Recording Magazine’s Room Acoustics Series Part 5

28 September 2011
Audio Production
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Recording Magazine’s Room Acoustics Series Part 5

Recording Magazine sends out a newsletter to its subscribers every few weeks. The newsletter is (coincidentally) titled “Sound Advice” and this month it features the fifth in a series about room acoustics. Room acoustics is one of the biggest concerns for Recording Magazine readers. I know that this is also a big issue for those of you in the voiceover world. I asked permission to reprint this newsletter (and will ask to reprint the others in the series as well) so that those of you with home studios can also benefit from the information. I want to personally thank Brent Heintz, VP/Associate Publisher for granting permission, allowing me to share this great information with you.

Please visit Recording Magazine‘s website and Facebook Page.

Catch up or skip ahead: Part 1, Part 2, Part 3, Part 4, Part 6, Part 7, Part 8.

Here is the fifth newsletter in the series on Room Acoustics:

Welcome to Sound Advice on Acoustics! For the past several months we’ve been dealing with the basics of controlling bass buildup in rooms, using ratios of room dimensions and active bass trapping and other room treatments. Bass is by far the hardest problem to deal with in tuning a room, and with it under control, we can now turn our attention to the mids and highs. Read on…!So far we’ve considered the effects of low-frequency waves in the room. There are other issues, related to the propagation of mid and high frequencies. When a mid- or high-frequency sound wave moves through a room, it eventually hits one of the various boundary surfaces (walls, ceiling, floor). When this occurs, the sound wave is either absorbed, transmitted, or reflected.

Absorption means that some or most of the wave’s energy is converted into heat. Transmission means that the wave has some of its energy passed—through the wall, for example. Reflection means that most of the wave’s energy is directed back into the room.

This will happen repeatedly as the sound wave hits various surfaces until all its energy is eventually dissipated.

A room where all the surfaces are completely absorptive sounds dead and unnatural, and is unsuitable for music-making or listening. Such a room, called an anechoic chamber, would be used for testing purposes, as in the manufacture of loudspeakers for example, to isolate the sound quality of the speaker under test from the room effects. An overly reflective room is also not ideal—too many reflections tumbling around the room obscure the clarity of music or speech (see below).

An ideal room strikes the right balance between the original sound from the source (i.e. instrument or loudspeaker) and the amount of reflected sound present. Furthermore, it is desirable for the reflected sound to be distributed as evenly as possible throughout the room, providing a comfortable and pleasant sense of ambience (liveness). This even distribution of reflected sound energy is called , and is generally desirable, as we’ll see. But along the road to this ideal room, there are many pitfalls that we’ll now examine.

Reflection of sound waves is the behavior we’ll be most concerned with in the next few paragraphs. Just as happens with lower frequencies, when mid- and high-frequency waves reflect back into a room, the positive and negative peaks of the direct (original) sound waves and those of the reflections will cancel and reinforce. This happens because the reflections are delayed in time relative to the direct sound, causing their positive and negative peaks to be offset from those of the direct sound, which results in the interference (see Figure 5 for an illustration of this).

In describing this, if we express the duration of a single cycle of a wave in measurements of phase, the delayed reflections as shown in Figure 5 can be said to be “out of phase” with the original sound. This phase-induced delay, or phase shift, is inevitable in any normal reflective environment.The short wavelengths of mid and high frequencies means that these cancellations and reinforcements occur more frequently all throughout the room rather than being clearly localized to specific broad areas as with standing waves. In a typical room, many complex interferences like this at higher frequencies result in changes in the frequency balance of sound in that room, as illustrated on a frequency response graph in Figure 6. This is called a comb filter response.

While this resulting frequency response may look very ragged, in actuality our hearing systems tend to average out and largely gloss over these subtle, myriad cancellations and reinforcements, and instead may perceive this as a not unpleasant coloration of the sound in a casual listening environment.

However, a recording studio is not a casual listening environment. For example, we depend on what we hear in the control room to make important decisions about the way the sounds in a recording blend and balance. We need to hear exactly what’s in the recording, not a “pleasantly colored” reproduction. Consequently, we need to exert some degree of control over any such effects that impact the neutrality of the monitoring environment.

With all this knowledge under our hats, it is time to look at another fundamental (excuse the pun) aspect of acoustics: how we hear. Next time: a crash course in how our ears and brains interpret sound. See you then!

4 responses on “Recording Magazine’s Room Acoustics Series Part 5

  1. […] up or skip ahead: Part 1, Part 2, Part 3, Part 5, Part 6, Part 7, Part […]

  2. […] up on the series: Part 1, Part 2, Part 3, Part 4, Part 5, Part 6, Part […]

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