Noise coming from outside of the house is really annoying especially when it is already late at night. You can use various ways depending on how high is the level of sound that is entering through the door. Try it by closing the door and listen to the sound in order to determine the correct soundproofing technique. But for any reason that you need some soundproofing for your door below are some things that you can try:
Replace the door
A typical door is usually a hollow core door. Sound easily passes through with this kind of door and you must replace it with a medium density fiberboard door or MDF door. You don't have to worry since these kinds of doors are not expensive and it has a good sound blocking quality. You can also use a solid wood exterior door or fiber filled doors. If you are renting a place and your landlord won't allow you to replace the door due to uniformity reasons, ask if you can change the door while you're renting the place and just place the original door back when you move out. You can also add another door which is opening the other way if you're landlord doesn't agree with the idea of changing the door.
Door sweep seal
Door sweep seals are available from hardware stores but first you need to measure the width of the door so you could purchase one which has the right length. Transom seal or door sweep are not only good in blocking sounds but also keeps the temperature inside. You can also use this if there is a huge gap underneath your door and remember to choose one that has a rubber flap. It is easy to install since it comes with proper screws and you only need a screwdriver to mount it at the lower back side of the door. With this the bottom part of the door will be sealed but you can still conveniently open and close the door.
Sound proof doormat
You can also try some sound insulation available in the market. First you have to measure the surface area of the door and then you can attach a sound proof doormat at the back of you door. It must also cover the seam of the door to seal it and you can attach it with adhesive or if you want to remove it from time to time you may opt for one with adhesive back.
Hanging an acoustic curtain to your door also helps prevent sound from coming through. Try screwing curtain rod brackets and you can hang the acoustical curtain from there. For maximum results you can also try attaching another acoustical curtain at the other side of the door. Acoustic curtains also works for sliding doors and open doorways. You can do so by placing on both sides of the doorway, though it can't block the sound completely it can still help reduce the sound.
Sound commonly leaks at the perimeter of the door but you can treat it using foam gaskets. These are ordinary thermal sealing gasket foam strips you can find in a hardware store. You can attach these at the door jamb to close the gap but you must still provide some space to be able to open and close the door.