Støy fra talende personer på spisesteder Jens Holger Rindel NAS Høstmøte, oktober 2009 Trondheim

2 Noise from speaking persons in eating establishments Jens Holger Rindel NAS Autumn Meeting, October 2009 Trondheim

3 Outline  Speech level in noise, the Lombard effect  Theory - A prediction model  Verification examples  Recommendations  Conclusion

4 Speech level in noise  When many people are talking at the same time in a social gathering, typically in a reverberant room, it is a common experience that it can be very noisy  Restaurants and canteens are typical examples of rooms where noise from speech can be a problem  However, until now very little is known about the influence of the reverberation time on this kind of noise

5 The Lombard effect  People adjust the speech level according to the ambient noise level (found by Lombard 1911)  The effect can be quantified by the parameter c  c is the speech level rise due to increased ambient noise (dB/dB)  Values of c in the range 0,5 – 0,7 dB/dB have been suggested in the literature

6 Description of speech levels (Lazarus, 1986) Speech level, 1m, dB(A)Description 54 – 59Relaxed 60 – 65Normal 66 – 71Raised 72 – 77Loud 78 – 83Very loud 84 – 89Shouting

7 Collected results L SA re L NA (Lazarus, 1986)

8 Theoretical model – Sound power of speech Lazarus concluded that the Lombard effect starts at a ambient noise level of 45 dB(A) and a speech level of 55 dB(A) Relation between SPL and sound power level in a free field, distance r, directivity factor Q Assuming distance r = 1 m, directivity factor Q = 2 for speaking person, we get:

9 Theoretical model – Noise level in room Assuming a diffuse sound field and equivalent absorption area A, the sound pressure level of the noise from N S speaking persons is: Insertion of the previous equations yields: where c is the speech level rise due to increased noise (dB/dB) A / N S is the absorption area per speaking person (m 2 )

10 Noise level as a function of absorption area and number of speaking persons Parameter c (dB/dB)

11 Noise level and speech level Parameter c = 0,5 dB/dB

12 Theoretical model Insertion of reverberation time without people T 0 (s), volume V (m 3 ), absorption per person a, and total number of persons N: Final model for noise from speech: where g = N / N S

13 Example 1 Food court, Vol. = 7228 m 3, RT = 1,3 s Ref.: Navarro & Pimentel (2007), Applied Acoustics 68, pp Parameter: g Number of persons per speaking person

14 Example 2 Food court, Vol. = 3133 m 3, RT = 0,9 s Ref.: Navarro & Pimentel (2007), Applied Acoustics 68, pp Parameter: g Number of persons per speaking person

15 Example 3 Canteen, Vol. = 1235 m 3, RT = 0,47 s Ref.: Tang et al. (1997), JASA 101, pp Parameter: g Number of persons per speaking person Measurement A: First half of lunch break Measurement B: Second half of lunch break

16 Example 1 Food court, Vol. = 7228 m 3, RT = 1,3 s Ref.: Navarro & Pimentel (2007), Applied Acoustics 68, pp g = 3 persons per speaking person

17 Example 2 Food court, Vol. = 3133 m 3, RT = 0,9 s Ref.: Navarro & Pimentel (2007), Applied Acoustics 68, pp g = 3 persons per speaking person

18 Example 3 Canteen, Vol. = 1235 m 3, RT = 0,47 s Ref.: Tang et al. (1997), JASA 101, pp Measurement A: First half of lunch Measurement B: Second half of lunch g = 3 persons per speaking person

19 Needed absorption area per person Noise level, L NA (dBA) Absorption area per person (m 2 ) Speech effort Relaxed Normal Raised Loud

20 Needed absorption area per person Noise level, L NA (dBA) Absorption area per person (m 2 ) Speech effort Relaxed Normal Raised Loud 26 m 2 7 m 2 2 m 2

21 Recommendations  With less than 2 m 2 absorption per person the noise level may exceed 79 dBA and the speech effort is loud or shouting  With m 2 absorption per person the noise level may be in the range 67 – 78 dBA and the speech effort is raised  7 m 2 absorption per person is needed to obtain a noise level below 67 dBA and a speech effort of normal  26 m 2 absorption per person is needed to obtain a noise level below 55 dBA and a speech effort of relaxed

22 Conclusion  The noise level due to speech may be reduced by 6 dB if the reverberation time is halved –not by 3 dB as for ordinary noise sources  This is due to the Lombard effect, which can be quantified by the rate c = 0,5 dB/dB in the cases of eating establishments studied here –c is the speech level rise due to increased ambient noise  The major uncertainty is due to the arousal level of the gathered people, here expressed by the parameter g –g : Number of persons per speaking person –g = 3 was found to give the best agreement in most cases studied here