Infrasound Properties of Sound Below 20 Hz and Ways to Measure It.

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InfrasoundProperties of Sound Below 20 Hz and
Ways to Measure It.

• Jerry BrownEssex Systems Oct. 2004

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A Roadmap

• My client has asked that I not discuss why we
  were interested in infrasound.
• So, I will confine my discussion to
• The general characteristics of infrasound with
  particular emphasis on how it differs from higher
  frequencies.
• How the environment interacts differently with
  it.
• The design of an unusual infrasound microphone
  based on measuring particle motion with a
  micromachined sensor.
• First, we need to review a few basic principles.

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Audible Sound
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The Audible Frequency Range

• 
  From http//home.tir.com/ms/concepts/concepts.ht
  ml
• Infrasound is everything below 20 Hz, the lower
  limit of human hearing. Wavelength at 20 Hz is
  16.6 meters (54 feet).

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Another Way to Look at It

• From
  http//home.tir.com/ms/concepts/concepts.html
• The Music portion of this graph must refer only
  to adult varieties or it would extend higher and
  further left.

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The Decibel
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dB Levels of Familiar Sounds
COMMON SOUNDS NOISE LEVELS (dB) EFFECT
Jet engine (near) 140 Jet
takeoff (100-200 ft.) 130 Threshold of
pain Thunderclap (near) 120 Threshold of
sensation Power saw (regular gt 1 Min)
110 Permanent hearing loss Motorcycle
90 Very annoying Many industrial
workplaces 85 Hearing damage begins (8
Hrs) Average city traffic noise 80 Annoying.
Vacuum cleaner 70 Intrusive. Normal
Conversation 60 Quiet Air conditioner
50 Comfortable Whisper 30 Very quiet Normal
breathing 10 Just audible 0 Threshold of
hearing

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Plane Waves
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Particle Motion in a Sound WaveThe graph is
animated. Click on it.

• The properties which are sensed by microphones
  are
• Particle velocity, u, or Pressure variation, p
• Important parameters are phase velocity,
  wavelength and frequency.

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Typical Values at 1000 Hz 90 dB(Plane Wave)

• Particle velocity is quite small compared to
  phase velocity (which is 331 m/s at STP). For
  1000 Hz at 90 dB (a very loud sound)
• Peak particle velocity 1.5 mm/sec
• Peak pressure 0.63 Pa or 6.2 x 10-6 atm.
• Peak displacement 0.23 micron
• Wavelength 0.331 meter.

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Values at 20 Hz 90 dB(Plane Wave)

• At 20 Hz and 90 dB (16 meters from the source).
• Peak particle velocity Same (1.5 mm/sec)
• Peak pressure Same (0.63 Pa)
• Peak displacement 12 micron
• Wavelength 16.6 m
• Not audible below 70 dB

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Values at 1 Hz 90 dB (Plane Wave)

• Finally, at 1 Hz and 90 dB (assuming that you are
  at least 300 meters from the source).
• Peak particle velocity Same (1.5 mm/sec)
• Peak pressure Same (0.63 Pa)
• Peak displacement 0.2 mm
• Wavelength 331 meter

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Problems With Standard Microphones and Amplifiers

• Microphone preamps and amps and are usually ac
  coupled and roll off below 20 Hz. It can be
  difficult to find all the coupling caps and get
  rid of them.
• Condenser microphones have a vent hole to
  equalize atmospheric pressure changes. This
  usually acts like a high-pass filter that cuts
  off below 20 Hz.

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The Pressure Microphone

• Capillary vent usually limits low frequency
  response.

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Alternative to the Pressure Microphone

• There are a few laboratory-type condenser
  microphones that will go down as far as 2 Hz.
• With infrasound, the wavelengths are so long that
  observations are almost always within a
  wavelength of the source. This is known as the
  near field.
• And pressure may not be the best variable to
  measure in the near field.

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The Near Field

• The near field is used in connection with sources
  that produce diverging spherical waves. Most
  real-world sources produce sound like this rather
  than plane waves.
• In the near field there is enough curvature in
  the wave front to affect the acoustic impedance
  of the air (Pressure/Velocity)

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Spherical Wave

• This illustrates a portion of a spherical wave.
• The graph is animated. Click on it.

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Why the Near Field is Different

• The motion of a few particles is exaggerated in
  this view to show that the particle spacing is
  changing circumferentially at the same time that
  its changing radially.
• The graph is animated. Click on it.

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The Affect on Pressure Variation

• So, unlike the plane wave, the spacing between
  particles in a spherical wave is changing both
  longitudinally and laterally.
• Both types of particle motion cause pressure
  variation in the wave. And the pressure
  variations partially cancel one another.

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Ratio of Pressure to particle velocity in the
near field relative to plane wave.

• Pressure diminishes in relation to particle
  velocity near the source.

