Acoustics of classrooms, restaurants and offices

1. Acoustics of classrooms, restaurants and offices Eng.Ivaylo Hristev

2. Contents • Terms and requirements: • Reverberation time • Echo criteria • Noise floor • Sound power in a room • Cocktail party effect • Classroom requirements • Restaurant requirements • Office spaces requirements

3. Reverberation time Reverberation time is one of the most common acoustic criteria. Requirements are usually posted in terms of Rt.

4. Echo Echo is called a single reflection arriving at listeners position after certain time. This time delay causes the echo to be heard as separate sound.

5. Echo criteria Haas echo criteria; depends on the level and time difference.

6. Echo criteria Reflections and their perceivable effect, afterM.Barron.

7. Noise floor – criteriaNR NR (noise rating) is a noise criteria used widely in Europe. In USA NC (noise criteria) is used instead. Both criteria set maximum allowed noise level for 1000 Hzallowing higher SPL at low frequencies, and lower SPL at high frequencies.

8. Noise floor – criteriaNR

9. Noise floor – criteriaNR Real life measurements in a TV studio

10. Sound source directivity Q Q=8 Q=1 Q=2 Q=4 When sound source radiates at open space (4π) its directivity is said to be 1; radiating at half space (2π) is equal to directivity of 2. Lowering the radiation angle to ½, directivity increases twice.

11. Sound power Here: SPL – Sound pressure level SWL – Sound Power Level r – distance between source and receiver Q – sound source directivity Rc – room constant S – total absorption area – averaged absorption coefficient

12. Speech intelligibility

13. Speech transmission index STI is measured with special noise signal and gives objective information about speech intelligibility in a room.

14. Alcons

15. Cocktail party effect Every doubling the number of people increase the reverberant sound pressure level with 3 dB and halves the conversation distance. In practice people may choose to talk louder, which further raises the noise level. Solution – decrease the reverberant SPL by adding absorption.

16. Acoustic requirements for classrooms Requirements for unoccupied classroom( volume ≤ 250 м³) USA 0,4 - 0,6 s UK 0,5 - 0,8 s Italy 0,6 - 0,8 s Switzerland 0,5 - 0,7 s France 0,4 - 0,8 s Germany 0,45 - 0,6 s Sweden 0,5 - 0,6 s 20% increase of Rt is allowed for 125 Hz Octave band Noise level: below NC30.

17. Typical solutions for classrooms Step 1 – provide sound absorption on ceiling

18. Typical solutions for classrooms Step 2 – ensure low frequency sound absorption

19. Typical solutions for classrooms Step 3 – ensure speech comfort for lecturer by providing reflections above him

20. Typical solutions for classrooms Step 3 – get rid of backwall late reflections; remove echo possibilities

21. Acoustic requirements for offices Controlling reflections

22. DL2 , DLf and sound distribution curve in open plan space Sound level, dB DL2 – rate of decay of SPL per doubling the distance DLf – excess SPL with respect to free field 60 55 Without acoustical treatment 50 DL2 45 Background sound DLf 40 With acoustical treatment 35 Free field 30 2 m 4 m 8 m 16 m Distance (d) from the speaker, metres

23. Acoustic requirements for offices Key elements of acoustic design in open plan offices: Absorption in the room at least as much as the floor area Space divider screens – at least 1,5 meters height Noise floor – as high as NC 35-NC 40; possibly use sound masking system. Open plan office acoustics online calculator: http://legacy.spa.aalto.fi/software/fioh-officemodel/

24. Questions ?