(Hopefully) Interesting port thoughts

[biff]

Some here might be familiar with the cherry bomb glass pack mufflers and after seeing an ad something occured to me - the basic design was a double walled tube, with the inner tube perfed, and the space between was stuffed with fiberglass or rock wool. Theory is that the higher frequencies are absorbed and the lower passed through. One of the non-beneficial attributes of a port is alleged to be passing frequencies above the intended bandpass. My experience with current sub is it seems more locatable unplugged, but with better extension than when ports are plugged. Perhaps the distance between perfs, or ratio of solid surface versus wool could effect bandpass. How would lining a tube with felt, wool etc. influence the output? It certainly could diminish port resonances, reduce chuffing - what else?

[Wolf]

Re: (Hopefully) Interesting port thoughts

Some here might be familiar with the cherry bomb glass pack mufflers and after seeing an ad something occured to me - the basic design was a double walled tube, with the inner tube perfed, and the space between was stuffed with fiberglass or rock wool. Theory is that the higher frequencies are absorbed and the lower passed through. One of the non-beneficial attributes of a port is alleged to be passing frequencies above the intended bandpass. My experience with current sub is it seems more locatable unplugged, but with better extension than when ports are plugged. Perhaps the distance between perfs, or ratio of solid surface versus wool could effect bandpass. How would lining a tube with felt, wool etc. influence the output? It certainly could diminish port resonances, reduce chuffing - what else?

Lining the inside diameter with foam has proven beneficial for board members before. I used teh adhesive Foamie for the Dragon-Foal project. It had a dip in simulation due ot the resonance, but I don't seem to hear it with this employed.



Not a new idea, but a reasonable thing to try.
Later,
Wolf

[billfitzmaurice]

Re: (Hopefully) Interesting port thoughts

My experience with current sub is it seems more locatable unplugged,

The problem isn't the port, it's insufficent internal damping of the cabinet. In theory you shouldn't have any directional frequencies coming out of the port if the crossover limits the bandwidth to below the directional frequencies, but theory doesn't account for the above bandwidth harmonics generated by the motion of the cone. Those harmonics are created on both sides of the cone, so if the box is insufficiently damped they can escape via the port. Filtering those harmonics is the main benefit of downward and/or rearward facing ports and drivers. Lining the duct won't do much, as the length of the duct is far too short compared to the wavelength of the problematic midbass harmonics, and the lining thickness must be compensated for in the port area.

[curt_c]

Re: (Hopefully) Interesting port thoughts

Some here might be familiar with the cherry bomb glass pack mufflers and after seeing an ad something occured to me - the basic design was a double walled tube, with the inner tube perfed, and the space between was stuffed with fiberglass or rock wool. Theory is that the higher frequencies are absorbed and the lower passed through. One of the non-beneficial attributes of a port is alleged to be passing frequencies above the intended bandpass. My experience with current sub is it seems more locatable unplugged, but with better extension than when ports are plugged. Perhaps the distance between perfs, or ratio of solid surface versus wool could effect bandpass. How would lining a tube with felt, wool etc. influence the output? It certainly could diminish port resonances, reduce chuffing - what else?

Both the common muffler and vented speakers are applications of a Helmholtz resonator. The extraneous noises (above the tuning frequency) coming out of a port generally have 3 causes:

Port noise caused by excessive air velocity.
This can be aleliorated by using a larger port diameter and/or flared ends. It's been my experience that using a port with a generous flare will allow for a smaller than otherwise required port diameter without audible 'chuffing'. This noise appears to be caused by the abrupt change in acoustic impedance at either teminus of non flared ports.

Port noise caused by driver enclosure resonances.
This would include standing waves and other non-attenuated frequencies generated in the passband of the driver. IME, lining the enclosure is not as effective as placing stuffing near the center of the enclosure. Polyfill does little to attenuate bass frequencies, but does an acceptable job at higher frequencies.

Port noise caused by the port self-resonance.
Like blowing on a pop bottle, any tube as a resonant frequency. This is not generally an issue, as most designers will ensure the port resonsnce is above the passbend of the driver, but it is possible when long lengths are used.

