When most people consider a speaker to design into their assembly to get the highest output, most are only concerned with, how much space do I have for the speaker, and how much power do I have available. There are many more factors that come into play. A big factor is the sensitivity rating of the speaker. The sensitivity (output SPL) rating of a speaker gives you a rough idea of how loud the speaker will play with a specific amount of power at a specific distance.
Consider a speaker with this sensitivity rating:
85 dB / 1 watt / 1 meter
This speaker will produce sound at 85 dB, 1 meter away, when it is given an input power of 1 watt. Typically the input frequency is 1 kHz or an average of specific frequencies (e.g. 800Hz, 1.0KHz, 1.2KHz, 1.5KHz). Depending on the type of enclosure along with other factors the speaker may not produce 85 dB but it's still a useful spec for comparison with other speakers.
You have to double the input power to produce a 3 dB increase in sound output (assuming the speaker is not reaching its limits). Therefore we can produce a table for how loud the speaker will play with a specific starting power:
|Power in Watts||Volume in dB|
You can see how it starts to take a lot of power to make a speaker play very loud.
When you ask yourself how much power you need for your system you need to ask yourself how loud you want your system to play and plan accordingly. Going with higher power amps or more sensitive (efficient) speakers will make your system play louder, but you can push your speaker too hard and cause failures later.
Another consideration is the measuring distance. A speaker with the sensitivity rating of 85 dB / 1 watt / 1 meter, is the same as a speaker rated at 91 dB / 1 watt / 50 cm, and or a speaker rated at 105 dB / 1 watt / 10 cm. This is due to a 6 dB increase with each halving of the measured distance, and a 20 dB increase with decreasing the distance to 1/10 of the original. This gives us a table like this:
Keep in mind that this works in reverse as well. A speaker rated at 95 dB at 10 cm will measure 75 dB at 1 meter and a speaker with a rating of 100 dB at 1 meter will measure 106 dB at 50 cm or 84.44 dB at 6 meters.
Now that we have done the math, let’s look at where this leaves us.
If you need a measured sound pressure level of 80 dB at 3 meters, and you have 4 watts available from your amplifier, the needed sensitivity at 1 watt and measured at 1 meter would be:
80 dB / 4 watts / 3 meters -- 3 meters to 1 meter is a 9.54 dB increase to 89.54 dB
89.54 dB / 4 watts / 1 meter -- 4 watts to 2 watts is a 3 dB decrease to 86.54 dB
86.54 dB / 2 watts / 1 meter -- 2 watts to 1 watt is a 3 dB decrease to 83.54 dB
So, to get 80 dB at 4 watts measured at 3 meters requires a speaker with a sensitivity of 83.54 dB at 1 watt measured @ 1 meter.
So in the end the basic answer to the question of what sensitivity you need for your speaker is based on how loud you want your system to play, how much power you have available, and the required distance.