No compromises front loaded double 18” cab

[Ossian Ott]

Re: No compromises front loaded double 18” cab

What is the opinion on b6 aligned bass reflex cabs by the experts around here? I've heard mixed opinions but if they are properly designed you could drop the tuning frequency to about 0.7. A 2x18 could then be made very small and still keep up both in spl and extension. Is there any drawbacks with the b6 alignment method if the cabinet is very well designed?

 

[Peter Morris]

Re: No compromises front loaded double 18” cab

What is the opinion on b6 aligned bass reflex cabs by the experts around here? I've heard mixed opinions but if they are properly designed you could drop the tuning frequency to about 0.7. A 2x18 could then be made very small and still keep up both in spl and extension. Is there any drawbacks with the b6 alignment method if the cabinet is very well designed?

The simple answer is there is no free lunch – If you have a standard reflex enclosure you can make it smaller (B6 alignment) still get the same output and extension by using EQ. You will not exceed Xmax … BUT … that 6 dB of boost needed to get the alignment flat will mean you need up to 4 times the power for the lowest frequencies … and probably exceed the thermal rating of the driver trying to achieve the same output.

If you don’t make the box smaller then you can tune the box lower and your speaker will have more bass extension and still have a flat response with the B6 alignment boost (6dB), but it becomes very easy to exceed Xmax. You will be able to go low but your maximum output will be compromised.

If you model all of this you will see what happening, but be careful, the 6 dB EQ boost added in many of the simulation programs does not include 6 dB boost in the power calculations.

 

[Jack Arnott]

Re: No compromises front loaded double 18” cab

I notice alot ocf the wheels used are the 3" blue ones....

Definitely use the 4" Blue Guitel's. There is a huge difference between the 3 and 4, even just getting the box started on a ramp is so much easier.
I have some subs that came with only three or three and a half, and I curse them every single time I use them.

 

[John Halliburton]

Re: No compromises front loaded double 18” cab

Definitely use the 4" Blue Guitel's. There is a huge difference between the 3 and 4, even just getting the box started on a ramp is so much easier.
I have some subs that came with only three or three and a half, and I curse them every single time I use them.

Agreed. I have one rack dolly that still has 3.5" Colsons, and I have the 4" waiting to swap. It makes a big difference. My two 1/4 by 1/2 dollies have 5" casters and a pair of ribs running on the underside to stiffen it up-both make a difference when they are really loaded.

Best regards,

John

 

[Uwe Riemer]

Re: No compromises front loaded double 18” cab

The simple answer is there is no free lunch – If you have a standard reflex enclosure you can make it smaller (B6 alignment) still get the same output and extension by using EQ. You will not exceed Xmax … BUT … that 6 dB of boost needed to get the alignment flat will mean you need up to 4 times the power for the lowest frequencies … and probably exceed the thermal rating of the driver trying to achieve the same output.

If you don’t make the box smaller then you can tune the box lower and your speaker will have more bass extension and still have a flat response with the B6 alignment boost (6dB), but it becomes very easy to exceed Xmax. You will be able to go low but your maximum output will be compromised.

If you model all of this you will see what happening, but be careful, the 6 dB EQ boost added in many of the simulation programs does not include 6 dB boost in the power calculations.

IIRC not all B6 alignments go for the smaller enclosure and compensate this with more power.
The vented design usually ends up anyways as 6th or 8th order HP, so wouldnt be it a good idea to account for that at the enclosure design stage.
Aiming for perfect B6 is questionable IMO, because the driver changes with high power.

 

[Jim McKeveny]

Re: No compromises front loaded double 18” cab

What is the opinion on b6 aligned bass reflex cabs by the experts around here? I've heard mixed opinions but if they are properly designed you could drop the tuning frequency to about 0.7. A 2x18 could then be made very small and still keep up both in spl and extension. Is there any drawbacks with the b6 alignment method if the cabinet is very well designed?

I will knowingly expose myself here, but WTF: Higher-order bandpass boxes tend to exhibit group delay results that may be regarded as undesirable by a subset of "speaker people".

Flame away!

 

[Max Warasila]

Re: No compromises front loaded double 18” cab

I don't mean to be rude, but did my question get lost in the shuffle?

 

[Art Welter]

Re: No compromises front loaded double 18” cab

I will knowingly expose myself here, but WTF: Higher-order bandpass boxes tend to exhibit group delay results that may be regarded as undesirable by a subset of "speaker people".

