Digital Sound & Music: Concepts, Applications, & Science, Chapter 4, last updated 6/25/2013
49
When you intentionally apply the precedence effect, you have to keep in mind that comb
filtering still applies in this scenario. For this reason, it’s usually best to keep the arrival
differences to more than five milliseconds because our hearing system is able to more easily
compensate for the comb filtering at longer time differences.
The advantage to the precedence effect is that although you perceive the direction of both
sounds as arriving from the direction of the first arrival, you also perceive an increase in
loudness as a result of the sum of the two sound waves. This effect has been around for a long
time and is a big part of what gives a room “good acoustics.” There exist rooms where sound
seems to propagate well over long distances, but this isn’t because the inverse square law is
magically being broken. The real magic is the result of reflected sound. If sound is reflecting
from the room surfaces and arriving at the listener within the precedence time window, the
listener perceives an increase in sound level without noticing the direction of the reflected sound.
One goal of an acoustician is to maximize the good reflections and minimize the reflections that
would arrive at the listener outside of the precedence time window, causing an audible echo.
The fascinating part of the precedence effect is that multiple arrivals can be daisy
chained, and the effect still works. There could be three or more distinct arrivals at the listener,
and as long as each arrival is within the precedence time window of the previous arrival, all the
arrivals sound like they’re coming from the direction of the first arrival. From the perspective of
acoustics, this is equivalent to having several early reflections arrive at the listener. For example,
a listener might hear a reflection 20 milliseconds after the direct sound arrives. This reflection
would image back to the first arrival of the direct sound, but the listener would perceive an
increase in sound level. A second reflection could also arrive 40 milliseconds later. Alone, this
40 millisecond reflection would cause an audible echo, but when it’s paired with the first 20
millisecond reflection, no echo is perceived by the listener because the second reflection is
arriving within the precedence time window of the first reflection. Because the first reflection
arrives within the precedence time window of the direct sound, the sound of both reflections
image back to the direct sound. The result is that the listener perceives an overall increase in
level along with a summation of the frequency response of the three sounds.
The precedence effect can be replicated in sound reinforcement systems. It is common
practice now in live performance venues to put a microphone on a performer and relay that
sound out to the audience through a loudspeaker system in an effort to increase the overall sound
pressure level and intelligibility perceived by the audience. Without some careful attention to
detail, this process can lead to a very unnatural sound. Sometimes this is fine, but in some cases
the goal might be to improve the level and intelligibility while still allowing the audience to
perceive all the sound as coming from the actual performer. Using the concept of the precedence
effect, a loudspeaker system could be designed that has the sound of multiple loudspeakers
arriving at the listener from various distances and directions. As long as each loudspeaker arrives
at the listener within 5 to 30 milliseconds and within 10 dB of the previous sound with the
natural sound of the performer arriving first, all the sound from the loudspeaker system images in
the listener’s mind back to the location of the actual performer. When the precedence effect is
handled well, it simply sounds to the listener like the performer is naturally loud and clear, and
that the room has good acoustics.
As you can imagine from the issues discussed above, designing and setting up a sound
system for a live performance is a complicated process. A good knowledge of amount of digital
signal processing is required to manipulate the delay, level and frequency response of each
loudspeaker in the system to line up properly at all the listening points in the room. The details
Previous Page Next Page