Audio Visual Design Guidelines

Loudspeaker selection and placement

59 views November 19, 2019 aetm 0

An effective electroacoustic design is essential to assist presenters in communicating spoken word and other programme material to the audience. 

Many institutions will develop a preference for one or more narrow ranges of speakers based on performance, voicing and reliability, but the final choice of speakers should always be reviewed for suitability in each space. 

Design of the speaker system will be informed by the need to achieve:

  • Effective coverage of the listening area(s)
  • Acceptable uniformity of SPL throughout the listening area
  • Good intelligibility and high gain before feedback for amplified voice; and
  • Reproduction of programme audio appropriate to the identified use cases

Typical performance criteria for a practical sound reinforcement system are

  • Voice
    • SPL of 65dB(A)
    • Intelligibility in the ‘excellent’ (0.75+) range (STI per IEC60268:16) 
  • Programme
    • SPL of 85dB(A)
  • Uniformity (AVIXA A102.01:2017 Audio Coverage Uniformity in Listener Areas
    • Coverage envelope of 6dB

 

Typical design approaches

Space type Practical design strategy
Seminar <50P Amplified voice is not expected in most small spaces, so amplification is typically limited to programme sound.

In small rooms, this is usually from a pair of speakers at the front or the room, or a sound bar where flat panel displays are used. In flexible/collaborative spaces with multiple displays or no single teaching space an array of ceiling speakers may be more appropriate. 

Seminar ≥50P Where microphones are used, acoustic feedback is an undesirable side effect which must be prevented. 

‘Front’ programme speakers are usually augmented or replaced with ceiling speakers to provide more uniform distribution of amplified voice. Uniformity allows technical staff to better commission the sound system to provide adequate SPL and high gain before feedback

Video conference The aim of any video conference space is to provide as natural a conversational environment as is practical. Depending on room size, programme and ‘far end’ sound are reproduced through a combination of

  • Front speakers to localise the ‘far end’ audio; and
  • Ceiling speakers to reduce overall SPL whilst improving intelligibility and uniformity

In larger conferencing spaces, ceiling speakers may be wired in multiple circuits to reflect the number and placement of microphones and improve Acoustic Echo Cancellation (AEC) processing

Lecture theatre/Auditorium Larger spaces require a bespoke approach which responds to the physical acoustics and best supports identified uses.

  • Voice and programme are typically localised to the front of the room, but may require separate speaker systems
  • Additional speakers with similar voicing to those at the front may be added in one or more locations deeper in the room, allowing. SPL uniformity to be managed. 

The audio feeding these speakers should be delayed so the sound arrives slightly later than that from the front, ensuring listeners still perceive sound as localised to the front of the room.

 

Speaker Types and Application

Care must be taken in the design to use speakers appropriate for the task, and to place them to best effect. The manufacturer’s datasheet is informative but must be read closely so calculations are based on realistic data. Designers should particularly review the following items and inspect polar plots or supplementary data to reach a decision.

  • Sensitivity
    • Generally expressed as xxdB 1W/1m, or “What SPL will be measured on-axis at 1m from the speaker for a 1W input”
  • Dispersion
    • The vertical and horizontal distribution at a nominated frequency (normally 1-2kHz – in the voice range). Lower frequencies tend to be omnidirectional and dispersion narrows with increasing frequency
    • Many professional ‘box’ speakers use a rotatable waveguide on the high frequency element to permit vertical or horizontal mounting.
    • Line arrays tend to have wide horizontal dispersion (above 150°) but very narrow vertical (10-20°)
    • Ceiling speaker dispersion depends on construction, and commonly ranges between 90 and 150°. 
      • Ceiling speakers with narrow dispersion should be considered in spaces with specular walls/façade to focus energy downwards
  • Impedance
    • Speakers are ordered for direct, low impedance connection (4-16Ω) or use on a high impedance line at 70/100V. 
    • Speakers rated for 100V will work fine on a 70V amplifier, though all power tappings are halved. Many commercial speakers are labelled with both 70V and 100V power ratings
    • A 70V speaker cannot always be assumed suitable for use on a 100V line – designers should check before specifying 
  • Frequency response
    • The frequency response of a speaker is expressed as the range of frequencies able to be reproduced e.g.: 85Hz-15kHz
    • On its own, this frequency range provides an incomplete picture – we also need to know the tolerance applied. 
      • This is often the point 6dB below the on-axis reference level, but some manufacturers measure at the -10dB points
      • Cinema and other more critical speakers may be described with a tolerance of ±3dB of mean
      • Loudspeaker polar plots are very informative if the printed data is not explicit. These present a graph of level between 0° and ±90°, helping designers understand the on- and off-axis frequency response of each particular unit for each of the key frequencies.

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