Designing Dual 18" vented boxes: ED19O.2 vs. SS RL-p18

[Mike P.]

A couple of thoughts, just my opinion. You want loud and deep but your modeling does not reflect the subs and amps capabilities. With a pair of the subs in 21.7 cu.ft tuned to 15 hz and with 4.5 db of boost applied, the subs will handle 650 watts each. This assumes the D2 sub wired for 4 ohms and one sub to each channel of the amp. With a Hi-Pass filter applied at 16 hz the subs never exceed Xmax.

Which brings us to porting. There is no issue with an air speed of 26 m/s, many a sub has been designed and built with this speed with no problems. Two six inch ports would have an air speed of 36 m/s, too high. A 10 inch diameter port 35.75 inches long would be just under 26 m/s. You could expect 124 db down to 15 hz with one cabinet. Now that's loud and deep! Should you go this route, I suggest you build one cabinet first, it may well be enough for your needs.

[nibix kfrundi]

Wow Mike. I am clearly missing something in my WinISD file of this build. As you can see from my prior posting, my specs are completely different from what you have determined. I could learn a lot by examining the file that you have made for this driver and this proposed cabinet. Is it possible for you to send them to me so that I can open them and examine them on my computer?

Thanks,

Geoff

[nibix kfrundi]

After thinking a bit about it, I understand the principle of keeping the two drivers independent in the cabinet and driving them from each of the channels in the EP2500. So with the D2 wired in series, I am at 4 Ohm and then driving each driver with 650 W. This is exactly what dradius had in mind. I would still like to examine the driver and project files for this build on WinISD, as I cannot generate the same graph that Mike sent in his last post, but at least a bit of the mystery is receding. Again, many thanks.

[Mike P.]

The file is a non auto calculate version with the parameters you listed from the E.D. website. Note that the Re stays at 3.5 ohms since each sub has its own channel at 4 ohms and the input power for both is 1300 watts. here is the file:

Elemental Designs 19Ov.2 Non AC.wdr

Here is the project file:

E.D.19O.2 - 21.7 Cu.Ft..wpr

[SteveCallas]

nibix wrote:

I have also finished reading the excellent book by Floyd Toole (
Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms
Amazon ) and in the interest of good bass for as many as possible, have elected to position two subwoofer enclosures in the middle of the front and the middle of the back wall.

Don't assume any particular placement will be the best in your room or you may be setting yourself up for failure. Every room is different and you really have to try it out and take a few measurements to be sure.

Also, while some manufacturers may do it, the last thing I would ever want to do is apply EQ boosting around the tuning region of a ported sub. Excursion is already at its greatest just above tuning, impedance is at its lowest at tuning (and cone movement is next to nothing, so the voice coil is getting pretty hot), and then the driver becomes susceptible to unloading below tuning. Not really the greatest place to apply some boosting.


Mike is leading you on the right path - get increased low end output through an increase in sensitivity. You do this by using a larger enclosure. The larger enclosure also allows you to use more adequate porting.

[nibix kfrundi]

Mike,

Those are very helpful. I have a few questions scattered through my ramblings below:

Why do you only specify one voice coil in the driver file?

I am glad that you have confirmed that the system input power on the signal tab is the power into the speaker from the amp. I found the following statement from the help file on this subject to be very confusing:

Term "power" should more correctly be voltage. This term "power" comes from definition by Richard Small, who defined the input power to be P=Eg²/Re (this is to simplify things, because impedance varies greatly depending on the frequency and the box), where Eg is RMS output voltage of your amplifier, and Re is DC resistance of voice coil. Actually, driver's impedance will vary greatly (see impedance graph), depending on the frequency, so this is generally not the power which is taken from the amplifier.

There are also absolutes in the help file that are good to know can be exceeded, such as:

Very important here is to make your ports large enough, so that peak air velocity won't exceed 5% of sound velocity in air. That is, about 17 m/s assuming normal environmental conditions. You can check it by using "Rear port - Air velocity" graph, by first setting simulation power to anticipated maximum power, see "signal"-tab. In this case, Pe is 125 Watts.

I wonder how many people base their understanding of WinISD and this subwoofer design primarily on this help file? It certainly lead me down some wrong pathways. I just found the excellent tutorial by Andrew Vaserfirer here. His recommended upper limit of air velocity of 20-25 m/s is closer to our value of 26 m/s.

