Wheelock MX-8 Mixer Specifications download pdf (Page 8)

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Distance and the 6 dB rule

Simply put, doubling the distance from the speaker means you lose 6 dB SPL, cut the distance in half, and you

gain 6 dB. This means that each time you double or half the distance you lose or gain 6 dB. In the above example,

anappliancedelivers90dB@64feetwith3.8wattsofappliedpower.Double the distance to 128 feet and the SPL

drops 6 dB to 84 dB. Doubling once again to 256 feet means 78 dB, and so forth. Of course, the reverse is also true:

Cutting the distances in half yields gains of 6 dB each time you do it.

The distance factor becomes especially important when laying out a system using horns. Ceiling speakers typically have

toprojectsoundonlyafewfeettothelistener’sears,butahornmayhavetoprojectsound30,60,100feetormore.

So it’s important to understand how a horn’s output is affected by distance and applied power. A cost effective, well

designed system balances amplier power, speaker placement and speaker efciency.

How loud?

OK, here’s the point of the entire last section: The paging volume should be at least 6dB over the background (am-

bient) noise level. This makes sense. The page level is competing with the background noise level and if the page

isn’t loud enough, no one can hear it!

Howdoweknowwhatthebackgroundnoiselevelis?Mostofces,lobbies,departmentstores,banksandrestaurants

arefairlyquiet(55-70dB)andcanbepredictablydealtwithusingceilingorwallspeakerstappedorsetat1wattorless.

Thisisusuallymorethanenoughpowertogeneratesufcientvolume.Noisierenvironmentsaregenerallyindustrial

areas or large expansive areas such as warehouses. The speakers usually chosen for these areas are horns because

theyaremoreefcient(louder)thanatypicalconeceilingspeaker.

The wise installer will want to know precise background noise levels in order to properly design the paging system to

overcomethesenoiselevels.Inthesesituations,therststepistomeasuretheambient,orbackgroundnoise.Don’t

guess — use a sound level meter! You can purchase an inexpensive, yet effective sound level meter and it is money

wellspentifyouwanttodesignapagingsystemthatworks.Byeliminatingtheguesswork,youwillbe“righton”with

yourlevelsandsubsequentselectionofequipment.Forbestresults,takeSPLreadingsduringpeaknoisehourssothe

systemcanbedesignedfor“worstcase”conditions.

Here are some examples of the most commonly encountered commercial background noise levels:

Once again, this gives us a starting point for our “6 dB over ambient” paging system goal.

Typical Noise Levels

Location dB

Machine Shop 90

Manufacturing(Noisy) 80

Printing Shop 80

Assembly Line 75

Supermarket 75

Transportation Waiting Room 75

Ofce(Noisy) 70

Shipping/Warehouse 70

Restaurant 70

Department Store 65

Bank(PublicArea) 65

Ofce(Quiet) 55

Ofce/DoctorsWaitingRoom 55

Hotel Lobby 55

Doctors Exam Room 55

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Wheelock MX-8 Mixer Specifications Page 8

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