BASICS OF ROOM ACOUSTICS

Transcription

1 BASICS OF ROOM ACOUSTICS Study aid to learn Communication acoustics, VIHIAM 000 Fülöp Augusztinovicz, professor Department of Networked Systems and Services december 13., Budapest

2 Main issues Notion of small and large rooms (as compared to wavelength) From reflections to reverberation The notion and characteristics of reverberation Direct and diffuse sound field Notion and characteristics of sound absorption Characteristics and parameters of sound absorbers Relationship of sound absorption and reverberation Reverberation time Typical RTs, measurement and design

3 What is small, what is large?

4 Various models in room acoustics Low frequency: wave acoustics dimensions comparable to wavelength Medium to high frequency: geometrical acoustics High frequency: statistical acoustics dimensions much larger than the wavelength Előadás címe 4 Előadó Neve BME Hálózati Rendszerek és Szolgáltatások Tanszék

5 Reflections l 2 l 1 l 3 l 4

6 An important descriptor: the impulse response p li τ i = c τ 1 τ 2 τ 3 τ 4 τ

7 What is the effect of reflections? Early reflections (< 50 ms, not too many) one cannot distinguish between direct and reflected sound > Haas-effect increased loudness advantageous, no amplification is needed Masking level - db Masking time - ms Előadó Neve Előadás címe 7 BME Hálózati Rendszerek és Szolgáltatások Tanszék

8 What is the effect of reflections? Medium reflections can be heard separately generates spaciousness make music performances more enjoyable different pieces require different acoustic environment Late reflections plenty of them, of more or less continuous (but not uniform!) distribution deteriorates or even hinders speech intelligibility Előadás címe 8 Előadó Neve BME Hálózati Rendszerek és Szolgáltatások Tanszék

9 The role of reflections in the ancient theatres As imagined in the 19th century: (Pierers Konversationslexikon, Stuttgart, 1891)

10 The role of reflections in the ancient theatres And the reality: Ruins of the Dyonysos theatre at the foothill of Acropolis in Athens

11 Marmor thrones, dedicated to nobilities The throne of the priest of Dionysos Eleutherios (the god of wine). 4th century B.C. Előadó Neve Előadás címe 11 BME Hálózati Rendszerek és Szolgáltatások Tanszék

12 Elements of the ancient greek theatres

13 Amphytheatre [sound refraction and absorption]

14 Amphytheatre Orchestra (place of the dancers): floor reflection!

15 Masque masque: a sound amplifying, impedance matching element

16 There is nothing new under the sun: the ancient Roman theatre Thde odeion of Herodes Atticus in Athens Előadó Neve Előadás címe 16 BME Hálózati Rendszerek és Szolgáltatások Tanszék

17 Impulse response Notion: response to an ideal Dirac impulse Measurement (in earlier times) by a handgun by spark source by continuous random noise by swept sine x Kossuth u. 4 Szint (FS:1) Távolság [m]

18 Impulse response / 3

19 Measured impulse response

20 Echogram

21 Transition from direct to reverberant field Direct sound field 6 db / double distance rule Reverberant sound field In principle spatially independent Influencing parameters Room size (Room shape, to some extent) amount of sound absorption in the room Előadás címe 21 Előadó Neve BME Hálózati Rendszerek és Szolgáltatások Tanszék

22 Energy considerations The source fills up the system with energy, the absorption of the walls sinks After sufficient time energy balance is reached

23 What is sound absorption? Conversion of energy: from acoustic (movement of air particles due to wave phenomena) to mechanic (movement of solid structures) and to heat (increased temperature of air particles and solid structures, due to friction) Description: sound absorption coefficient Előadás címe 23 Előadó Neve BME Hálózati Rendszerek és Szolgáltatások Tanszék

24 Energy balance at a reflection Impinging power, P in Reflected power, P refl Absorbed power, P abs Pin = Prefl + Pabs + Ptrans 1 = ρ + α + τ Transmitted power, P trans

25 Sound absorbing mechanisms / 1 1. The simplest model: Rayleigh s porosity model S s r 4 = S s + s S 2

26 Sound absorbing mechanisms / 2 2. The structural frame (skeleton) is also set into motion: 3. Flow resistence - extra energy deduction - friction dissipation

27 Descriptors Makro level descriptors Sound absorption coefficient, α Weighted sound absorption coefficient, α w Influencing factors Porosity Flow resistence Tortuosity

28 Typical sound absorbers Glasswool made of super slender wool added with phenolic resin binder by pressing, heating and solidifying. Can be covered with Pwc cloth or aluinium foil. Rock wool made of volcanic rock, typically basalt and/or dolomite (nowadays also from recycled materials made from slag ) Plastic foam Előadás címe 28 Előadó Neve BME Hálózati Rendszerek és Szolgáltatások Tanszék

29 Some typical sound absorption curves

30 The effect of flow resistence and thickness Cloth covered glasswool plate of various thickness and flow resistance, measured in a reverberation room

31 Measurement of sound absorption / 1 Reverberation chamber method

32 Impedance tube method Measurement of sound absorption / 2

33 Sound absorption tables Előadás címe 33 Előadó Neve BME Hálózati Rendszerek és Szolgáltatások Tanszék

34 Effect of air gap behind the absorber

35 Effect of perforation

36 Practical example: (sound absorbing) suspended ceiling

37 Spatial elements

38 Membrane resonator

39 Helmholtz-resonator m a ω = c a 0 1 m c a a

40 H-resonators in practice

41 Regions of the sound field Direct field and reverberant (or diffuse) field L p L p 1 = LW + 10log + 2 4r π 4 R Lp - LW [db] log r Távolság a forrástól [m]

42 Back to the description of the reverberant sound field Notion of the reverberation time, T: time period, in which the energy content of the sound field reduces to one millionth. Expressed in SPL this corresponds to 60 db decrease.

43 The founder of modern room acoustics Auditorium Fogg W. C. Sabine ( )

44 Reverberation time T60 V V V = = 0.16 R Aα A1 α1 + A2α Anα n A α + A α A α α = n n = i= 1 A1 + A A n n α Aα R = Aα 1 α n i= 1 Aα i A i i Influencing parameters: directly proportional to the volume V of the room, inversely proportional to the average sound absorption the total surface of surrounding walls A α and

45 Room constants, typical values Volume α=0,35 α=0,3 α=0,2 α=0,15 α=0,07 [m3] Auditorium Cinema Thatre, opera Concert hall Church R Aα = 1 α Aα

46 Reverb (auralization) dry recording ( ) h τ Reverbera ted sound

47 Reverberation of text

48 Room acoustical modelling

49 Computer aided acoustical modelling

50 Computer aided acoustical modelling