PUBLIC ADDRESS/PAGING SYSTEM/AUDIO-VISUAL · PDF fileSYSTEM/AUDIO-VISUAL SPEAKER PLACEMENT AND WIRING GUIDELINES ... “Successful reinforcement ... • For successful reproduction

PUBLIC ADDRESS/PAGING SYSTEM/AUDIO-VISUAL SPEAKER

PLACEMENT AND WIRING GUIDELINES

Bob HertlingSupervising Communications Engineer

RCDD, OSPRI Telecommunications Systems Contractor License

#4848PARSONS

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Overview

WHAT THIS PRESENTATION IS…• A general discussion of basic concepts and key

concerns related to system design and installation.

WHAT THIS PRESENTATION IS NOT…• A detailed design guide for actual installations.

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Topics of Discussion

A. Importance of correct spacing/location

B. Basic speaker characteristicsC. Acoustic considerationsD. Determining coverage areasE. Speaker layoutsF. Electrical considerations

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A. Importance of Correct Spacing/Location

Most Public Address (PA)/Paging/Audio-Visual (A-V) systems can be considered to be “distributed reinforcement” sound systems in that they provide “real time” amplification of an audio source (e.g. live or recorded audio announcements or media audio) to listeners in a served area using a number of individual speakers.

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A. Importance of Correct Spacing/Location (cont.)

“Successful reinforcement systems must be loud enough (sufficient acoustic gain), possess clarity (provide a low percentage of articulation loss of consonants in speech), and cover the listeners with uniformity, while avoiding the coverage of areas devoid of listeners.” (quote from Sound System Engineering, Don Davis, page 11)

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A. Importance of Correct Spacing/Location (cont.)

Speaker spacing and location are a key part of ensuring that the PA/Paging/A-V system provides the required degree of coverage.

Poor spacing and/or bad location selection can result in a PA/Paging/A-V system that provides, at best, marginal, or at worst, no useful information to listeners !

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B. Basic Speaker Characteristics1. Most voice announcing sound systems use two basic

speaker types – coaxial cone and horn.

2. Speakers are considered to be transducers, in that they convert electrical energy into mechanical energy that results in the generation of sound.

3. Most typical coaxial cone and horn speakers utilize electromagnetic drivers to transform the electrical energy input to a mechanical energy output – they can be considered to be a specialized form of linear electric motor.

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B. Basic Speaker Characteristics (cont.)

Typical coaxial cone speaker components:

1. Frame – provides mechanical rigidity and mounting for all components.

2. Voice coil – performs the conversion from electrical energy to magnetic energy.

3. Pole piece – of magnetic material, is attracted and repelled by magnetic fields generated in the voice coil.

4. Diaphragm – usually of paper or plastic, the mechanical movement of this item by the pole piece produces sound.

5. Spider – connects the diaphragm to the frame and to the pole piece for stability while still allowing movement required to produce sound.

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Typical coaxial cone speaker

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Typical horn speaker components:

1. Frame – provides mechanical rigidity and mounting for all components.

2. Voice coil – performs the conversion from electrical energy to magnetic energy.

3. Pole piece – of magnetic material, is attracted and repelled by magnetic fields generated in the voice coil.

4. Diaphragm – usually of paper or plastic, the mechanical movement of this item by the pole piece produces sound.

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Typical horn speaker components (continued):

5. Spider – connects the diaphragm to the frame and the horn and to the pole piece for stability while still allowing movement required to produce sound.

6. Horn – usually a double or in some cases a triple folded arrangement, aims and focuses the sound in an arranged directed pattern.

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Typical Horn Speaker

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The following acoustical parameters are important considerations in speaker selection and spacing/location:

1. Audio frequency response – measured in Hertz.

2. Sensitivity – measured in decibels (dB) Sound Pressure Level (SPL).

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BACKGROUND INFORMATION ON SPL:

a. SPL can range from 0 dB (threshold of hearing for a typical person) to 120 dB (threshold of pain for a typical person).

b. A subway train entering a station typically can generate 90 dB SPL measured at 20 feet.

c. An average person’s voice at a conversational level generates 70 dB SPL measured at 1 foot.

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BACKGROUND INFORMATION ON SPL (continued):

d. SPL represents sound energy intensity – commonly referred to as loudness.

e. An increase of 10 dB SPL is perceived by a typical listener as doubling the volume of the sound.

f. SPL is an acoustical/mechanical measurement, NOT an electrical measurement !

