what does 'port tuning' mean?

[Anthony]

what is this all about?

i am not a techie, so language for the village idiot please

 

[Guest]

tunning the reflex port on a speaker to ensure it fills in at certain frequencies, different diameters and tube lenghts are the adjustable dimensions, changing these "tunes" the port to the designs required parameter.

Adding resistance to a port, with foam, socks or straws, and can be done at home, if you find your speakers too boomy near a wall for example, this is also a form of port tuning.

HTH

 

[Anthony]

ok. correct me if i'm wrong please, but if i've understood correctly:

- bass port tuningis when the designer will adjust the diameter and length of the tubes in the cabinet to get the frequencies they want emanating from the bass port.

- and damping just reduces the quantity of sound emanating from the port by stuffing something in it

sorry for being so technologically dim on this, just learning as i go.

 

[Guest]

pretty much it. I detect another question coming on however, you can also retune your own ports, a bunch of drinking straws effectively reduces diameter, and if they are left quite long can alter port loenght too, cheap trick, to play with, but be careful you don't put too far in and dmage something, port lenght can also be adjusted by using a cardboard tube just smallenough to fit in port, you can also cut inside edge at an angle. Both cheap and easy to do.

 

[meninblack]

IIRC it's not so much sound coming out of the reflex port as changing the sound coming from the bass driver by changing the resistance to (backward) movement due to compression of the air in the box. At high frequencies it's an insignificant effect, becoming more and more significant at the lower frequencies.

Changing the portcan also effect the resonant frequency of the cabinet, which changes the sound.

 

[Anthony]

aahh, the light is started to come on, if only dimly

so how come when you look inside a cabinet you are not looking down a tube but just into the cabinet itself? where are the tubes from the drivers, or are there no tubes (except in transmission line speakers)? is it the size of the port that matters, or position, or both?

answers breed further questions:?

 

[Guest]

usually it's just space filled with wadding behind drivers, port is inseted in hole, it's size lenght and resistance to air is what gives it it's properties, sometimes the drivers are sealed from each other and have other ports or remain sealed, again this is all tuning, transmission line is basically imagine speaker box was ten feet deep (ala pmc fb1), but very narrow in vertical dimension, tapering away from driver, now fold this up into a domestic speaker box, and fill with foam and or wool like wadding as above, and you have a transmision line.

 

[Anthony]

thanks peeps

p.s. must change that avatar i use. it;s making me feel sick to look at.

 

[Guest]

you mean you understood that, fook, could you explain it back to me then, coz I can't make head nor tail of it

 

[Anthony]

well, i think i understand. maybe that is good enough

 

[Guest]

Anthony wrote:

well, i think i understand. maybe that is good enough

allways worked for me

 

[Killahertz]

Passing swiftly over previous 'answers'

Port tuning could be one of two thing: that (rightly) mentioned regarding the damping of output from the port - ie as a simple means of reducing port efficiency, and thus reducing overall in-room response. Or, it could be that at the design stage - matching the port to the enclosure to the drive unit (in this case a bass or mid-bass).

If the latter, then it is primarily about efficiency. Efficiency of (increased) output, and also efficiency of extension - ie maximising the 'depth' of bass response. How does it achieve this efficiency?. Simply by taking advantage of the 'work' done by the rear of the drive unit - bear in mind that the'normal' output from the front of thedrive unit is matched by an equal but opposite'output' from the rear.

Now, also bear in mind that this rearward output is opposite to that from the front (ie in opposite phase, or phase shifted by 180 degrees). So, that if it were simply allowed out of a 'hole' in the enclosure it would 'sum destructively' with the front-ward output, ie they would cancel each other out.

The role of the port, then, is to generate an output (energised from the rearward 'work' of the bass/mid-bass drive unit) that is in phase with that from the front, so that they 'sum constructively', or combine. Hence the increase in efficiency, and (with careful 'placement' of the tuning frequency), the potential increase in extension.

How the port does what it does is a little more complex. Very basically it contains it's own volume of air, or air mass. This is entirely seperate from that in the enclosure, but itworks with that in the enclosure - ie they are interdependant. This air mass (through design, ie sizing) has a resonant frequency, usually chosen to coincide with that of the drive unit. Above resonace the port 'appears' as a solid bung, ie it has no function. At and around resonance the air mass is made to vibrate (basically as a result of varying air pressures within the enclosure as a result of the motion of the drive unit), and does so in phase with the output from the front of the drive unit, increasing efficiency. Below tuning the port appears as a hole, output from it and front of the drive unit cancel, and output drops off rapidly.

At and around tuning, the port 'loads' the drive unit, meaning that the drive unit's motion is controlled, with most of the output coming from the port. This is how plugs and bungs are used to 'tune' the port's response. They limit the control of the drive unit, at the same time as limiting output from the port.

As for transmission lines, well, they are a different kettle of fish all together. One thing is for certain, though - they are not like ported speakers in their operation.

Hope, this, erm, helped?