Microphones and Room Acoustics and Their Influence on Voice Signals

1. Microphones and Room Acoustics and Their Influence on Voice Signals Svante Granqvist 1, Jan Švec 2 1 Department of Speech, Music and Hearing (TMH), Royal Institute of Technology (KTH), Stockholm, Sweden 2 Groningen Voice Research Lab, Department of Biomedical Engineering, University of Groningen, The Netherlands

2. Recording voice • Microphones • SPL calibration • Room acoustics / Noise • Recommendations

3. Microphone directivity • Two common directivity patterns • Omnidirectional • Picks up sound from all directions • Cardioid • Mainly pics up sound from the front of the microphone; good for suppression of ambient noise and reverberant sound • Omnidirectional is preferrable unless ambient sound is a problem

4. Microphone frequency response • Omnidirectional measurement microphones can be flat within a dB 20-20 000 Hz • Microphones for stage/studio use often have a peak around 5-10 kHz • Cardiod microphones have a distance dependent bass boost (proximity effect)

5. Microphone recommendations • First choice should be an omnidirectional electret or condenser microphone • If ambient noise or reverberation is a problem • Consider using a head mounted omnidirectional microphone • Consider a directional microphone • at least 30 cm from the mouth or, • at the distance which gives flat response • Make sure that you fully understand the consequenses before using head mounted directional microphones • Do not use dynamic microphones

6. SPL calibrationcalibrator • A calibrator is a device that is attached to the microphone and generates a well-defined SPL (usually 94 dB, 1kHz) • Standard for measurement microphones • Make sure the calibrator fits tight on the microphone • Never, ever use it for directional microphones!

7. SPL calibrationloudspeaker & level meter • Generate a test tone with a loudspeaker or use a sustained vowel • The level at the microphone is measured by a level meter close to the microphone

8. SPL calibration Level meter at 30 cm, mic at mouth • Can be used for headmounted microphones • Automatically accounts for the short distance between mic and mouth • Results in calibration as if level had been measured at 30 cm

9. SPL calibration Recommendations • Take the time to do level calibrations! • A simple calibration with the voice and a level meter is often enough • Calibration is not difficult, still it often goes wrong • Keys to successful calibration: • Verification • Experience / establisment of a standard procedure

10. Room acoustics

11. Room acousticsRecommendations • Use an acoustically treated room to reduce reverberation • Put the microphone well inside the reverberation radius • Put the microphone even closer for soft voice • Beware of reflective surfaces

12. Five points to bring back home • Directional microphones suffer from a proximity effect, use omnidirectional microphones if possible • SPL calibration is cool • Voice SPL needs to be reported with a distance (e.g. 30 cm) • 30 cm is the smallest acceptable distance for sound level meters and microphones not fixed to the head • At distances larger than the reverberation radius, voice is always altered by the reflected sound

13. Big nonos • Measure H1-H2 with a head mounted cardioid microphone • Record LTAS of the voice using a microphone at a distance of 2 metres • Use a calibrator on a directional microphone • To say: “I don’t need SPL calibration for this specific study”

14. Thank you!