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A Lecture

Sound Waves

compressions and rarefactions in air pressure

fluctuations of air pressure carried through air

rarefied gascompressed gas wavelength

Sound Waves

Wavelength () is measured from crest-to-crest

or T

horizontal axis could be:

space or time

p

r

e

s

s

u

r

e

Wavelength () is measured from crest-to-crest or trough-to-trough, or upswing to upswing.

For traveling waves (sound, light), there is a speed (c)

C = f Frequency (f) refers to how many cycles pass by per second

measured in Hertz (cycles per second)

associated with this is period: T = 1/f

Sound waves in air are longitudinal.

Our ear ignores changes slower than 20 Hz

Eardrum cant respond faster than about 20 kHz Eardrum cant respond faster than about 20 kHz

Thus we can only hear sounds with frequencies ranging from

20 Hz to 20,000 Hz

Infrasonic Waves

Sound waves below 20 Hz are called infrasound.

Infrasonic waves travel with the sound speed which is 343 m/s for air of

20C.

C = (331.3 + 0.606 T) m/swhere T is temperature in degree Celsius.

The propagation of infrasound is controlled by the effective sound speed,

which is a function of the temperature and wind along the source-

receiver trajectory

Ceff = C + v cos

The amplitudes of infrasonic waves are small with respect to

the ambient pressure and vary between milli-pascals (Pa) to

tens of Pa.

On the Earths surface, Standard Atmospheric Pressure is On the Earths surface, Standard Atmospheric Pressure is

101,325 Pa = 1.01325 bar

1 Pascal = 10 Bar

Infrasonic waves are of long wavelengthWavelength of 10s period signal ~ 3.3km

Wavelength of 100 Hz signal ~ 3.3 m

Infrasound propagates with little attenuation over great distances

For example with same initial amplitudes, a 10.0 Hz signal For example with same initial amplitudes, a 10.0 Hz signal can travel a distance of 100 D with same attenuation compared to that of a 100 Hz signal at distance, D (all other parameters remaining unchanged).

Infrasound can be detected on the Earths surface by specialized instruments operating in the passband from ~ 0.01 to 10 Hz, called Microbarometer.

A Microbarometer

Noise Reducer

Spatial filtering and not frequency filtering.

Sample an area larger than the turbulence scale of the wind.Coherency length of wind ~ 10cm. Reduce wind effect by averaging the pressure field over an area.

Dimension of noise reducer should be < < wavelengths of infrasound.

Atmosphere: Temperature Profile

Infrasonic Arrivals

Besides Direct Arrivals(Speed ~ 340 m/s)

There are three regions in the atmosphere where from

infrasonic waves may turn back towards the surface

(subject to favourable conditions)

Speed

From the troposphere 320m/s 330 m/s

From the stratosphere 280 m/s 310 m/s

From the thermosphere 220 m/s 270 m/s

Seismic & Infrasound Signals (Mining Events)

Infrasound from Other Sources

Lightning

Helicopter

Howitzer (at 5 Km)

Rocket Launch (various stages)

1st stage burnout: 6.4s

2nd stage ignition: 16.0s

2nd stage burnout: 19.5s

3rd stage ignition: 25.0s

3rd stage burnout: 57.4s

4th stage ignition: 76.0s4th stage ignition: 76.0s

4th stage burnout: 94.6s

Bolide signal

Volcanic signal

Mt. Erebus, located approximately 30 km from the infrasound site, is an active

volcano producing occasional small eruptions.

Microbaroms Associated with

Marine Storms

microbaroms are

narrowband waves with

a 5 second period,

observed world-wide

and produced by and produced by

marine storms.

can be the dominant

signal in the winter

infrasound data

Arrays

When a signal crosses the array, small differences in the

arrival times of the signal at the individual array elements are

used to calculate the velocity and direction of the signal.used to calculate the velocity and direction of the signal.

The accuracy of the velocity and direction determinations

increases with the diameter of the array.

On the other hand, the diameter of the array cannot be made

larger than 3 km since the shape of the signal must not

significantly change from one array element to the other