1. Morset Sound Development Title: A practical comparison of the ”new” swept-sine technique and MLS in measuring room impulse responses and acoustical parameters Lars H. Morset Morset Sound Development Trondheim, Norway

2. Morset Sound Development My background Studied acoustics at NTNU (prof. Krokstad, now prof. P. Svensson), now doing Ph.D on violin acoustics Started Morset Sound Dev. 1997, developing WinMLS – a sound-card based software for audio, acoustical & vibrational measurements. Old MLS: Maximum Length Sequence New MLS: Measurements based on Log. Sine- sweep

3. Morset Sound Development Background Theory Assumes our system is linear and time- invariant (LTI) –linear: a*x + b*y = if we double the input signal to the system, the output signal is also doubled if we superpose two signals –time-invariant: system does not change as function of time –the impulse response completely describes a LTI system

4. Morset Sound Development Are our conditions really LTI? No, a loudspeaker is not completely linear –it will have some distortion (harmonic), increasing with output level a room is not completely time-invariant –the air will move –temperature changes –time-variance usually a bigger problem for large rooms

5. Morset Sound Development MLS and linearity Harmonic distortion cannot be separated from the impulse response –the loudspeaker is never completely linear harmonic distortion will happen and it is increases with the output level, SPURIOUS PEAKS –Solution: Do not use too high output level –but a high output level is desired... Want maximum SNR Size/cost of loudspeaker

6. Morset Sound Development MLS and time-variance MLS is not robust against time-variance –rooms are never time-invariant –usually a problem for high frequencies –longer measurement time does not increase SNR when we have time-variance Sampling frequency input/output has to be exactly the same to avoid loss of quality –not a problem for “clocked” hardware –always a problem for remote measurements

7. Morset Sound Development MLS “remote” loop-back measurement (pre-filtering used) Problems with high-frequency

8. Morset Sound Development MLS “remote” results continues

9. Morset Sound Development How do we deal with this in general? Loudspeaker non-linearity can be kept to a minimum by using high quality elements and avoiding using high output levels Time-variance in rooms –turn off ventilation –make sure people are not moving around during measurements –avoid temperature gradients Perform remote measurements with device that has the same clock

10. Morset Sound Development A logarithmic swept sine “solve” these problems It has to be post-processed correctly! –Not the same as traditional swept sine A higher loudspeaker output levels can be applied Robustness to minor time-variance. –Make sure a single sweep only is used Remote measurements can be performed without loss in quality –Make sure a single sweep only is used

11. Morset Sound Development Swept sine Remote results Swept sine Remote results

12. Morset Sound Development Occupied measurements Seems to still be a problem measuring large halls (London conference on auditorium acoustics-2002) –We now have the solution! Do not want cables on the floor (remote) Multi-channel measurements (24 channels) Very good SNR obtained with 5-20 seconds log-sine sweep –The log-sweep is not that bad to listen to

13. Morset Sound Development What is today's “best” measurement technique? In the general case of measuring room impulse responses and acoustical parameters, a long (5-40 seconds), single sweep should be applied –removes uncertainties with harmonic distortion and time- variance –are left “only” with transducer uncertainties and impulse response post-processing For measuring sound systems in small rooms (studios), the sweep length can be as low as 1 sec. –In this application, MLS can also successfully be used

14. Morset Sound Development 2 Log-sweep measurements

15. Morset Sound Development How can I myself test these methods? You need this hardware: –PC with soundcard (usually built-in) –Microphone –Loudspeaker Evaluate WinMLS in 30 days for FREE by downloading from www.winmls.com –Inexpensive versions for personal usage Have information/CDs available here