Port Design Compromises

[pick]

I am working on a couple of new ported subwoofer designs. I continue to struggle with the same issue I have always come up against for high-excursion drivers in ported designs. This issue is the balance of port size, port air velocity, and port resonance. I have been on forums and have read articles for years, concluding that there are many conflicting opinions.

I see many comments about the maximum acceptable port velocity being < 10 m/s, < 26 m/s, and < 34m/s. In my opinion, < 10m/s is unworkable. This limit imposes physical requirements on the design that result in something nobody wants in their living room. This limit also often results in port resonance at low frequencies. Many have said they worry more about port chuffing/compression and do not worry much about port resonance. I’m not sure I agree. The same high-power, high-excursion conditions that generate high air velocities also generates harmonics. If port resonance occurs below 2x crossover frequency, the largest harmonics will be allowed to sing.

I am currently running a subwoofer that generates port velocities in the mid-upper 30’s m/s at about 22Hz at max excursion. I can tell you the chuffing is audible when driven with a 22Hz sine wave input. This effect, however, is never really observed during normal listening. Max excursion happens well above the driver’s thermal power limit, so the chuffing is never sustained and is likely more a short-duration transient signal issue and may be masked by the actual signal which is quite loud. In current designs, I consider these air velocities a little high, and I shoot for something below 30 m/s.

I am looking to use a slot port about 1.5-inch x 14.75-inch. It will have about 0.5-inch round over on each end to take the sharp edges off (I realize more would be better, but I’m trying not to complicate the build). To get 20Hz tuning, I end up with 1^st port resonance around 180Hz and max air velocity of about 30 m/s (this is at 44mm p-p excursion). This includes a high-pass filter to limit excursion below the port frequency. It seems to me this will have to be a fair compromise.

Am I out of my mind? Are people here really building < 10m/s ports?

[Brian Steele]

Forget about simple vented designs. Look up "offset driver transmission line" designs. Basically the whole box is converted into one big vent, and you can choose to reduce the impact of the first out of band resonance by offsetting the driver accordingly.

[billfitzmaurice]

]Am I out of my mind? Are people here really building < 10m/s ports?

Some do, I don't see the need. 18-20m/s is adequate. Where slot ports are concerned they make port velocity worse. There is a region of high friction between the port air mass and the port walls that extends inward from the port walls some 1/4 to 1/2 inch. For all intents and purposes that region detracts from the effective port area. It's why ports in general tend to deliver a lower Fb than the calculations predict. In the case of slot ports that high friction region occupies more of the port than with less oblong shapes, which results in even lower Fb than calculated and higher port velocity as well.

[jhollander]

I think 28 m/s is acceptable pot velocity. I also think you should pick the max SPL/ voltage at the drivers x-max at design frequency. Chances are you can use a reduced voltage/ SPL and use a smaller port. Third most subs use a separate amp and can be low passed. So use a sealed design and forget the port.

[Wolf]

Model for output, xmax, port mach, and extension. Do not base box models on the drivers' power handling specs, as those are normally thermal ratings and not related to xmax and mechanical abilities in applied boxes.

Later,
Wolf

[pick]

Some do, I don't see the need. 18-20m/s is adequate. Where slot ports are concerned they make port velocity worse. There is a region of high friction between the port air mass and the port walls that extends inward from the port walls some 1/4 to 1/2 inch. For all intents and purposes that region detracts from the effective port area. It's why ports in general tend to deliver a lower Fb than the calculations predict. In the case of slot ports that high friction region occupies more of the port than with less oblong shapes, which results in even lower Fb than calculated and higher port velocity as well.

I was hoping you would be here. I have seen you around in many discussions about port designs. I have seen you describe this effect before, but now I am thinking more about it. In previous ported builds, I have always started with my port a few inches too long during initial assembly. I then make measurements to see how big the box really is and what its tuning frequency is compared to the design calculations. I then remove and cut the port incrementally until the right mix is achieved. This is obviously easier with round pipe ports than it is for slot ports. On slot ports, I seem to end up with a shorter port than predicted. This supports your statement. It would seem I should be able to use this information. I should be able to increase the port height slightly to account for the effective reduction of port area caused by the high-friction region and achieve the originally intended result. The trick then would be deciding how much to increase the height.

Would it be safe to assume you would tell me round tube ports are better than slots?

[pick]

Model for output, xmax, port mach, and extension. Do not base box models on the drivers' power handling specs, as those are normally thermal ratings and not related to xmax and mechanical abilities in applied boxes.

Later,
Wolf

Agreed. I calculate port air velocity at xmax, which is always occurs at higher than the rated power in these designs.

[pick]

I think 28 m/s is acceptable pot velocity. I also think you should pick the max SPL/ voltage at the drivers x-max at design frequency. Chances are you can use a reduced voltage/ SPL and use a smaller port. Third most subs use a separate amp and can be low passed. So use a sealed design and forget the port.