Flame away!

The B6 is basically a "too small" bass reflex needing a 6 dB LF boost to flatten out the response.
Turns out that my last cabinet re-build, cramming two 15" in a slot loaded push-pull arrangement in the the same box previously used for 2 Lab 12" (the design in the stickies in this DIY section) is "kinda" a B6 (QTS is too high), well more a B10...
http://www.diyaudio.com/forums/subwoofers/255010-compact-2x15-ppsl-using-dayton-pa385-8-drivers.html

To quote DJK:

"Take a driver with a Qts=0.312 and put it in a sealed box sized to make it a D2 alignment. Now vent it to Fs and it becomes an SBB4, with the best transient response of all the vented alignments. Now add a Q=2 high-pass filter at Fs, and you now have a B6. It still has the transient response of the SBB4 because the box volume and tuning have not changed, you have just applied some EQ, mechanically it is the same. Fb=Fs=Faux.

The Qts=0.312 is the intercept of two equations determining box size and low-frequency cut-off. The box size equation says that the size is proportional to the square of the Qts, so a Qts=0.3 can run in a box about half the size of a Qts=.4 driver. The problem is the bass response rolls off at a higher frequency the lower the Qts is. The two equations have an intercept at Qts=0.312, yielding the best bass extension and minimum box size, and allowing you to build a B6 alignment."

Higher-order bandpass boxes are different, there is no direct output of the driver, only output from 1 or more tuned resonant chambers.
Their bandwidth is limited, and the rapid phase change at the top of the bandpass does make alignment to the top cabinets "difficult".

Art

 

[Art Welter]

Re: No compromises front loaded double 18” cab

As a casual reader of this I'd like to ask a question about having four ports - why? I'd assume it has something to do with larger volumes of air with the same tuning, right?

Large displacement drivers need a large "slug" (or slugs) of air, or the port(s) will be "blown out" at high excursion, "port compression".
Four corner triangle ports will brace the cabinet well, and are a near optimal use of the cabinet frontal area. They also provide symmetrical cone loading, which reduces cone rocking motion. Rocking can cause distortion, and even voice coil rubbing and destruction.
Four round ports require less frontal space than one port of the same volume, and are easy to figure with most calculators, as corner ports tune a bit lower than the volume would suggest.

 

[Jim McKeveny]

Re: No compromises front loaded double 18” cab

Large displacement drivers need a large "slug" (or slugs) of air, or the port(s) will be "blown out" at high excursion, "port compression".

...Which seems bi-directional on its face. Should not venting have eased/radiused exits AND entrances?

 

[Max Warasila]

Re: No compromises front loaded double 18” cab

Large displacement drivers need a large "slug" (or slugs) of air, or the port(s) will be "blown out" at high excursion, "port compression".
Four corner triangle ports will brace the cabinet well, and are a near optimal use of the cabinet frontal area. They also provide symmetrical cone loading, which reduces cone rocking motion. Rocking can cause distortion, and even voice coil rubbing and destruction.
Four round ports require less frontal space than one port of the same volume, and are easy to figure with most calculators, as corner ports tune a bit lower than the volume would suggest.

That question was from a long time ago - my question was why the math to calculate port length was not working to calculate the proper length for the cabinet design Luke used. The math says each should be 25+" long, but in reality, the effective port length is closer to 10". What gives?

 

[Peter Morris]

Re: No compromises front loaded double 18” cab

IIRC not all B6 alignments go for the smaller enclosure and compensate this with more power.
The vented design usually ends up anyways as 6th or 8th order HP, so wouldnt be it a good idea to account for that at the enclosure design stage.
Aiming for perfect B6 is questionable IMO, because the driver changes with high power.

Yes not all B6 alignments do that .... I was more or less talking about alignment 15 described by Thiele, most people talk about that one as "the B6"

http://www.northreadingeng.com/A_N_Thiele_Vented_Part_I.pdf

 

[Art Welter]

Re: No compromises front loaded double 18” cab

That question was from a long time ago - my question was why the math to calculate port length was not working to calculate the proper length for the cabinet design Luke used. The math says each should be 25+" long, but in reality, the effective port length is closer to 10". What gives?

You don't mention what math was used, that sounds like too much of a difference for the math to have been correct.

I have found simulations to be perhaps 30% off on length depending on the actual port shape, location, and entrance used, so I always build long and cut down to the desired tuning for a prototype.

