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

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

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 dont need SPL calibration for this
  specific study

14
Thank you!