Modern measurement techniques in room and building acoustics

Transcription

1 Das Messen in der Raum- und Bauakustik Michael Vorländer Institut für Technische Akustik RWTH Aachen Modern measurement techniques in room and building acoustics Introduction Modern versus classical methods a conflict? (MLS, sweeps, ) New challenges in impact sound Correct measurements? -> GUM Conclusions

2 Modern vs classical methods Building Acoustics - what do we measure? Sound levels (energy density) Reverberation times (1/3 octave bands) Sound intensity Classical methods in building acoustics ISO 140 (Sound insulation) ISO 354 (Sound absorption) ISO (Impedance tube) ISO (Flanking transmission)... and ISO 3382 for room acoustics

3 What do we measure? Level Decay How? stationary random noise direct measurement

4 Modern measurement techniques deterministic signals indirect measurement Modern measurement techniques basic result Impulse response? Integrated impulse response (Schroeder plot)

5 Integrated impulse response p 2 t Stationary case : = < = )d ( 0) ( τ τ p N t h C h 2 t Switch off : Impulse response = = t t p N d p d p N t h τ τ τ τ τ τ )d ( ] ) ( ) ( [ ) ( Decay curve t = 0 Integrated impulse response = t p N t h τ τ d ) ( ) ( Decay curve Expected decay = average decay curve (infinite averages) (smooth monotic curve)

6 Modern measurement techniques in room and building acoustics Introduction Modern versus classical methods a conflict? (MLS, sweeps, ) New challenges in impact sound Correct measurements? -> GUM Conclusions No! Modern measurement techniques in room and building acoustics Introduction Modern versus classical methods a conflict? (MLS, sweeps, ) New challenges in impact sound Correct measurements? -> GUM Conclusions

7 Measurement of impulse responses F s(t) S(f) * h(t) room, wall, LTI... H(f) s'(t) S'(f) F s s '( t) = s( t) * h( t) = ( τ ) h( t τ ) dτ S' ( f ) = S( f ) H( f ) Measurement of impulse responses F s(t) S(f) * h(t) room, wall,... H(f) s'(t) S'(f) F Method 1: Spectrum division, inverse FFT S'( f ) -1 H ( f ) = h( t) = F { H ( f )} S( f ) Important: S(f) broadband spectrum

8 Example 1: 2-channel FFT technique s(t) LTI- System FFT DIV H(f) FFT IFT h(t) Measurement of impulse responses F s(t) S(f) * h(t) room, wall,... H(f) s'(t) S'(f) F Method 2: Deconvolution h ( t ) = s '( t ) s 1 ( t ) s 1 ( t) = F -1 S 1 ( (matched filter, FIR filter) f )

9 Example 2: white sinusoidal sweep s 1 ( t) = s( TRep t) Measurement of impulse responses F s(t) S(f) * h(t) room, wall,... H(f) s'(t) S'(f) F Method 3: cross correlation h( t) = s'( t) s( t) = s'( t) s( t) = s'( τ ) s( t + τ ) dτ Important: s(t) correlation signal

10 Example 3: s MLS MLS technique ( t) h( t) smls ( t) = smls ( t) smls ( t) h( t) = Φ ss ( t) h( t) measured cross correlation Zeit FHT Zeit Modern measurement techniques -1 S'( f ) h( t) = F S( f ) h ( t ) s 1 '( t ) s ( t = ) ; s ( t) = F S( f ) h( t) = s'( t) s( t) = s'( t) s( t) Formulations mathematically equivalent ( white spectrum signals) Differences in crest factor (peak to rms), numerical precision, performance of A/D hardware,...

