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Old 18th October 2004, 11:06 PM   #1
Baronty is offline Baronty  United Kingdom
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Default Seas 2Way.

I know i posted a first build type thread the other day (3way)
but i decided to start a bit simpler so im gonna make some 2way bookcase style speakers for a mate.

Im thinking seas 27TFFC tweeter and a P17RCY MidWoofer.

This will be in a 7.3L vented enclosure tuned to 66Hz with a 6.8cm vent 28cm long giving a vent mach of 0.04 (calculated in WinIsd Beta).

I was thinking of using a 2nd order LW crossing at 3KHz with a zobels network on the midwoofer.

I have attached and image of the xover and zobels network i designed for it and a SPL plot for the P17RCY from WinIsd.

If someone could have a look and check that i havent gone horribly wrong it would be very much appreciated.



Oh and baffle step effects - how much should i worry?
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File Type: gif 2way 2ndlw seas.gif (33.8 KB, 614 views)
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Old 19th October 2004, 12:02 AM   #2
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I would have used a 12l box tuned to 44ish Hz.
even if I went for a 7l box I wouldnt tune it above 55Hz
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Old 19th October 2004, 07:46 AM   #3
Nisbeth is offline Nisbeth  Denmark
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12 liter box and 44-45 Hz tuning would be my suggestion as well


/U.
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Old 19th October 2004, 08:12 AM   #4
Baronty is offline Baronty  United Kingdom
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Why? - I have ploted the different curves in WinIsd and 7.3 @ 66Hz remains flatter for ~100hz longer - admittedly the rate of roll off is steeper but the F3 point is 75.18hz compared to 91.24Hz for the same driver in a 12L enclosure tuned to 44Hz.

Can you explain why you recommend 12l / 45Hz? Im new to this and not sure what im missing here

Ive ploted the curves in winisd and attached them below

Green - 12L / 44Hz F3 @ 91.24hz
Red - 7.3l / 55Hz F3 @ 79.92Hz
Blue - 7.3L / 66hz F3 @ 75.18Hz
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File Type: gif comparison p17rcy.gif (36.0 KB, 482 views)
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Old 19th October 2004, 05:35 PM   #5
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Default Intent

Not sure of your intentions here, but what you have modeled has no bass. If you wish to pair this up with a subwoofer as a satellite, then the response is appropriate. Otherwise, it will be all midrange and highs, with no balance.

I think the other posters were suggesting some kind of alignment to extend the low end response.

Tim
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Old 19th October 2004, 06:41 PM   #6
Baronty is offline Baronty  United Kingdom
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Im trying to model a simple twoway full range speaker in a small enclosure. when you say it has no bass - at what point does the mid / midbass become bass / sub-bass?

The WinIsd plots above show only a 10 - 20Hz extention from my original plot (blue line) and the suggested 12L / 45Hz enclosure (green line) is an F3 75.18 really produce poor bass response?

The blue curve maintains a 92dB profile to a lower freq than the green

Sry still kinda new to all this - just trying to understand how to extend the bass repsonse while maintaining a relatively flat profile.

All the help is greatfully appreciated
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Old 19th October 2004, 11:03 PM   #7
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Default Response

Couple of basics might help - highly simplified. Bass extends from 20 Hz up to aound 200 Hz. Most commercial speakers run to +- 3dB from 30Hz and below, for the large floor standers, to 60 or 70 Hz for the small bookshelf/mini-monitors, in terms of bass extension. Music can run to very low frequencies with a big pipe organ, like 16 Hz, but generally speaking, low E on the electric bass guitar is good enough, or, at or near 40 Hz. So to reproduce 95 % or more of all music, that should be your target.

The Thiele-Small parameters are basically an electrical means to measure what a woofer is capable of doing. For sealed enclosures, look for Qts values from .35 to .50. Since Qts is a rough reflection of damping, a sealed box generally requires more excursion, therefore less motor strength, and lower damping than a woofer used in a ported box.
Ported enclosures usually contain woofers with Qts values from mid .30s on down. When Qts drops into the .20s, the woofer is said to be highly damped, which works well for efficiency but not so well for excursion, where the motor strength is actually working against a reasonable freedom of movement.

For the short term, to get extension near 40 Hz, look for Qts around low .30s, Xmax in the 6mm or more range, Fs in the 30s Hz or below, and efficiency as high as you can get. Peerless has affordable woofers that should meet yor needs, also Vifa, but the Seas are really quite good. The metal cones offer more of a challenge to a beginning DIYer due to the resonances and break-up problems at higher frequencies that must be successfully dealt with in the crossover. See Siegfried Linkwitz's "Orion" dipole design.
One popular Seas design is the Thor MTM, which may give you some idea of the specs to look for, and the woofers are paper.

The box size will increase at a rate of 8 times for every octave you drop in bass extension while maintaining the same SPL(dB), so it will not be possible to achieve low frequency extension, low power use, and small box size simultaneously, Hoffman's Iron Law says, so pick two.
Most people cheat on the power end of it by simply giving it more gas, failing to realize the result of this is rising compression and distortion, but that is another day.

Speaker-building is a collection of compromises, but at least you get to pick those right for you.

Tim
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Old 19th October 2004, 11:49 PM   #8
Baronty is offline Baronty  United Kingdom
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cheers for the pointers - ive been using the T-S parameters and WinIsd to calculated Spl and freq response but im still having great difficultly finding a driver.

As you said its a matter of compromise but i really cant go for an enclosure of more than 20L.

This restricts a number of drivers i would have other wise chosen - the Vifa c20WJ-19 for example which maintains the 91dB SPL and can extend bass response down to an F3 of 33.1Hz unfortuneately this require a cab or over 124L

Im having great difficulty in find a driver that retains the sensitivity while having a extended bass response.

Any driver suggestion? - I have modelled the majority of the ones i have been able to find readily availiable and it appear that most with a 90+spl and an f3 of below 40hz require a cab of 50L and upwards
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Old 20th October 2004, 12:49 AM   #9
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Default Efficiency

Baronty,
Try the 850122 Peerless 7". The Qs are a little high, the efficiency pretty low, but that is the cost of the box size remaining fixed if we still want to attain somrthing close to 40 Hz.
See how it models in a 20 l ported enclosure tuned to 40 Hz.
Fs is 38 Hz, so not much more there. Handles power well though, and has 5mm xmax, so the numbers look OK.
It will work in either a sealed or ported enclosure. While you have not mentioned the dreaded sealed box, they do actuallt work well. Just not as efficient.

Tim

Edit: The Thor has magnesium woofers.
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Old 20th October 2004, 07:08 AM   #10
manek is offline manek  India
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try the vifa P17wj-00-08 6.5" mid/bass
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