Not specifically in the help file, but accessed from here after a google search states:

A high groupdelay will result in a boomy/muddy sound. The groupdelay is created by the basreflexport and is thus one of the downsides of using basreflex cabs.
Most people however are used to this sound since basrelfex is used widespread in daily live.
Not only the highed of the groupdelay is to be considerd. The shape is also important. It's better to have a slowly forming round peak then a sudden needle-like peak that's there for just a few frequencies..Generally the groupdelay will reach it's maximum around the tuning frequency

A rule of thumb coming from Hifi is: frequency x groupdelay = 400 (max). Less being preferable. At 40 Hz that would be 10 miliseconds. A little bit higher (up to 600) around the tuning frequency can be overcome.
From this rule of thumb you can clearly see that the highed of the groupdelay becomes more important at higher frequencies and less important at lower frequencies. This is because the sensitivity of your ears increases as the frequency rises.
A high groupdelay in the 80 Hz and up area for instance will ruine the "kick'. Making it sloppy having less impact. Making the volume bigger or tuning the port lower, will increase the groupdelay. As you lower the tuning frequency the groupdelay will shift down along side with it.

Andrew doesn't talk about group delay in his tutorial. The group delay of the cabinet design that you attached previously reaches 80 ms at 12 Hz. This is partly influenced by the high pass filter, which is certainly important for keeping cone excursion under Xmax. Perhaps at this frequency the group delay is barely audible and will not muddy the sound. How do you look at group delay and manage it for a "tight" sounding bass response?

With the parameters that you have worked out for the cabinet and driver, it is interesting to see how little difference appears between a sealed design and a ported design. For a sealed design, a slightly smaller box (19.6 cu ft) reduces the cone excursion to be similar to the vented design. Everything else equal the group delay falls to 40 ms at 12 Hz and the difference in SPL is a maximum of 6 dB at 16 Hz and only 2 dB at 30 Hz. I applied a 4th order low pass filter at 60 Hz to both designs as well.

They look like this:



What do you think is the audible nature of this difference?

Again, many thanks for all of the insights.

[nibix kfrundi]

Steve, I have read many of your posts and learned a lot from them. Thanks for weighing in.

SteveCallas wrote:

Don't assume any particular placement will be the best in your room or you may be setting yourself up for failure. Every room is different and you really have to try it out and take a few measurements to be sure.

I do understand that the room will have a substantial influence. I will try to maintain flexibility of placement. What do you think about having two cabinets versus one. It seems like two offers more flexibility in tuning to the room, and it should enable a more even bass distribution across the listeners.

Also, while some manufacturers may do it, the last thing I would ever want to do is apply EQ boosting around the tuning region of a ported sub. Excursion is already at its greatest just above tuning, impedance is at its lowest at tuning (and cone movement is next to nothing, so the voice coil is getting pretty hot), and then the driver becomes susceptible to unloading below tuning. Not really the greatest place to apply some boosting.

In Mike's design, the cone excursion maxs (22 mm) at 11 Hz, has a local nadir (6 mm) at 15 Hz and then climbs to 18 mm at 22 Hz before falling at higher frequencies. Impedance is 4 Ohm at 15 Hz. I'm afraid I don't know what "unloading" is, but it doesn't sound good. However our flat FR curve doesn't look nearly so flat without the 4.5 dB parametric boost. I hate to give that up. Now with a 40 cu foot cabinet, I regain all of the low end boost and my port shortens up nicely.

Let's see, 40 cu ft would be 72" x 60" x 20". That might stretch the limits of practicality. Is there a blend of cabinet volume and amplifier boost that makes you more comfortable near the tuning frequency?

Incidentally, the impedance and the cone excursion are the same when comparing the larger cab with no eq to the smaller cab with eq boost. Am I still risking an unloading event with the larger cab?

[SteveCallas]

What do you think about having two cabinets versus one. It seems like two offers more flexibility in tuning to the room, and it should enable a more even bass distribution across the listeners.

Absolutely, I'm all for multiple subwoofers.

How large an enclosure can you live with? How about 400 liters?

[Mike P.]

Why do you only specify one voice coil in the driver file?

The file uses the parameters for 2 coils wired in parallel, I just didn't enter that it had 2 coils. It doesn't affect the modeling.

Group delay in speaker design is one thing, in subwoofers it's another story. It's not something that you would notice.

I'm not able to see the graph you posted above due to a bad internet connection.

As for boosting the low end, with a Hi-Pass filter you're within Xmax, cone excursion and impedance doesn't drop below 4 ohms. I don't see this being a issue with program material. Just don't play a 15 hz sine wave with 1300 watts and your sub will be fine.

[nibix kfrundi]

SteveCallas wrote:

Absolutely, I'm all for multiple subwoofers.

How large an enclosure can you live with? How about 400 liters?

I should be well over that. The 21.7 cu ft design that we have been kicking around is 615 liters and the monster 40 cu ft design that I mentioned in my last post would be 1133 liters.