Question: For a [20W amp](http://www.adafruit.com/datasheets/MAX9744.pdf), what are the determining characters, apart from speaker Impedance and Power rating, that affects a selection.

Example Specs -  

![enter image description here](https://i.stack.imgur.com/redtT.jpg)  

(source - <http://www.madisoundspeakerstore.com/approx-2-fullrange/aurasound-nsw2-326-8a-120-2-full-range-with-solder-pads/>)

What are these  

- Power capacity(RMS)  

- Power capacity(peak)  

- Maximum Excursion  

- Resonant Frequency  

- Total, Electrical, Mechanical Q  

- Xmax  

- Compliance 

Any other notes. 

Also, can I use this speaker.

Thanks.

Answer: 
Short answer: yes you could use this speaker, but maybe you shouldn't.

**Power Capacity**

Simply put, RMS power capacity = safe power capacity.

Peak power ratings are the marketing departments' favorite, because peak power of even the smallest speakers can be up to 1000 Watts!!! (For one microsecond. Right before being engulfed in flames.) If you do not know how peak power rating is measured, it is a useless number.

*"But wait, this speaker is 15W RMS, and my amp is 20W..."*

Trust me, your amp will *never* output 20 Watts under usable circumstances. If you were to turn it all up, and somehow protection wouldn't kick in, your music will sound as if you're farting trough a megaphone. It sounds horrible, even if you're into dubstep.

All in all, based purely on power rating this speakers and amp will play nice together. Don't turn your amp up all the way, and it will use those 'spare watts' to give you a nice, dynamic output.

**Maximum Excursion**

This is how far the speaker cone (the moving part) can travel before irreversible damage occurs. It's what makes people upload movies of their subwoofer to Youtube. For your design, you should not worry about this.

**Resonant Frequency**

If you keep pushing a decent sized tree at just the right speed, and you will notice that your relatively low force is able to move the tree remarkably easy. This is because you keep adding force to the tree at it's resonance frequency.

The same goes for speakers. Now, it's no problem at all if the resonant frequency appears in output signal every once in a while, but it's not the best idea to constantly use a speaker near or at its resonant frequency. Simplified, the speaker could shake 'out of control', sounding bad and even damaging itself.

**Frequency Range**

As Naz already pointed out, the frequency range of this speakers is limited. Normally, it is described as the range between the two point where the sensitivity of the speaker (how well it can reproduce the frequencies) drops a relative 3dB. Here, - again, marketing - it is measured as the range between the 10dB drops in sensitivity.

So, *could* you use this speakers? Yes. *Should* you use this speakers? That is up to you. My advice is to look around for other (slightly bigger) loudspeakers, as this will probably improve the frequency range, especially towards the lower end. The magic words are 'fullrange driver' here. Do not expect earth rattling bass from your amplifier, however, when paired with the right speakers you will be able to produce a very nice sound.

Good luck!
Wow!. Never seen such detailed description of a speaker.
Power capacity: I would say that's the nominal/normal, long term power of the speaker. That is, how much power you provide to a speaker.
Peak power capacity would be the maximum short term power/spike that the speaker can handle before it burns.
Maximum excursion: maximum amplitude the speaker's diaphragm can travel (in/out).
Resonant frequency: what ever... check out this [link](http://www.eminence.com/support/understanding-loudspeaker-data/). 

You probably can use this speaker. Depending on what you are going to connect it to: output impedance and power of the amplifier - that's the minimum you need to consider. See the frequency range 250-15k, that's the mid range. It will miss deep low frequencies and high frequencies. So, this would better go in combination with other speakers to cover a broader range of hearing band. Good systems range 20-25kHz, but you can not get a single speaker that will cover the entire range. Therefore, you can usually see two-three speaker system.