What are these holes for in this driver/transcuer (AKG K81DJ) ?

I am in the process of modding my AKG K81DJ and noticed that the back of the driver/transducer has a set of holes. Sorry couldn't take a better picture the holes but I am asking about the 4 spots that are on the metal back of the driver. One of them was completely open and the other three were covered with tape and a dab of some type of glue. I peeled the tape slightly on one of the sealed ones to confirm it was indeed a similar type of hole, and it is. What I am more curious about is that why are some hole sealed and just one of them not ? What would happen if I close that one too ? Or open all 4 of them ?

Anyone know ?

The main mod is to apply some rope caulk on the inside of the plastic cases to prevent unwanted vibrations that color the sound. There is already a "blu tac" mod for this headphone. I am planning on taking this further by applying a layer of fiberfill to damp backwave reflections. This idea came to mind because the driver as well as the mounting plate has semi transparent holes and orifices that would let those backwave reflections pass through. Actually I did finish these mods last night and the improvements in sound were obvious. The bass is more tighter and the mid bass resonances are gone.

I also removed the removable foam pad from inside the ear cup and that has resulted into more distinct highs.

Anyone else have any takes on the driver holes I am asking about ? I guess the only way to find out is to try out and see what happens!

Quote:

Originally Posted by civilmonkey 

All holes open = muddy / more bass.  All holes closed = not as much bass.  I played around with it and found that

 

a) they are turned pretty well (and best to my ears) from the factory

b) although there was a difference, there wasn't that must of a difference.

Thats the kind of information I was looking for. I knew there had to be someone other than me around here who's played around with that.

However, my experience was not so subtle. What I experienced was that opening and closing just one hole had a significant impact on the bass, and also on the mid bass and midrange performance.

When all holes were closed the bass dropped but it was kind of a clean 'just-enough' type of bass. An added bonus of this was that the low midrange cleaned up a lot, probably because the extra bass was not creeping up on the low midrange any more. Vocals were clear and not overly heavy and chesty as they used to be. I would go as far as saying it was no longer a dark sounding headphone!

When just one hole was opened up, it changed things. The extra oomph in the bass came back but at a price - it was that boomy muddled bass and it messed up the low mids too.

So if you are a basshead and like warm/dark sound then you dont need to mess with the holes at all. But if the heady bass gets to you and you'd rather give up the bass quantity for quality, and for clear vocals then seal all those holes up!