Sound Intensity

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Sound Intensity

• Theory and practice

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Agenda

• Sound Intensity
• Theory, Limitations
• Sound Intensity Instrumentation
• 2144, 2260
• Sound Power measurements
• ISO 9614-2, 2260 benefits
• Noise Source Location

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What is Sound Intensity

• Definition of Sound Intensity
• Sound Intensity is the time-averaged rate of
  energy flow per unit area
• The sound intensity vector equals the
  time-averaged product of the instantaneous
  pressure and the corresponding instantaneous
  particle velocity at the same position

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Sound Power from Sound Intensity

P ?s I . ds
??
??
P
P ( W ) I
4 ? r 2
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Effects of External Sources
? SI . dS W
? SI . dS 0
Stationary Sources
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Sound Power
LW LI 10 . log S/S0
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The Sound Power (Intensity) Standards
Point Measurements Sweeps
ISO 9614 Part 1 ISO 9614 Part 2
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Measurement Surfaces
Hemisphere Box
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Why Use Sound Intensity?

• Less sensitive to background noise
• Gives directional information
• Enables segmentation of source
• Isolates the object under investigation
• Quality control of measurement

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Pros and Cons of Sound Intensity

• Pro
• Less demands to measurement site ? save site
  preparation costs
• Identifies causes of problem ? saves time when
  solving problems
• Contra
• More complex instrumentation ? higher instrument
  and training costs
• More measurements ? may increase measurement time

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What is Sound Intensity used for?

• Sound Power Determination
• Non-standard survey (indication)
• Measurement according to standard
• Data gathering for modelling
• Noise Source Location
• Source location
• Source ranking
• Noise mapping
• Building Acoustics
• Sound Reduction Index
• Leakage detection
• Sound absorption
• Sound Field Investigation

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Where is Sound Intensity used?

• Industry
• Research development
• Product noise labelling(CE marking)
• Environment
• Noise modelling
• Noise reduction in buildings
• Noise reduction of existing installations
• Universities
• Education
• Research

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How is Sound Intensity Measured
Sound Intensity in one direction r is measured
with two closely spaced microphones
The average pressure is measured as
The average particle velocity is measured as
The Sound Intensity is
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Theoretical Limitations at High Frequencies

• Finite Difference Approximation Errors

Accuracy within 1dB Spacer Limit 50 mm up to
1.25 kHz 12 mm up to 5 kHz)
) without correction for resonance's at high
frequency
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Limitations at Low Frequencies

• The Sound Intensity is proportional to the phase
  change over the spacer
• A phase mismatch of the analyzing system
  introduces an error to the calculated sound
  intensity

The phase mismatch error is most severe at low
frequencies
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Intensity Probe Directivity (Intensity)
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Pressure-Intensity Index

• Sound Intensity Sound Pressure
• for sound propagating along the probe axis in one
  direction only (free field)
• Sound Intensity lt Sound Pressure
• for sound propagating at an angle to the probe
  axis
• for diffuse sound fields

Pressure-Intensity Index Lp - LI
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Pressure-Residual Intensity Index

• A Residual Intensity ? 0 W/m2 will be measured
  when exactly the same signal is input to the two
  microphones - due to the phase mismatch of the
  analyzing system
• The Residual Intensity varies with the pressure
  level
• The Pressure-Residual Intensity relates to the
  phase mismatch

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Dynamic Capability

• Dynamic Capability ? Pressure-Residual Intensity
  Index - K

• Often used values K 7 or K 10 (1 dB accuracy
  for K 7)

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Improving Low Frequency Limit

• Low Frequency limit depends on
• Dynamic Capability (phase match and spacer)
• Pressure-Intensity index of measurement
• Lower the Frequency limit by
• Decreasing the Pressure-Intensity index by
  measuring closer to the source or by putting
  additional absorbtion material into the room
• Improving the Dynamic Capability
• Increase the spacer
• Improve the phase matching

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2260 - Calibration

• Make a pressure calibration of the two
  microphones using
• the 4231 Sound Level Meter Calibrator
• the 3541 Sound Intensity Calibrator

• Enhance the phase match of the system using
• the 3541 Sound Intensity Calibrator

• Verify the Pressure-Residual Intensity index
  using
• the 3541 Sound Intensity Calibrator

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Useful frequency range
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Summary on limitations

• High Frequency limit depends on
• Spacer distance
• Microphone dimensions
• Low Frequency limit depends on
• Dynamic Capability
• Pressure-Intensity index of measurement
• Dynamic Capability depends on
• Spacer distance
• Phase match between channels
• Signal to Noise Ratio depends on
• Frequency
• Averaging time

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Sound Power Measurements

• Determine Sound Power using standards
• ISO 9614-1 (not directly supported by BZ 7205)
• ISO 9614-2
• ECMA-160
• ANSI s12.12

