In live sound, be it for speech- or music-based applications, we spend a lot of time considering what equipment to use and how it should be deployed to achieve the desired result of an intelligible and pleasing listening experience for the audience.
Many factors affect these decisions, but arguably the most important is the nature of the space in which we plan to operate, everything from the choice of microphones, the capabilities of the control surface, and the nature and positioning of the loudspeakers are heavily influenced by what happens when our carefully crafted sound is let loose in the wild.
Acoustics is the branch of physics concerned with the properties of sound, but we more commonly use it to describe the properties of a room or building that determine how sound is propagated.
When sound issues forth from a generator, be it an instrument or a loudspeaker, its behavior is initially very easy to predict – we know that it will decay over distance as the energy dissipates over an increasing area. However, once it starts to encounter physical objects, things get complicated very quickly. So let’s have a look at this behavior and break it down so we can better understand it.
When sound energy encounters a boundary, be it a wall, floor or ceiling, various things will happen depending on the nature of the boundary and the stiffness of its surface (Figure 1).
The most common result is reflection but part of the energy can also be absorbed (and converted into heat) as well as transmitted though the boundary to continue its journey. If the object is relatively small the energy can also be refracted around it. The degree to which the sound energy is reflected, absorbed, transmitted or refracted is dependent on the frequency of the sound and the angle at which it strikes the boundary.
High-frequency sounds require less energy to generate their shorter wavelengths which means they’re more likely to be reflected or absorbed whereas low-frequency sounds, particularly those who’s wavelengths exceed the width of the boundary, are more likely to be transmitted. This kind of behavior is easily observed simply by standing outside a room where sound is being generated or amplified, you will always tend to hear more of the lower frequency content.
Transmitted sound can be a major problem for music venues as it represents a potential noise nuisance to neighbors, but this should hopefully be addressed in the design and construction (or adaptation) of the venue, or simply by keeping the amplitude below an acceptable value. The bigger issue for the mix engineer is the sound reflected inside and the reverberation that ensues.
If we look at the behavior of a theoretical point source of sound (i.e., where sound energy propagates equally in all directions) in a perfectly square room, we can see a symmetrical pattern of direct and reflected sound energy developing over time. If the source continues to generate sound the system quickly reaches a state of equilibrium where the sound energy being produced equals the energy being dissipated, thus filling the space with a sustained field of sound energy.
In Figure 2, if we stand at point 1, we will hear the direct sound from the source at a high level but none of the reflected sound from the room.
Alternatively, at point 2 all we will hear is the reflected sound and none of the direct sound. In between those two extremes is point 3, which is where the level of the direct sound equals the level of the reflected sound – this is called the critical distance. If we move beyond the critical distance the reflected sound quickly overtakes the direct sound, making it increasingly less intelligible.
Therefore, in order to provide a valid listening experience for the audience, it’s important to ensure they’re within the critical distance.
If we turn off the source, the sound will continue to bounce around losing energy until it dissipates and stops completely. This is what we perceive as reverberation. If we were to measure the time it takes to drop by 60 dB (a level chosen as a benchmark of inaudibility), we would arrive at a figure which defines the reverb time of the room (sometimes called RT60). A small room typically has a reverb time below 1 second, a concert hall is more likely in excess of 2 seconds, and a church can easily be far beyond 6 seconds.
To make things even more complicated, different materials reflect sound to different degrees. The amount of sound energy absorbed by a given surface is called the absorption coefficient which specifies a number between 0 (the perfect reflector) and 1 (the perfect absorber, often defined as an open window).