Building Acoustics

Oscillating systems 29

above, this may be a serious limitation in using MLS as opposed to the swept sine
technique.

0 5 10 15 20 25

0

0.005

0.01

0.015

0.02

0.025

0.03

Line number

|X(f)|

a)

b)

-5000 0 5000

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

tau (sample)

R (tau)

Figure 1.23 Example of maximum length sequence (MLS). a) The first 25 lines in the spectrum of the sequence
shown in Figure 1.22. b) Autocorrelation function.

Finally it should be pointed out that, since the MLS has a flat frequency or line
spectrum, the autocorrelation function is very near to a Dirac δ-function. Figure 1.23
shows the spectrum and the autocorrelation function for the sequence partly shown in
Figure 1.22. For the spectrum, only the first 25 lines are shown. (How many lines are
there?)