BASICS OF ROOM ACOUSTICS Study aid to learn Communication acoustics, VIHIAM 000 Fülöp Augusztinovicz, professor Department of Networked Systems and Services fulop@hit.bme.hu 2017. december 13., Budapest
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
What is small, what is large?
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
Reflections l 2 l 1 l 3 l 4
An important descriptor: the impulse response p li τ i = c τ 1 τ 2 τ 3 τ 4 τ
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
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
The role of reflections in the ancient theatres As imagined in the 19th century: (Pierers Konversationslexikon, Stuttgart, 1891)
The role of reflections in the ancient theatres And the reality: Ruins of the Dyonysos theatre at the foothill of Acropolis in Athens
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
Elements of the ancient greek theatres
Amphytheatre [sound refraction and absorption]
Amphytheatre Orchestra (place of the dancers): floor reflection!
Masque masque: a sound amplifying, impedance matching element
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
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 10-5 8 6 Kossuth u. 4 Szint (FS:1) 2 0-2 -4-6 -8 0 100 200 300 400 500 600 700 800 Távolság [m]
Impulse response / 3
Measured impulse response
Echogram
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
Energy considerations The source fills up the system with energy, the absorption of the walls sinks After sufficient time energy balance is reached
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
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
Sound absorbing mechanisms / 1 1. The simplest model: Rayleigh s porosity model S s r 4 = S s + s S 2
Sound absorbing mechanisms / 2 2. The structural frame (skeleton) is also set into motion: 3. Flow resistence - extra energy deduction - friction dissipation
Descriptors Makro level descriptors Sound absorption coefficient, α Weighted sound absorption coefficient, α w Influencing factors Porosity Flow resistence Tortuosity
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
Some typical sound absorption curves
The effect of flow resistence and thickness Cloth covered glasswool plate of various thickness and flow resistance, measured in a reverberation room
Measurement of sound absorption / 1 Reverberation chamber method
Impedance tube method Measurement of sound absorption / 2
Sound absorption tables Előadás címe 33 Előadó Neve BME Hálózati Rendszerek és Szolgáltatások Tanszék
Effect of air gap behind the absorber
Effect of perforation
Practical example: (sound absorbing) suspended ceiling
Spatial elements
Membrane resonator
Helmholtz-resonator m a ω = c a 0 1 m c a a
H-resonators in practice
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] 0-5 -10-15 -20-25 10 20 50 100 200 500 1000 2000 5000 10000-30 -35 log r -40 1.0 10.0 100.0 1000.0 Távolság a forrástól [m]
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.
The founder of modern room acoustics Auditorium Fogg W. C. Sabine (1868-1919)
Reverberation time T60 V V V = 0.16 0.16 = 0.16 R Aα A1 α1 + A2α 2 +... + Anα n A α + A α +... + A α α = 1 1 2 2 n n = i= 1 A1 + A2 +... + 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
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
Reverb (auralization) dry recording ( ) h τ Reverbera ted sound
Reverberation of text
Room acoustical modelling
Computer aided acoustical modelling
Computer aided acoustical modelling