What do you mean after rise?
Take a 4 ohm speaker and put it on the ground. Hook up some measuring equipment and play a tone through it at a frequency not really near its resonant frequency and it should read around 4 ohms. Now, put it in a box and do the same thing. It won't read around 4 ohms; it will be slightly higher. Why? A speaker has what is called impedance: anything that resists movement is called impedance. A resistor has impedance, as do inductors and capacitors. Your speaker has 3 types of impedance: electrical impedance, aka, what the voice coil rating is (4 ohms maybe), mechanical impedance, aka, how the moving parts of the speaker limit its movement and finally the third which is called radiating impedance which is a function of how well the speaker can move due to the air (a small 2" speaker has a far lower radiating impedance than a large 21" subwoofer just as it's easier to wave a toothpick quickly than it is an oar). When you put a speaker in an enclosure, that changes the radiating impedance.
These changes are dependent on the size and tuning of the enclosure. Putting a speaker in an enclosure with a higher impedance than free air will cause the impedance to rise at certain frequencies. I'm trying to keep this as simple as possible, so if it doesn't make sense, please ask and I'll clarify. Think about it like this: it's easy to compress the air that you're sitting in right now with your hand, but try to compress a bag filled with air. Once that pressure increases, the impedance increases. Increase the size of the bag, and the impedance drops.
Please note that this is an extremely rough and not entirely scientifically nor mathematically accurate description, but it's just to help you understand what impedance rise means. I could give you a fleshed out response, but you wouldn't understand what I was talking about since you're just starting in audio //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
