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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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 www.speech.kth.se/~svante/pevoc5

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

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

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

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

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

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 www.speech.kth.se/~svante/pevoc5

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

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

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

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

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! www.speech.kth.se/~svante/pevoc5

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 www.speech.kth.se/~svante/pevoc5

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 www.speech.kth.se/~svante/pevoc5

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 www.speech.kth.se/~svante/pevoc5

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” www.speech.kth.se/~svante/pevoc5

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

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

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 www.speech.kth.se/~svante/pevoc5

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

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

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

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

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

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 www.speech.kth.se/~svante/pevoc5

Perceptual evaluation Omni-directional electret/condenser microphone Reduce background noise as much as possible Microphone distance 5-50 cm Well within rr www.speech.kth.se/~svante/pevoc5

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! www.speech.kth.se/~svante/pevoc5

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

These were my recommendations You may find reasons to not follow them But they better be good...  www.speech.kth.se/~svante/pevoc5

Questions? This presentation is available on the web www.speech.kth.se/~svante/pevoc5 svante.granqvist@speech.kth.se