Loudspeaker Choice and Placement. D. G. Meyer School of Electrical & Computer Engineering

Transcription

1 Loudspeaker Choice and Placement D. G. Meyer School of Electrical & Computer Engineering

2 Outline Sound System Design Goals Review Acoustic Environment Outdoors Acoustic Environment Indoors Loudspeaker Choice and Placement Summary / Conclusions

3 Sound Reinforcement System Design Goals evenness of coverage intelligibility (articulation loss of consonants) ratio of direct sound field to reverberant sound field gain before feedback SPL at furthest listening position frequency range/response smoothness of frequency response curve locality of reference headroom

4 Factors Which Complicate Sound System Design reverberation / echo early / late arrivals room surfaces (absorption) room geometry seating characteristics variable fill empty room full room

5 Review In acoustics, the ratios most commonly encountered are changes in pressure level, measured in db-spl: db-spl = 20 log 10 (p/p o ) where p o = 20 µn/m 2 As distance from a sound source doubles, the db-spl decreases 6 db (this is called the inverse square law) Adding/subtracting db levels: SPLa ± SPLb = 10 log 10 [ 10 db-spla/10 ± 10 db-splb/10 ] Doubling acoustic power corresponds to a 3 db increase in SPL Doubling perceived loudness corresponds to a 10 db increase in SPL

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7 Sound Power Levels Sound power levels (db-pwr) are calculated as: db-pwr = 10 log 10 [ (acoustic power) / watt ] The standard reference for power in audio work is watt (1 milliwatt), called 0 dbm, which corresponds to volt across 600Ω Example: The power in watts corresponding to +30 dbm is: 30 dbm = 10 log 10 [ x / 10-3 ] = 10 log 10 [x 10 3 ] = 10 log 10 [x] + 10 log 10 [10 3 ] = 10 log 10 [x] = 10 log 10 [x] x = 1 watt

8 Acoustic Environment Outdoors SPL decreases 6 db for every doubling in distance Sound energy dissipates due to mechanical effects (greater dissipation at higher frequencies) Relative humidity affects sound absorption (greater at higher frequencies) Temperature gradient affects sound refraction Ground surface causes reflection and absorption of sound

9 Sound Refraction Due to Temperature Gradient

10 Acoustic Environment Indoors Consider enclosed space with internal volume V and total boundary surface S Each surface s i has an absorption coefficient a i The average absorption coefficient is a avg = [Σs i a i ]/S The room constant R = (S a avg )/(1-a avg ) The room will possess a reverberation time RT 60 (time in seconds for steady-state sound to attenuate 60 db)

11 Determination of RT60

12 Acoustic Environment Indoors The sound arriving at a listener s ears has (at least) three distinct divisions: direct sound early reflections reverberant sound The direct sound undergoes no reflections and follows inverse-square-law level change (-6 db per doubling of distance) The reverberant sound tends to remain at a constant level if the sound source continues to put energy into the room at a regular rate

13 Sound paths in a concert hall Time relationship of direct and reflected sounds Comparison of direct, early, and reverberant sound fields in an auditorium

14 Acoustic Environment Indoors This gives rise to several basic sound fields: near field free field reverberant field far field The near field is typically defined to be within a distance of twice the largest dimension of the sound source In the far free field, inverse-square-law level change prevails In the reverberant (diffuse) field, sound energy density is nearly uniform

15 Critical distance (Dc) Sound fields in an enclosed space

16 Critical Distance Dc (the critical distance ) is the point at which the direct sound and the reverberant sound are both at the same level Dc determines the maximum acoustical separation, hence maximum acoustic gain Dc can be controlled by changing R (the absorption of the room surfaces) or the Q of the loudspeaker (higher Q, more directive, less reverberant energy) Dc determines the required directivity of the loudspeaker in an existing room

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18 Intelligibility Several methods have been devised for assessing the speech intelligibility of sound reinforcement systems Articulation loss of consonants (%ALCONS) is one method of determining the articulation score of an enclosed space If ALCONS < 10%, intelligibility is very good Between 10-15%, intelligibility is sufficient ALCONS of 15% is considered a practical working limit

