When we listen to music either live or through a sound system, those sound waves we hear are interacting with the environment all around us. Music will sound different when played in a recording studio compared to a purpose-built venue, or a living room.
So just what are the rooms around us doing to our music, and how does it affect our experience?
Reflections and reverberation
Sound bounces off of solid objects, and this includes room surfaces. When you pop a balloon in a church, and you hear a long decay of the sound after the event, this is the effect of sound bouncing around the room and back to your ears. This effect is called ‘reverberation’.
The time it takes for the sound of your clap to decay is called the Reverberation Time, and it is measured in seconds.
The reverberation time in a room is proportional to its volume. The bigger the room, the longer its reverberation time – hence one reason why the sound of your clap takes so long to decay in a church or concert hall.
Direct sound vs reflected sound
The first sound that reaches your ears is the ‘direct sound’. This is sound that has travelled in a straight line between the source and the listener. Let’s create a timeline of sound reflections, and call this time zero.
After the direct sound arrives, reflected sounds (which took a longer path to get to your ears), start arriving shortly (milliseconds) after. As the reflected sound waves travel further and bounce off of walls, ceilings and objects, they lose some energy, so they are generally quieter than the direct sound. The effect of sound energy being lost to objects and room surfaces is called sound absorption and will be the topic of another article. This reflectogram shows us the direct sound arriving at time zero, and the reflections following shortly after, each getting steadily quieter.
Depending on when these reflections arrive at your ears, and how loud they are, this can alter your perception of the music.
0 – 1 millisecond
Reflected sound arriving in the first millisecond after the direct sound reaches you, can alteryour perception of where the sound came from. So if you have more reflections arriving from the left in the first millisecond, you will perceive the sound source as being slightly to the left of where it actually is. This is more relevant to critical audio listening rooms, control rooms and mixing studios, where uneven reflections can alter the perception of left and right channel panning (known as the stereo image).
0 – 5 milliseconds
Reflected sound arriving in the first 5 milliseconds is generally perceived as being one entity with the direct sound. This effect is known as fusion.
5 – 50(ish) milliseconds
At approximately 50 milliseconds, the fusion effect begins to break down. This point in time is called the ‘fusion breakdown threshold’. Reflections received in this time are still generally fused with the direct sound, but have the effect of making the sound ‘fuller’, and creating a sense of intimacy, or presence.
Note that this 50 millisecond threshold varies widely with the type of sound you are listening to. Here are some guideline values for different sound sources.
*The threshold for music can vary depending on the style of music, among other factors. A figure of 80 ms is often used in designing spaces for classical music, while a shorter period of 50 ms (or less) has been suggested in spaces for amplified music.
The following timeline might help you make sense of the overall picture.
Envelopment
One of the key experiences that listeners appreciate in the best concert halls is ‘envelopment’. This is the feeling of being wholly consumed by sound from all directions.
Envelopment occurs when early reflections (those in the first 80 ms in the case of live classical music), arrive at the listener from directions other than the direction of the source. While this includes reflections from above and behind, the typical way to measure this is using a quantity called Lateral fraction (LF),which is the proportion of the energy which arrives from the side compared to direct from the source.
Late reflections (after the breakdown of fusion)
Late reflections are those arriving after ‘fusion breakdown’, that is, after the 35 to 100 ms boundary where reflections are fused into the direct sound. These late reflections are perceived as reverberation – this is the sound you hear decaying in a church or concert hall after you clap your hands together or pop a balloon.
Clarity
A commonly used metric in venue design is Clarity, or C80. This is the ratio of early reflections (before 80 ms) to late reflections (after 80 ms).
If the Clarity is too low, the listener might have trouble distinguishing musical notes from one another.
What’s the difference between reverb and echo?
Reverb is typically perceived as the decay of an earlier sound, where each reflection has (on average) less energy than the last.
An echo is perceived as a distinct reflection (like a repetition) of the initial sound. Echoes can occur if a certain reflection is substantially louder than the reflections immediately before it, or where there are too few reflections in the space to form a nice, steady decay (i.e. the reverberation density is too low).
Putting it all together for design of music rooms
How does all this relate to the design of a critical listening room, a music studio, or venue? It depends on the situation.
In a music studio or audio suite, we generally want to eliminate (or substantially reduce the level of) the very earliest reflections. From a pyscho-acoustic perspective, this will avoid unwanted shifting of the stereo image, and from a room response perspective, this will reduce the effect of comb filtering, providing a more even response at the listening position. An old heuristic called the 20-20 rule states that we should reduce reflections in the first 20 milliseconds by 20 dB. The overall reverberation time of a mixing suite or control room might be 0.3 to 0.4 seconds, with most of the reflections coming from further away, adding warmth to the sound without causing comb filtering or stereo imaging issues.
In a venue for amplified popular music, a reverberation time of between 0.5 and 1.0 seconds might be suitable. These venues use sound reinforcement systems, so don’t necessarily need all the reflections to maintain the volume and strength of the sound like in a classical music venue, and artists can use electronic reverb if they want to create that texture. A longer reverberation time in these venues than in studios provides a sense of envelopment, which correlates strongly witha positive listener experience. However, in contemporary music with a strongbeat or bassline, too long a reverberation time also risks reducing clarity between notes.
In a venue for classical music, reverberation times as long as 3 or 4 seconds are not unusual. Classical music is often composed with this acoustic in mind, and the same music performed in a roomwith a shorter reverberation time may feel lacking.
Summary
Rooms and objects reflect sound.
Direct sound is the first sound to arrive at our ears, having travelled in a straight line from the sound source.
The early reflections to arrive at our ears fuse with the direct sound, and alter our experience of it.
The late reflections sound like a separate entity – a decay of the original direct sound.
The boundary between early and late reflections is variable and depends on what it is we arelistening to (speech, popular music, classical music).
References
Acoustics and Psychoacoustics, David M. Howard and Jamie Angus, 2017