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3. Dispersion angle – measured in degrees.

For the purposes of this presentation, the following parameters for a typical ceiling mounted 8-inch coaxial speaker will be used:

• Frequency response – 60 Hz – 16 kHz. • Sensitivity – 97 dB/1 watt/1 meter SPL. • Dispersion angle – 50 degrees off axis.

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4. Frequency Response – range of audio frequencies which the speaker can faithfully reproduce.

• For successful reproduction of speech in a PA/Paging system, the accepted minimum frequency response of all components within the system is typically 350 Hz – 5 kHz.

• For A-V systems, the accepted minimum frequency response of all components within the system can be as broad as 20 Hz – 20 kHz.

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5. Sensitivity – the on-axis (directly in front of or below the speaker) loudness produced by the speaker in dB SPL measured at a specific distance (usually 1 meter) with an specified electrical power input (usually 1 watt).

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6. Dispersion Angle – this is the angular value within which the SPL is not more than 6 dB below the on-axis level (the Sensitivity) for the speaker’s overall frequency response or a specific frequency specified by the speaker manufacturer.

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C. Acoustic ConsiderationsAcoustic considerations for speaker spacing/location include:

1. Space dimensions and configuration (length, width, height, circular, rectangular, etc.).

2. Ambient noise (ranges from low as in an office environment to high in an industrial environment).

3. Surface characteristics (reflective – concrete, ceramic tile and similar surfaces or absorptive –carpet, fiber ceiling tile and similar surfaces)

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C. Acoustic Considerations (cont.)

For the purposes of this presentation, the following parameters will be used:

1. Space dimensions (includes length, width, and ceiling height) and configuration (rectangular).

2. Ambient noise (medium to high)

3. Surface characteristics (reflective)

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D. Determining Coverage Areas

1. Obtain space measurements and prepare plan and elevation views.

2. Note existing surfaces and finishes.

3. Identify areas not to be covered.

4. Identify special conditions (e.g. open archways to stairwells or other spaces; abrupt changes in dimensions, such as ceiling heights).

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D. Determining Coverage Areas (cont.)

5. Calculate the base coverage area for an individual speaker based on the manufacturer’s specifications.

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1 meter

97 dB SPL

50 degree dispersion angle

1 watt input

ELEVATION VIEW

PLAN VIEW

Radius (r) = tan50x1 = 1.1918 meters

SPEAKER


6. Once the speaker’s base coverage area has been determined, the next step is to include the following measurements into the calculations to determine the location specific coverage area:

a. Distance from speaker to listener ear height.

b. Desired SPL at listener ear height.

7. For the purposes of this presentation, the following parameters will be used:

a. 12-foot ceiling height from the floor and 5-foot listener ear height.

b. Minimum of 75 dB SPL and maximum of 95 dB SPL at listener ear height.

NOTE: The level of 95 dB SPL at listener ear height is typically the maximum allowable to avoid the potential for hearing damage !

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D. Determining Coverage Areas (cont.)8. Items to consider:

a. If the reference distance for the initial SPL calculation is doubled, the SPL will decrease by 6 dB. (e.g. base coverage area SPL at 1 meter is 97 dB at 1 watt, at 2 meters the SPL will be 91 dB at 1 watt.).

• This principle is also known as the Inverse Square Law.

b. If the reference input electrical power for the initial SPL calculation is doubled, the SPL will increase by 3 dB. (e.g. base coverage area SPL at 1 meter is 97 dB at 1 watt, at 1 meter the SPL will be 100 dB at 2 watts.)

c. For this example, the measured ambient noise at within the area to be covered ranges from 55 to 60 dB SPL.

d. The SPL at listener ear height should be 15-20 dB above the ambient noise level.

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9. Calculate the location specific coverage area for an individual speaker using data from items 1-3.

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91 dB SPL

50 degree dispersion angle

1 watt input

ELEVATION VIEW

2 meters(6.6 ft)

PLAN VIEW

Radius (r) = tan50x2 = 2.3836 meters(7.8 ft)

SPEAKER


10. Review the calculations:

a. Do the results provide the necessary SPL level at listener ear height ?

b. If yes, the next step is to determine the speaker layout within the space.

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c. If no, there are additional steps which must be taken before proceeding with the layout:

(1) Increase or decrease the height of the speaker above the floor.

(2) Increase or decrease the input electrical power to the speaker.

