Fine Tunes #4
It all started back in the cave. Humans have a directional sense for sound because below 2kHz or so, the brain detects the time delay between the signals reaching our ears. No delay means that the sound is coming from directly in front of you. If the sound emanates from everywhere, it's probably an audio reviewer. This information is processed by the audiophile brain during the first 800 microseconds (0.8ms) of the transient; that's the maximum time delay between the ears of most people, including reviewers, and is achieved when a soundsource is directly to the listener's side.
Allen's treatise explains that it's only after the initial recognition of direction that the perception of tonality begins. "This has recently been proven in scientific studies, and it's believed to be a critical part of our survival, historically. In other words, we first locate the origin of a sound, a potential danger for example, and then try to identify what it was that made the sound." Probably that pesky reviewer again.
To achieve a good stereo soundstage, you've got to eliminate as many early reflections as possible. This has a secondary benefit, says Perkins. "According to a psychoacoustic phenomenon called the Haas effect, the brain prioritizes the first sound wave to avoid confusion if the reflections are low enough in amplitude. The result is, if the speakers measure flat under anechoic conditions, the brain will register flat behavior even if test equipment shows severe deviations in frequency response because of room reflections."
Let's start with a dedicated listening room like the one illustrated in fig.1. The ideal speaker positions here are at the two center points of an ellipse touching the walls of the room. The best listening position is 1-3' from the rear wall. "Make sure your system is wired in correct phase!" warns Allen. "The direct sound from the speakers reaches the listener before the side-wall reflections. You'll have better soundstaging and hear the speakers' tonal balance without room-related interference. The basic formula is, the distance from the speaker directly to the ear should be at least 5' less than the distance from the speaker to any reflective surface to the ear. The secondary sound must arrive at least 5 milliseconds after the primary one. For example, let's take a speaker-to-ear distance of 6'. The speaker-to-wall distance is 5', and the wall-to-ear distance is 8', totaling 13'. 13' minus 6' equals 7', so that should work."
As you'll see from fig.1, Allen prefers long-wall placement, as this allows for maximum speaker separation. He explains that the close proximity of the listener's head to the rear wall has two effects. At the room boundaries—the walls—the room nodes are suppressed, as sound pressure is high but velocity low. So sitting in the maximum pressure area gives the best perception of deep bass. And, as the reflections there are shorter than the circumference of the listener's head (even mine!), the brain can't sense the time delay. "When the brain can't localize reflections," Allen explains, "it ignores them." That's for sure—just ask Kathleen!
Fig.1 Direct vs Reflected Sound
Imagine being in a noisy public place and chatting with a companion. You're easily able to follow the conversation, even though a recording made from your listening position would sound like random noise. "Hearing your name spoken several feet away, you can change your focus and 'listen in' on the other conversation," says Perkins. "Our brain does this automatically all the time. It filters out the annoying natural resonances of a room, facilitates speech intelligibility, identifies potential dangers, and so on."
Footnote 1: You'll find a copy of "Principles and Techniques of Speaker Placement" on Immedia's website.