Click here to purchase the entire book in PDF format. Precedence Effect Stand about 10 m from a large flat wall outdoors and clap your hands. You should hear an echo. It'll be pretty close behind the clap (in fact it ought to be about 60 ms behind the clap...) but it's late enough that you can hear it. Now, go closer to the wall, and keep clapping as you go. How close will you be when you can no longer hear a distinct echo? (not one that goes ``echo........echo'' but one that at least appears to have a sound coming from the wall which is separate from the one at your hands...) It turns out that you'll be about 5 m away. You see, there is this small time window of about 30 ms or so where, if the echo is outside the window, you hear it as a distinct sound all to itself; if the echo is within the window, you hear it as a component of the direct sound. It also turns out that you have a predisposition to hear the direction of the combined sound as originating near the earlier of the two sounds when they're less than 30 ms apart. This localization tendancy is called the precedence effect or the Haas effect or the law of the first wavefront [Blauert, 1997]. Okay, okay so I've oversimplified a bit. Let's be a little more specific. The actual time window is dependent on the signal. For very transient sounds, the window is much shorter than for sustained sounds. For example, you're more likely to hear a distinct echo of a xylophone than a quacking duck. (And, in case you're wondering, a duck's quack does echo [Cox, 2003]...) So the upper limit of the Haas window is between 5 ms and 40 ms, depending on the signal. When the reflection is within the Haas window, you tend to hear the location of the sound source as being in the direction of the first sound that arrives. In fact, this is why the effect is called the precedence effect, because the preceding sound is perceived as the important one. This is only true if the reflection is not much louder than the direct sound. If the reflection is more than about 10 - 15 dB louder than the direct sound, then you will perceive the sound as coming from the direction of the reflection instead[Moore, 1989] Also, if the delay time is very short (less than about 1 ms), then you will perceive the reflection as having a timbral effect known as a comb filter (explained in Section 3.2.4). Also, if the two sounds arrive within 1 ms of each other, you will confuse the location of the sound source and think that it's somewhere between the direction of the direct sound and the reflection (a phenomenon known as summing localization)[Blauert, 1997]. This is basically how stereo panning works.