Recording Magazine’s Room Acoustics Series Part 6
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 sixth 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.
Here is the sixth newsletter in the series on Room Acoustics:
Welcome back to Sound Advice on Acoustics! For the last few installments, we’ve been talking about the basics of room acoustics: how sound moves, room dimensions, and systems for analyzing and dealing with low-frequency problems. As we start to consider the mids and higher frequencies, with the transmission, reflection, and absorption of sound as it travels around our room, we have to have an understanding of how our own hearing systems interpret this barrage of direct and reflected sound. Here we go!
When a direct sound reaches our ears followed by a reflection, how we perceive these two sounds is determined by the arrival time of the reflection. If it arrives less than about 50 milliseconds (thousandths of a second) after the original sound does, we perceive only the original direct sound, but the interference effects contributed by the delayed out-of-phase reflection will color the timbre of that sound.
Our auditory system’s perceptual fusing of the direct and reflected sounds, under the conditions described above, is called the precedence effect (or Haas effect)—the ear integrates all reflections within ~50 milliseconds of the first arrival (direct sound). But if a reflection arrives more than ~50 milliseconds later than the direct sound, it is not perceptually fused with the direct sound and is instead heard as a discrete echo.
The actual threshold of the shift in perception from integration to echoes is gradual between about 25–50 milliseconds, and varies depending on the nature of the original sound. For example, for short percussive sounds with sharp attack transients, the perceptual fusing of direct and reflected sounds may break down at only 20 milliseconds of delay, or even less. (A musician calls this “flamming.”)
The delay, or gap, between the direct sound and the first reflection, as well as the spacing of the other (integrated) early reflections determines the acoustic character (our sense of the size and shape) of the room. If there are enough reflective areas, the later reflections, instead of being heard individually as echoes, build up in density and form reverberation, which continues after the direct sound stops, dying away gradually. Reverberation time (RT60) refers to the time it takes for this reverberant tail to decay by 60 dB. In control rooms this property is usually kept to a minimum by design.
Another aspect of our hearing in regards to direct and reflected sound is described by the “Law of the First Wavefront.” This says that when we hear a direct sound followed by early reflections, our auditory system not only integrates them, but also determines the localization of this combined sound from the direction of the first arriving sound.
So if a sound wave originates from a loudspeaker in front of us and slightly to the left, followed a few milliseconds later by a reflection from the right, we identify everything as coming from the loudspeaker. However, if the reflected sound is about 10 dB or so louder than the direct sound, the localization towards the direction of the first arrival breaks down, the perceived image shifts, and the direct/reflected sound then appears to be spread out between the actual sources.
Another directional aspect of our auditory perception is that reflections from the same direction as the (direct) source can be 5–10 dB louder before being detected than reflections originating from other directions. This is so because the direct sound masks the reflections coming from the same direction. So strong reflections from the sides of the room (lateral reflections) can be more problematic than those from the front.
The presence of early reflections, echoes, and reverberation in a room is normal and adds fullness and a sense of spaciousness to music, but in a control room, too much of this can be a problem. Recordings being monitored already contain recorded ambience, or else they may have artificial ambience added to them, but either way we need to hear the reflections in the recordings more than the ambience of the control room itself. And of course, reflection-based effects like image shifts and colorations of the direct sound also obscure aspects of the recorded sound like panning and tonal balance, so reflected sound must be tightly controlled to insure a good monitoring environment.
If we’re going to try to control or eliminate certain reflections in a room, we should trace their pathways as they travel through the room. When a sound wave is reflected off a room surface, there is a well-known rule which describes the propagation of that reflected wave: “The angle of incidence is equal to the angle of reflection.” This means that for whatever angle a sound wave strikes a reflective surface, it will bounce off that surface at an equal but opposite angle (see Figure 7). You can see this for yourself by bouncing a flashlight beam off a mirror—and this is actually the basis for a handy means of dealing with these reflections that we’ll discuss next time (which we call the “mirror trick”).