I have been using 4 small ports for 30 years, in many many different designs. Typical activity is to drill four 1/4 inch holes to start with and then play music with bass and gradually increase the port size until you start getting the shavings blown out of the holes. This is your smallest effective port size and it will chuff badly under heavy acoustic load.
Typical size will end up at about 3/8 inch, almost regardless of box size. My current system uses a 0.8 cu ft box with a Dynavox 9 inch and auto carpet underlayment pad half way up and around all four inner sides and the back. With the mid range cup, the volume is about 0.74 cu ft. These holes are in the MDF and are rounded on both ends and are not thicker than the MDF 3/4 inch either.
Frequency response is flat to 35 Hz and due to the extra resistance the driver stays linear out to 107 dB, though you can begin to hear the ports at that ridiculous level, still very very smooth and melodious bass.
You can hear into a mass of bass viols quite easily. Of course the driver has been EnABL'ed so it is already more linear and stable than an ordinary woofer. Just try it, you can always plug up the holes later. Neither more nor less than four holes ever seemed to work out.
Bud
Typical size will end up at about 3/8 inch, almost regardless of box size. My current system uses a 0.8 cu ft box with a Dynavox 9 inch and auto carpet underlayment pad half way up and around all four inner sides and the back. With the mid range cup, the volume is about 0.74 cu ft. These holes are in the MDF and are rounded on both ends and are not thicker than the MDF 3/4 inch either.
Frequency response is flat to 35 Hz and due to the extra resistance the driver stays linear out to 107 dB, though you can begin to hear the ports at that ridiculous level, still very very smooth and melodious bass.
You can hear into a mass of bass viols quite easily. Of course the driver has been EnABL'ed so it is already more linear and stable than an ordinary woofer. Just try it, you can always plug up the holes later. Neither more nor less than four holes ever seemed to work out.
Bud
I've made this small "letterbox" speakers as an experiment and they sound very good they are 4 Ohm's delievers a nice bass playing with my old sansui qr 6500 the port has the same area as if it were round. The port length is also the same just made alternative - se next picture.
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Hi,
The Q of the port will be lower, that's the point - lower Q broader spread. By increasing the size of the box to 'compensate' for the lower output level, you are pushing the Q of the response straight back up
The reynolds number is interesting - it would dictate that the greatest flow conditions have the greatest shear boundary, and therefore the highest resistance - ie greater than critically damped. This helps reduce IMD in a speaker simply because unlike a single large port, the cone in effect operates in an environment closer to a sealed cabinet, thanks to the resistance of the air in the portlets.
However, this transition would occur at a point below useful output, unless the portlets were very small indeed ( a few mm or so), but I know of no modelling techniques for that in this specific instance.
The briggs leaky box (an early example of an aperiodic design) used 'pegboard' for one of the cabinet sides (usually the back).
Goodmans developed a 'resistive port' as a direct result of that early research.
Have fun
Owen
The Q of the port will be lower, that's the point - lower Q broader spread. By increasing the size of the box to 'compensate' for the lower output level, you are pushing the Q of the response straight back up
The reynolds number is interesting - it would dictate that the greatest flow conditions have the greatest shear boundary, and therefore the highest resistance - ie greater than critically damped. This helps reduce IMD in a speaker simply because unlike a single large port, the cone in effect operates in an environment closer to a sealed cabinet, thanks to the resistance of the air in the portlets.
However, this transition would occur at a point below useful output, unless the portlets were very small indeed ( a few mm or so), but I know of no modelling techniques for that in this specific instance.
The briggs leaky box (an early example of an aperiodic design) used 'pegboard' for one of the cabinet sides (usually the back).
Goodmans developed a 'resistive port' as a direct result of that early research.
Have fun
Owen
"The briggs leaky box (an early example of an aperiodic design) used 'pegboard' for one of the cabinet sides (usually the back)."
The Wharfedale cabs my father had in the early 60s were about 4.5 cub feet. The rear panel had many 1/2" or so wide slots, about 1" apart, taking up most of the surface area. The sides and back were lined with heavy felt. Inside were the S10RSDDs.
The early Wharfedales had large magnets, long travel voice coils (3/8" each way) very linear suspension, heavily damped, low resonance, and retained high efficiency.
The only complaint I had with them was the metal dustcap sounded harsh on SS amps, (and now I'm ducking for cover)
Fed with a Pioneer SM161 (PP 6BM8s) would on occasion be used in a mid sized church hall. I still have those drivers. Unfortunately one went O/C a few months back.
The other pair I have were extensively modified about 1978, and still going strong.
