New Reference Listening Room for Two-Channel and Multi-Channel Stereophony (Design - Measurement - Modelling) Éva Arató-Borsi, Tamás Póth Hungarian Radio, Technical Department Andor T. Fürjes Technical University of Budapest Hungarian Section of AES 104th AES Convention, May 1998
Contents 1. Introduction 2. Design Considerations 3. The Measured Sound Field Parameters 4. Listening Test 5. Modelling 6. Conclusion 104TH AES CONVENTION, MAY 1998 2
1. Introduction � The need… to test different techniques by listening tests all over the world needs to improve the listening conditions � The demand… to achieve a special environment for listening conditions � The design of a new reference listening room in the Hungarian Radio � The aim to achieve the requirements given by recommendations. � 2-channel � 5-channel 104TH AES CONVENTION, MAY 1998 3
2. Design Considerations � Basis of the design considerations EBU recommendation: the last version EBU Tech 3276 "Listening conditions for the assessment of sound programme material: monophonic and two-channel stereophonic" � The room geometry the listening room was designed into an existing room Floor plan and the 3- dimensional view 104TH AES CONVENTION, MAY 1998 4
3. The Measured Sound Field Parameters For practical reasons the room is used for talk studio too compromises � The room dimensions: length 7.8 m width 7.05 m height 4.85 m 55 m 2 area: 267 m 3 volume: � All dimensions suit the recommendation: � proportions of the room � floor area � volume Since these calculations assume perfectly rigid walls and rectangular shape… …a finite element model was created to investigate the low frequency behaviour of the room (SYSNOISE). 104TH AES CONVENTION, MAY 1998 5
3. The Measured Sound Field Parameters � The choice of the acoustical elements on the basis of the calculations placements of the elements � Low-frequency absorbers � Diffusers � Wide-band absorbers 104TH AES CONVENTION, MAY 1998 6
3. The Measured Sound Field Parameters The properties of the sound field produced by the loudspeakers in the listening area determines the quality of the listening environment. � The sound field parameters specified by the EBU: A Direct sound B Early reflections C Reverberant field D Operational room response curve E Background noise � The measurements… with the MLSSA analyser of DRA Lab. 104TH AES CONVENTION, MAY 1998 7
3. The Measured Sound Field Parameters A Direct sound Loudspeakers: � FL,FC, FR - Genelec 1038A � SL,SR - Genelec 1032A …suit the specifications B Early reflections For the two - channel stereophonic: Acceptable level of the early reflections: -10dB after the direct sound up to 15-20msec . Front - left Front - center Front - right ETF (Waterfall) Energy - Time - Frequency response at the reference listening point. 104TH AES CONVENTION, MAY 1998 8
3. The Measured Sound Field Parameters The results of the subjective tests show… …the influence of the early reflections around the surround loudspeakers can be disturbing. Acceptable level of the reflections?… …need for further investigations. The design objective: to avoid the strong early reflections for every channel. Surround - left Surround - right ETF response at the reference listening point - from surround speakers. 104TH AES CONVENTION, MAY 1998 9
3. The Measured Sound Field Parameters C Reverberant field The recommended nominal value 0.2 sec < T m < 0.4 sec THE MEASURED REVERBERATION TIME T 60 [s] 0,6 EBU references 0,5 0,4 T60[s] 0,3 0,2 0,1 0 10 12 16 20 25 31 40 50 63 80 1k 1,2 1,6 2k 2,5 3,1 4k 5k 6,3 8k 100 125 160 200 250 315 400 500 630 800 1k 1.25k 1.6k 2k 2.5k3.15k 4k 5k 6.3k 8k 0 5 0 0 0 5 0 0 0 0 5k k k 5k k f[Hz] The measured reverberation time 104TH AES CONVENTION, MAY 1998 10
