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How to design the slope of an auditorium?

Updated: Aug 7

Introduction


In this blog, we will talk about why, if at all, should there be a slope for audience seating in an auditorium, and what are the things that you should consider while designing the said slope should you choose to have one (Spoiler: you always should choose to have one).





Why are we talking about this? Why is it important for people to know this?


While designing auditoriums, we have oftentimes seen people unable to decide how the seating should be. Clients usually depend upon architects and designers and therefore architects and designers should have definitive answers to these questions. It is therefore important to understand the logic behind making these decisions, and this video will help you in doing exactly that.


After going through this video, 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.


Is there an aspect to the slope in an auditorium other than sight-line clearance?


It is almost taken for granted that an auditorium will have a sloping floor. It is intuitive to think why the slope exists, the first that comes to our minds is the sight lines which will make it possible for the people sitting in the farther rows to have decent visibility of the stage.


Did you also know that there is an acoustical impact of the way the audience is positioned? There is something known as grazing sound incidence. It literally means what the name suggests. Sound waves will graze a surface which is parallel to them when the distance is sufficiently near. This results in undue absorption of sound over a wide frequency range.


One needs to take this absorption into account whenever the path of the sound grazes the audience (whether the slope is present or not). The grazing becomes much more apparent when a slope exists.


How to determine the angle of the slope?


Now that it is established that a sloped seating is a good idea, I have observed many a times a lack of certainty in deciding the slope. In the following drawing, α is the angle of the slope, S is the source, a is the distance from source to the first sear row, b is the distance from source to last seat row, d is the distance between seat rows, e is the source height above head of listener in first row, h is the sight line clearance at the last row.




With this formula, you can now see how far back in the hall it is possible to maintain the slope. Now ‘h’ should be between 8cm and 12cm to ensure good visibility and avoid grazing sound attenuation.


But this causes the over height near the source to be much higher than the design goals. We don’t want that because that makes the last row reach much high above the ground level and it also decreases the volume of the hall which is necessary for having good reverberance in the room.


The best way possible to tackle this issue is to have a constant over height for each row. In the following drawing, a person's head in the nth row is elevated Hn above the heights of the heads in the first row if Hn is chosen according to the formula given below the drawing.




Now that this has been decided, it isn’t possible to have an actual smooth curving floor. We need to design steps that lead to such an apparent curve. We cannot change the height of the risers of the steps, hence we need to change the length of the treads of each step.



From this formula, it can be noted that ‘h’ isn’t simply a function of the step height but the distance between the next row. You can observe this effect when you go to an auditorium or try standing behind someone tall.


With this knowledge, you can formulate the treads of each row to achieve the best possible shape for the auditorium seating.


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.

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