LECTURE 4 Room acoustics

Transcription

1 Rak Building Physics Design 2 ACOUSTICAL DESIGN Autumn 2015 LECTURE 4 Room acoustics Matias Remes, M.Sc. FISE A acoustics

2 Room acoustics Huoneen hyvällä akustiikalla tarkoitetaan sellaisia äänisuhteitten ominaisuuksia, että huoneessa esitetty puhe ja musiikki kuuluu korvaan kauniina, luonnollisena ja selvänä huoneen jokaisessa kohdassa. Diplomi-insinööri U. Varjo 1938

3 What is the significance of room acoustics?

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9 Significance of room acoustics The purpose of room acoustical design is to control the propagation, reflection and attenuation of sound within a space Direct sound Reverberant sound (reflections) Useful and harmful reflections Sound attenuation and absorption, diffusion Design goals according to the use of space, for example: Speech Good speech distinction (e.g. auditorium) / good speech privacy (e.g. open plan office) Music Appropriate reverberation and sense of space in the audience, stage acoustics which support music making

10 Significance of room acoustics Room acoustical design Design of sound reflections Design of sound absorption Design of the shape and geometry of the space Room acoustical design maximising the amount of sound absorbing material E.g. in a lecture hall the performer must be able to speak without restarining ones voice and so that the audience can distinguish what is being said Need for both sound absorbing and reflecting surfaces! Succesful room acoustics is, thus, a combination of the geometry of the space and the absorptive and reflective properties of materials

11 Significance of room acoustics Examples of design goals Movie theater Hearing the sound track in the way the movie makers have intended it to be heard Concert hall Good spatial impression (sound surrounds the listener), sense of intimacy, warm sound color, adequate clarity, etc. Restaurant Peacefull acoustical environment (communication from short distance) Open plan office Speech sound distract concentration speech privacy between work places Factory Noise level may cause hearing damage design of effective sound absorption and noise blocking screens

12 Reflection of sound Sound reflects at simplest as light: angle of incidence = angle of reflection (specular reflection), this applies when Sound wavelength is adequately smaller than the dimensions of the object causing the reflection The reflecting surface is even (not sound scattering) and hard (not sound absorbing) Sound reflection is complicated phenomenon and depends on frequency and the properties of the reflective surface

13 Reflection of sound Significance of reflections Example: sound suddenly stops in a large concert hall Number of reflections occuring within the first 1 s is about 8000 As the number of reflections increases, there is a reverberant sound field in the space in which the listener cannot distinguish single reflections from one another; in large spaces this occurs after about 100 ms Sound field in a space comprises of three distinguishable parts: Direct sound Early reflections Reverberant sound field

14 Sound field in a room Direct sound from source to listener 2. Early reflections within ms after direct sound 3. Gradually attenuating reverberant sound field

15 Refelection of sound

16 Sound field in a room Direct sound, early reflections and reverberant sound and their relations determine how sound is perceived in a space Kuvat: Rossing et al. 2002, Beranek 2004

17 Reflection of sound Significance of reflections Sound perception is affected by the level of reflections, their delay in relation to direct sound and the direction from which they reach the listener Strong reflections with adequate delay are heard as separate echoes (disturbance) If the delay between direct sound and early reflections is appropriate (about ms), the reflections increase the loudness of sound (perceived sound level) important in the design of speech and music spaces Lateral reflections (reaching the listener`s ears from the sides) add to the sense spatial impression and broadening of the sound source crucial in the design of concert halls The effect of single lateral reflection to sound perception (Barron 2003)

18 Reflection of sound The effect of basic room geometry Rectangle: Lateral reflections occur in the entire space Fan-shape: Reflections scatter and are directed mainly to the rear part of the space (not in the middle) Round: Reflections from concave surfaces cause sound to strongly focus on some parts of the space

19 Reflection of sound Example: sound focusing

20 Reflection of sound Effect of surface geometry

21 Reflection of sound Example: reflecting surfaces in a concert hall

22 Reverberation time Significance of reverberation time The reverberation time in a space correlates rather well with the perceived clarity of speech or music: long reverberation the syllables in speech or separate musical notes attenuate slowly and mask each other

