Introduction
Consonance Acoustics is an acoustical design company. The journey of this company started by trying to learn why it was difficult to listen in some places than others and we are now here to discuss the whys and the whats of acoustics and related sciences.
Let's talk about the common mistakes we have observed people make while designing auditoriums and later face the walk of shame. Believe us, we ourselves have faced the walk of shame and therefore want no one to go through that.
Why are we talking about this? Why is it important for people to know this?
Architects usually create a vision for a space and then design it accordingly, but oftentimes some trivial acoustical mistakes can be avoided before moving on towards designing and this blog will discuss what are those mistakes and give a possible workaround for those acoustical defects. After going through this blog, you can be extremely confident about presenting your design to the clients as you will have considered some easy but extremely important aspects that are more often than not ignored by other designers.
1. Curved surfaces are the first thing that one should avoid while designing an auditorium. It is a BIG NO NO.
Can you guess which world-famous performance hall had this issue?
The Royal Albert Hall!
The Royal Albert Hall had really poor acoustics when it was built. When you take the tour, they tell you about it and later show you the treatment. Amongst other issues, one of the bigger issues was the dome.
Oftentimes it is easy to forget the utility of a space and people later repent when someone comes to them and charges a huge refitting bill.
It is in fact easy to judge why a curved surface will be bad news. Have you burned leaves using a magnifying glass? Then you know how the curved surface is able to refract the incoming waves onto a single focal point. Whenever there is a curved surface, waves falling on it will reflect in a concentrated way towards a point. This creates focal points for sounds in an auditorium creating an imbalance in the sound.
The best way to work around any curve is to add an absorbing or a diffusing layer on top. Royal Albert Hall’s dome has these amazing discs that hang from the ceiling as a defense against this problem.
There are also diffusers called Quadratic Residue Diffusers and Primitive Root Diffusers (A.K.A. Skyline Diffusers) that diffuse sound.
As you can see, their shape, which although looks random, helps in creating a surface that interacts with incoming sound waves which are scattered back in all directions. It is of importance to note that this happens for specific frequency bands which is dependent on the shape and size of these diffusers.
2. Confusing wood/plywood as an absorbing material
Materials with a surface density greater than 10 kg/m2 have negligible sound attenuation. Plywood has a density of around 500 kg/m3 to 650 kg/m3. Which means, unless the plywood is below 15mm, it will not even begin to absorb anything. Additionally, sound absorption mechanisms differ for different frequency bands.
Imagine installing a framework on the walls, inserting rockwool and later applying an 18mm plywood board on top of that and calling that a sound absorbing panel. It should be called a money waste panel.
Sound absorption works differently for different frequencies of sound. To explain simply, the high-pitched sounds are absorbed by perforated absorbers which let air come inside them and then obstruct its flow. Common examples of such absorbers are perforated gypsum panels, rockwool panels which have fabric wrappings, fabric upholstered auditorium seats, drapes etc. Low pitched sounds are absorbed by vibrating membranes which are slightly complicated than perforated absorbers. They need a low surface density of the top layer.
Neither of the absorbers can therefore have plywood as their top layer.
3. Having a long balcony overhang
The level of reverberant energy below the balcony overhang is determined by the relation between the height of the overhang and the depth of the seating below the overhang. This energy if too low, creates a dead sounding space and hence is a problem for listeners, especially in the case of musical performances.
Best way is to simulate the reverberation, but a general rule of thumb can be to have the height greater than twice the depth (H>2D) for theatres greater than or equal to the depth (H>D) for concert halls.
4. Having exactly parallel surfaces
It is common to have halls shaped as a shoe box, add sloped stairs. Having the side walls parallel to each other causes an annoying effect called the flutter echo. Say there are two sources of sound which play the same sounds with some time delay. Humans can’t perceive that there are two sources if the time delay is below 50 ms. This is called the just noticeable difference (JND).
For echoes to be heard, we have known since school that a distance of more than 17 metres is required. This is because the speed of sound is 340 m/s and the JND of humans is 50ms. Which means 340 m/s x 0.05 s = 17 m is the distance that will be needed so that the reflecting sound is heard again after 50ms.
Now, even if the distance between two parallel surfaces is less than 17 m, the multiple rapid reflections in the same direction will mean that there are some reflections that will occur after the interval of 50 ms. These will be heard like flutters as they will be spaced quite close to each other in terms of time intervals. They cause a decrease in clarity of speech and other audio aspects.
There are a few ways to avoid this from happening. Absorbing sound only on one of the parallel surfaces can stop flutter echoes from happening. Also adding sound diffusers can help. Even if the surfaces have an angle of 3 degrees between, it sufficient to avoid flutter echoes from happening.
Conclusion
With the knowledge of what not to do, you can incorporate these simple techniques in your design and deliver the perfect product to your clients. We have observed that the clients don’t really know of these issues until they occur, having prepared a solution definitely sets you apart. Unfortunately, we can’t cover stuff in detail in this blog, but you are welcome and encouraged to contact us to know more about auditorium design.
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