A tutorial on acoustic measurements for the non-technician

1. A tutorial on acoustic measurements for the non-technician Svante Granqvist Royal Institute of Technology (KTH) Dept of Speech Music and Hearing (TMH) Stockholm, Sweden svante.granqvist@speech.kth.se

2. Today’s topics • Sound and microphones • Room acoustics • Calibration • Recommendations www.speech.kth.se/~svante/pevoc5

3. Conclusions • Use omni-directional electret or condenser microphones whenever possible • Do not use directed (e.g. cardioid) microphones unless you really need the directivity • Especially not close to the speaker • Avoid dynamic microphones • Place the microphone within the reverberation radius of the room • Keep noise level low • Establish a routine for level calibration www.speech.kth.se/~svante/pevoc5

4. What is sound? • Demo of sound field • Sound pressure (pascals, Pa) • Sound pressure level, SPL (decibels, dB) • Particle velocity (metres per second, m/s) • Particle velocity level? Rarely! www.speech.kth.se/~svante/pevoc5

5. Sound pressure • Simple relation to sound intensity • Our ears are mainly pressure sensitive • Simple relation to distance (~1/r) • Doubled distance => halved SP <=> SPL: -6dB • Pressure has no direction • Pressure sensitive microphones are omni-directional (no directivity) • So: how do they make directed microphones? www.speech.kth.se/~svante/pevoc5

6. Particle velocity • Particle velocity has a direction • So it can be used to create directivity! • Particle velocity only => figure of eight • Mainly sensitive in two directions www.speech.kth.se/~svante/pevoc5

7. Cardioid • Particle velocity and sound pressure combined => cardioid • Mainly sensitive in one direction www.speech.kth.se/~svante/pevoc5

8. Directivity • Omni-directional (SP only) • Directed (involves particle velocity) • Figure of eight • Cardioid • Super-cardioid • Other special directivity patterns • Great! ...or is it? www.speech.kth.se/~svante/pevoc5

9. Directed microphones • We are primarily interested in sound pressure • ...but also measure particle velocity • ...then PV and SP have to be proportional to one another! • Are they? www.speech.kth.se/~svante/pevoc5

10. NO! www.speech.kth.se/~svante/pevoc5

11. Particle velocity • Particle velocity is proportional to sound pressure, but only in the far field (~1/r) • In the close field, it differs! (~1/r2) • The limit between far and close field depends on frequency www.speech.kth.se/~svante/pevoc5

12. Particle velocity • Particle velocity exhibits a bass lift in the close field • proximity effect www.speech.kth.se/~svante/pevoc5

13. Proximity effect (cardioid mic) www.speech.kth.se/~svante/pevoc5

14. Proximity effect • Demo www.speech.kth.se/~svante/pevoc5

15. Manufacturers’ data sheets Cardioid www.speech.kth.se/~svante/pevoc5

16. Manufacturers’ data sheets Cardioid www.speech.kth.se/~svante/pevoc5

17. Manufacturers’ data sheets Cardioid www.speech.kth.se/~svante/pevoc5

18. Manufacturers’ data sheets Omni-directional www.speech.kth.se/~svante/pevoc5

19. Manufacturers’ data sheets • Frequency responses are mostly measured in the far field, even for microphones that obviously are intended to be mounted in the close field • You have to add the proximity effect for directed microphones to those curves! www.speech.kth.se/~svante/pevoc5

20. Proximity effect (cardioid mic) www.speech.kth.se/~svante/pevoc5

21. Manufacturers’ data sheets Cardioid www.speech.kth.se/~svante/pevoc5

22. Manufacturers’ data sheets Cardioid www.speech.kth.se/~svante/pevoc5

23. Manufacturers’ data sheets Cardioid www.speech.kth.se/~svante/pevoc5

24. Manufacturers’ data sheets Omni-directional www.speech.kth.se/~svante/pevoc5

25. Demo • Proximity effect: • Hear the bass lift from the directed microphone www.speech.kth.se/~svante/pevoc5

26. OK, point taken, he doesn’t like directed microphones • But then, why are there so many directed microphones out there? • Music industry, broadcasting, stage use etc: • A bass boost of a few dBs does not matter much or might even be desired (sound ”better”) • Noise supression may be more important than a flat frequency response • Most recordings do not have a scientific purpose www.speech.kth.se/~svante/pevoc5

27. Transducer type • Electret/condenser • Can easily be made to have flat response • Cheap electret microphones (< €30 ) can be of sufficient quality • Requires battery/power supply • Sensitivity may decrease towards end of battery life • Dynamic • Difficult to acheive a flat response • Good dynamic microphones are expensive • Rarely purely pressure sensitive (even though data-sheet may say so) • No need for battery/power supply www.speech.kth.se/~svante/pevoc5

28. Bottom line... • Use omni-directional, electret/condenser microphones for scientific purposes! • Make sure batteries are fresh or use some other type of power supply www.speech.kth.se/~svante/pevoc5

29. Room acoustics • In a room sound originates from: • the sound source, directly • or from reflections at the walls www.speech.kth.se/~svante/pevoc5

30. Room acoustics www.speech.kth.se/~svante/pevoc5

31. Room acoustics • Reverberation radius, rr • The distance where reflected and direct sound are equally loud • Less absorbtion => stronger reflections => smaller rr www.speech.kth.se/~svante/pevoc5

