An example may help:
Customer Question: “How far will your loudspeaker throw”?
Engineer Answer (with a question unfortunately): “What SPL do you require”?
The reason the second question is asked is to determine the performance requirements for the venue.
Customer: “This is a softball field and we have measured crowd sound pressure levels at 90 dB, and the crowd is 75 feet from the location of the loudspeaker mounting position.”
Engineer: “Great! The loudspeaker is rated at 1 watt at 1 meter to produce a sound pressure level of 99 dB, and it has AES rated power handling of 1,000 watts continuous. So, 75 feet is 22.8 meters, and that means that at 1 watt of input, the SPL will be reduced by 27.1 dB (spherical spreading loss only; i.e., a 6 dB loss for every doubling of distance, and no atmospheric losses are assumed at this distance, although there may be some based on actual wind and humidity conditions).
Now, if we take the 1 watt at 1 meter level of 99 dB and reduce it by the distance of 75 feet, we see that this loudspeaker will produce a sound pressure level of 71.9 dB with a 1 watt input (99 dB minus 27.1 dB).
The customer has specified the crowd level of 90 dBA (this is loud, but typical for many sporting venues) and since we would like to have 3 dB or more level above the crowd in order to make our announcement, play music, or make other program material intelligible, we calculate that we will need approximately 93 dB of SPL. This will require a power input to the loudspeaker of 132 watts.
The loudspeaker will “throw” 75 feet, but we only know this because we know what SPL we need to achieve at this distance (without knowing the necessary SPL, we have no way of answering the question).
Customer Question: “But we also have a baseball field where we have to mount the loudspeaker on the scoreboard in center field and we need to have good intelligibility at the back of the stands behind home plate…So, will this loudspeaker ‘throw’ 450 feet?”
Engineer Answer: “Will the crowd be talking at normal speech levels or screaming?”
Customer: “What’s the difference?”
Engineer: “Well, let’s see…Assuming spherical spreading only (6 dB loss for every doubling of distance), the SPL at 450 feet (137 meters) will be reduced by almost 43 dB (42.7 dB to be exact), so the SPL at 1 watt of power input to the loudspeaker will be only 56.3 dB.
“Now let’s add some power and see what happens. If we’re trying to get 3 dB above average speech levels, we will need about 63 dB since normal “talking” is about 60 dB. That means we would need 4.7 watts of power…No problem, the loudspeaker will “throw” 450 feet without any issues.”