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Bandpass and LIGO

Ospac has a very accurate band pass filter that can be used to reduce rumble or high frequency hissing. It is so accurate that it can be used to remove noise from the gravitational wave detection experiment LIGO.

Fetch data and generate raw sound

The LIGO experiment detected a special event of which the raw data can be downloaded:

gzip -d H-H1_LOSC_4_V1-1126259446-32.txt.gz
gzip -d L-L1_LOSC_4_V1-1126259446-32.txt.gz

This data represents the signals from two "laser microphones" located in Hanford and Livingston, USA, which are separated from each other by about 3000km. The microphone have shown to be able to detect planes flying over them as well as seismic waves. But apart from that, they should also be able to detect gravitational waves as they were predicted by Einstein. Using ospac you can listen to the recording with 4096Hz sample frequence (they also provide files with 16384Hz sample frequency):

ospac --raw --ascii 4096 H-H1_LOSC_4_V1-1126259446-32.txt \
            --ascii 4096 L-L1_LOSC_4_V1-1126259446-32.txt \
            --output ligo-raw.wav
ospac --raw ligo-raw.wav --plot ligo-raw.ppm

Raw ligo data

Listen to the raw ligo data

Band pass filter

This already sounds fascinating, but the actual signal is covered by noise. Using Audacity we ca have a look at the spectrum:

Raw ligo data spectrum

Within the interval from 80Hz to 300Hz there seems to be a valley of less noise, as this was hinted by Ligo). Using the --bandfilter of ospac with a very thin transition interval of just 10Hz, we can have a closer look at this less noisy segment:

ospac --raw --ascii 4096 H-H1_LOSC_4_V1-1126259446-32.txt \
            --ascii 4096 L-L1_LOSC_4_V1-1126259446-32.txt \
            --bandpass 80 300 10 --normalize \
            --output ligo-filter.wav
ospac --raw ligo-filter.wav --plot ligo-filter.ppm

Raw data band filtered 80-300Hz

Listen to the filtered ligo data

Isolate the final result

What's was at 16s? We can isolate this event using the skip filter:

ospac --raw --ascii 4096 H-H1_LOSC_4_V1-1126259446-32.txt \
            --ascii 4096 L-L1_LOSC_4_V1-1126259446-32.txt \
            --bandpass 80 300 10 \
            --skip --skip-level 0.5 --skip-order 0.95 \
            --normalize \
            --output ligo-event.wav
ospac --raw ligo-event.wav --plot ligo-event.ppm

Isolated event in the raw data band filtered 80-300Hz

Listen to the isolated event in the filtered ligo data

These are two massive black holes well beyond our sight that fell into each other generating gravitational waves that reached Earth.

Apart from this, you can very well use the --bandpass filter to reduce low frequency rumbling or high frequency hissing. For this, you should most probably use wider transition intervals for faster processing.

This text originally appeared on in German.