TimDec

TimDec is a program that measures the reverberation time of rooms.

Disclaimer: So far this software is in a beta stage, and the results have not yet been verified against other methods. You may use the program at your own risk, and you should as always examine the results for accuracy. Since I am still developing the software, I am very interested in having screen dumps of your measurements, in order to learn how the software behaves for different room conditions. Please send your comments and/or GIF JPG or TIF files to timdec@tolvan.com .

TimDec measures the decay time by playing bursts (typically 1 s long) of noise through a loudspeaker, and record it with a microphone. During the burst a diffuse field will build up in the room, and when the noise is turned off, the diffuse field will decay. TimDec chops the recorded signal into windows, which are run through a fast fourier transform (FFT). TimDec plays and records several cycles of this noise burst and calculates an average, in order to achieve smooth graphs of the reverberation.

The loudspeaker should preferably, but not necessarily be omni-directional. If it is not, it should be directed away from the microphone. The microphone should be omnidirectional, and placed far away from the loudspeaker and typically not next to a wall.

The results are displayed in the different panels of TimDec as follows:

TimDec main window. Here the FFT parameters are set, as well as the duration of the noise burst and the silence before and after the burst. The greyscale image is similar to a spectrogram, with the amplitude of the signal shown in a grey scale. The noise burst can be seem between 1 and 2 s, and the short "tail" after 2 s represents the reverberation.

TimDec decay window. Here a "slice" taken at approximately 1550 Hz from the grey scale image in the main window is shown. The reverberation decay after 2 s is obvious. The green line has been added manually by clicking with the mouse, and the reverberation time corresponding to the line slope can be seen on the status bar.

Timdec reverberation time window. TimeDec can also automatically extract the reverberation time from the decay graphs, and does so for each frequency of the FFT. The blue lines seen in the decay window are set by the "initial drop" and "additional drop" entries in this window. The result is a graph over how the reverberation time varies with frequency. In cases when the reverberation time cannot be measured with these settings, a time 0 s is displayed. The grayed part of the curve represents frequencies below the Schröder frequency, at which reverberation time is not a well defined entity.

Timdec reverberation radius window. Timdec can convert the reverberation time to reverberation radius if the room volume is known.

TimDec response window. This graph shows the frequency response of the loudspeaker, room and microphone. Given that an omnidirectional microphone with a flat response is used, this graph can be used to measure the in-room response of the loudspeaker. The blue curve shows an average over the duration of the burst, the red curve shows the response at a time selected in the main window, in this case at about 0.5 s.

Note: Since TimDec plays white noise with a lot of the energy concentrated at the high frequencies, there is a risk of damaging the tweeters of a full-range loudspeaker system. Please take the appropriate measures to avoid tweeter damage. This is mostly synonymous with keeping the sound at a sane level.