Features

Acoustics271Sound System Design12 mSPL at a certain DistanceDelay time T Centralised Sound SystemThe sound pressure level of conventional speakers decreases = 12 with / 340 increasingdistance as:= 0.045s When a sound is combined with an optical event, then the sound should+ 0.01 In a centralised sound system the speakers are installed at the same a place.p = p 0- 20 x log (d) [dB SPL] (4)come from the direction of the event. A sound coming from another directionas the event would irritate the spectator. cove r age ang l ecove r age anp: sound pressure Mainsuppo rti nglevel at distance p 0: SPL at 1 m d: distance [m]sp ea ke rs( d e l a y )Often the event is at the front of a room and sometimes there is a stage.In other words: When doubling the distance sp ea (x2), ke then rs the sound pressure In many cases the speakers will be placed at the left ea and r l e right ve l he side i gh of t thelevel reduces to a quarter (1/4), or by 6 dB.The relation between some distances and decreases of sound pressure levelis as following:event’s place. The speaker should be mounted high to obtain a not too loudSPL near the speakers.In case of a deep room supporting speakers can be installed at the walls orDistance [m] 1 2 3 4 5 10 20 50 100ceiling in the direction of the sound propagation. A smart solution is to senda delayed audio signal to the speakers in the rear, then the event will still beDecrease of 0 6 9.5 12 14 20 26 34 40 heard from the event’s place.sound pressureCalculation of the delay time:level [dB]T = d/340 + 0.01s [s] (6)T: delay time [s] d: distance from front speaker to delay speaker [m]Table 5: Decrease of SPL with the distance; reference distance: 1 mExample: A speaker produces a SPL of 105 dB at 1 m. What is the SPL at adistance of 5 metres? ‡ SPL = 105 dB - 14 dB (from table 3) = 91 dB.SPL at certain power and distance12 mDelay time TIn a project the SPL must be calculated at a given power and distance to the= 12 / 340 + 0.01speaker(s). So the formulas for SPL at a certain power and SPL at a certain= 0.045sdistance will be combined. The SPL at a given power and distance can be Mainsuppo rti ngcalculated as:sp ea ke rs( d e l a y )p = p n+ 10 x log(P) - 620 W x log (d) 3 [dB W SPL] (5) 0~6 W 60 Wsp ea ke rsp ar kingp: sound pressure level [dB] p n: sensitivity of the speaker [dB]d: distance from speaker P: power (supplied to the speaker) [W]Example: A loudspeaker shall be installed in a room. The longest distanceto the audience is 8 metre. The loudspeakers has a sensitivity of 90 dB/Wmand 30 watts input power. What SPL can be reached most far away from10 m40 mthe loudspeaker?Figure 7: Frontal sound system with supporting (delay) speakersw are hous eS al e s a reaDecentralised Sound Systems8m8mIf no optical event is combined with the sound or in large areas with a24 mlow ceiling height (e.g. a supermarket) a frontal sound system is neithernecessary nor useful. A better solution is a decentralised (distributed) soundsystem in which the speakers are placed where needed.Ceiling Sound Distribution 6 W Systems 3 W 0~6 W 60 W5 mThe ceiling speakers should be arranged evenly over the area, i.e. with thesame distance between each other. Important for the design of such a systemis the required intelligibility. 4 m T It ech. depends on:10 m6.4 m- ceiling heightr oo mFigure 6: Distances of the speakers in the example- speaker’s coverage anglew are houCalculation:- purpose of usage (quality of sound)SPL = 90 dB SPL+ 10 x log(30) - 20 x log (8)The higher the speaker, the wider the covered listening area (ear level= 90 dB + 15 dB - 18 dB = 87 dB SPLheight) which is approx. 1.5 metres above the floor.8mFor a good intelligibility the audibility of 6 kHz should be good at each place.Applying the tables 2 and 3:A reasonable intelligibility is reached at 4 kHz, but BGM can be worse.SPL = 90 dB SPL + 15 dB (table 2) - 12 dB (table 3) - 6 dB (table 3)= 87 dB SPL4 m