The purpose of this lab is to find out what the reverberation time (RT60; Rossing ch. 23) is in the lecture hall. This was measured under five different conditions:

1. at 200 Hz. with the mic in the middle of the room (the normal mic placement);

2. same as above, but with the mic about 2 feet from the speaker (!)

3. same as (1), but with the frequency raised to 5000 Hz.

4. same as (1) (frequency 200, mic in middle of room), but this time with the curtains drawn in front and back of the room.

5. same as (3) (frequency 5000, mic in middle of room), curtains drawn as in (4).

Your assignment, due Thursday Nov. 10, is to measure the reverberation time ("RT60") of the room in each of these cases. Did increasing the frequency increase or decrease the reverberation time? Did drawing the curtains increase or decrease it? Which of these had a stronger effect on reverberation time? By how much did moving the mic right next to the speaker affect it?

Here are the five runs:

5000 Hz, mic in center of room, curtains drawn

5000 Hz, mic in center of room, no curtains

200 Hz, mic in center of room, curtains drawn

200 Hz, mic in center of room, no curtains

5000 Hz, mic close to speaker, no curtains

In each graph, the horizontal axis is tenths of a second, and vertical axis is decibels.

In the first of the five runs, some lines were addded to demonstrate how to find reverb time. To find the reverberation time, we ask the question: over how much time does the decaying portion of the sound level drop by 60 dB? The answer is the difference between the points of interstection of the vertical and slanted lines with the time axis.

The data collection in this lab was done in class, Nov. 3.