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Old 14th March 2002, 04:03 PM   #1
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Question Subwoofer question

I want to build a subwoofer but was wondering wich design is the best :
- Down Firing or Front Firing.
And :
- Sealed or Vented.

I will by a plate amp (like the one at PE) from my local store.
It will be drived by a 10" driver.

Thanks for suggestions.

MisterB
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Old 14th March 2002, 07:57 PM   #2
Super is offline Super  United States
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There is no way to say that either sealed or vented is "best", each has its tradeoffs. A lot of it depends on the parameters of that driver.

Some speakers that arent designed to be downfiring can actually become damaged over time due to "cone sag". If you aren't sure that the driver has a suspension designed to be used in a down firing configuration, make it front firing.
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Old 15th March 2002, 12:20 PM   #3
Wizard of Kelts
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Sealed versus Vented? A little background.

Making deep, forceful bass is all about moving air. The deeper the frequency, the more air must be moved. At the same volume level, a loudspeaker has to move FOUR times, (not two times), as much air 20 Hz. as it has to move at 40 Hz. 40 Hz. is one octave higher than 20 Hz.

The amount of air a loudspeaker can move is determined by the area of it's cone times the length of it's back-and-forth movement, (cone excursion). Many of the new 10" subwoofers, for instance, can move more air than the 12" AND 15" woofers of years past because they have much greater cone excursion, even though the 10" obviously has less cone area.

In the lowest octave that the loudspeaker has to play, there is a significant difference in the behavior of the sealed system versus the cone system. If the sealed box and the ported box are both expected to play down to 20 Hz, at 40 Hz there will be a remarkable phenomenon begin to occur.

If we run a signal generator down the tones from 40 Hz on down to 20 Hz, we will see the sealed box move back-and-forth to a greater degree. The lower we go, the more the sealed system has to increase it's back-and-forth motion. At stated previously, at 20 Hz it needs to have FOUR times the back-and-forth movement that it does at 40 Hz.

The ported system? Different story. As we run the test tones down from 40 Hz to 20 Hz, we see that that the ported system does NOT require any greater back-and-forth movement at all. At 20 Hz, the cone of the ported system will NOT need to travel any more than it did at 40 Hz. Why? The port of the ported system tunes the air in the box to a certain frequency. At frequencies within an octave of that tuning frequency, the air in the box is conditioned to react to just a little excitation by the loudspeaker cone. So to speak, the air in the ported box is more "sensitive" to excitation by the cone than the air in the sealed box. This phenomenon only last for the octave above the tuning frequency.

For the lowest octave, where air moving ability is needed the most, the following equivalents are true. Assuming both the sealed and ported systems are set up correctly:

1) A 10" speaker in a ported box is equivalent to a 20" inch speaker, (with equal cone excursion), in a sealed box.

2) A 10" speaker in a ported box is equivalent four 10 inch speakers of equal cone excursion in a sealed box.

3) A 10" speaker in a ported box is equal to a 10" speaker of four times it's cone excursion in a sealed box. So if you have a 10" speaker with a half inch cone excursion, any 10" speaker with a cone excursion than 1/8"-a piddling amount-will produce more bass.

Normally, for any given enclosure volume, you will require a different woofer for a sealed system than you will for a ported enclosure. The "Thiele-Small parameters"-important electrical measurements-will have to be different. Speakers are available that are suitable for either enclosure type for just about any box volume.
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Old 15th March 2002, 12:57 PM   #4
Wizard of Kelts
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Why then, do we have such things as sealed box subwoofers?

First, the fact that the lowest frequencies come out the port of the vented speaker means that there is a slight delay in the delivery of those frequencies into the room. Many speaker enthusiasts feel that this muddies the bass.

If you examine the output of a single pulse for both systems, it becomes clear that the sealed system has a slightly cleaner response than the vented. It has been illustrated that at the lowest frequencies, the ear is less sensitive to this situation than at midrange frequencies-that is why there is no such thing as a vented midrange speaker. Nonetheless, it is undeniable that the sealed system produces a snappier response on the chart. Whether you can hear it or not is a matter of individual ability.

Also, deep bass requires large enclosures. A Dayton 12" subwoofer, for instance, requires a 10 cu. foot enclosure to go flat down to 16 Hz. Some owners consider this size impractically large. So they put the speaker in a smaller box. This reduces the amount of volume output possible in the lowest octave. Reduced volume output in the lowest octave requires less air being moved. With less air being required to be moved, the speakers in the sealed boxes can handle it. The speakers in the closed box could not handle the bass outputs that would be possible in a larger box.

