Estimates based on a model of room acoustics. Arthur Boothroyd 2003 Used and distributed with permission for 2003 ACCESS conference

Transcription

1 Estimates based on a model of room acoustics Arthur Boothroyd 2003 Used and distributed with permission for 2003 ACCESS conference

2 Basic model Direct signal (level falls by 6 db per doubling of distance) Early reverberation (early reflections) (level re talker output depends on talker directionality (Q), room volume and RT) Effective signal level (ES) the higher of the two ~ Actual noise (external and internal sources) (level varies with proximity to source) Late reverberation (level re early reflections depends on RT) Effective noise level (EN) the higher of the two ~ Effective s/n ratio (s/n) = ES EN (-15dB to +15dB) s/n Speech Audibility Index (SAI) ( 0% to 100%) SAI+(language & listener variables) speech perception

3 Critical Distance Distance at which levels of direct signal and early reverberation (early reflections) are equal (zero degrees azimuth) Within critical distance direct signal dominates Beyond critical distance early reverberation dominates Beyond 3 times critical distance direct signal negligible Direct Signal dominant Early reflections dominant Effective signal Direct Signal negligible Early reflections db Distance Critical distance Direct signal

4 Estimating critical distance (for Q=4) Room Volume Reverberation time (seconds) (cubic ft.) Arthur Boothroyd Used and distributed with permission for 2003 ACCESS conference Critical distance (feet)

5 Late reverberation Interferes with perception, therefore classed as noise Equivalent noise level re early reverberation depends on reverberation time (RT) Beyond 3 times critical distance effective s/n ratio depends only on difference between early and late reverberation and, therefore, only on RT.

6 Late reverberation expressed as an equivalent noise level RT Room volume (cubic feet) (seconds) Arthur Boothroyd Used and distributed with permission for 2003 ACCESS conference Equivalent noise level (dbspl)

7 Effective signal-to-noise ratio (s/n) and Speech Audibility Index (SAI) for listeners at 3 or more times critical distance RT (secs) s/n (db) SAI(%) Arthur Boothroyd Used and distributed with permission for 2003 ACCESS conference SAI = (s/n+15)/30*100

8 Definitions Talker output long-term average level of the talker s speech measured at 1 foot (30 cm) from the mouth (here assumed to be 72 dbspl). Talker directionality tendency of the talker to radiate sound in a forward direction compared with other directions. Q a formal measure of directionality. Energy radiated in forward direction = Q times the average radiated in all directions (here assumed to be 4 in the frequency region of most importance to speech perception). Direct signal the speech signal that travels in a straight line from talker to listener without undergoing any reflections. Reverberation the persistence of sound in a room because of repeated reflections from the boundaries and other surfaces. Reverberation time (RT) - a formal measure of reverberation. RT = the time taken for the sound level in a room to fall by 60dB after the source of sound is turned off. RT is frequency-dependent. Early reverberation (or early reflections) those reflections of the talker output that arrive at the listener early enough to be integrated with the direct signal, and with each other, so as to enhance perception. Critical distance the distance from the talker at which the levels of the direct signal and the early reverberation are equal. Late reverberation those reflections of the talker output that arrive at the listener too late to be integrated with the early reflections and that degrade perception. Effective signal logarithmic sum of the direct signal and early reflections. Effective noise logarithmic sum of the actual background noise and late reverberation. Speech Audibility Index percentage of the useful information in the effective speech signal that is not masked by the effective noise. (SAI = ((effective signal effective noise) + 15)/30*100.