23 Reverberation time Significance of reverberation time Too short a reverberation time is not desirable because in an overly damped space there are no useful reflections! In addition to appropriate reverberation time, good room acoustics provides that The space has appropriate size and shape Sound absorbing and reflecting surfaces are positioned correctly Two viewpoints: room acoustics experienced by the audience and by the performer For the audience, it is important to have useful sound reflections form the performer to the audience For the performer, it is important that the stage acoustics supports the performer`s activity

24 Reverberation time vs. use

25 Reverberation time vs. use Tila V / hlö [m 3 ] Kokoustila 3 5 Auditorio, teatteri 4 6 Musiikkiteatteri, ooppera 5 8 Kamarimusiikkisali 6 10 Konserttisali 8 12 Kirkko 10 14

26 Reverberation time Sabine equation Sabine equation: V RT60 0, 161 A A n 1 S1 2S2... ns n isi i1 Sabine equation assumes that the sound field in the room is diffuse, i.e., at any point in the room sound can arrive from any direction and the field remains the same throughout the room This is an idealisation that does not hold perfectly true in real rooms

27 Reverberation time Notes on Sabine equation Sabine equation can be used with good accuracy in rooms which are sufficiently reverberant Sabine equation is most accurate in a reverberant room where the average absorption coefficient is < 0,25 1) In very absorbent rooms the Sabine equation gives erroneous results Additional requirements for Sabine equation to yield accurate results: The room geometry should be simple (cube-like) and the room should be quite small The absorption material should be evenly distributed on the room surfaces Calculation error increases in large and complex spaces In rooms where all the absorption material is positioned only on one surface, Sabine equation yields shorter reverberation time than is the case in practice 1) F.A. Everest, K.C. Pohlman, Master Handbook of Acoustics, 2009

28 Reverberation time Eyring-Norris equation Eyring-Norris equation: V RT60 0,161 S ln 1 average where V is room volume, S is the total surface area of the room and α average is the average absorption coefficient: average Sii S i

29 Reverberation time Notes on Eyring-Norris equation Eyring-Norris equation can be used in more absorptive rooms where average absorption coefficient is > 0,25, e.g., studios However, sound absorption coefficients that are commonly available and published by material manufacturers are Sabine coefficients (measured in a reverberation chamber and calculated using Sabine equation) and can, thus, be directly applied only to the Sabine equation For this reason, Sabine equation is the usual choice in acoustical design and is also used on this course Other researchers have suggested alternative reverberation formulas, e.g., Fitzroy, Millington, Hopkins-Striker...

30 Reverberation time Air absorption Taking account of air absorption, the Sabine and Eyring- Norris equations can be written as: Sabine: RT 60 V 0,161 A 4mV Eyring-Norris: RT 0, S ln1 mv V average 4 where m is the air attenuation coefficient (some values: m = 0,009 at 2 khz; m = 0,025 at 4 khz; m = 0,080 at 8 khz) Air attenuation is only significant in large spaces above 2 khz depends on relative humidity of air, absorption increases at low humidity

31 Air absorption T = 20 C, RH = 50 %

32 Room modes Sound field within a room is comprised of room resonances, called room modes Room mode = characteristic resonance of the room Three types of modes: axial, tangential, oblique The amount and spacing of room modes changes with frequency At low frequencies there are only a few room modes and the modes are sparsely spaced, as a result of which the reverberation time and sound level can vary considerably in different points in the room (consider the placement of a subwoofer in a living room) At high frequencies the number of room modes gets so high and their frequencies are so close to one another, that single room modes cannot be distinguished sound field approaches the idealisation of diffusivity

33 Room modes The frequency, below which the sound field in a room is not diffuse (socalled Schröder frequency) depends on reverberation time and volume: Example, typical dewlling room: T = 0,5 s ja V = 30 m 3 f s = 260 Hz The room modes of a rectangular room can be calculated based on the dimensions of the room (L x, L y, L z ): where l, m, n are integers V T f s Z Y X lmn L n L m L l c f

