochure and in the reference book. Or you can schedule a meeting with us.