qgis - Masking/removing polygons from shapefile

I have a shapefile containing the road traffic noise of a city.

The shapefile has 5 columns, where 'noise_in' (noise interval) ranging from 1-5 is included.

I want to remove/mask the road noise from a specific area.

My problem is that the noise intervals covers more than just this area, e.g. if I remove a polygon from the file, covering the area of interest, it will also remove the noise from an adjacent area, which I'm not interested in.

The attribute table looks like this:

The road noise for the area looks like this:

I would like to remove all the road noise from this area, meaning all 5 noise intervals. Unfortunately, I have no available date for recalculating the road noise for the area.

I don't have a polygon for each area.

Here's an example of "Identify Features":

The red highlighted areas are two different polygons covering large areas of road noise, and I'm only interested in the big blue "vein" in the middle.

I'm using QGIS 1.8 Lisboa for MAC

Thanks in advance

Answer

It appears that you have a shapefile that has overlapping polygons. I'd save each by the 5 road noise categories and then clip out the noise categories you wish to remove.