C

[Wolf]

Re: (Hopefully) Interesting port thoughts

The problem isn't the port, it's insufficent internal damping of the cabinet. In theory you shouldn't have any directional frequencies coming out of the port if the crossover limits the bandwidth to below the directional frequencies, but theory doesn't account for the above bandwidth harmonics generated by the motion of the cone. Those harmonics are created on both sides of the cone, so if the box is insufficiently damped they can escape via the port. Filtering those harmonics is the main benefit of downward and/or rearward facing ports and drivers. Lining the duct won't do much, as the length of the duct is far too short compared to the wavelength of the problematic midbass harmonics, and the lining thickness must be compensated for in the port area.

If you think I'm that naive, you're mistaken. I DID factor the actual diameter in the simulation after the lining was added, or it wouldn't be the length that it is. In the case of a 2-way, the frequencies are not eliminated. I know Biff mentioned a sub, but.. You can only stuff a vented box so much before it's no-longer a vented box. Facing it to the rear only changes the directionality of the emitted resonances, and does not filter them out of teh sound spectrum entirely. As for the length of the port being a factor, it's the diameter that actually causes the cancellation at it's harmonic resonance. If you decrease the diameter, it will disappear in Unibox. Since it's the dimaeter, and I damped that distance, it does matter, and I can hear a difference. Using multiple small-diameter ports in bookshelf speakers actually is better than a single large-diameter for these reasons. The cancellation is not as severe, chuffing is still reduced, and you still hit your target.
Later,
Wolf

[billagee]

Re: (Hopefully) Interesting port thoughts

I did this several years ago just as Wolf shows. The port noise (mainly chuffing) from a 1.5" port was objectionable so I tried lining a 2" port with 1/4" foam and it worked very well. A 2" port without foam lining would have been prefered, but there was not enough room for the lenght at 2" diameter.

The main trade-off is that the port becomes more lossy (lower Qp), but I didn't find that objectionable. I have to wonder, though, if the surface texture of the foam had as much to do with the reduction of the chuffing as did the sound absorption by the foam. B&W uses dimpled ports to reduce turbulance noise.

I have some measurements of the port somewhere. I will post them if I can find them.

Bill

[billagee]

Re: (Hopefully) Interesting port thoughts

These are measurements I took some time back. I inteneded to extend the measurements to include greater foam thickness with larger ports just to get a better view of the effects, but as yet I haven't had time.



All measurements are nearfield.

Unlined port is 6" x 1.5"
Red is port
White is woofer
Blue is woofer + port

Lined port is 5.5" x 2" lined with 0.25" foam
Green is port
Yellow is woofer
Gray is woofer + port

The resonance of the unlined port (red) is calculated to be 957Hz and that is about where it is. The second harmonic even shows up, too. Replacing the port with one that is lined (Green) knocks the self resonance down 15 to 20dB.

Comparison of the port reflex action shows the increased damping of the lined port over the unlined, and is reflected in the woofer and port+woofer plots.

Bill

[Wolf]

Re: (Hopefully) Interesting port thoughts

These are measurements I took some time back. I inteneded to extend the measurements to include greater foam thickness with larger ports just to get a better view of the effects, but as yet I haven't had time.



All measurements are nearfield.

Unlined port is 6" x 1.5"
Red is port
White is woofer
Blue is woofer + port

Lined port is 5.5" x 2" lined with 0.25" foam
Green is port
Yellow is woofer
Gray is woofer + port

The resonance of the unlined port (red) is calculated to be 957Hz and that is about where it is. The second harmonic even shows up, too. Replacing the port with one that is lined (Green) knocks the self resonance down 15 to 20dB.

Comparison of the port reflex action shows the increased damping of the lined port over the unlined, and is reflected in the woofer and port+woofer plots.

Bill

Interesting in that it rolls off faster below tuning (as far as I can tell). This might actually be beneficial to smaller units with less Xmax. Very intriguing!!
Later,
Wolf

[billfitzmaurice]

Re: (Hopefully) Interesting port thoughts

I know Biff mentioned a sub, but..
Wolf

My response addressed the OP's question, which specifically concerned subwoofers. Yours, while not without value, are OT.

[biff]

Re: (Hopefully) Interesting port thoughts

I did mention my experience with my sub, but the query was more geared towards woofers that might have appreciable output in the port resonance range.

[billagee]

Re: (Hopefully) Interesting port thoughts

Interesting in that it rolls off faster below tuning (as far as I can tell).

Having so many plots on a single graph can be confusing. Sorry about that.

It is the unlined system response that rolls off faster (blue) while the damped port system response (gray) has a more shallow roll off. Taken to the extreme, a completely plugged port would result in a sealed, 2nd order response (and no chuffing ).

Bill