32. Room acoustics www.speech.kth.se/~svante/pevoc5

33. Room acoustics • Reverberation radius • At what distance is the direct sound as loud as the sound that has been reflected from the walls • Typical value 4 – 0.5 meters • Reverberation time • How long does it take for the sound level to drop by 60 dB? • Typical value 0.5 – 4 seconds www.speech.kth.se/~svante/pevoc5

34. Room acoustics • How to measure Reverberation time/radius • Several ways, one would be to record a ”bang” and see at what rate the sound level drops • The time for a 60dB drop corresponds to reverberation time • Calculate reverberation radius from this time: www.speech.kth.se/~svante/pevoc5

35. Bottom line... • Within the reverberation radius, conditions are similar to free field • Outside, reflections from the walls dominate the sound • So, put the microphone (well) within the reverberation radius! www.speech.kth.se/~svante/pevoc5

36. Level calibration • Most common method: • Record a signal with a known level i.e. a calibration tone • By relating the level of the calibration tone to the levels of the signals of interest, absolute calibration is acheived www.speech.kth.se/~svante/pevoc5

37. Calibration file, example Calibration tone ”The level was 89 dB” www.speech.kth.se/~svante/pevoc5

38. CalibrationCalibrator • Procedure: • Mount and start the calibrator (2-10 seconds) • Unmount calibrator and say the level of the calibrator • Advantages: • Stable calibration tone • No sensitivity to room acoustics or surrounding noise • Disadvantage: • Calibrator that fits the microphone required • Important that the seal is tight! www.speech.kth.se/~svante/pevoc5

39. CalibrationLoudspeaker + SPL meter • Procedure: • Beep at 1kHz ~ 80 dB (2-10 seconds) • say the level as read on the level meter • Advantages: • Stable calibration tone • Disadvantage: • Loudspeaker + signal source reqiured • Some sensitivity to room and surrounding noise www.speech.kth.se/~svante/pevoc5

40. CalibrationVoice + SPL meter • Procedure: • Sustain /a/ ~80 dB (5-10 seconds) • say the level as read on the level meter • Advantages: • No loudspeaker required • Calibration signal (voice) has approximately the same spectrum as the signals of interest • Disadvantages: • Hard to keep the level of the /a/ stable • Some sensitivity to room and surrounding noise www.speech.kth.se/~svante/pevoc5

41. CalibrationVoice + SPL meter • Procedure • Sustain /a/ ~80 dB (5-10 seconds) • say the level as read on the level meter • Advantages: • No loudspeaker required • Calibration signal (voice) has approximately the same spectrum as the signals of interest • Automatic compensation for microphone distance • Disadvantage: • Hard to keep the level of the /a/ stable • Only valid for this particular distance • Some sensitivity to room and surrounding noise • dB meter should be within rr www.speech.kth.se/~svante/pevoc5

42. >30cm >30cm Calibration,directed microphones ? • Only in the far field (>30 cm), but still within rr • Only for rough estimation of SPL • Never use SPL calibrators! • ”Don’t try this at home” www.speech.kth.se/~svante/pevoc5

43. Distance compensation • Sound pressure drops as ~1/r • Re-calculate SPL to appear as recorded at a different distance, e.g. record at d2=5 cm, but report at d1=30 cm. • Only for omni-directional microphones! • Formula: www.speech.kth.se/~svante/pevoc5

44. Bottom line... • Establish a routine for calibrations • Don’t calibrate directed microphones • Report SPL at 30 cm • Compensated or actual • Beware of the ”mixer” on most PC soundcards www.speech.kth.se/~svante/pevoc5

45. Recommendationsmicrophone and room acoustics • Depend on • the purpose of recording • the recording environment • Noise • Room acoustics Example of purposes: SPL Spectrum F0 Inverse filtering HNR Perceptual evaluation www.speech.kth.se/~svante/pevoc5

46. SPL • Omni-directional electret/condenser microphone • If noisy environment: • Try to attenuate the noise • Shorten microphone distance (10 cm to the side of the mouth) • Avoid directed microphones for this purpose! • Put the microphone well within the reverberation radius of the room (~rr/2) • Re-calculate or calibrate for 30 cm www.speech.kth.se/~svante/pevoc5

47. Spectral properties (spectrogram) • Omni-directional electret/condenser microphone • If noisy environment: • Try to attenuate the noise • Shorten microphone distance (5-10cm to the side of the mouth) • If background noise still is a problem a directed microphone can be used, but beware of the proximity effect and keep microphone distance constant! • Put microphone well within rr www.speech.kth.se/~svante/pevoc5

48. Spectral properties (LTAS, H1-H2, line spectra) • Omni-directional electret/condenser microphone • If noisy environment: • Try to attenuate the noise • Shorten microphone distance (5-10cm to the side of the mouth) • Do not use a directed microphone • Put microphone well within rr • Pay attention to reflective surfaces such as windows, manuscripts etc. Added proximity effect, cardioid at 5 cm www.speech.kth.se/~svante/pevoc5

49. F0, jitter/shimmer • Any decent microphone is OK, since periodicity is independent of frequency response • If noisy environment: • Try to attenuate the noise • Shorten microphone distance (5-10 cm) • Use a directed microphone • Check if F0 algorithm is affected by a bass lift! www.speech.kth.se/~svante/pevoc5

50. Inverse filtering • Omni-directional electret/condenser microphone flower < 10 Hz • Reduce background noise as much as possible • Never use a directed microphone • Microphone distance 5-10 cm • Within rr/10 • Pay attention to reflective surfaces such as windows, manuscripts etc. • Anechoic chamber is preferred Addition of reflection to the direct signal www.speech.kth.se/~svante/pevoc5