http://deepaksharma1992.blogspot.in/2013/05/what-makes-sound-of-siren-so-different.html

1. POLICE SIREN
ANALYSIS
Project Cover Story
Deepak Sharma and Mohit Yadav

2. Question that we answered with this project:
What’s so special in the sound produced by siren which grabs our attention immediately?
The Cover Story behind our project:
The analysis of police siren (Fig. 1) (Current 1Amp (at 12V D.C.), SPL 124dB (at 1m distance)
and size: 155mmX135mm) was done by placing it in institute’s project room and taking 10
readings using microphone and Data Acquisition system provided by national instruments.
Fig. 1 showing the siren used.
The readings were plotted at different distances in both parallel (X) and transverse (Y) to the
axis of siren. 10 reading were taking ranging from a distance of 1m to a distance of 8 m in X
direction and from a distance of 0 m to a distance of 3m in both positive and negative Y
direction. The 10 sheets showing up the SPL readings for different frequencies could be
found in the excel file attached. But, almost all the sheets gave the same curve (Fig. 2) for
change in SPL values with frequency and gave a general consensus that the significant part
of sound was seen in the frequency ranging from 1 kHz-5 kHz in which a human’s ear is
more sensitive to hear the sound. But, as we were not able to do much with this result (also
the SPL v/s Time result was not much clear in providing information). We went on with
comparing the siren’s .wav file and a song named “Tu jaane na” to see why the siren sounds
this very different.
X Axis
Y Axis

3. Fig. 2 showing experimental results.
While working with
the two
aforementioned
sound files (see Fig. 3
for SPL values
obtained), we saw the
difference in effective
components that our
ear attends to,
namely; Loudness,
Sensitivity to
difference frequencies
and pitch variation.
What we found was that due to the effectiveness of Cochlea, the inner part of ear which has
more than 60,000 Inner hair cells, the higher frequencies sensitize our brain before lower
frequencies. This explanation is well mentioned in Place theory which states that our
perception of sound depends on where each component frequency produces vibrations along the
basilar membrane. This basilar membrane consists of Cochlea, which acts as the home for sensors
(inner hair cells) (see Fig. 4). Also, the spectrogram plotted for both the sounds showed that sound
produced by siren had harmonics in it whereas the other song didn’t have much harmonics. This
could be seen in Fig. 5.
Fig. 4 Showing the Basilar membrane.
But, finally we were able to distinguish between both the sounds by looking at the 3rd
and
most important aspect of hearing; i.e. pitch perception and variation. We plotted pitch
contours and found that pitch variation for siren was much more than for the song, which
had a flat curve for the variation. The FFT (A), Spectrogram (B) and Pitch Variation (C) could
be seen in figure 5 and 6 for Police Siren and aforementioned song respectively.

4. Fig. 5: FFT (A), Spectrogram (B) and Pitch Variation (C) for Police siren
Fig. 6: FFT (A), Spectrogram (B) and Pitch Variation (C) for a melodious Song (“Tu Jaane na”)
Therefore, we could figure out finally that it’s the loudness, frequency range & harmonics in
sound and most importantly, the pitch variation which sensitizes us to be more attentive to
a piece of sound.
Now, we know the reason behind the peculiar perception siren’s sound that we have.
B
C
A
B
C
A