Styrofoam maybe. Could probably pick up some at a craft store and trim it to the proper size.
First choice would be foam (packaging material but cut to shape). Second - crumpled up paper towel. Just remember it may come out if the volume is turned up too high. Third would be womans nylons.
BTW socks worked for me when I had Bic speakers 30 years ago.
I use a rolled up cotton terry wash cloth with good results. It still allows the cabinet to vent, but nowhere near as much.
If you want to, "tone it down a little"; Go to a fabric/upholstery shop and get some (loose-not "felt")bonded dacron(http://cgi.ebay.com/Upholstery-Bonded-Dacron-Quilting-Batting-Padding_W0QQitemZ350220762258QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item518ac8bc92&_trksid=p3286.c0.m14&_trkparms=65%3A12%7C66%3A2%7C39%3A1%7C72%3A1205%7C293%3A1%7C294%3A50). You can experiment with different densities, by rolling it tighter/more loosely, and damp the port to your tastes. Most other suggestions will turn your cabinet into an acoustic suspension design, and completely ruin the system designer's alignment. Have you tried lifting them off the floor and/or moving them away from any corners?
I use upholstery foam about 1 inch thick that can be rolled up and placed in the port. It works well.
I used expandable rubber test plugs available from the plumbing department in any hardware store to plug ports in my PSB Stratus Goldi speakers. They happened to be a perfect fit and made an airtight seal. They come in 1-1/2, 2, 3, 4" sizes and if they don't fit for you you can wrap an undersized plug with strips cut from bicycle inner tubes to get close, then turn the wing nuts to expand the plugs to seal the ports. This is a cheap and easy solution unless your ports are on the front and you don't use covers. Talk about low WAF!
I had good results cutting soda straws to the length of the port and then simply packing it full. This reduces turbulence and overload without changing the tuning of the port. Using fabric, wool etc. impede the air movement is not only a gamble, worst case it can damage the speaker.
I've used filter wool refills from a tropical fish pet shop. You can buy large bags of it for a few dollars.
Try an Acoustic System pro resonator ( copper) for internal use. It brings a lot of good things !!!!!!!!!
The bonded dacron that I suggested will turn some of the acoustic energy in the port to thermal energy, and thereby damp it's resonance. Done correctly(not crushed or tightly packed), it won't impede the woofer's motion. It certainly will not damage the system in any way.
George Short and North Creek appears to favor the approach suggested by Samujohn: he shipped straws with the Eska kit I purchased (rear ported). I've not compared the straws with other materials, tho. John
I tried the straws before I tried the upholstery foam, because Stuart Tyler of ProAc used straws in the Response 1 speaker years ago. It did not sound right at all in the ported Dahlquist speakers that I was using. Foam did the trick.
FWIW - are you aware of what you are doing when plugging the port? Depending on the tuning, the port may be beneficial in that it protects the woofer from extreme excursion at ultra LF.
Not all ports are tuned to boost bass although in consumer speakers most are.
Thanks for the suggestions...I don't want to completely cut off the port, then I'd have little bottom end. I'm stuck with them in the position they currently are, a little more than 1.5 meters/4 ft from the back wall. I think the Dacron suggestion will be the first try, being that it won't completely plug the port's...there are two, bass and mid-bass, speaker is a Jamo D450, which has turned into quite an enjoyable speaker for me (no, I don't plan on listing them soon).
Thanks once again, L'wood
Want a bit less bass pull loudspeaker out from wall or corner. Why mess up the tuning your designer choise. And if you think its wrong sell the loudspeaker.
Lots of good suggestions so far.
On a more humorous note, one of the recommendations of stuffing the ports on the previously very successful Radio Shack monitors with Linnaeum tweeters was to use tampons. The obvious side joke was that the speakers caused one's ears to bleed.
I've tried some loose bonded dacron like Rodman suggested and it did work...eliminated some of the bloom. Thank you for the suggestion...had to order it on ebay cos HomeDepot and such had no idea what I'm talking about
Fabric and upholstery shops generally carry the stuff, as it's used in pillows, quilts, etc. Glad that it worked for you. Happy listening!
How large is the hole and how small do you want to make it. I can get you some armafex I use for pipe insolation in differant thicknesses and differant center hole sizes or you can order some on the internet. Type in google, armaflex pipe insolation. It only comes in 6 foot lengths though. Thickness will determin cost. You can also try Home depot for small sizes.