Definition: The expansion of sound in a room. Sound spreads in the air with an average speed of 340m/s (in solid substances a lot faster). Within a room, sound strikes reflection surfaces such as the floor, the ceiling or walls. This produces sound reflections and reverberations.
Human beings are able to quickly estimate the size of a room by using their ears and listening to the reaction time as well as the direction of sound reflections. Without being aware of it, we are constantly busy with this process during the day: every time we enter a new room, the acoustics change.
The amount of sound that is produced depends on the purpose of a room.
Sound sources can be speech, background noises, such as an air
conditioning or a humidifier, or disturbing noise caused by traffic or
neighbours. Music, in comparison to speech, has a much broader spectrum
of very low to very high frequencies, which means that the design of
suitable acoustics represents a real challenge. Generally, every purpose
requires an individually designed acoustic solution.
What are bad acoustics?
In most cases, we take acoustics for granted and don’t think much about it. This changes when acoustics disturb us when we are busy with a task: bad acoustics can, for instance, limit the speech intelligibility in a room and hinder other people in receiving our message. We might feel disturbed at work because our colleagues produce too much noise. It takes a lot of concentration to make a telephone call or to write a letter, and the additional strain caused by surrounding noise leads to tiredness.
Furthermore, it may happen that we interpret a noise in a wrong way, because the acoustics in a room don’t react in a linear way, or that we record a conversation, but are disappointed with the result, because reflections have disturbed the sound.
In a concert hall or a church, on the other hand, the room itself adds to the musical message and reinforces it. Not only for the musicians, but also for the audience, it can be very frustrating if the room climate in a concert hall is too dry, if there are reverberations, or if the size of the room isn’t adequate.
In conclusion, we can say that the acoustics of a room are bad when noise hinders it in its function.
What are good acoustics?
A room must reflect the noise that is being produced. These sound reflections help our orientation and tell us that we are actually in a room. A room that is totally free of reverberations and absorbs all noise isolates us completely from the world around us, and for most people it is a strange experience to hear only the noise produced by them.
The design of perfect acoustics depends on the goal.
When all sound is absorbed equally, high as well as low frequencies, a room sounds balanced and pleasant. When people talk in a room, we want the sound to fade as fast as possible, so that we can understand what is being said. Therefore, the reverberation time in small rooms, such as offices, should not exceed 0.6 seconds.
But when we listen to classical music, we want the sound to linger on for a while, so that the different sounds mix. A concert hall, therefore, has an average reverberation time of 2 seconds.
A teacher wants a quiet classroom and needs to be understood well by his pupils, so the maximal reverberation time for classroom has been set at 0.8 seconds.
These standards defining good acoustics are documented in the NEIFN/ISO 3382 specifications.
For more information on acoustics in classrooms including a video click here..
How can the acoustics of a room be measured?
The main acoustic parameters of a room, such as the reverberation time and reflections, can be determined by using a sound source and special measuring instruments. When we make an acoustic measurement, we use a loudspeaker that sounds in all directions as well as a signal pistol that serves as a sound source.
With the help of a measuring microphone that is sensitive at all sides, the “answer“ of the room in the form of direct sound, reflections, echoes and reverberations can be registered and saved on a computer, tablet or smartphone. On this basis, we calculate values that are necessary to define the acoustic starting position of a project.