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

Recording voice Microphones SPL calibration Room acoustics / Noise Recommendations

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

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

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

SPL calibration calibrator 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!

SPL calibration loudspeaker & 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

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

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

Room acoustics

Room acoustics Recommendations 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

Five points to bring back home 1.Directional microphones suffer from a proximity effect, use omnidirectional microphones if possible 2.SPL calibration is cool 3.Voice SPL needs to be reported with a distance (e.g. 30 cm) 4.30 cm is the smallest acceptable distance for sound level meters and microphones not fixed to the head 5.At distances larger than the reverberation radius, voice is always altered by the reflected sound

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”

Thank you!