3. The Measured Sound Field Parameters D Operational room response curve Definition: the sound pressure level produced by the loudspeakers at the reference point. FL 20 FR FC 15 Front - 10 � Left ( FL ) A [dB] 5 � Center ( FC ) 0 � Right ( FR ) -5 -10 25 40 63 100 160 250 400 630 1k 1,6k 2,5k 4k 6,3k 10k 16k f [Hz] 20 SL SR 15 Surround - 10 � Left ( SL ) A [dB] 5 � Right ( SR ) 0 -5 -10 20 25 31, 40 50 63 80 100 125 160 200 250 315 400 500 630 800 1k 1,2 1,6 2k 2,5 3,1 4k 5k 6,3 8k 10k 12, 16k 20k 20 31.5 50 80 125 200 315 500 800 1.25k 2k 3.15k 5k 8k 12.5k 20k 5 5k k k 5k k 5k f [Hz] Measured operational room response curves 104TH AES CONVENTION, MAY 1998 11
4. Listening Tests (5-channel) Center Front Left Right 4 m 3.4 m 3.4 m 3.4 m 3.3 m 3.3 m Surround 4.6 m Left Right Position of speakers and listeners � Test procedures and equipment � Yamaha DSP-E492 commercial Dolby Pro Logic decoder with Dolby Surround encoded music/effects � Tascam DA88 8-channel digital recorder with discrete 5- channel recordings 104TH AES CONVENTION, MAY 1998 12
4. Listening Tests (5-channel) Center Front Left Right 4 m 3.4 m 3.4 m 3.4 m 3.3 m 3.3 m Surround 4.6 m Left Right Position of speakers and listeners � Subjective results � Dolby Surround: better if surround speakers are turned to face each other � Discrete 5-channel: better if surround speakers are turned to listeners � Surround speakers shall sound as diffuse as possible � Spatial impression is less position dependent 104TH AES CONVENTION, MAY 1998 13
4. Listening Tests (5-channel) � Subjective and objective parameters ??? Impulse Response Processing Objective parameters � modified M-factor: ( ) ( ) = − M 10 log k t 10 log k t 10 2 1 10 2 2 t ( ) ∫ 2 p t dt ( ) = 0 k t ∞ 2 ∫ ( ) 2 p t dt t � Centre time: ( ) ∞ ⋅ 2 t p t dt ∫ s = t 2 p max 0 104TH AES CONVENTION, MAY 1998 14
5. Modelling � Why computer aided modelling? � prediction in design phase, � shorter design period, � low costs; � modelling posteriorly helps improvement of modelling procedure; � experiments with “virtual” set-ups. � Modelling with computers - review � numerical solutions (FDM, FEM, BEM) � geometrical room acoustics (Image-source models, Ray- tracing, Cone-tracing, Beam-tracing) 104TH AES CONVENTION, MAY 1998 15
5. Modelling � Modelling technical rooms - problems � relatively small sizes (architecture and furniture together, near-field modelling of sources and reflections, limits of geometrical acoustics) � special surfaces and materials (diffuser, absorber, etc.) � special requirements (what parameters, measuring surface material and sound source properties - no high resolution data available) � Low Frequency Modelling (SYSNOISE) � acoustic modes � positioning of low-frequency sources � effect of the “rigid” wall 104TH AES CONVENTION, MAY 1998 16
5. Modelling � High Frequency Model � why developing a new software… Basic Assumptions � beam-tracing � beam surfaces are exact pieces of the wave front S � boundaries may be curved - beam distortion � diffuse reflection means ϕ ’ the change of solid angle of the beam ϕ S S’ ϕ � “sampling” in detection of ∆ t ⋅ c reflections 104TH AES CONVENTION, MAY 1998 17
5. Modelling � Results of Modelling predicted measured frequency response energy-decay � Improvements � correct diffuse reflection model � correct diffraction model - sampling a very coarse approach � introduction of phase � near-field model of sources and reflections � precise data about sources and materials 104TH AES CONVENTION, MAY 1998 18
6. Conclusion � Requirements � Measurements � Subjective Test � Modelling � Next… 104TH AES CONVENTION, MAY 1998 19
5. Modelling � High Frequency Model � why developing a new software… Basic Assumptions � beam-tracing � beam surfaces are exact pieces of the wave front S � boundaries may be curved - beam distortion ϕ ’ � diffuse reflection means ϕ S S’ the change of solid angle ϕ of the beam 104TH AES CONVENTION, MAY 1998 17
5. Modelling � Results of Modelling predicted measured frequency response energy-decay � Improvements � correct diffuse reflection model � correct diffraction model � introduction of phase � near-field model of sources and reflections � precise data about sources and materials 104TH AES CONVENTION, MAY 1998 18
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