WinMLS Acoustical

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• WinMLS - Acoustical
• Measurements

Lars H. Morset Dr. Johan L. NielsenMorset
Sound Development Trondheim, Norway
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Introduction

• General about us and our measurement system
• Theory (Impulse Response and MLS)
• Structure of the software
• Sound system calibration measurement example
• Room Acoustics
• Parameters according to ISO 3382
• Perform demonstration measurement
• What are your needs in acoustical and vibrational
  measurement systems?

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Morset Sound Development

• A small Norwegian firm
• established 1995, WinMLS work started 1997
• Dr. J. L. Nielsen involved in room acoustics part
  
• Good connections with the university acoustics
  group, consultants and Genelec, Finland.
• Purpose
• Main Development and sale of WinMLS
• Some consultancy work
• Hardware solutions for our customers
• Adapting the software for our customers

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Mission of Morset Sound Development

• Morset Sound Development is developing and
  selling professional windows-based software for
  integration in user-friendly systems for reliable
  acoustical measurements (currently based on PC
  and sound card) at low expense.

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Why we developed WinMLS?

• PC sound card is inexpensive open hardware
• New measurement methods take over...
• New equipment needed
• Suited for PC/sound cards (problem initial time
  delay)
• Existing MLS-systems had weaknesses
• MLSSA (1 channel, 12 bits, DOS-based)
• Some others Must go through several steps to
  obtain a measurement or plot the data.
• Not flexible enough for the professional user

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Advantages of WinMLS

• Flexible, but still User Friendly
• Made by acousticians for acousticians
• Open Runs on most sound cards gt Flexible and
  low-cost
• Get results after one click
• By loading setups (default setups available)
• Minimum of tedious procedures required
• Support several file formats, measure initial
  time delay, run from Matlab, calibrated
  measurements, ...

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Hardware

• PCI/ISA-card (2-24 channels)
• Fsmax 215 (48) kHz, 24 (16) bit (SNRgt100 dB)
• Portable system Laptop
• Built-in sound chip
• Good enough quality for several applications
• Mini-jack plugs not optimum
• USB
• An extra box needed
• VXPocket PCMCIA card (we sell this card)
• 2-4 (8) channels
• High quality plugs
• Selectable Microphone/Line input
• 2 channel card used by gt50 professional WinMLS
  users
• 4 channel card is new, has just been tested
  (synch.)

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Market Segments

• Some User Segments
• Consultants in Sound Systems/Room Acoustics
• Research and Education
• Sound System Engineers and Installations
• Audiophiles
• Each segment need customized interfaces
• Do this by using setups
• Measurement all settings relating to performing
  a measurement
• Post-processing all other settings

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Using WinMLS2000

• Start software, explain setups
• Measurement
• Post-processing
• open example measurement
• active plot (settings), active measurement
• overlay, solo,
• open several measurements
• Glossary of terms in help-file

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The Impulse Response
Time domain
y(t)
x(t)
y(t) x(t)h(t)
Linear system x', x'' inputs e y', y''
outputs a x' b x'' a y' b
y' Time-invariant system x(t-t) y(t-t)
Frequency domain
Y(?) X(?)H(?)
H(?) Fh(t)
Relationship t ?
Used on permission from N. Prodi
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MLS (Maximum Length Sequence) Method
DETERMINISTIC SEQUENCE MADE OF ZEROS AND ONES
WHICH IS TERATED TO IMPROVE THE S/N
TO FULLY EVALUATE THE REVERBERATION TAIL THE
SEQUENCE SHOULD BE LONGER THAN THE SUPPOSED REV.
TIME.
THEORY
x(n), y(n) discretized signals R correlation
operation Rxy(n) Rxx(n) h(n) since x(n)
MLS one has Rxx(n) ?(n) (like WN !!) Rxy(n)
?(n) h(n) h(n)

h(t)

Generator
MLS x(n)
y(n)
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Hadamard

tramsform
Correlation input - output
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h(n)
Maximum length of the impulse response

• MLSSA max order 16 216-1 points
  1.5 s
• WinMLS max order gt21 221-1 points
  47.5 s
• sr 44.1 kHz

Used on permission from N. Prodi
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Emphasis to improve SNR
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Frequency Resolution

• Displayed freq res Sample rate / FFT Size
• Sample Rate 48 kHz
• FFT Size 214 16,384 points
• Displayed freq res 48k / 16k 2.93 Hz
• Impulse resp length MLS period / Sample rate
• MLS Period 214-1 16,383 points
• Impulse resp length 16k / 48k 341 ms
• True frequency resolution 1 / Imp resp length
• True frequency resolution 1 / 0.341 2.93 Hz

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Time Windowing

• Used to remove room reflections from the impulse
  response when computing Frequency Response
• Different shapes
• Full, Half, (Flat top)
• Rectangular, Cosine, Hanning, Hamming,
  Blackmann-Harris, Kaiser-Bessel and others
• New True freq res 1 / Window imp resp length
• New true frequency resolution 1 / 0.035 28.57
  Hz
• Demonstration of time windowing in WinMLS...

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Radiation Setup

• Radius 1.05 m (far field)
• Resolution 22.5 degrees-gt 1.5 kHz is limit for
  measuring directivity.
• Fast (15 min) using 8-channel MLS and
  PC-controlled turntable.
• Automatic method for saving makes this possible