I posted this before on the issue of wattage and dBs.
Sound level is measured in terms of decibels (dB). Volume is referred to as sound pressure level (SPL). In tests, the average person can detect a 3dB change in volume. 3dB is just enough difference that you can barely tell the volume has changed. A 10 dB change in SPL is perceived as twice as loud. With that in mind we can move on to what is required in terms of power (measured in watts) for an amp to create a 3dB change.
Amplifier Output - It takes 2 times the power from an amp to change the volume 3dB. In other words if an amp is producing 1 watt of power it needs to increase to 2 watts of power to make a 3dB change. This is a ratio of 2:1. By the same token if the amp is producing 50 watts of power it will need to increase to 100 watts to produce a 3dB change. 100 watts would take 200 watts for that same change.
How much power would it take to Increase the SPL 10dB? It will take 10 times the power to increase the SPL by 10dB! In other words, if your amp was producing 50 watts of power you would need to increase it to 500 watts to achieve a 10dB increase. As you can see it takes a lot of power to get a small increase in volume.
Speaker surface area is one factor that effects volume. This concept is not as simple as it first appears. If we double the surface area we will increase the SPL by 3dB. Here is an example. If you have a 50 watt amp with one 12" speaker and you add another 12" speaker you will get the magic 3dB increase. You would have the same SPL as a 100 watt amp with one
12" speaker. To get the next 3dB increase we need to double the surface area again so we would need four 12" speakers. Having 4 speakers will give us a 6dB increase in SPL compared to 1 speaker. Sounds like the same system as the power ratio above doesn't it. Here is were the complex part comes in. If we double the 4 speakers to 8 speakers you would think that there would be a 9dB increase in SPL compared to one speaker, right? Nope. What we get is only a 6dB increase compared to one speaker. Huh? We have now introduced a new factor to this equation...Phase Cancellation. In short the distance between the speakers causes the sound to reach your ears, from some of the speakers, at a different time . This has the effect of canceling some of the sound. So...More speakers are better up to a point.
Speaker efficiency also plays a role in the optimizing of SPL. How well a speaker can convert electrical energy into mechanical energy is called speaker efficiency. If you look at the specification sheet of a speaker it will contain information of this type. It is described as, "Output measured at one watt - at one meter", and then a dB value is given. What this means is a one watt signal is played through the speaker and then its SPL is measured at a distance from the speaker of one meter. Because we know about the 3dB increase concept described above we can compare the SPL of different speakers to determine which one can produce the most SPL. The larger the SPL number the more efficient and louder the speaker will be. It will usually be less expensive to get a more efficient speaker(s), than a larger power amp, to produce the same or greater SPL.