It should be noted that some high quality subwoofer manufactuers, such as REL, have both sealed and vented models in their best line of subwoofers.

If all this sounds complicated, maybe the best thing to do is to tell us what size you plan to make your subwoofer box, what subwoofer speakers you have your eye on and the people on the forum can explain your choices to you. Have you downloaded a loudspeaker program yet?

Oh, yes, I thought I would just mention that Super is right about up/down versus front firing. The long excursions of today's subwoofers make it necessary to make certain that the manufacturer says up or down firing is possible.
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Old 15th March 2002, 02:36 PM   #5
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Default a few questions...

hey kelticwizard,
Quote:
At 20 Hz, the cone of the ported system will NOT need to travel any more than it did at 40 Hz. Why? The port of the ported system tunes the air in the box to a certain frequency. At frequencies within an octave of that tuning frequency, the air in the box is conditioned to react to just a little excitation by the loudspeaker cone.
you mentioned an octave above the tuning frequency the excursion of the woofer will not be any greater than the start of the octave, ok, so what happens below the tuning frequency? I know that below the tuning frequency, excursion becomes uncontrolled, I am just wondering If there was any certain rate of increase of excursion, just looking at it all I would take a wild guess at x8 but I may be wrong....

Also, what about resonant frequency, is it bad to tune a box to below the woofers Fs? if so, how does it affect it?
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Old 15th March 2002, 04:25 PM   #6
Wizard of Kelts
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Andrew:

More about the excursion below Fb later.

As to the question of tuning a box below the speaker's free air resonance, the answer is yes, you can do it. In fact, Thiele came out with a whole list of alignments that do just that. For various reasons, it is somewhat more desirable to find a speaker with an Fs that is below or at the desired lowest frequency we want the speaker to play, but we are not speaker manufacturers and cannot get drivers custom made for our enclosure needs, can we? We have to go with what is available. Fortunately there are a lot of drivers available.

If possible, try not to make the Fb too far below the speaker's Fs. If you have driver with an Fs of 30 Hz, a Qts of .4, and a Vas of 2 cubic feet, and you put it in a box with a VB of 2 cubic feet and tune it to 21 Hz, then the response will be 9 dB down at 21 Hz, and the entire bass will droop. You will need a big amp to make up the difference, but it is one way to get big bass from a comparatively small box. Forget about getting anything beneath 21 Hz on this speaker-big dropoff.

If I increase the Vb on this speaker to 3 cu feet, I will be 6 dB down at 21 Hz and my bass will droop a lot less.

If you are going to increase your box volume over Vas, and tune your box to below Fs, ideally your Qts should be above .4. but you can get away with anything being slightly off in ported boxes if it is not too severe.

If I had my druthers, I druther have my speaker's Fs at or below my box Fb. But I would not abandon a project if I could not find a speaker to fit that requirement.

Have you downloaded a speaker design program? Test it and see what curves you get.

By the way, do you have anything specific in mind?
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Old 16th March 2002, 06:17 PM   #7
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kelticwizard

I guess I'll jump in here with a couple of design options I've been considering - perhaps they could be used for discussion or illustration.

Both are suggested alignments from Parts Express:

1.) Dayton MKII 10 inch - 1.4 cubic foot EBS tuned to 25 Hz - anechoic F3 bout 28
2.) Dayton MKII 12 inch - 2.0 foot sealed - thinking I'd EQ round 20-25Hz maybe as much a 5-6 dB

I am dealing with the all important SAF here, so my options are limited and musical fidelity is paramount to me, but I do want some serious low end.

Seems based on modeling with WinISD that #1 is more of a gentle rolloff ported design that a real EBS. Does anyone have opinions of the relative performance for these two designs??

thanks
peter
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Old 16th March 2002, 11:12 PM   #8
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Quick question here:

I built an enclosure from a design originally meant for a downfiring setup...will there be any problem if I simply lay the box on its side for a front firing setup? I would rather lay it sideways, because it fits better in my room that way. Also, I want to be able to see the sub. It looks cool.

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Old 17th March 2002, 12:39 AM   #9
Super is offline Super  United States
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As long as no ports are blocked and it isnt in danger of tipping, theres no problem in turning it on its side. Go for it.
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Old 17th March 2002, 01:10 AM   #10
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Cool! I asked this before, but I didn't fully understand about wiring the voice coils in parallel. I have two leads from the amp, + and -, would I split each lead into 2, like with a y-cable, then attach two + to each + connector on the sub and the 2 - to each -connector?

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