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Measuring Particle Velocity
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The Microflown

• It happens that there is a sensor that is
  uniquely suited to measuring particle motion.
• Its made by a company called Microflown
  Technologies, located in the Netherlands.
• http//www.microflown.com/ .

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• Platinum is deposited on a silicon substrate. It
  is then masked and etched to form two closely
  spaced wires. Each wire is 1 mm long by 5 microns
  wide.

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• Current is applied to heat the wires to 200 C to
  400 C. Air flow in the plane of the wires shifts
  the temperature profile causing a temperature
  difference which is sensed by measuring their
  resistances.
• Microflown makes sensors capable of covering the
  entire audio range. The units used for this work
  have a range of dc to several kHz.

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Complete MicrophoneDesigned Mfgd by Essex
Systems
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Signal Processing
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3-Channel Microphone AmplifierDesigned Mfgd
by Essex Systems
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Amplifier with Laptop
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Data Acquisition Display(Testpoint software)
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Calibration

• Calibrating a particle motion sensor poses unique
  problems.
• Following a suggestion from Microflown, speakers
  are placed at the ends of a short cylinder and
  driven 180 degrees out of phase to create a
  moving plug of air. At infrasonic frequencies the
  velocity of the air mass is affected very little
  by its inertia.
• Two laser triangulation sensors measure the
  speaker cone velocities.

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Calibration
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Sensitivity

• The flow sensitivity of the Microflown bridge is
  approximately .7 volt/m/sec
• So for 1Hz at 90 dB with dual sensors the signal
  is

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Noise

• The noise floor for the entire system, in terms
  of velocity, is 1 micron/sec (1 to 40 Hz). Almost
  all of that is from the sensor.
• The noise is undoubtedly related to the high
  temperature of operation.
• It behaves like semiconductor flicker noise.

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How Good is It for Infrasound?

• If a 12 inch speaker (backside closed) is
  vibrating with an amplitude of 1 mm at 1 Hz the
  pressure amplitude at a distance of 1 m will be
  20 dB (barely a whisper if it were at 1 kHz) .
• The particle velocity will be 36 microns/sec,
  well above the microphones 1 micron/sec noise
  floor.

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But, Thats Not the Whole Story

• When we began making measurements, it became
  apparent that environmental noise is an
  overwhelming factor.
• At first, a lot of time was spent looking for
  electrical shielding problems.
• But eventually, it became clear that the
  microphone responded to air motion that isnt
  ordinarily associated with sound.

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Pictures From a Schlieren Camera

• The following two pictures were made in the
  laboratory of
• Dr. Gary S. SettlesProfessor of Mechanical
  Engineering Director, Gas Dynamics Lab     
  phone [phone]Penn State
  University                fax (814)
  865-0118301D Reber Bldg.                     
  email gss2_at_psu.eduUniversity Park, PA 16802
  USAhttp//www.mne.psu.edu/psgdl

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Ambient Turbulence

• This is a picture of air turbulence caused by hot
  air (from the man, a CRT and a heat vent in the
  back). This was done with a large Schlieren
  camera that makes small differences in air
  density visible.

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Even Convection From Our Bodies is a Factor

• In this Schlieren photo you can see the
  convection currents caused by body heat.

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Its Like Listening to a Conversation in a
Hurricane

• The picture that began to emerge is that ambient
  air flow caused by ventilation, heat and movement
  of people in a room were overwhelming the
  particle motion sensor. Drafts from an air
  conditioning system would create signals so large
  that the electronic amplifiers would saturate and
  quit responding.
• For an infrasonic sensor a quiet air
  conditioned room with people in it is really
  noisy.

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Acoustic Filtering

• Eventually, it was recognized that most of the
  turbulence noise in a quiet room is below 1 Hz.
• By trial and error we found that a fine mesh
  screen over the microphone aperture can act as a
  low frequency acoustic filter to screen out much
  of the turbulence noise.
• Fabric from womens nylon hosiery was used at
  first.
• Later this was replaced by stainless steel mesh
  that you see in the present design.

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Measurement Without a Screen

• The graph below shows typical recordings with and
  without a mesh screen. Red without, blue with.

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Results With a Screen

• Here is the blue line of the previous graph at
  different scale. Its a recording of body sounds
  with the microphone a few inches from the chest.

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More Filtering

• The low frequency cutoff of the microphone
  amplifier was set to 0.7 Hz with a 4th order high
  pass filter.
• Further filtering was done in software using an
  FFT algorithm.

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Making the Infrasound Audible

• Its hard to look at an infrasound recording and
  make much of it.
• So, we developed a software tool to convert a
  recording to audible sound.
• Two transformations were required to convey all
  the information to the ear.
• First, a constant amplitude wave is produced with
  a frequency that is proportional to the slope of
  the infrasonic wave.
• Then this wave is amplitude modulated in
  proportion to the amplitude of the original wave.

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Some Interesting FM Sounds