Assignment 8: Calculating Total Absorption 1. Based on the geometric measurements provided and the acoustical data measured, calculate the total absorption in this room in each octave band. (Hint

Sound System at BPC

The goal of any sound reinforcement system is to provide all of the listeners with the same experience. To achieve this goal, all of the listeners should be within the critical distance of a loudspeaker. You know from last week's lesson that critical distance increases with directivity. This means that sound reinforcement loudspeakers benefit from a high level of directivity (a high value of Q).

You also know that the sound level from a point source decreases at a rate of 6 dB per doubling of distance, inside of the critical distance. This means that using a point source loudspeaker will result in a significant decrease in sound pressure level as listeners get farther from the loudspeakers. One way to reduce this effect is to use a loudspeaker that is more like a line source.

The BPC sound reinforcement system consists of two linear arrays of loudspeakers. These are vertical arrays of smaller loudspeakers that together approximate a line source. An added benefit of the array is that it provides a high level of directivity. Together, these attributes provide fairly even sound pressure level coverage, and a long critical distance.

The arrays in the BPC have to be hung high enough above the stage for performers to walk under them. Unfortunately, this means that they are unable to reach all the way to the back seats on the orchestra level, as they are in the shadow of the balcony. Under-balcony delay loudspeakers provide reinforcement in this area. They are known as delay loudspeakers, because the signal sent to them is delayed to match the time it takes the sound from the linear arrays above the stage to travel to their position. If they were not delayed, the sound from these loudspeakers would arrive at the listeners before the sound from the arrays, creating a distracting echo.