34 Room modes The number of modes (mode density) increases with increasing frequency, at low frequencies mode density is low From room acoustics point of view, room modes are the more problematic the smaller the size of the room Room modes must be considered in the design of, e.g. Control rooms Recording studios

35 Room modes Standard deviations of reverberation times measured in 50 empty rooms in a dwelling Each point represents the reverberation time calculated from measurement conducted in 12 points in a room In small rooms sound field is not diffuse at low frequencies, thus the variance in reverberation time increases towads low frequencies

36 Absorption vs. reflection

37 Sound absorbing materials General Sound absorption three absorption mechanisms: Porous materials (P) Resonant absorbers (R) Membrane / panel absorbers (M) Typical absorption behaviour in the figure

38 Sound absorbing materials Porous materials effect of placement Sound absorption of porous materials is based on thermal losses caused by friction in the pores of the material At the surface of a rigid structure (e.g. wall, roof) sound pressure is at maximum and particle velocity at minimum Maximal particle velocity occurs at 1/4λ distance from the surface of the structure to achieve effective absorption, there should be absorbing material at this distance

39 Sound absorbing materials Porous materials Porous material absorbs sound most effectively when the thickness of the material is at least four times the sound wavelength: d 1/4λ Example: mineral wool 20 mm: 20 mm 1/4λ λ 80 mm f 4290 Hz Note: low-frequency sounds have long wavelength (e.g. 100 Hz 3,4 m) thin layers of porous material do not much absorb low frequencies! /4 /4

40 Sound absorbing materials Porous materials Absorption coefficients of mineral wool Note the effect of suspension height

41 Sound absorbing materials Porous materials examples of use

42 Sound absorbing materials Porous materials examples of use

43 Sound absorbing materials Perforated panels (resonant absorbers) Perforated panels act as resonant absorbers; the absorption is based on mass-spring resonance caused by the air in the hole acting as mass and the air in the background airspace acting as spring Absorption is most effective at the resonance frequency of the mass-spring system Resonance frequency and absorption coefficient are affected by: Thickness of the airspace and filling with porous material Size, amount and geometry of holes Thickness of the panel Resonance frequency is typically at mid frequencies ( Hz), below and above which absorption coefficient decreases Absorption coefficient of perforated panel absorbers can be increased by filling the background airspace with porous absorption material

44 Sound absorbing materials Perforated panels 1,0 0,8 Absorptiosuhde 0,6 0,4 Reikäala 7 %, ilmaväli 30 mm Reikäala 17 %, ilmaväli 30 mm Reikäala 17 %, ilmaväli 200 mm Reikäala 17 %, ilmaväli 200 mm, jossa 50 mm mineraalivilla 0,2 0, Keskitaajuus [Hz]

45 Sound absorbing materials Panel absorbers Structure of a panel (or membrane) absorber: a closed airspace behind an impervious (not perforated) panel which can be filled with porous material Absorption coefficient is highest at low frequencies around the resonance frequency Resonance frequency depends on the surface mass of the panel and thickness of the airspace: 60 f0 m`d Note: at high and mid frequencies panel absorbers are sound reflecting structures!

46 Sound absorbing materials Panel absorbers 1,0 0,8 Absorptiosuhde 0,6 0,4 0,2 0, Kipsilevy 13 mm, mineraalivilla 50 mm Kipsilevy 13 mm, mineraalivilla 100 mm Kipsilevy 13 mm, ilmaväli 100 mm 2 x kipsilevy 13 mm, mineraalivilla 50 mm Keskitaajuus [Hz]

47 Sound absorbing materials Miscellaneous materials and structures

48 Hard surfaces 1,0 0,8 0,6 0,4 0,2 0, Absorptiosuhde Lakattu puu 69 mm Maalattu betoni Muovimatolla päällystetty betoni Rapattu tiili Keskitaajuus [Hz]

49 Chairs and audience 1,0 0,8 Absorptiosuhde 0,6 0,4 Puu- tai muovituolit Pehmustetut istuimet Yleisö pehmustetuilla istuimilla 0,2 0, Keskitaajuus [Hz]

50 Air absorpion Effect on reverberation time In large spaces reverberation time decreases at high frequencies because of air absorption Example curve: Savonlinna hall 2,5 2,0 1,5 1,0 0,5 0, Jälkikaiunta-aika T [s] Keskitaajuus [Hz]

51 Effect of vapour barrier on absorption

52 Effect of vapour barrier on absorption

53 Classification of absorption materials According to EN (classes A-E) Measured absorption coefficient is compared to a reference curve, the sum of unfavourable deviations 0,10 Note: definition of absorption class does not consider frequencies below 200 Hz!

