Tweakers' Asylum

Slot loading a ported / Rb box? how much longer does the port 'seem to be'?

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Timbo in Oz

(A)

how deep should the slot / gap be?

Warmest
Timbo in Oz
The Skyptical Mensurer and Audio Scrounger
And gladly would he learn and gladly teach - Chaucer. ;-)!
'Still not saluting.'

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For a round port - jnorv 15:50:00 01/08/07 (4)
ending in a baffle, the accepted "extra" length is one radius. For a square port, you could either use the radius of the equivalent circular area. For a high aspect ratio rectangular port, I would use 1/2 the small dimension. It is all just a guess. The "correct" to tune a port is to measure the impedance as a funtion of frequency. the port tuning is at the minimum point between the two maximums.
Jim N
- If you ARE right I can only ever add 3 cms to a 6cm bi.d. port! Even IF it and the driver are slot loaded!? - Timbo in Oz 16:24:00 01/08/07 (3)
  I had (incorrectly?) deduced (deluded?) that a slot-loaded port should seem half as long again*, times 1.5! [Or was it one fifth / times 1.2?].
  IE that is where it's impedance minima would shift down to, lying between the two 'new' peaks. the new operating frequncy being that of a port one and half times as long. Nope?
  That is, in a port in a baffle that is 38cms in diameter and (both port, driver and baffle) face the floor at a distance of let's say 2 (or 3) inches!
  What factors help me determine the height of the loading slot?
  So adding 3cm (half the i.d. of the port) to 15cms the current length, and that slot-loading will only ever add 3cm's to a 6cm i.d. port. Giving 18cm's as the effective port length of the existing system facing down onto a 60cm diam. paver loading the 38cm diam round baffle with a 5cm gap.
  Correct?
  AND, ..... Even if I make the port longer by deepening the baffle by 5 cm, still gives a total effective port length of 15 + 5 + 3 => 23cm's.
  I doubt that's going to change the Fb much of a smallish 23litre enclosure. I need to lower the Fb from 38 to 27hz, to 2/3rds at least.
  
  Warmest
  Timbo in Oz
  The Skyptical Mensurer and Audio Scrounger
  And gladly would he learn and gladly teach - Chaucer. ;-)!
  'Still not saluting.'
  - here's a ref, to the 1.5 times theory - Timbo in Oz 18:26:29 01/08/07 (2)
    And I think Bullock says something similar.
    
    Warmest
    Timbo in Oz
    The Skyptical Mensurer and Audio Scrounger
    And gladly would he learn and gladly teach - Chaucer. ;-)!
    'Still not saluting.'
    - http://www.audioasylum.com/audio/speakers/messages/194659.html (Open in New Window)
    - After reading your previous post (below) - jnorv 19:34:58 01/08/07 (1)
      I have better idea of what you are after. If I get it right, you are trying to get credit for the distance from the center line of the port to the outside edge of the baffle using the floor as the other wall. If the port is located anywhere near the edge of the baffle, I think that that improvement on length would be small, not to mention the fact that the "duct area" is now much bigger. It is an interesting question, not unlike, "how close can I place a rear loaded port to the wall?". I think you are stuck with measuring the impedance curve and going from there by trail and error.
      Jim N
      - Thanks for that! and you could also try over at Inmates - LONG - Timbo in Oz 00:35:57 01/09/07 (0)
        
        In Reply to: After reading your previous post (below) posted by jnorv on January 8, 2007 at 19:34:58:
        
