morset sound development title: a practical comparison of the ”new” swept-sine technique and mls...
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Morset Sound Development
Title: A practical comparison of the ”new” Title: A practical comparison of the ”new” swept-sine technique and MLS in measuring swept-sine technique and MLS in measuring room impulse responses and acoustical room impulse responses and acoustical parametersparameters
Lars H. Morset
Morset Sound Development
Trondheim, Norway
Morset Sound Development
My backgroundMy 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
Morset Sound Development
Background TheoryBackground 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
Morset Sound Development
Are our conditions Are our conditions really really LTI?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
Morset Sound Development
MLS and linearityMLS 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
Morset Sound Development
MLS and time-varianceMLS 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
Morset Sound Development
MLS “remote” MLS “remote” loop-back measurement loop-back measurement
(pre-filtering used)(pre-filtering used)Problems Problems withwith high-frequency high-frequency
Magnitude Frequency Response - Title
Frequency [Hz]100 1 000 10 000
Pre
s. [P
a] /
unca
l. [v
olts
], dB
-45
-50
-55
-60
-65
-70
-75
-80
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-90
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-100
...\FromLatitudeToLynx\MLSSeq15Avg4.w mb
...\FromLatitudeToLynx\MLSSeq13Avg16.w mb
...\FromLatitudeToLynx\MLSSeq17Avg1.w mb
Morset Sound Development
MLS “remote” resultsMLS “remote” results continues continues
Magnitude Frequency Response - Title
Frequency [Hz]100 1 000 10 000
Pre
s. [P
a] /
unca
l. [v
olts
], dB
-40
-45
-50
-55
-60
-65
-70
-75
-80
-85
-90
-95
-100
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-110
...\FromLatitudeToLynx\MLSSeq15Avg1.w mb
...\FromLatitudeToLynx\MLSSeq17Avg1.w mb
...\FromLatitudeToLynx\MLSSeq13Avg16.w mb
...\FromLatitudeToLynx\MLSSeq13Avg1.w mb
...\FromLatitudeToLynx\MLSSeq15Avg16.w mb
Morset Sound Development
How do we deal with this in general?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
Morset Sound Development
A logarithmic swept sine “solve” these A logarithmic swept sine “solve” these problemsproblems
• 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
Morset Sound Development
Swept sine Swept sine Remote resultsRemote results
Morset Sound Development
Occupied measurementsOccupied 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
Morset Sound Development
What is today's “best” measurement What is today's “best” measurement technique?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
Morset Sound Development
2 Log-sweep measurements2 Log-sweep measurements
Reverberation time (T30)
Frequency [Hz]63 125 250 500 1000 2000 4000 8000 16000
T30
(s)
1
0,8
0,6
0,4
0,2
58.2
55.6
59.2
60.061.5
61.8
64.6
67.8
60.3
61.0
57.9
59.1
58.462.7
62.0
63.5
67.2
61.1
...\TimeVariance\Sw eep21dot5StartAt30HzShort.w av
...\TimeVariance\Sw eep21dot5Day2Short.w av
Morset Sound Development
How can I How can I myself myself test these methods?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