Non-permanant window treatments

I'm currently using a spare bedroom as a practice space for my band. (3 peice funk band, bass, guitar, and drums) All of my neigbors are fine with this except the one directly to the side of the room. I live in a brick, non-insulated home.. I've found that the biggest area that the sound is transfering is the two windows. I'm not interested in ripping the windows out or replacing them just yet, but there is a large sill area where I could build up a panel with maybe gaskets on the outside to sit into the window frame. and maybe some sort of fastening system on the wall or frame How much could I expect to lose db wise this way? would it be worth my time? any design ideas. I'd preferably like to keep the cost as minimum as possible because I'll need to buy tools aswell, but my neighbor has already agreed to contribute "a few hundred" which I assume is in the 200ish dollar range so I'd say my materials budget is around 250 per window

Aloha and welcome to the forum, Try using the search function above for "window plug" or "windows." Maybe check the "stickies" sections as well as there is something on the forum that covered this awhile ago. Hope this helps, Aloha 8)

Thanks much. I'd found knightfly's diagram post this evening, I've been actively search but still haven't found any sort of talk about what kind of db ballpark it would be for dampening the sound. I figure I could built it reasonably enough price wise, but if I'm only going to gain 3db of volume reduction it might not be worth the expense because it wouldn't improve my neighbor situation much. Also, I apologize for not searching more thoroughly. It's become a very time-dependant thing, so I figured I'd go ahead and post while continuing to search

there's a couple of factors to consider - what is the transmission loss of the walls? versus the windows? if you have the windows set in solid brick walls, then you want to try to match the TL and resonant frequencies of that brick wall. which probably means you either need a frame to place a sealed brick "panel" orver the window, or something with enough mass to match it. maybe looking at how studio doors are constructed with dense wood, lead, drywall, MLV, etc... to achieve 40db TL at lower frequencies would be what you need to construct. smaller but similar.