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(3) Select a different speaker – usually a speaker with a smaller dispersion angle can provide a greater SPL output at 1 meter than one with a wide dispersion angle.

(4) In this case, the calculations must be redone, before proceeding with the speaker layout !

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E. Speaker Layouts

Once the final coverage area for an individual speaker has been determined, the next step is to evaluate and select a speaker layout methodology.

Two basic patterns exist:

1. Square

2. Hexagonal

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E. Speaker Layouts (cont.)

1. Square pattern

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2. Hexagonal pattern

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• The choice of pattern depends on the best fit between the space dimensions and the speakers.

• Also, the pattern orientation can be rotated as needed to fit the shape of the space.

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Three basic spacing methodologies exist:

1. Edge-to-edge

2. Minimum overlap

3. Edge-to-center

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1. Edge-to-Edge Spacing

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SPEAKER SPEAKER


• The spacing distance is equal to 2r (r = radius of the coverage area).

• Requires the least number of speakers to cover a space.

• It will leave some gaps in coverage.

• It is not recommended for locations with poor acoustics or significant background noise.

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2. Minimum Overlap Spacing

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SPEAKER SPEAKER


• The spacing distance is equal to r times 1.414 for speakers utilizing a square pattern and r times 1.732 for speakers utilizing a hexagonal pattern.

• Requires additional quantity of speakers than the edge-to-edge spacing methodology.

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• It will cover gaps in the coverage area (but may not be sufficient in many cases).

• It may still leave areas where announcements are not audible or intelligible in areas with poor acoustics or significant background noise.

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3. Edge-to-Center Spacing

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SPEAKER SPEAKER


• The spacing distance is equal to r.

• Requires additional quantity of speakers than both the minimum overlap and edge-to edge spacing methodologies.

• It utilizes the highest speaker density commonly used for PA/Paging systems.

• It is the best methodology for areas with poor acoustics or significant background noise.

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F. Electrical ConsiderationsOnce the speaker spacing and location issues have been addressed in the design, the next step is determining the electrical requirements and constraints. 1. For systems using a VoIP/Ethernet-based

distribution methodology, these factors can include:

• Horizontal cabling length limits (100 meters/295 feet) per the TIA standards.

• Powering availability (Power over Ethernet limits and/or availability of local 120VAC power).

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F. Electrical Considerations (cont.)2. For systems using a constant voltage

distribution methodology, these factors can include:

• Total audio power requirements.

• Audio circuit configuration.

• Audio circuit sizing.

• Amplifier loading.

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F. Electrical Considerations (cont.)

Audio Power Requirement Determination

a. These systems utilize nominal voltage level audio output circuits from audio power amplifiers –typically this voltage is 25 or 70.7 volts, but in some instances, this voltage could be 100 volts or higher.

b. The speakers are wired in parallel to the audio output circuits via multi-tap matching transformers at each speaker location.

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c. The matching transformers are used to match the impedance of the speaker voice coil (typically 8 ohms) to the high impedance of the constant voltage audio output circuits of the amplifiers and allow, through the multi-taps on the primary of the transformer, selection of the power in watts to be provided to the speaker.

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d. Matching transformers can be purchased with taps as low as ¼ watt up to values as high as 15 watts and with various levels in between.

e. The selected transformer must always match the speaker voice coil impedance and must not allow the power to exceed the speaker manufacturer’s maximum, otherwise damage to the system may result !

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f. Calculating the audio power required first involves obtaining all of the individual speaker power requirements based on the matching transformer tap settings and adding them up (e.g. 25 speakers each tapped at 1 watt equals 25 watts).

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Audio Circuit Configuration

a. Determine the number of circuits required to connect the speakers to the amplifiers.

(1) Zoning requirements, separation of spaces within the same zone, and raceway configuration are just three of the factors to be considered in determining the number of circuits required.

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(2) Assign speakers to each circuit. This is best done by utilizing one-line and/or riser diagrams with each speaker uniquely identified to its location on the plan and elevation drawings.

(3) Ensure that circuit connections are polarized (+/-) correctly !

• Incorrect polarity can create an “out of phase” condition where speaker outputs can interfere with each other, reducing levels and/or creating distortion.

