Wheelock MX-8 Mixer Specifications download pdf (Page 13)

Spacing and forward projection

Experience has shown that the most effective side to side spacing of a typical horn is about 40 feet. This is determined

bythearerateanddispersionpattern,whicharexedandconstant.

Thefront-to-rearspacingrequiresalittlemorethought,becauseweareactuallydealingwiththeforwardprojectionof

thehorn.Thisprojectionisdeterminedbytwovariables:

 •TheSPL(soundpressurelevel)thatthehorniscapableofataspeciedwattage

• The ambient noise that this SPL has to compete with in order for the audio to be heard

Remember, a page has to be at least 6 dB above the ambient noise level in order to be heard effectively. Generally,

forhornssuchasWheelock’sSA-H5,SA-H15(self-amplied)orST-H15B,ST-H30(70volt)tobeeffective,front-to-rear

spacing should be as follows:

 •128feetforquiettomoderatelocations(50-74dBambient)

 •64feetfornoisylocations(75-85dBambient)

 •32feetforverynoisylocations(86-96dBambient)

Thefollowingdiscussionshouldmakethisclear.Let’ssayyouarebiddingajobandthisparticularsituationcallsfor

paging in a noisy production area. You take out your trusty SPL meter and get readings of 85 dB throughout the area.

Now,weknowthatforthepagetobeheard,weneedatleast6dBmorethanambient,or91dB.Youhavealready

decided upon a 70 volt system, and you will be using Wheelock’s ST-H15B horn. Wheelock provides the following per-

formance data for the horn:

ST-H15B Performance

Feet on

Axis

Sound Pressure Level (dB @ 1K Hz)

Full Power

(15W)

1/2 Power

(7.5W)

1/4 Power

(3.8W)

4 120 117 114

8 114 111 108

16 108 105 102

32 102 99 96

64 96 93 90

128 90 87 84

1 2 ... 8 9 10 11 12 13 14 15 16 17 18 ... 23 24

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Wheelock MX-8 Mixer Specifications Page 13