        Tah Jim,
        the thread at Inmates is down below,
        titled "My matured plans ...."
        As you will read, I will be tweaking / morphing / cloning* the main pair of Model 3 'spheres too, over time! B&W's rear trumpet-port, cum flow-restrictor on the mid-range spheres in the N series?! They'll die-off slowly, -6db by ~35 - 40 hz? overdamped/sealed hybrid / aperiodic?
        In short - phase one is an active-xovered 2.5 way adaptation - to provide baffle-dip shelf compensation - using a 2nd matching sphere with midbass diver only. Deeper bass extension is also now very possible due to extra bass-power handling, allowing extension boost / 2nd order assist, with LESS cone excursion (Xmax) than the previous M3 sphere alignment could achieve.
        And, any other eq I might add, ;-)} !
        All this, but WITHOUT adding an entire set of filters in with the feed to the main pair and on out to my ears! This is now very coherent and transparent, and is not to be futzed with in the pre-amp's active circuit.
        IE I hope to gain on the swings - while the roundabouts matter rather LESS than usual! I hope, ;~), X fingers!.
        I think your main points would also hold even with a 'predicted response' from a model on a PC.
        Once I find out what the new Fb and, depending on how much it all adds up to - along with room gain - I can determine whether I need 'extension boost', if at all.
        MIND, I AM going to be active filtering - for cone bounce - JUST below the final Fb, anyway!!
        Keep in mind that this bass only sphere will be driven separately, via an active filter - fed at spkr power level, from the mid-bass tube amp, via 'the HP xover and polarity switch' box, each has a 4PDT relay in there!
        to provide variable - BDSC shelf boost. Yes, OEM these '8inch 2-way' spheres had NO low pass!!!!??? But, very open mouthed!
        So, this 'black box' has several duties, an active, user adjustable, bandpass, bass ELF Shelf and eq, filter set. Stereo, or dual mono?
        i) an adjustable (190 down to 95? ) 'low-pass' filter - being 4th order or steeper, making a nice flat blend easier to achieve.
        ii) an adjustable shelving-response filter operating from the low-pass point, on-down to 'DC'.
        DC <--------------\
        for adjustable (tunable) baffle-diffraction=dip compensation! - ? - a level shelf that can give me coverage from zero audible compensation (off) to @ 1.5db, 2db, 3db, 4.5db.
        Viz
        _______ to -------- on up in level, feeding into
        
        iii) a 2nd order High Pass filter with - adjustable - gain (Q really) at the filter /'Fb boost' freq'y - And boost frequency adjustible over an octave (18 to 36hz - I ween - for this job) IE from boost at Fb to flat at LP. Viz
        |\_____
        |
        with the bump/boost at Fb, as adjusted for freq and boost.
        OR 'flat' but still High pass filterd below Fb
        |---------|
        from Fb to LP.
        This ensures that we can have a 6th order High-Pass acoustic response at Fb virtually killing ELF cone-bounce below that. Switchable? - just to show cone bounce STOPping on LP's!? ;-).
        
        iv) Up to 3 'down only type / if cheaper' - parametric eq'rs - switchable in and out, variable centre and Q/width@ - for bass peak supression anywhere in the filter's entire pass-band from 180 or so down to the Fb filter/booster. @I don't know how deep these notches might need to be.
        v) Finally A master-gain control feeding a buffer for matching to power amps?
        I plan to use a nice little integrated 40p.c. SS amp for driving the spkrs - at first. So, I might not need v)!
        Note that the main pr only get 20 watts in the entire mid/bass range and 'neigbours pissed-off loud' is achievable, so that should be enough!
        I will be able to make the new bass sphere's baffle a circle of 60 cm diameter/ 30cm radius. With the smaller baffle of the sphere sitting in-to it, to a depth of 19mm at least - like a big gelato orange on a flat plate'!?
        And I can also adjust the acoustic resistance - of the slot bewteen the baffle and the same sized paver - by closing off say 60% of the slot's circumference with one sand-snake / draft excluder tube per channel.
        The baffle will also add 5 cms depth - at a minimum - to the port making it 20 cm's in actual length. IF I 'jig-saw out'* the more central bits of the concrete baffle where the driver rebates and holes are, we can also ADD some Vb which helps Fb fall a bit as well!
        The BIG baffle will be made of layers of MDF and Chipboard and solid door! the down-facing sphere will be 'gripped' at its sides by a layer of MDF/chip at least
        Coming outwards - (down!) - from each sphere's flat circular baffles face (*edges left only?) there will be a ring-layer of 25mm coreboard laminated both sides, a 3/4/5 mm MDF 2 layer ring at least, and then the 25mm MDF laminexed on the glue side. To the sides of the sphere is the 19mm 'wide-ring layer' - a cut-out that fits around/grips the sphere sides just behind the old concrete baffle. will resist rocking of the sphere. It will have another matching sphere on top of it, mind.
        The new baffle will be pressure-glued (?#) with water-based contact cement, except for the laminex interface, which may require an active etching cement, or some roughing up!
        I have inherited an old screw-press from a picture-framer /silk-screener / DIY guy, so laminating panels together will now be a snap. I've used bags of sand and concrete blocks for this before!
        Dave Berriman at HFN&RR uses layers of 3MM MDF to apply two / 3 layers of sheerforce damping to each cabinet wall, in his DBn kit designs. Viz the DB7.
        I can vouch that this layering can be very cost effective, if not time effective!
        Wish me luck!
        
        Warmest
        Timbo in Oz
        The Skyptical Mensurer and Audio Scrounger
        And gladly would he learn and gladly teach - Chaucer. ;-)!
        'Still not saluting.'
        
        My matured plans for a 2nd pr of Model 3 spheres. (Open in New Window)

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