Recommended amplifier power is exactly that, an amplification range measured in watts that’s best suited to the speaker. This doesn’t mean that amplifiers outside of the recommended range won’t work; they just won’t work well. In other words, they’re not recommended.
For example, an 100W amplifier will work with a 20-60W bookshelf speaker. However, there’s so much extra power that even a modest increase in volume could result in blowing your speaker drivers (not good). Inversely, a 15W amp will also work with the same pair of speakers – just don’t expect a great deal of volume let alone detail.
So, it’s important to match the right amp power with the right recommended amplifier power range. And that’s as easy as scrolling to the bottom of each speaker’s product page and checking the specifications.
Sensitivity translates to speaker volume; the higher the sensitivity, the louder the speaker. This is an equation of converting amp power, watts, (W) into speaker volume, decibels (dB). For example, with a sensitivity rating of 88dB, 1W of amp power is required to produce 88dB of volume at 1m from the speaker.
Now, for every 3dB of increased volume, you need to double the amp power. So, for 91dB you need 2W, for 94dB – 4W, 97dB – 8W, 100dB – 16W, 103dB – 24W, 106dB – 48W, so on and so forth. You get the point. It’s easy to see how going loud requires watts, quickly.
So, while you might have the recommended amp power to match the speaker, sensitivity will still determine just how loud you can go. And as a rule of thumb, 84dB is considered poor, 88dB is considered good, 92dB and above is considered really good.
From this baseline, you can determine for yourself how loud you’re likely to go and the sensitivity required to get you there.
Increased cabinet volume creates… you guessed it, increased sound volume. Larger chassis also allow for larger drivers, which generate better projection to give you a more robust audio experience. So, in the end you have to weigh up a few things: does my listening room demand some extra volume; would I like some extra volume; can I fit the cabinets in my space; and can I afford to go bigger? After all, more speaker more money.
Now, when you accomodate for cabinet size, power handling, and sensitivity, you’re pretty well covered. These are the essentials when considering what kind of bookshelf speaker will work for you. But, if you want to dive just a little deeper, you might consider port design, driver configuration, and whether you want to add speaker stands. And all of that is covered in Part II.