I considered going with a sealed design. I have a hard time getting on board with this idea based on previous measurements in the intended listening space. The room is not sealed and has a wall open to the entire first floor, resulting in little room gain. Lower extension is needed down to low frequencies. Also, I am reluctant to use a linkwitz transform, consuming so much amplifier power and cone excursion to offset the higher F3. Ported designs have served me best here

[jhollander]

Room pressurization is minor compared to the room gain by boundary reinforcement. You might want to play around with Jeff Bagby's Diffraction and Boundary Simulator. Any boost or filter should be applied pre amplifier power. Your receiver might use a pre out that is filtered or your plate amp has filters. My Crown uses a DSP low pass. What are you planning to use?

[tvrgeek]

Port restrictions are what killed my current attempt at a BP-4 sub. Went back to sealed. My ears tell me velocities need to be under 10m/s, but to control port resonances, ports less than about a foot. I have not found drivers with the parameters to fit those constraints in a BP-4.

I design for 90 dB. Damn loud. If port velocity is higher at a higher SPL, I am sure I would not hear it as I would not be in the room.

Not a fan of the LT if you model cone excursion. I have found I do not want exceed 1/4 X-max if I want the distortion to be livable. Many don't care about bass distortion and consider 10 % or higher to be acceptable. If you read Siegfried, he advocated it for a gentle music system, NOT for high power special effects HT. EQ should be used to fine tune, not as a magic fix-all. If your design needs more excursion for your measured target, you need more sd.

To add to Wolf, do the model to get you close for as prototype, but do not trust any model for the in room result. You have to measure. Not only do rooms have more gain than most think, but your perception of "right" rolls off the lowest octave. A system that measures flat to 20 would leave me screaming from the room! After pulling what was left of my hair out, I am re-affirming my preference for low Q subs. he more the better. If I can ever get a BP-4 to work, I will change my mind as the acoustic filtering LP seems to be a great advantage.

[billfitzmaurice]

Would it be safe to assume you would tell me round tube ports are better than slots?

Very safe, although I don't use them, as they don't add any structural integrity to the cabinet. With direct radiating ported enclosures I use corner ports. They add bracing to the baffle, top, bottom and sides, as seen in the rear view picture below. They also equalize the internal pressure throughout the cab to eliminated cone rocking at long excursions.


In a sub this configuration might not allow long enough ports. What I do in that case is to make the cab down firing, to minimize the footprint. It also reduces the audibility of chuffing.

[pick]

Room pressurization is minor compared to the room gain by boundary reinforcement. You might want to play around with Jeff Bagby's Diffraction and Boundary Simulator. Any boost or filter should be applied pre amplifier power. Your receiver might use a pre out that is filtered or your plate amp has filters. My Crown uses a DSP low pass. What are you planning to use?

I have used Jeff's tools for crossover design, but I have not tried the boundary simulator. I do, however, take a lot of measurements. The current setup is a 12-inch TC-1000, 3.2CF, 22Hz tuning, 500W plate amplifier, mini-DSP. I attached three images. This one shows subwoofer-only at the primary listening position with DSP.


This one shows the measurement without DSP used to set the DSP filters. This is an average of 6 measurements taken at 4 seat position on my couch, 1 standing at the couch, and 1 standing at room center. You can see I have a large peak in room response around 32Hz related to the front to back dimension of the room (about 18-19 feet). There is not a great deal of room reinforcement at the lowest frequencies. If I were to go to a sealed design, I would likely need to boost the output below 30Hz. The third image shows total in-room response at the primary seating position.


I plan to use the same DSP with an NX6000 amplifier driving two subs.


My motivation to build something new (besides just wanting to do it) is to use 2 subs to even out response throughout the available listening area.

[Dukk]

I pay zero attention to port resonance as it has never caused me issue.

The issue I see is the aspect ratio of the slot vent is way off. The more square it is the less drag there is and the more efficient it will be. Ideally, one should try for 6:1 or less. This is not always practical but 10:1 is way out there.

[pick]

I pay zero attention to port resonance as it has never caused me issue.

The issue I see is the aspect ratio of the slot vent is way off. The more square it is the less drag there is and the more efficient it will be. Ideally, one should try for 6:1 or less. This is not always practical but 10:1 is way out there.

Yes, you are right. I left out one detail. There is a divider in the slot. The port is actually implemented as two ports, each being 1.5-inch x 6.625-inch x 35-inch.

[Wolf]

TVR-
Of course measuring is a good idea. What makes you think I don't? If the sim is made with bad data, garbage-in makes garbage-out.

I tend to model for 100dB, not 90dB, as this does not give you enough headroom for larger rooms. 110dB is better for HT sub simulations to prevent failures. Then you set the mach to less than 17m/s if able, but 28m/s max. If it is within these boundaries, the lower volume usage will be acceptable.

It greatly depends on your room and setup if a 20Hz extension is not going to work as well as an F3 of 40Hz. My Overdrive10 bandpass sub is likely close to flat to 25Hz. It sounds great! Never heavy, one-noted, or BOOOOOOOOOOOOOOOOM.

Later,
Wolf