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F. Electrical Considerations (cont.)Audio Circuit Sizing

a. Usually, a minimum of #16 AWG wire is specified for audio output circuits between the amplifier outputs and the speakers.

b. In a 70.7 volt PA system, #16 AWG wire is limited to a maximum safe current of 6 amperes, resulting in a maximum power capacity of 420 watts at a maximum distance of 90 feet, assuming a 0.5 dB (12.5 %) line loss.

c. In some cases, wire size may have to be increased to meet the power and/or distance limitations within a circuit.

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WIRE SIZE

OHMS PER

1000’ LOOP (R)

MAX SAFE AMPS

(I)

MAX SAFE

POWER (W)

MAX LENGTH (FT) AT

10W

MAX LENGTH (FT) AT

15W

MAX LENGTH (FT) AT

20W

MAX LENGTH (FT) AT

30W

MAX LENGTH (FT) AT

40W

MAX LENGTH (FT) AT

60W

MAX LENGTH (FT) AT 100W

# 16 8.0 6 420 3600 2400 1800 1200 900 600 370

# 14 5.2 15 1000 5600 3800 2800 1900 1400 950 560

# 12 3.2 20 1400 9100 6200 4600 3100 2300 1600 910

# 10 2.0 25 1750 9900 7300 5000 3700 2500 1450

# 8 1.28 35 2450 7800 5700 3900 2280

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Length of Two-Wire 70.7-volt Line Delivering Various Values of Power at 0.5 db (12.5 %) Loss

(courtesy Altec Lansing)


Amplifier Loading

Manufacturer recommendations/best practices:

• The connected load should not exceed 80 percent of the amplifier power rating – for a 100 watt rated amplifier, the total connected load should not exceed 80 watts, including circuit losses.

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F. Electrical Considerations (cont.)CODE AND AHJ CONSIDERATIONS

1. In all cases, system wiring must comply with all applicable codes and standards !

2. In the US, NFPA 70, the National Electrical Code (NEC); Article 640 (Audio Signal Processing, Amplification, and Reproduction Equipment), contains the primary governing requirements for PA/Paging/A-V systems to be enforced by the AHJ.

3. Article 640 also contains references to Article 725 (Class 1, Class 2 and Class 3 Remote-Control, Signaling and Power-Limited Circuits).

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F. Electrical Considerations (cont.)CODE AND AHJ CONSIDERATIONS (CONT.)

Question…Are these NEC Articles relevant to your system implementation, regardless of whether it utilizes a VoIP/Ethernet architecture or a constant voltage distribution methodology ?Answer …IT DEPENDS ON THE AHJ’s INTERPRETATION !!

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F. Electrical Considerations (cont.)CODE AND AHJ CONSIDERATIONS (CONT.)Reasons…

• The NEC currently does not differentiate between the two types of systems.

• Article 640 does allow the use of Class 2 or Class 3 power-limited wiring as defined in Article 725, provided the amplifier assemblies are listed and marked for use with Class 2 or Class 3 power-limited wiring – This is typical for amplifier assemblies having output power no greater than 100 watts, in order to meet the supplied power limits defined in Article 725.

• Article 725 specifically does NOT allow audio circuits using Class 2 or Class 3 power-limited wiring to occupy the same cable or raceway as other Class 2 or Class 3 power-limited circuits.

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CODE AND AHJ CONSIDERATIONS (CONT.)Reasons (cont.)…• Many AHJ’s also prohibit audio circuits using Class 2 or

Class 3 power-limited wiring from occupying the same cable or raceway with Communications circuits as defined in Article 800.

Result…• These systems may be required to utilize cabling and

pathways that are partially or totally independent of other ICT cabling and infrastructure within a premise !

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Useful References and Organizations

• Sound System Engineering by Don and Carolyn Davis, 1984, Howard W. Sams

• Handbook for Sound Engineers – The New Audio Cyclopedia by Glen Ballou, Editor, 1988, Howard W. Sams

• Commercial Sound Guidelines, Dukane Communications Systems Document # 427-19-00006(01)

• Speaker Guide, TOA Electronics Document # L-SPRKGUIDE

• National Systems Contractors Association (NSCA)– www.nsca.org

• InfoComm– www.infocomm.org

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Questions ?Contact Information:

Phone:401 - 439 – 0335 (cell)

E-Mail:[email protected]@parsons.com

THANK YOU !

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Documents

PUBLIC ADDRESS/PAGING SYSTEM/AUDIO-VISUAL · PDF fileSYSTEM/AUDIO-VISUAL SPEAKER PLACEMENT AND WIRING GUIDELINES ... “Successful reinforcement ... • For successful reproduction