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Why Siren sounds the way it dioes?


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Why Siren sounds the way it dioes?

  1. 1. POLICE SIRENANALYSISProject Cover StoryDeepak Sharma and Mohit Yadav
  2. 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 10readings 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 theaxis of siren. 10 reading were taking ranging from a distance of 1m to a distance of 8 m in Xdirection and from a distance of 0 m to a distance of 3m in both positive and negative Ydirection. The 10 sheets showing up the SPL readings for different frequencies could befound in the excel file attached. But, almost all the sheets gave the same curve (Fig. 2) forchange in SPL values with frequency and gave a general consensus that the significant partof sound was seen in the frequency ranging from 1 kHz-5 kHz in which a human’s ear ismore sensitive to hear the sound. But, as we were not able to do much with this result (alsothe SPL v/s Time result was not much clear in providing information). We went on withcomparing the siren’s .wav file and a song named “Tu jaane na” to see why the siren soundsthis very different.X AxisY Axis
  3. 3. Fig. 2 showing experimental results.While working withthe twoaforementionedsound files (see Fig. 3for SPL valuesobtained), we saw thedifference in effectivecomponents that ourear attends to,namely; Loudness,Sensitivity todifference frequenciesand pitch variation.What we found was that due to the effectiveness of Cochlea, the inner part of ear which hasmore than 60,000 Inner hair cells, the higher frequencies sensitize our brain before lowerfrequencies. This explanation is well mentioned in Place theory which states that ourperception of sound depends on where each component frequency produces vibrations along thebasilar 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 soundproduced by siren had harmonics in it whereas the other song didn’t have much harmonics. Thiscould 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 3rdandmost important aspect of hearing; i.e. pitch perception and variation. We plotted pitchcontours and found that pitch variation for siren was much more than for the song, whichhad a flat curve for the variation. The FFT (A), Spectrogram (B) and Pitch Variation (C) couldbe seen in figure 5 and 6 for Police Siren and aforementioned song respectively.
  4. 4. Fig. 5: FFT (A), Spectrogram (B) and Pitch Variation (C) for Police sirenFig. 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 insound and most importantly, the pitch variation which sensitizes us to be more attentive toa piece of sound.Now, we know the reason behind the peculiar perception siren’s sound that we have.BCABCA