WinMLS - Acoustical

MeasurementsLars H. Morset & Dr. Johan L. Nielsen

Morset Sound DevelopmentTrondheim, Norway

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?

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

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.

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

Advantages of WinMLS

• Flexible, but still User Friendly– Made by acousticians for acousticians

• Open: Runs on most sound cards => 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, ...

Hardware• PCI/ISA-card (2-24 channels)

– Fsmax= 215 (48) kHz, 24 (16) bit (SNR>100 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 >50 professional WinMLS users• 4 channel card is new, has just been tested (synch.)

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

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

The Impulse Response

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°)

Time domain

y(t) = x(t)*h(t)

Systemunder testt t

Unit pulse ( ) Impulse response of the systemi

Frequency domain

Systemunder test

White noise Transfer function (Modulus)

Y() = X()H()

H() = F[h(t)]Relationship t :

y(t)x(t)

Used on permission from N. Prodi

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.

THEORYx(n), y(n) discretized signalsR correlation operation

Rxy(n) = Rxx(n) *h(n)

since x(n) = MLS one hasRxx(n) = (n) (like WN !!)

Rxy(n) = (n) *h(n) = h(n)

h(t) Generator MLS x(n) y(n)

Hadamard

tramsform

h(n)

Correlation input - output

Maximum length of the impulse response• MLSSA max order 16 216-1 points 1.5 s*

• WinMLS max order >21 221-1 points 47.5 s **sr = 44.1 kHz

Used on permission from N. Prodi

Emphasis to improve SNRMagnitude Frequency Response

Measured - 17:06:34, 11Oct2000 Plotted - 17:06:38, 11Oct 2000Comment:

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Magnitude Frequency Response

Measured - 17:03:03, 11Oct2000 Plotted - 17:03:07, 11Oct 2000Comment:

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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

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...

“Radiation” Setup

• Radius 1.05 m (far field)• Resolution 22.5 degrees

-> 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