19 ALCONS A scale of the percentage of the articulation loss of consonants. A formula derived by V.M.A. Peutz based on distance, RT 60, Volume of air in the room, the N factor, and the Q of the source. A measure of how difficult it is to understand someone in a room. The lower the number, the better for speech intelligibility. Source: Church Audio & Acoustics Glossary

20 ALCONS The measured percentage of Articulation Loss of Consonants by a listener. An ALCONS of 0% indicates perfect clarity and intelligibility with no loss of consonant understanding, while 10% and beyond is growing toward bad intelligibility, and 15% typically is the maximum loss acceptable. Source:

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22 Causes of Reduced Intelligibility Poor signal-to-noise (SNR or S/N) Excessive reverberation Long/delayed reflections Loudspeaker misalignment Misequalization Q too low (beamwidth too wide) Distance from source

23 Designing for Acoustic Gain Sound (at listener s ears) must be sufficiently loud (25 db above ambient noise level at mid-frequencies in rooms with RT sec) Sound must reasonably approximate audio spectrum produced by source Sound must reasonably approximate a ratio of direct-to-reverberant sound within the constraints of acceptable articulation loss

24 Effective Acoustic Distance Based on room characteristics and ambient noise level, can calculate the maximum physical distance between a talker and listener with no sound system Want this equivalent acoustic distance (EAD) to be established at the farthest (most distant) listening point EAD is the perceived (rather than actual) distance between the sound source and the listener

25 Basic parameters of a single-source system Distances involved in NAG calculation

26 Acoustic Gain Calculations Based on EAD and farthest listening point, can calculate the needed acoustic gain (NAG) of the sound system Needed acoustic gain calculation: Level at actual/farthest distance = Face-to-Face level desired (db) + 20 log 10 (Face-to-Face Distance = EAD) 20 log 10 (Actual/Farthest Listening Distance) NAG = (Face-to-Face level desired) (Level at actual/farthest distance)

27 Acoustic Gain Calculations Need to add to this a feedback stability margin (FSM) of (at least) +6 db The number of open microphones (NOM) reduces the system gain (doubling the number of open microphones reduces the system gain 3 db) Potential acoustic gain calculation: PAG = 20 log 10 D 1 20 log 10 D log 10 D 0 20 log 10 D S FSM 10 log 10 NOM

28 Acoustic Gain Calculations Major points to remember: Doubling/halving distance yields 6 db change 10 db change corresponds to doubling of perceived loudness PAG can be improved by making the loudspeaker-to-microphone distance (D 1 ) as large as possible making the loudspeaker-to-listener distance (D 2 ) as small as possible making the talker-to-microphone distance (D S ) as small as possible limiting the number of open microphones

29 Electrical Power Requirement When SPL goal at a given listening distance known, also need: Sensitivity rating of loudspeaker (typically spec as 1m on-axis with input of 1 electrical watt) Acoustic level change/attenuation between loudspeaker and farthest listening position Example: 90 db program level at listening distance of 32 m outdoors Loudspeaker sensitivity measured as 110 db Acoustic level change = 20 log (32) 30 db Add 10 db for peak (program level) headroom SPL required at source is = 130 db Need 20 db above 1 watt, or 10 (20/10) = 100 W

30 Transmission

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32 Transmission

33 Loudspeaker Choice and Placement central cluster + excellent coverage + high intelligibility + high gain before feedback + smooth frequency response + good locality of reference cluster needs to be large for long, narrow room potential for interference in driver overlap regions hard to hide architecturally ugly hanging mess

34 Central Vertical Line Array

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49 Loudspeaker Choice and Placement split source / point and shoot + best if multi-channel + high intelligibility + potential solution for challenging room geometries + generally more aesthetically pleasing (but not always) potential for creating large interference zone potential for loss of locality of reference potential for limited frequency range over which directional control is possible

50 Split Source / Point and Shoot

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56 Loudspeaker Choice and Placement distributed / delayed + good solution for large, absorptive rooms with low ceilings + potential solution for challenging room geometries + potential solution for reinforcing distant zones requires digital delays / multiple amplifiers (expensive) potential for loss of locality of reference generally not well suited for rooms with high ceilings (or that are highly reverberant)

57 Distributed / Delayed

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64 Summary / Conclusions There is no universal, one size fits all solution to loudspeaker selection When selecting loudspeakers for sound system design project, compare and contrast offerings from multiple manufacturers be prepared to defend your choices