Loudspeaker ramblings

I’ve never really thought too much about getting sound out into the world. Everything just…works. I have an old Samsung mp3 player, it plays music through speakers into my ear. I can adjust the volume easily.
Same for my beloved Droid phone. It outputs both music and voices.

Last summer I built something that ran off a 12 volt -12 amp-hour battery. The sound output was through an 8 ohm speaker. Not only was the sound not very loud, but the battery ran flat after 2 nights.
Oops.

I will take the opportunity for a do-over! I know- very exciting….SPEAKERS!!!! Amplifiers!!!! Noise!!!!
A little research turned up the following: speech is at 60 dB, vacuum cleaners at 70 dB and a chainsaw is 110 dB.
And over 192 dB can kill you. WTF????

For what I’m doing, I’m OK with vacuum cleaner loudness, so I would design for 90 dB to give some space above what I need.

In order to figure out my power calculation (so that I don’t end up with dead batteries this summer) I need to know what my power calculation in Watts is, as it relates to dB.
Looks like I need about 4 – 8 Watts of power to run my speaker, depending on the type of speaker.

A speaker isn’t completely a resistive load -the value of a speaker is measured in inductance, which varies with frequency and resistance. The impedance of the speaker is related to the frequency. The graph below illustrates an 8 ohm speaker as it relates to frequency. I think this graph is interesting. It clearly shows that 8 ohms is an average and the impedance of the “8 ohm” speaker is all over the map.