[redbook]

 Hello. I was just wondering if anyone has experimented with their reflex ports to better control bass in a room. For 24 years  I have braced and mass loaded my JBL Ti 120 speakers. They are on concrete stands. They are driven by a great Bryston 4bst amp through 6 ft. of Audioquest CV4 wire. The last thing I have done is to narrow the ports with a foam plug . Everything has suddenly snapped place. Mids are clearer and the bass is tighter and focused. Anyway , thanks far reading this and I hope my efforts may help someone ................

[mcgsxr]

An interesting experiment, and I am glad it worked for you.

Was this the first time you tried this - ie did you try other changes to the port or was this the first?

I think (not a speaker designer, just a DIY guy for years) you essentially raised the effective frequency of the port by reducing the size of it - this likely coincided with a reduction of output at a particularly nasty frequency for your room.

Glad it worked!

I have played around with port tuning, but only for car subs.

[Letitroll98]

Yes, the ol' stick a pair of rolled up socks in the port trick, and variations of same.  Sticking a smaller cardboard tube inside of the port tube, sometimes also a bit longer, can tame a boomy system.  All of these things alter the tuning frequency of the port and can improve systems designed to have a bass hump, which I think might be the case with the JBL Ti 120.  So your fix of using a foam plug should be right on point. 

Did you narrow the port with a cylindrical plug, e.g. reduce the diameter of the port, or did you restrict the airflow with a full plug? 

[Æ]

Yes, the ol' stick a pair of rolled up socks in the port trick, and variations of same.  Sticking a smaller cardboard tube inside of the port tube, sometimes also a bit longer, can tame a boomy system.  All of these things alter the tuning frequency of the port and can improve systems designed to have a bass hump, which I think might be the case with the JBL Ti 120.  So your fix of using a foam plug should be right on point. 

Did you narrow the port with a cylindrical plug, e.g. reduce the diameter of the port, or did you restrict the airflow with a full plug?

If you reduce the diameter of the port, you change the Helmholtz frequency, which may or may not be good thing. You have to be cautious when reducing the port diameter, if you do so, the air velocity inside the port increases, which can lead to turbulence, audible port noises, chuffing.
Nothing like a BIG port.

[Æ]

Hello. I was just wondering if anyone has experimented with their reflex ports to better control bass in a room. For 24 years  I have braced and mass loaded my JBL Ti 120 speakers. They are on concrete stands. They are driven by a great Bryston 4bst amp through 6 ft. of Audioquest CV4 wire. The last thing I have done is to narrow the ports with a foam plug . Everything has suddenly snapped place. Mids are clearer and the bass is tighter and focused. Anyway , thanks for reading this and I hope my efforts may help someone ................

Yes, the bass would be tighter at the sacrifice of some lower bass. If you really want to see what is happening, you need to run an impedance curve/plot. Your loudspeakers may not have been "tuned" properly to begin with. Improper tuning is often an attempt to provide more bass than is really there. Impress the consumer.

[roscoeiii]

Chuffing?

[redbook]

  I didn't mention that the "plug" isn't a plug  ! It is a length of foam paint roller! The port diameter is reduced from about 3" to 1" . The overall length is the same as original. I play around with pulling the roll out further by 1/2" or so and that is an audible difference as well. Too far--to much bass....too for in....the bass becomes somewhat over damped and hard on some lower frequencies. Due to the great 12' woofer the deeper bass is not lacking when the notes go down (IMG)  This whole process of going to the speakers and back to the chair a million times has been a great physical workout as well   LOL........... Thanks for the replies folks.                            

[redbook]

 Oh, by the way..no chuffing  ( chuffing being the audible sound of air rushing out from a opening).

[Æ]

Chuffing?

http://dictionary.reference.com/browse/chuffing

[JohnR]

Interesting tweak. In general a smaller port will lower the tuning frequency, I'll post a sim later. However the foam paint roller is probably providing some sort of acoustic damping as well. Interesting.

[JohnR]

Bear in mind that this is an illustration only - the specific effect will depend very much on the actual driver. With that said, this graph shows the effect of reducing port size (while keeping its length the same, thus lowering the tuning frequency).

[redbook]

 Thank you JohnR.That chart really shows what I experience in my listening area. This has been a hit and miss operation (as all my other tweaks) base on what I've learned since 1968 plus gut feelings. Like I said, mass loading these Ti 120s have done wonders for bass tightness and the cross brace must have helped as well. I'm just happy that JBL provided an enclosure that allows me to experiment.Thanks again.

[roscoeiii]

So we've covered port size a bit, but I am still curious about the effects of port tubes. Any rules of thumb for tube length. Advantages or disadvantages of flared tubes vs. cylinder shaped ones?

[audioengr]

Here is my experience:

I purchased some ribbon speakers, intending to use them at shows.  They were not expensive, but had good drivers and construction.  I redesigned the crossover and they sounded good, but not really dynamic in the bass.  Then , I decided tojust pull out the tuning port a bit, first 1/2 inch and then 3/4 inch.  Magic!  At 3/4 inch they just exploded with dynamics.  Sounded like a different speaker.  Moral of the story is to fine tune all ports, because they likely need it.

Steve N.

[redbook]

 Exactly my result. It has been a long road for a short journey   .....

[Æ]

So we've covered port size a bit, but I am still curious about the effects of port tubes. Any rules of thumb for tube length. Advantages or disadvantages of flared tubes vs. cylinder shaped ones?

Flared tubes are used for their aerodynamic advantage.

http://koti.mbnet.fi/jahonen/Audio/Papers/AES_PortPaper.pdf