54 Classification of absorption materials

55 Sound diffusing structures (diffusors) [Salter et al. 1999]

56 Concert halls Subjective and objective parameters The characteristics of sound perceived in a hall can be describes subjectively (e.g. warm sound, three dimensional sound ) Difficulty: how to define accurately what different subjective descriptions mean Common terminology to be used by acousticians and musicians has been developed ( subjective parameters ) which define the different sound characteristics (e.g. Beranek) Objective parameters are measurable quantities which try to describe the subjective perceptions as well as possible

57 Concert halls Objective parameters Reverberance (Kaiuntaisuus) Reverberation time RT 60 Early decay time (Varhainen jälkikaiunta-aika) EDT, has been found to correlate better with subjective impression than RT 60 ) Clarity (Selvyys) Clarity C80 (describes how much of the sound arrives at the listener ms after direct sound) Definition D50 Spatial impression (tilan tuntu) Lateral Energy Fraction LF (describes hthe amount of reflections reaching the listener from the sides, laterally) Loudness or strength of sound (Äänen voimakkuus) Strength G Warmth, brilliance (Äänen lämpimyys ja kirkkaus) Bass Ratio Treble Ratio Stage support (lavatuki) Support ST1 (correlates with how the performers are able to hear each other on stage)

58 Concert halls Connections between parameters Subjektiivinen ominaisuus Objektiivinen mittaluku

59 Concert halls Sound field in a concert hall Acoustics experienced by the audience Ideally the audience should be able to hear the performace in the same way in every point of the hall (nod acoustically bad seats) The balance between different players / singers should be good and not dependent on the listening position Acoustics experienced by the performer(s) Performers should be able to hear each other Room acoustics shoud act as a continuation of the performers instrument: the performer should be able to hear the acoustics of the space properly and the acoustics should support the performers efforts

60 Concert halls Design issues Volume and basic shape What is performed? Rock / classical? Full-sized syphony orchestra / chamber music? Music theatre / opera? Multi-functionality? Seating capacity (audience, performers) Also other than room acoustical issues must be considered! Sound insulation to surrounding spaces Noise control HVAC Theater equipment, lighting etc.

61 Concert halls Example: Wien, Musikverein (1870)

62 Concert halls Example: Berlin Filharmonie (1963)

63 Concert halls Example: Finlandiasali (1971)

64 Concert halls Example: Sibeliussali (2000)

65 Auditoria Most important room acoustical consideration: speech intelligibility Essential to good speech intelligibility: Direct sound path from speaker to listener (other audience members must not be in the way to block the direct sound) raised floor on the audience area Directioning of early reflections towards the audience (to strengthen the direct sound and, thus, improve speech intelligibility) positioning of sound reflecting and absorbing surfaces Appropriate reverberation time (remember the masking effect from previous slides )

66 Auditoria Objective parameters Objective parameters describing speech intelligibility: Speech Transmission Index STI Rapid Speech Transmission Index RASTI STI depends on the sound level of speech, reverberation time, reflections, distance between speaker and listener and background noise level STI is based on distinction of speech syllables in a space, value range STI = 0 none of the syllables are heard correctly STI = 1 all the syllables are heard correctly In auditoria and other speech spaces the goal is to achieve a high value of STI (good speech intelligibility), whereas in spaces such as open plan offices the goal is as low STI-value as possible (good speech privacy between work stations)