11 Energy compression by: -spectrumdivision - deconvolution - cross correlation 1 Relative signal power 0,8 0,6 0,4 0,2 0 signal after processing signal 0 0,2 0,4 0,6 0,8 1 t / T background noise Impact into standardisation: ISO Classical method vs. modern methods Impulse measurement technique (Excitation, spectral requirements, level and linearity, stability and time-invariance, integration limits, averaging, noise compensation,...) Annex A: Example of MLS Annex B: Example of Swept-sine

12 Errors caused by nonlinearities After Müller and Massarani (J. Audio Eng. Soc. 2001) Fig. 10. Measurement of room impulse response in a reverberant chamber with 1, 10 and 100 synchronous averages. Left: with MLS, right: with sweep of identical coloration and energy. The curves are compressed to 1303 values, each of them representing the maximum of 805 consecutive samples. Modern measurement techniques in room and building acoustics we can measure as accurately as we want! yes, if LTI is fulfilled? the remaining sources of errors are related to the acoustic field, to loudspeakers and to microphones, these errors are more or less systematic (-> GUM)

13 Modern measurement techniques in room and building acoustics Introduction Modern versus classical methods a conflict? (MLS, sweeps, ) New challenges in impact sound Correct measurements? -> GUM Conclusions Impact sound level of a person walking L n L walker =?

14 Interaction force Y source v 0 F=? Y receiver L n L walker =? Calculating L walker TAPPING MACHINE FLOOR WALKER L n Y hammer Y floor Y walker F hammer - floor F walker - floor L walker

15 walker Measurement setup Body to be measured F v F Shaker Y walker shaker walker Measurement setup

16 walker Mobility [db re. 1 ms -1 N -1 ] Walker and floor mobilities Weise (2003) - subject walking Scholl (2001) - leg of subject sitting (AJ) Miwa (1975) - average of 20 subjects standing Miwa (1975) - subject standing on one leg Miwa (1975) - subject standing on heels Miwa (1988) - leg of subject sitting Watters (1965) - subject standing on high heeled shoes Jeon (2006) - subject lying in hammock Brunskog (2003) - timber joist floor mass of 80 kg infinite concrete floor of 20 cm 8 barefoot subjects measured on the setup shown before Frequency [Hz] L n, L walker, L ap and all the others TAPPING MACHINE FLOOR WALKER L n Y hammer Y floor Y walker F hammer - floor F walker - floor L walker New rating schemes? Flexibility to characterise various sources? (walking, jumping, housing equiment)

17 Modern measurement techniques in room and building acoustics Introduction Modern versus classical methods a conflict? (MLS, sweeps, ) New challenges in impact sound Correct measurements? -> GUM Conclusions Why are uncertainties relevant? Round Robin III Measurements - Studio PTB ITA Farina PTB97 PTB T30 [s] Frequency [Hz]

18 GUM Guide to the Expression of Uncertainty in Measurement Standardised methodology to treat uncertainties Guideline to develop uncertainty intervals Requirement in many measurement standards in acoustics GUM procedure (2005)

19 GUM procedure & application E X C I T A T I O N D/A Quant.Err. Rec.Level Sweep MLS Impulse Averaging / Signal s length Transmission path Ampl. Loudspeaker Room Mic. Preampl. A/D Deconv. Distortion Gain Error Directivity Distortion Position (accuracy for reposition) Position (on Stage, prox. to surfaces) Transfer Function (Equalisation) Background Noise Geometry ITDG (Prox. to surfaces) Climate (Temperature, Humidity) HVAC (Air Movement) Boundary Conditions (Openings) Person in Room Setup (Variable Acoustics, Stage) Directivity Distortion Sensitivity Gain Error Distortion Position (accuracy for reposition) Position (on stage, prox. to surfaces) Orientation (Figure 8, Head) Calibration Error Quant.Err. I M P U L S E R E S P O N S E Applying GUM to room acoustics Identification of influence factors is challenging Influence factors not always directly measurable Complex mathematical operations (2Ch-FFT) RIR not a simple in-between quantity Search for a simple and practical model

20 Room acoustical measurement model Linear model with sources of error Linear uncertainty dependence graph Experiments Determining correction factors using special measurements Turning the loudspeaker on a turntable Displacement of the loudspeaker Displacement of the microphones Background noise LTI-assumption correct?

21 One example: Source rotation Source rotation

22 Another example: Receiver position Receiver position

23 Source position Uncertainty budget of a single measurement

24 Reduced uncertainty budget due to averaging Conclusion New methods are powerful (ISO) Coming soon: New approaches in impact and structure-borne sound in buildings GUM Strategy to reduce uncertainty GUM in sound insulation (Wittstock, PTB)