There are pictures on the net of the OBs. For some reason I did not save them. Could even be in the full range gallery, or a Japanese site.
Regards,
Geoff.
Edit: the bit about OBs was for another thread. More coffee needed
The Wharfedale cabs my father had in the early 60s were about 4.5 cub feet. The rear panel had many 1/2" or so wide slots, about 1" apart, taking up most of the surface area. The sides and back were lined with heavy felt. Inside were the S10RSDDs.
The early Wharfedales had large magnets, long travel voice coils (3/8" each way) very linear suspension, heavily damped, low resonance, and retained high efficiency.
The only complaint I had with them was the metal dustcap sounded harsh on SS amps, (and now I'm ducking for cover)
Fed with a Pioneer SM161 (PP 6BM8s) would on occasion be used in a mid sized church hall. I still have those drivers. Unfortunately one went O/C a few months back.
The other pair I have were extensively modified about 1978, and still going strong.
There are pictures on the net of the OBs. For some reason I did not save them. Could even be in the full range gallery, or a Japanese site.
Regards,
Geoff.
Edit: the bit about OBs was for another thread. More coffee needed
Owen,
Interestingly Bass Box Pro did predict the bass frequency extension from the 10 mm or so size of the four ports, and worked well with the 19 mm port length and radiused edges.
The rest of what you say is exactly my experience, though actual diameter of ports can be very touchy and I "tune" the Q using exact diameter pieces of 0.26 mm mylar. Also, bass performance is the most linear, in a qualitative sense, with one port smaller by this piece of plastic than the other three.
Bud
Interestingly Bass Box Pro did predict the bass frequency extension from the 10 mm or so size of the four ports, and worked well with the 19 mm port length and radiused edges.
The rest of what you say is exactly my experience, though actual diameter of ports can be very touchy and I "tune" the Q using exact diameter pieces of 0.26 mm mylar. Also, bass performance is the most linear, in a qualitative sense, with one port smaller by this piece of plastic than the other three.
Bud
Port velocity depends on how much power you feed it
It's been a while since I used the standard version of WinISD. The Pro version plots port velocity and it's good to keep it under 17 m/sec or so to avoid chuffing. I think the Standard version doesn't plot it but just indicates "green" if it's low enough.
The thing is you need to feed the speaker a realistic power level to check port velocity. The default is 1 watt so you need to boost it up in the settings. Your small port(s) are guaranteed to chuff at any reasonable power level and it won't be "green" any more. As far as the software is concerned, it just looks at total port area. It doesn't care if there are several small ports or one big port. Either way your port area is way too small to be useful. Even if you want to do an aperiodic box, you're going to need a lot more total port area.
It's been a while since I used the standard version of WinISD. The Pro version plots port velocity and it's good to keep it under 17 m/sec or so to avoid chuffing. I think the Standard version doesn't plot it but just indicates "green" if it's low enough.
The thing is you need to feed the speaker a realistic power level to check port velocity. The default is 1 watt so you need to boost it up in the settings. Your small port(s) are guaranteed to chuff at any reasonable power level and it won't be "green" any more. As far as the software is concerned, it just looks at total port area. It doesn't care if there are several small ports or one big port. Either way your port area is way too small to be useful. Even if you want to do an aperiodic box, you're going to need a lot more total port area.
here's a vent calculator for DOS prompt which might handle the tuning aspect of multi-holes - if it will run (?)
cope and paste to browser
(http://home.earthlink.net/~buddhaboy2/VENTWRK.EXE)
I think the technique may conditional use -Baruch's patent and commercial speaker was spattered with holes and Rayl's :^)
J.J. Baruch US2766839 "Loudspeaker System" Filed March 16th 1953
vent velocities with large woofer can be very high and a 15" in box with 3/8" holes can make a nice breezy fan with sinewave at fb
Karlson used distributed slits in the twelve inch enclosure which did the same job as a bunch of holes - it could generate harmonics below it passband vs a single non-choking vent.
cope and paste to browser
(http://home.earthlink.net/~buddhaboy2/VENTWRK.EXE)
I think the technique may conditional use -Baruch's patent and commercial speaker was spattered with holes and Rayl's :^)
J.J. Baruch US2766839 "Loudspeaker System" Filed March 16th 1953
vent velocities with large woofer can be very high and a 15" in box with 3/8" holes can make a nice breezy fan with sinewave at fb
Karlson used distributed slits in the twelve inch enclosure which did the same job as a bunch of holes - it could generate harmonics below it passband vs a single non-choking vent.
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