RULE - THERE"S NO REPLACEMENT FOR DISPLACEMENT !!!!!
Originally Posted by rollomat
Would you go ported? I heard that sealed ones offer more punch while ported ones dig deeper? Also a myth?
But I also have a general question: Is it correct that the punch is in frequencies from 50 Hz to 100 Hz? Do all subwoofers that offer for example 105 dB in the range from 50 Hz to 100 Hz have the same punch? Volume = Punch? Or are factors like the size and weight of the speaker important as well?
The larger the cone and the wider the X-max travel of the driver, the more air a sub displaces and the more effect it will have by better coupling to the room air and inducing a pressure wave (sound). That's why 18" drivers with 34mm Xmax each way can move so much air. (please watch first video in above post - JTR Captivator vs Sleeping Bag)
Here is some info the Master List of bass movies with frequency charts, a graphic showing common musical instruments, their frequency range and where they impact you. Below that is a stock figure from a report about which parts of the body resonate with which frequencies creating greater sensation of "chest slam etc..." So while chest slam
may occur best at 50-100 Hz
, that feeling of head pressure
doesn't resonate until 20-30 Hz
Ported subs have more output than a sealed sub but roll off dramatically after the port tuning frequency. Sealed subs have less output than ported but they roll off gradually into the single digits (with room gain). You can add multiple sealed subs to compensate for the loss in output.
Theoretically, every doubling in displacement (two subwoofers) or doubling in power (twice the watts) equals approx. +6 dB increase in SPL volume. This is however dependent on wether the subs are co-located (in the same position), or located in different locations where room acoustics might cause them to cancel each other out at certain frequencies. Seaton Sound's F18's are designed to be stacked together on either side or under the screen so this is not an issue.
Does that mean ported subs are better? Not necessarily so. It depends on the size of your room, the extension of the subwoofer, and other things like room acoustics and slab vs raised subfloor.
The frequency wavelengths produced by subwoofers are often much longer than the longest room dimension of your home theater. Therefore your room and the subwoofer are coupled as a system producing low bass. If you have a reasonable sized room, at some point the boundary reinforcement of the room gives some room gain. The smaller the room the higher up this gain starts and the more SPL it contributes at the lowest frequencies. The larger the room the lower this gain starts and the room may contribute very little room gain.
With a room size at 25' x 15' x 9.5" you are trying to pressurize a volume of 3,563 cu ft
which is a medium sized room. Your longest dimension will be be the diagonal axis between the corner ceiling and opposite corner floor:
(25x25) + (15x15) + (9.5x9.5) = 940.25
Diagonal = square root of 940.25 or 30.66 ft
Room gain is normally 12dB/octave starting at the frequency where the longest room dimension is 1/2 wavelength. A wavelength in feet is the speed of sound (1130 feet per second) divided by frequency. In your room the longest dimension is 30.66 ft, so room gain will start at 18.43 Hz
(30.66' x 2 = 61.32' and 1130/61.32 = 18.43 Hz.) and be up +12 dB by 9.2 Hz
.. Gain increases every octave below that frequency so you could be up +24 dB by 4.6
Hz (an octave lower equals one half the frequency, an octave higher equals double the frequency).
But remember, these are all 'theoretical numbers' representing a best case scenario. Many factors influence room gain from sub placement, location of nearby surfaces, floor or wall construction (slab/raised, drywall/stud or cinder block/basement HT) so real world results will usually be lower.
Sealed subs normally roll off gradually below 20+ Hz and a really good design may have extension down to the single digits. However the output of a sealed sub us usually down down 10-15 Db at 10 Hz and down 20-30 dB by 5 Hz. So in your room (all things being perfect
), room gain will compensate for the roll off on TWO
Frequency Response - One Sub
18 Hz.....0 dB.........0 dB...........0 dB
9.2 Hz...-15 dB......+12 dB.......-2 dB
4.6 Hz...-30 dB......+25 dB.......-5 dB
Now if you want to get a flat frequency response down to10Hz and below you are going to need more sealed subs. Each doubling of sub displacement gets you up to +6 dB output across the bandwidth. So two sealed 18's in your room would theoretically look like this:
Frequency Response - Two Subs
18 Hz.....0 dB.........0 dB...........0 dB
9.2 Hz...-9 dB........+12 dB.......+3 dB
4.6 Hz...-24 dB......+25 dB.......+1 dB
This assumes that you have two subs with extension to 4.6 Hz and which are strategically placed to eliminate phase cancellations in an ideal room that's perfectly sealed (no leaks through ceiling recessed fixtures, wall outlets, under door jambs etc.).
In in a very large room where the longest dimension is 49 ft, don't expect to get any appreciable room gain until approx. 11.53 Hz (49 ft x 2 = 98 Ft, 1130/98 = 11.53 Hz.). This could also be true if your theater opens up to other areas of the house like a stairwell, pass through to a kitchen.
Therefor, if you have an average sized room then using a sealed sub (or multiples thereof) will give you the best of both worlds - more output from multiple drivers plus more extension supported by room gain. Effectively the room gain compensates for the roll off in a sealed sub and you can get flat extension into the single digits.
But if you don't have a reasonably sized room volume, or can only afford to buy one or two subs versus four to eight sealed subs, then ported may be your best bet.
The reason why we're all so excited about the JTR Captivators
is because there are other ported subs on the market that have the SPL volume but they're only tuned to 20 Hz and roll off below that. There are also a few ported subs with sub 15 Hz extension but they do not put out a lot of dB's at the lowest frequencies. The JTR Captivators promises to give us the best of both worlds Volume AND extension to 15 Hz.