The Science Behind Auditorium Acoustics
How we hear a speech, a lecture, a piece of music or a video clip, can depend on the type of space we are in and its set-up. Acoustics of a room have a significant factor on the quality, volume and reception of sound, and it is of priority in any form of auditorium. The science behind auditorium acoustics depends on which factors must be combined to create the best sound to fit the purpose and its context of usage.
Levels of sound must be considered to cast the best sound over an audience. Auditorium sound speakers need to produce a level of approximately 65 dB, reaching every seat in the room. Without this, there is risk of large, sudden bursts of sound. It will be a high priority to avoid electronic distortion, as not only is this uncomfortable for an audience, but it also inhibits their ability to listen clearly.
Natural Noise Levels
In an empty auditorium, it is expected that the natural noise of the room should measure 20 to 30dB lower than the speaker level heard. When we are talking of this ‘natural’ noise in the room, we are factoring in everything being switched on: the lights, sound system (without anyone using it,) and air con. When audiences move into the room, breathing and rustling (without talking,) would bring the dB levels up to around 35, in a good auditorium.
Reverberation in an auditorium disappears with time, when the sound is absorbed by different surfaces and interactions in the room. In some rooms, it might take longer for the sound to dim and this is called a more ‘live’ space. In contrast, if a room is very absorbent of the reverberation, it is said to be acoustically ‘dead.’
Using a combination of absorption (to reduce noise in the room,) and diffusion (to break up any sound waves that aren’t absorbed) is the solution to controlling reverberations in a room. It helps to create a balance between the ‘live’ and ‘dead’ sound-out of the space. This can be achieved through absorption or diffusion panels being applied appropriately to auditorium walls.
2 seconds is around the optimal reverb time in a medium-sized, general-use auditorium.
In a room, direct sound is noise that can be heard from the source itself, such as from a speaker’s mouth. The further the person is away from us, the more likely we are to hear them with ‘reflected’ sound (as the sound bounces from objects in a room before it reaches us.) In an auditorium space, this means that the direct sound reaches each listener from around 0.01 to 0.2 seconds.
Fullness is when there is a lot of reflected sound and ‘clarity’ is achieved when the is more prominent direct sound. Fullness is more desirable with ‘large’ performances or a piece with many people involved. Meanwhile, clarity is best for performances involving singular speech- or a lecturer speaking in an auditorium space.
Warmth and Brilliance
Subjectively, ‘warmth’ describes a softness to the music, whilst ‘brilliance’ refers to a ‘brighter’ and more clear sound. Each of these wants to be used in moderation to achieve the best sound. This balance is achieved by juggling the Reverberation Time’s (RT) low frequency, to the RT’s high frequency.
As the namesake suggests, sound can be more ‘intimate’ if it is heard directly and the distance of listeners is short. This is usually typical of small rooms, however, and not in that of an auditorium. This said, many large spaces benefit from reflecting screens being used close to the source of the sound, as these then reduce the time delay. By using reflecting screens, a sense of sound intimacy can be restored between speaker and listeners.
It’s all in the balance:
To start with, creating great sound in an auditorium is a case of a very fine balance. On the one hand, the ring of live performances or speakers will allow an audience to hear better in the space, however, there’s a fine line between a controlled ring and then producing an uncomfortable echo! The fullness and clarity can be dependent on performance or speaker, but again, finding what’s ‘right’ for the auditorium space you have will be key.
There are many factors that a sound technician or acoustic specialist would carefully consider and play around with to meet the purpose at any given time, by a hall’s users. If you’re using an auditorium space, consider seeking advice to best maximise your acoustics when you utilise the room.