WinMLS - Acoustical Measurements Lars H. Morset & Dr. Johan L. Nielsen Morset Sound Development Trondheim, 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 Senest i går- idrettshall som ikke ble målt

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) Portable system: Laptop 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

Transfer function (Modulus) The Impulse Response Time domain System under test t Unit pulse ( d ) Impulse response of the system i x(t) y(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  System under test White noise Transfer function (Modulus) Y() = X()H() H() = F[h(t)] Relationship 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. 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) 6 Hadamard tramsform Correlation input - output 6 h(n) 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 SNR

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