 

[Art Welter]

Re: No compromises front loaded double 18” cab

...Which seems bi-directional on its face. Should not venting have eased/radiused exits AND entrances?

Large radii exits and entrances do help keep the port working well at high velocity.

One more option that makes reality further from simulation...

 

[Ossian Ott]

Re: No compromises front loaded double 18” cab

The simple answer is there is no free lunch – If you have a standard reflex enclosure you can make it smaller (B6 alignment) still get the same output and extension by using EQ. You will not exceed Xmax … BUT … that 6 dB of boost needed to get the alignment flat will mean you need up to 4 times the power for the lowest frequencies … and probably exceed the thermal rating of the driver trying to achieve the same output.

If you don’t make the box smaller then you can tune the box lower and your speaker will have more bass extension and still have a flat response with the B6 alignment boost (6dB), but it becomes very easy to exceed Xmax. You will be able to go low but your maximum output will be compromised.

If you model all of this you will see what happening, but be careful, the 6 dB EQ boost added in many of the simulation programs does not include 6 dB boost in the power calculations.

Interesting, thank you Peter.

What is creating the heat in the voice coil, the heavy movements inside the magnetic field or merely the current applied to it? Is what I guess would explain the phenomenon to me. I was under the impression the movement caused the heat but it seem to be quite the opposite, ironic to mislead myself like that lol.

Does the typical thermal increase of with and without b6 alignment show in the simulations? I don't know how to use hornresp

The box that use this technique that caught my interest was Tony Wilkes q18 which seem to be a nice design.

 

[Peter Morris]

Re: No compromises front loaded double 18” cab

Interesting, thank you Peter.

What is creating the heat in the voice coil, the heavy movements inside the magnetic field or merely the current applied to it? Is what I guess would explain the phenomenon to me. I was under the impression the movement caused the heat but it seem to be quite the opposite, ironic to mislead myself like that lol.

Does the typical thermal increase of with and without b6 alignment show in the simulations? I don't know how to use hornresp

The box that use this technique that caught my interest was Tony Wilkes q18 which seem to be a nice design.

The heat come from the DC resistance of the voice coil and the current thorough that resistance.

Here is my best guess of the Q18 at 1000 watts. In this case it has 4dB less output than a "standard" alignment at 50Hz

Also note that where the maximum power boost occurs (35Hz) is where one of the minimum impedance dips occurs ... i.e. maximum thermal stress.

 

[Art Welter]

Re: No compromises front loaded double 18” cab

Interesting, thank you Peter.

What is creating the heat in the voice coil, the heavy movements inside the magnetic field or merely the current applied to it? Is what I guess would explain the phenomenon to me. I was under the impression the movement caused the heat but it seem to be quite the opposite, ironic to mislead myself like that lol.

Does the typical thermal increase of with and without b6 alignment show in the simulations? I don't know how to use hornresp

The box that use this technique that caught my interest was Tony Wilkes q18 which seem to be a nice design.

Ossian,

Unfortunately, moving paper back and forth is not very efficient in terms of sound production, and heat is the byproduct in the motor assembly.
Peter's simulation shows the "nice design" would require 4000 watts to produce 124 dB at 35 Hz.
130 dB is equal to 10 acoustical watts, assuming no thermal compression, it would take 16,000 watts to produce that SPL with the q18, 15,990 watts lost (turned) mostly to heat.
The voice coil would no doubt be lost to heat in a second or so, as the impedance minima is also at the excursion minima, less movement to "fan" heat away from the motor.
The normal size bass reflex would loose 3990 watts to heat.
A 6 dB more sensitive horn design would loose 990 watts to heat.

There is a big power/heat penalty to go low, loud and small.

Art

 

[Jim McKeveny]

Re: No compromises front loaded double 18” cab

Ossian
There is a big power/heat penalty to go low, loud and small.
Art

Choose 2, at least for now. Maybe active servo-style solutions will rear their heads again....with their own unique parameters.

 

[Malcolm Brown]

So .. Been reading a while. Thinking about a quad of these boxes for next season. What is the consensus on the preferred driver? Luke settled on the Faital 1030 for it's round low end. Is there a better choice today ? I noticed that the B&C that was in the running is not on USSpeakers web site... Is there an equivalent ? Thanks, Mal

 

[Max Warasila]

I have been cooking up a prototype sub for a couple of months and found time to finish it. Complete with name brand components stolen from a regional audio provider! Going try loading them with RBX100s soon. Might have a better chance keeping up with the DIYs then.