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Keep in mind that a speaker's loudness level dies down much more quickly outdoors than it does indoors, following the inverse square law. Say you have your speakers set such that the level at your position on the deck, x meters away, is 75dB (quite loud). A person twice as far away from the speakers as you, 2x, would hear it as 69dB, about as loud as a busy street. Double that distance again to 4x, and you have 62dB, which is about the level of a normal face-to-face conversation.



This would certainly be true if the outdoor speakers were a spherical radiator. They're not though. At best, they're a hemispherical radiator for the majority of the frequency range. This would make them twice as directive as a spherical radiator resulting in a 3dB drop for every doubling in distance.

The most important thing in these outdoor installations is the effect of ambient noise. Say you have ambient noise of 70dB. And you want your noise margin to be at least 10 dB. We know the speakers are 88dB/W/3 feet efficient. The distance from your fence to the listening position is 30 feet. To ensure that the signal you hear is not submerged in noise, you'd have to get the SPL at 3 feet away to be around 90 dB (10log(30/3)+70+10). Contrast that to mounting them 6 feet away from you. You would only need 83dB for that or less than a quarter of the power. Mounting them by the deck would result in less sound power being radiated into the neighbors' yards.

If you mount them at the fence, you'll have wind and temperature gradients to contend with that lead to refraction effects which could result in sound being directed upwards. Then you'd need even more power. They would also not be protected.

So bottom line, mounting them at the fence is not a good idea.