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

2. Today’s topics Sound and microphones Room acoustics Calibration
Recommendations

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

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!

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?

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

7. Cardioid Particle velocity and sound pressure combined => cardioid
Mainly sensitive in one direction

8. Directivity Omni-directional (SP only)
Directed (involves particle velocity)
Figure of eight
Cardioid
Super-cardioid
Other special directivity patterns
Great!
...or is it?

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?

10. NO!

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

12. Particle velocity
Particle velocity exhibits a bass lift in the close field
proximity effect

13. Proximity effect (cardioid mic)

14. Proximity effect
Demo

15. Manufacturers’ data sheets
Cardioid

16. Manufacturers’ data sheets
Cardioid

17. Manufacturers’ data sheets
Cardioid

18. Manufacturers’ data sheets
Omni-directional

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!

20. Proximity effect (cardioid mic)

21. Manufacturers’ data sheets
Cardioid

22. Manufacturers’ data sheets
Cardioid

23. Manufacturers’ data sheets
Cardioid

24. Manufacturers’ data sheets
Omni-directional

25. Demo Proximity effect: Hear the bass lift from the directed microphone

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

27. Transducer type Electret/condenser Dynamic
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

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

29. Room acoustics In a room sound originates from:
the sound source, directly
or from reflections at the walls

30. Room acoustics

31. Room acoustics Reverberation radius, rr
The distance where reflected and direct sound are equally loud
Less absorbtion => stronger reflections => smaller rr

32. Room acoustics

33. Room acoustics Reverberation radius Reverberation time
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

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:

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!

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

37. Calibration file, example
Calibration tone
”The level was 89 dB”

38. Calibration Calibrator
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!

39. Calibration Loudspeaker + 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

40. Calibration Voice + 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

41. Calibration Voice + 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

42. Calibration, directed microphones ?
>30cm
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”

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:

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

45. Recommendations microphone 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

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

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

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

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!

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

51. Harmonics-to-noise ratio
Omni-directional electret/condenser microphone
Background noise must be lower than voice noise
Microphone distance 5-50 cm
Well within rr

52. Perceptual evaluation
Omni-directional electret/condenser microphone
Reduce background noise as much as possible
Microphone distance 5-50 cm
Well within rr

53. But you never know...
For example, the first intention may be to only extract F0
It might turn out, after the recordings are made, that the recorded material would be suitable for some other measurement, like SPL
Therefore, do it ”right” from the start!

54. 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

55. These were my recommendations
You may find reasons to not follow them
But they better be good... 

56. Questions?
This presentation is available on the web