Science Interviews

Interview

Sun, 10th Jan 2010

Analysing Acoustics and the "Cocktail Party Effect"

Jens Holger Rindel, Odeon A/S and Jorg Buchholz, Technical University of Denmark

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Chris- Now, in the past, buildings werenít necessarily designed with the acoustics in mind, which means if you take old structures , like railway stations or concert halls, and then you put in a fancy new electronic PA system, the results can be quite poor quality at best, or maybe unintelligible echoes at worst.  And thatís largely because you don't know beforehand how to compensate for the intricacies of the architecture and then the presence of people and the furniture.  But what if you could use a computer system to simulate what you would hear if you were sitting in any part of the building, listening to the sound system that you're planning to put in?...

NTNU National Concert HallJens -   My name is Jens Holger Rindel and I'm working for a company Odeon A/S in Denmark where we developed room acoustic software.  The software simulates the sound in a space and it can be used for concert halls, operas, theatres, open plan offices, industrial halls, and a lot of places where acoustics is important.

Chris -   So, is the basic premise then that if someoneís going to build a building or put in some infrastructure, without having to put in the infrastructure they can use your program to work out what it will sound like in that structure?  So, letís take an example of, if I'm building a concert hall and I want to work out where to put my speaker system, I can work out how best to arrange the speakers, so that everyone sitting anywhere in the concert gets the best reproduction of the sound?

Jens -   Yes and these results can be calculated, covering all possible positions of the audience, so you can easily see from the results how even the acoustics is in the hall and if there are any bad spots, that should be examined further.

Chris -   Is the program basically using a model of the structure?  So, do you have to feed in a sort of rendition of the arrangement of the building? Where the walls are, where the seats are, so that the program basically sees a virtual construction of the area that you're studying and it uses that to work out how the sound would be experienced within that structure?

Jens -   Yes, thatís correct.  The first step in the modelling is to make a virtual model of the space.  The most efficient way can be to simply have the architectís 3D model, then it may be transferred and imported to the Odeon acoustic software. Then assign the sound absorption from the surfaces and scattering of sound which has something to do with the roughness of the structures, and then it's ready to do the acoustic simulation.

Chris -   Have there been any situations where people are taking your Odeon software and using it to inform either the ground up creation of a building space or putting in sound systems in existing building spaces?

gare du Lyon stationJens -   Well, one example could be a recent use in the Copenhagen Railway Station.  It has recently got a new loudspeaker system and Odeon was used to predict the speech intelligibility that could be obtained for this new system.

Chris -   In other words, to combat the age old problem of, you can't hear the train wherever you're going is departing from platform 3, five minutes ago because you missed it?

Jens - Yes, exactly.

Chris -   So how would you go about solving a problem like that then?

Jens -   In this case, the problem could not be solved by bringing down reverberation in the hall because  it has a very large volume.  So, it was solved instead by introducing line array speakers which are very tall columns of speakers and this is exactly what we can model in the Odeon software.  This allows us to calculate how good the sound system is going to work and itís possible then to choose positions as a listener and try to listen to some sound samples, and thatís what they have prepared for this.  The first one represents that you are standing in the middle of this very big central station.  Thereís a lot of people around as background noise and, 2 metres in front of you, there's a lady talking to you directly...

Copenhagen Central Station Natural Sound

Chris -   So sheís clearly a mathematician as much as anything else, but what is that revealing?

Jens -   Well, you are able to understand her, but itís very obvious that this happens in a very big place and a very reverberant place.

Chris -   And so, what would be the consequence of putting in a whole bank of speakers into that space?  I mean, you just end up with echoes you can't understand presumably?

Jens -   Yes.  I then modelled the new speaker system, but before we listen to the whole system, I should like to give a second example, that we just turn on one of the speakers to give an idea of how the sound would be if you have a sound system which is not well planned...

Copenhagen Central Station Original Sound

Chris -   So thatís the experience weíve all had at the railway station, isnít it?  We just can't hear what the message is.

Jens -   Yes, thatís right.

Chris -   So, what can we do about this.

Jens -   We can then design a proper loud speaker system and with sufficient number of speakers, in this case  it was 20, itís much better than what you normally would experience in such a place...

Copenhagen Central Station Improved Sound

Chris -   So you must agree, that does sound a whole lot clearer.  The whole thing was actually done using a computer simulation of the station and then using that simulation to work out where to put the speakers, to achieve the maximum intelligibility for the people using the station.  I was talking there to Jens Holger Rindel who wrote the Odeon software that actually does that modelling. 

Chris - Now another person using this system is hearing researcher, Jorg Buchholz at the Technical University of Denmark.  He uses Odeon to recreate standardized noisy environments in his laboratory to try to get to the bottom of whatís called ĎThe cocktail party effect.í  This is the fact that even despite huge amounts of background noise, we can usually follow a conversation quite easily.  But people with hearing impairments actually find this really very difficult, but until we can find out why they find it so difficult, itís really hard to make hearing aids that can compensate.

Jorg -   Normal hearing people usually have no problem in communicating in cafeterias or other noisy places, but the hearing impaired often do have severe problems in such situations and hearing aids often do not really help and thatís exactly what weíre interested in.  Why are normal hearing people so good in doing that?  Why do hearing impaired people have these problems?  And why or how can hearing aids help in these situations?

Chris -   So, how are you trying to address that problem?  How can you solve it or investigate further whatís going on?

