Couple of things you could try:
Display a visible timer for tasks, eg online stopwatch or similar, and move on to the next task when time is up. Put focus back onto the students - "I've given you ten minutes to do activity x, if you've been talking instead of working you are now going to fall behind as we move on."
Get a chart that you can display on the board and divide into 2 columns - one smiley face and one sad face. Every time the majority of the class is working or quiet or asks a sensible question put a mark in the happy column, every time you countdown from 5 (without shouting) but they continue to talk put a mark in the sad column. I've done this before and it works really well as even if the students aren't listening/are too noisy to hear they will see what you're doing and others will shush them too. You can decide on some rewards/punishments at the end of each week, depending on whether there are more marks in the happy or sad column.
If attending detentions is a problem - do you ever teach them before break or lunch? If so just keep them back that lesson every week. Again, I've done this and keeping the whole class for even 5 mins in complete silence seems like a very long time for the students! Do not let them leave until there has been absolute silence for however long you've decided - ie if one person talks the time starts again.
Note: you will inevitably get some kids complaining "why are we all being punished, only person x was talking!" The way I've dealt with this in the past is to explain that they must work together as a class to meet your expectations. You can point out that as the collective noise level is too much, it's impossible for you to pinpoint the key noise-makers. Therefore, until every student takes ownership and quietens down you have no choice but to enforce collective punishments. As the noise reduces over time you can then make a point of starting to let the "good kids" go while others are kept behind.
Hope this helps.