67 Auditoria Significance of STI

68 Auditoria Example of positioning surfaces A surface above the speaker which reflects sound to the front and rear parts of the audience (1) Sound reflecting surfaces to the mid and rear parts of the audience so that the entire audience area receives reflections (2-5) Front surfaces of the hall sound reflecting (non-parallel walls on stage to avoid flutter echoes), lower parts of the walls sound reflecting to achieve useful reflections from the sides as well as from the ceiling (6) Absorbing surfaces to the upper parts of side walls and rear part of the ceiling, rear wall sound absorbing or scattering to avoid distracting reflections back to the front part of the hall and the stage (6) 1) 2) 3) 4) 5) 6) Absorboivaa pintaa Heijastavaa pintaa A

69 Classrooms Optimising speech intelligibility (SFS 5907) In classrooms, it is important that speech distinction from the treacher to pupils is good Recommended reveberation time 0,5...0,8 s When class A sound absorbing material is used (absorption coefficient > 0,9), the centre part of the ceiling should be left sound reflecting (absorption coefficient ), from which sound is reflected to the mid and rear part of the room this improves speech intelligibility If class C material is used, the material can be positioned on the whole ceiling surface because the material also reflects sound The amount of class A material needed is about 70 % of the floor area and for class C material correspondingly 100 % Absorption material should also be placed on the walls to avoid distracting flutter echoes

70 Screens Wall elements used for dividing space, do not extend to ceiling height (as opposed to walls), thus sound is diffracted over the screen Screens have many noise control applications: screens for blocking speech in open plan offices, noise blocking screens in factories, road noise barriers The sound attenuation of a screen, D [db], is determined as insertion loss: D = L p,1 L p,2 where L p,1 is the measured sound pressure level at listener position without the screen and L p,2 is the SPL with the screen in place

71 Screens In the free-field (a space with no sound reflections, e.g. outdoors), the sound attenuation by screen of a point sound source is given by the equation z D 10log where z is the distance difference and λ is sound wavelength (λ = c 0 /f)

72 Screens The effect of screen height H on sound attenuation when x 1 = x 2 = h 1 = h 2 = 1,2 m, calculated with equation on previous slide The mass of the screen is assumed to be infinitely large so the sound insulation of screen is insignificant Solid lines correspond to sound reduction index (mass law values) of two different walls (surface masses 5 kg/m2 and 20 kg/m2) Note: the sound attenuation of a solid wall is always significantly higher than that of a screen

73 Screens Effect of sound reflections on attenuation The sound attenuation values on the previous slide assume a space with no sound reflections; the values can be achieved by, e.g., outdoor noise barriers In practical rooms the sound attenuation of screens is even considerably lower because of reflections; screens mainly attenuate the direct sound from source to listener but have little impact on reverberant sound Attenuation is especially affected by sound absorption of the ceiling and wall surfaces close to the screen

74 Screens Effect of sound reflections on attenuation Example: factory hall with screen constructed around a work station with noisy activity Although the screen height was 6 m, the screen only attenuated sound by 1dB(A) to its surroundings Reasons: Ceiling and wall surfaces were sound reflecting, reverberation time of the hall was high Screen height was only about 1/3 of the room height

75 Open plan offices Acoustics is a key component of indoor climate in open plan offices The most distracting sound is speech heard from surrounding work stations Acoustical design goal is to maximise speech privacy between work stations, thus minimise speech intelligibility (note: opposite goal to e.g. classrooms) In open plan offices there are no sound insulating partitions, but only screens speech privacy must be mainly achieved by means of room acoustics [Haapakangas et al. 2007]

76 Open plan offices Good speech privacy between work stations requires that three factors are considered: 1. Direct sound path between work stations must be truncated using screens, preferrable scren height is > 150 cm 2. Sounds reflecting from room surfaces must be adequately attenuated sufficient amount of sound absorbing material correctly positioned on the surfaces of the room (ceiling, floor, walls, screens, furnishings) 3. Masking sound sufficiently high background sound level in the space to mask speech sounds, preferrably implemented artificially using loudspeakers to produce the masking sound, recommende sound level dba

77 Studios Reflectionfree zone at the mixer`s position

78 Studios Dolby surround 7.1 speaker layout

79 Studios Requirements for early reflections and reverberation time [2005 Dolby 5.1-channel music production guidelines]