Acoustics of classrooms, restaurants and offices Eng.Ivaylo Hristev

Contents 1.Terms and requirements: Reverberation time Echo criteria Noise floor Sound power in a room Cocktail party effect 2.Classroom requirements 3.Restaurant requirements 4.Office spaces requirements

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

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.

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

Echo criteria Reflections and their perceivable effect, after M.Barron.

Noise floor – criteria NR 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 Hz allowing higher SPL at low frequencies, and lower SPL at high frequencies.

Noise floor – criteria NR Noise rating curveApplication NR 25 Concert halls, broadcasting and recording studios, churches NR 30 Private dwellings, hospitals, theatres, cinemas, conference rooms NR 35 Libraries, museums, court rooms, schools, hospitals operating theaters and wards, flats, hotels, executive offices NR 40 Halls, corridors, cloakrooms, restaurants, night clubs, offices, shops NR 45 Department stores, supermarkets, canteens, general offices NR 50Typing pools, offices with business machines NR 60Light engineering works NR 70Foundries, heavy engineering works

Noise floor – criteria NR Real life measurements in a TV studio

Sound source directivity Q Q=1Q=2Q=4 Q=8 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.

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

Speech intelligibility

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

Alcons

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.

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.

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

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

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

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

Acoustic requirements for offices Controlling reflections

2 m4 m8 m Sound level, dB Distance (d) from the speaker, metres m Free field Background sound Without acoustical treatment With acoustical treatment DL 2 DL f DL 2, DL f and sound distribution curve in open plan space DL 2 – rate of decay of SPL per doubling the distance DL f – excess SPL with respect to free field

Acoustic requirements for offices Key elements of acoustic design in open plan offices: 1.Absorption in the room at least as much as the floor area 2.Space divider screens – at least 1,5 meters height 3.Noise floor – as high as NC 35-NC 40; possibly use sound masking system. Open plan office acoustics online calculator:

Questions ?