• Benefits of using Sound Intensity
• Results are determined within a given precision
• Results are trustworthy, repeatable and comparable

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ISO 9614-2

• Determination of sound power levels of noise
  sources using sound intensity - Part 2
  Measurement by scanning
• Define a measurement surface around the source
  under test and divide it into at least four
  segments
• Perform two scans at each segment
• Evaluate
• the repeatability of the two scans
• the dynamic capability compared to the p-I index
  of the measurement
• the presence of extraneous noise
• Calculate the total sound power by summing the
  results of all the segments

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ISO 9614-2 - Scanning
Required by the Standard
2260 Solution

• Perform scanning
• at a steady pace
• with probe pointing towards the source
• accurately following the scan path

• Use aural feedback
• Count no. of bips while moving the probe
  backwards and forwards
• Keep the eyes on the probe while listening to the
  progress of the measurement

• Aural feedback goes up one octave
• Status code S if scan time is less than 20 s

• Duration of any scan at least 20 s

• Soft key wizard to keep track of the scans

• Make two scans of each segment

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ISO 9614-2 - Repeatability
2260 Solution
Required by the Standard

• Repeatability
• Status code R for each frequency band
• Detailed view available

• Partial Power Repeat., Criterion 3
• Perform two scans on each segment, then check for
  each frequency band
• the difference between the two scans is within a
  certain limit

• For all segments
• if, in any one frequency band, the sum of the
  partial powers failing criterion 3 is more than
  10 dB below the sum of the partial powers
  satisfying criterion 3, then the total sound
  power can still be calculated for that frequency
  band

• Automatically checked by the software, status
  code R on total sound power

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ISO 9614-2 - Dynamic Capability
Required by the Standard
2260 Solution

• Evaluate FpI, Criterion 1
• FpI lt Ld for each frequency band
  (Pressure-Intensity index for the total surface
  shall be less than the dynamic capability)
• Two grades of accuracy

• Dynamic Capability
• p-I index checked against Dynamic Capability for
  each frequency band, in each segment, surface and
  total surface. Status code D for each frequency
  band
• Two grades of accuracy
• Detailed view available

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ISO 9614-2 - Extraneous Noise
Required by the Standard
2260 Solution

• Evaluate F/-, Criterion 2
• F/- lt 3 for each frequency band (Negative
  partial power indicator for the total surface
  shall be less than 3 dB)
• Only mandatory for Engineering grade
• Definition

• Extraneous Noise
• Extraneous Noise checked against 3 dB for each
  frequency band, in each surface and in the total
  surface. Status code E for each frequency band
• Detailed view available

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ISO 9614-2 - Calculation of Sound Power
Required by the Standard
2260 Solution

• Define your measurement surface directly on the
  screen of 2260
• Define the area of the segments
• Sound Power is automatically calculated for the
  displayed segment or surface

• Calculate the sound power from each segment by
  multiplying the sound intensity by the area of
  the segment
• Calculate the total sound power by adding the
  sound power from all the segments together

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Sound Intensity - The Situation Today

• There are problems
• Heavy equipment
• Many cables
• Results via PC
• Mains operated

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Sound Intensity - The Situation Tomorrow

• The solution
• Hand-held equipment
• Cable-free
• On-the spot results
• Battery operated

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A Real-world Intensity Analyzer

• The 2260 Investigator
• Out of the laboratoryand into the palm of your
  hand
• All equipment in one carrying case
• Only one person needed from scanning to final
  result

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Designed for Practical Field Measurements

• Feature
• Visual and aural feedback

• Benefit
• Better control,better measurements

• Flexible segmentation

• Focus on measurement
• Freedom to adapt to the measurement situation

• Visual data manager instead of a lot of file
  numbers

• Better overview of results
• Easy bookkeeping

• On-site calculation and display of results no
  need for a PC

• On the spot results
• All-in-one robust unit

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Sound Power
with the Hand-held Sound Intensity Analyzer

• Noisy environments
• Sound Power measurements on-site
• Low cost and easy to use instrumentationNo need
  for special test facilities such as anechoic
  rooms, etc.
• Standardized measurementsResults in accordance
  with ISO 9614-2, ECMA-160 or ANSI S.12.12
• Or non-standard

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Sound Power - Reporting using Noise Explorer
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Noise Source Location
with the Hand-held Sound Intensity Analyzer

• Simple on-line source location
• Lots of measurements
• easily collected in surfaces on screen
• Extensive guidance
• automatic selection of next predefined segment
• Contour maps
• directly on screen
• Ranking of surfaces number map of surfaces

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Noise Source Location
Reporting on a PC
Excel
Surfer
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Sound Intensity Made Easy

• The Hand-held Sound Intensity System
• A complete hand-held, single unit sound intensity
  system
• A system designed for practical field
  measurements
• Makes measurements easier and faster
• Gives you on the spot analyses of sound power and
  intensity
• ... at a price you can afford