Balliol College Oxford

The quality of sound that is consciously heard in a space is what traditional “acoustics” is all about, particularly if that sound is music. There are schools, theories, numerous research projects and multiple textbooks devoted to the topic of producing the prefect acoustic environment for listening to various types of music. I am not even going to touch the surface of this topic – if you’re interested in designing a concert hall or a high-end recording studio you’ll need to become familiar with concepts like “early lateral reflections”, “room modes” and much more. And you’ll be using modelling tools that are more sophisticated (and MUCH more expensive) than SoundSoup.

What I’d like to focus on is a single property that is useful for designing the sound in ANY space – reverberation. This is the degree to which sound bounces around the space before it is finally absorbed through interaction with the materials on the walls, ceiling and floor.

In a typical room, if you are more than a few metres from the sound source then most of the sound you are hearing has bounced off at least one surface before reaching you – so reverberation is important for your experience of the sound.

The simplest, and most useful, measure of reverberation is the Reverberation Time (RT). This is normally expressed as the time it takes for sound in a room to reduce by 60 decibels after it has been turned off – although actual standards define it in a slightly different way that depends on the slope of the sound reduction.  Typical RT values vary from less than 0.3 seconds (which might represent a recording studio or a space with lots of thick carpet and plush furnishings) to over 2 seconds (which might represent a cathedral).

Rooms come in all shapes and sizes, and the RT doesn’t always express everything about reverberation in the room. However, typical rooms are pretty close to a particularly simple model, called an “ergodic” or “Sabine” room, and for these rooms the RT:

  • is the same everywhere in the room; and
  • depends only on the volume of the room and the total amount of sound-absorbing material in the room (not on where the absorbing material is placed).

When modelling reverberation, SoundSoup assumes that the room is Sabine. The RT depends on the frequency of the sound, because absorbing materials are better at absorbing some sound frequencies than others. Usually the absorption is better (and so the RT is lower) at high frequencies.

The important thing to know is that reverberation affects the way sound is perceived in ANY room, not just when listening to music. Here are a few guidelines.

If the room has a low RT (less than about 0.5 seconds):

  • the sound will be experienced as “soft” and “comfortable”;
  • the room will feel small;
  • people will naturally speak more quietly;
  • speech will usually be understandable at distances of 4m or more.

If the room has a high RT (greater than 1 second):

  • the sound will be experienced as “jangly” or “buzzy”;
  • the room will feel larger;
  • people will naturally speak more loudly;
  • speech will only be understandable at short distances.

For completely different reasons, concert halls for unamplified music also need a high reverberation time, and recording studios need a low one. However, as I said above, I’m not going into design details for those spaces.

As an example, a café designed for patrons to come in from a busy street, order a takeaway espresso and leave might be designed with a high RT, to match the “buzzy” atmosphere. On the other hand, a café where patrons sit in an armchair, pull out their laptop and work on their next novel over a few skim lattes might be designed with a low RT, to promote a peaceful, relaxed atmosphere.

SoundSoup allows you to play with different levels of reverberation, by adding absorbing materials to your room, and listen to the result. You can add different materials to the walls, floor and/or ceiling – you’ll find that the level of reverberation depends on the type of material and the area over which you can spread it. You can also add room contents, like people, chairs and furniture, which also reduce the reverberation. SoundSoup-Free shows you a reverberation time (at a frequency of 500 Hz, probably the most useful frequency for voice sounds) while SoundSoup-Pro shows you the RT at all sound frequencies as well as more details of the absorption properties of your materials.

If your sound design incorporates the three fundamentals:

then you’ll be well on the way to taking charge of your acoustic environment.