CafeteriaJorg -   First, we have to understand what goes wrong and there are different aspects in it.  But for us, we are interested in real life stimuli which is usually quite complicated to look at because we can't follow a person around the city all the time.  So what we do is try to get the city situation or cafeteria situation into the laboratory, and then we can measure speech intelligibility in such environments or aspects like distance perception and so on.  We can measure in our laboratory space and this would be as close to real life as possible.

Chris -   I see.  So you're bringing the cafeteria or the noisy station or whatever, to the lab by creating it artificially, but you can put the hearing impaired subject in that environment and see how they react and respond to that environment, what they can hear in that environment, and then you can tweak that environment in very standard ways to work out what is going on.

Jorg -   Thatís exactly what weíre doing, so we have lots of loud speakers in the room.  We create signals using Odeon and we set the person in the centre of this loud speaker array and we even put some Ė as we call it a Ďmaster hearing aidí on.  So this is a computer platform which we can fully control.  So, we have the entire system, we can basically control the acoustic environment, the sound sources in there, but also the signal processing in the hearing aid that the subject is listening through in that environment.

In-ear hearing aidChris -   And presumably, what that will enable you to do is to then work out what aspects of the acoustic environment are bad for a standard hearing aid or bad for a hearing impaired person and work out how to adjust the hearing aidís function to compensate, so the person hears better?

Jorg -   Thatís exactly what we do.  For example, early reflections in a room are very, very dominant and in many cases, people consider this as disturbing, but actually for speech, it helps and hearing aids often destroy these early reflections which might not be the most good thing to do.

Chris -   Is that a discovery you've only just made, so that hasnít actually filtered down into the hearing aid market yet effectively?  Is that something that has yet to break-in and be implemented?

Jorg -   Hearing aid companies try these things, but I think it is now the time when you have this transition, where we basically try to go from the simple laboratory into a more realistic environment because  thereís a difference in how hearing impaired people with hearing aids perform in a laboratory, so how we fit , the audiologist fit hearing aids, and how they then are used in real life. This gap has to be bridged somehow and thatís what we do now.  So we have the environments Ė the loudspeaker-based environments, the simulation software and so on, so that we now can do these things and the hearing aid industry is picking up on that right now.

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This is one of the great frustrations in modern life, and I don't know why it has not been easily solved. I have a couple of ideas. The new system would include a microphone that would pick-up ambient noise and do one or two things.

For instance, the new system could simply narrow and shift the announcement frequencies AWAY from the dominant ambient noise frequencies. Second, the announcement system could actively suppress the ambient noise with already extant sound suppressant technology, then superimpose the announcement.

I think there is really really big money to be made by anyone who can invent and patent a simple 'black box' device that could be spliced into the existing system. For instance, let us consider the drive-in fast food ordering station.

The system already includes both sending and receiving circuits. Splice in the new black box and it uses suppression technology to reduce the ambient noise at the ordering station, and then adds clarification 'algorythms' to the output signal as well.

Another, less complicated approach is for the announcement system to create its own background noise then superimpose the announcement on top of it. Consider a simple, but rather loud 60HZ hum to overwhelm the ambient noise. Then superimpose the announcement.





I doubt any of this is very high tech.
litespeed, Sat, 6th Feb 2010

An even less Hi-Tek solution would be to train the announcers to actually speak properly. For example, allowing sounds to emerge from the mouth rather than the nose (or other orifices) would make an enormous difference. There used to be a whole science in this field. As I recall it was called Elocution http://en.wikipedia.org/wiki/Elocution

Sadly, it seems to have died out. For example, some people think it's quite appropriate to drop "H"s all over the place while inserting nonexistent "R"s whenever they feel like it - not to mention the tortured vowels. It's quite possible to retain an accent without totally butchering the language. Geezer, Sun, 7th Feb 2010

I think the best solution is an abundance of LCD display screens so that you can read the relevent information. syhprum, Sun, 7th Feb 2010

King's Cross station has some rather interesting column speakers about 4metres tall by 20-30cm wide, with 30-odd actual speakers in a row in each. These will tend to emit the sound in a "flat pancake" pattern, directing the sound energy usefully along the platform, but not up into the roof/ceiling where it would serve no useful purpose but would contribute a lot of echo.

Another solution often seen these days is just to have a very large number of very low-power speakers eg every 5-10 metres along the platform. Again you're concentrating the energy where it's useful, and not putting excessive noise into the environment to reverberate. techmind, Mon, 15th Feb 2010

Duct tape everyones mouth.

For more sensible solutions I could agree with syhprum. LCD's is probably the best solution. However, since the majority of us are walking with headphones and mobiles... Send the information to the mobiles. DrChemistry, Sat, 27th Feb 2010

Great stuff. The start of this thread is enticing because it says speakers in train stations. Another huge issue is the one off show or event that idiot event producers book into these "fancy, old historic barns" and expect the audio company to make everything sound like a nice conference room. We zoned the Union Station in St Louis with 10 lollipop speakers with time delay, but really worked to make sure the "zones" didn't overlap so time delay wasn't an extra issue to poor acoustics. Still sucked. The "hub-bub" effect pushed the spl between 800-2K well past the 75 db we were averaging when I scoped the room with my spectrum analyizer in every zone. We achieved really good zoning...almost no cross talk from overlap. But still is was a cluster ****. Another great process would be to educate the client to only use these great looking halls to feed or licquer people and keep the award shows, talking heads, auctions (the absolute worst)to acoutically dry places that have little or no standing waves. Put some screens up and puke sponsorship all night, but don't bring a mic in! Thomas A Frazee, Mon, 20th Sep 2010

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