sub woofer questions

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tosob

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sub woofer questions
« on: 16 Nov 2017, 01:15 pm »
I'm really new to a lot of this , so if the questions maybe  elementary .

In a setup using either the ob 7, x otica,  it calls for a sub-woofer for the bottom. I'm assuming that is for both channels. On a 7 channel avr that only has one sub out how do you split the signal on each channel  to both the powered woofer and the speaker for each channel?

Is there a noticeable difference in sound between the "H" and "W" subs?

If you make the "W" sub deeper , say to 18" and wider, making the sides 1.5" , will it effect the sound negatively.


Marty

mikeeastman

Re: sub woofer questions
« Reply #1 on: 16 Nov 2017, 01:59 pm »
You can use a Y splitter and feed both channels of the subs. Changing the dimension from the design will effect the sound negatively, Danny can tell you how much it will effect it.

Danny Richie

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Re: sub woofer questions
« Reply #2 on: 16 Nov 2017, 03:47 pm »
As Mike said using a Y splitter is no problem.

The difference in the H frame verses W frame is that the W frame makes the stack shorter. However it makes a larger unbraced side panel area that will need additional support or side wall thickness to handle the presser.

Making the box deeper will allow for slightly more output down low as the front to back wave separation is increased. However, this will also reduce the upper frequencies ranges that it can play up to. Increases the space between the sides will allow it to play higher up again. And it will also reduce some of the loading on the woofers keeping them from playing higher SPL's down low.

mlundy57

Re: sub woofer questions
« Reply #3 on: 16 Nov 2017, 07:29 pm »
When connecting open Baffle stereo subs to an AVR you have a couple options. Neither include connecting the OB subs to the AVR’s subwoofer output if you plan on using the AVR to listen to music and watch movies. It’s the watching certain movies part that causes problems. These OB subs are not designed to deal with the heavy infrasonic (below 20Hz) frequencies from movie special effects that are encoded in the LFE (subwoofer) channel. These special effect signals can cause the drivers to bottom out.

While OB subs connected to the Sub Out channel of an AVR will be just fine when listening to music or watching movies that do not have much in the way of special effects, if you watch a movie that has a lot of couch shaking, window rattling special effects the OB subs will not be able to handle it. They will go crazy and bottom out. For example, I set a pair H-Frames up awhile back and they were just fine until Star Wars VII was played. When the Star Destroyer started flying past the planet in the opening scenes the drivers started bottoming out. At first, I thought I had gotten the wiring for the servo circuit messed up but I had not.  I called Brian at Rythmik and he told me what I have discussed above.

The two ways to hook the OB subs to the AVR include using the high level (speaker level) inputs or the low (line) level (RCA) inputs from the subwoofer’s A370 plate amp.

If you want to use the high-level inputs on the plate amp you would connect a second pair of speaker cables to the L & R main speaker terminals on the AVR. Do not reassign the surround back channels to Main and then connect the speaker wires from the A370 to the surround back speaker binding posts (I blew up an amp doing this once).

If you want to use the A370’s RCA inputs, connect the “Line In” jacks on the A370 to the L & R main speaker pre-amp outputs on the AVR.

Set your main speakers to Full Range, Large, or whatever your AVR calls it to send a full range signal to the main speakers.

The LFE channel on an AVR is for a home theater sub designed for this purpose. Danny’s standard or sand box sub would be a great choice for this duty. If you are going to use your system for both music and movies you need the OB subs as part of the Left & Right main speakers and a sealed or ported HT sub for the LFE channel.

Mike

tosob

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Re: sub woofer questions
« Reply #4 on: 17 Nov 2017, 12:10 am »
Thanks everyone for thew replies. A couple of more questions.

For a 2 channel system, one amp per channel? From what I've seen in other posts that is what it looks like people are doing. The amp show Right and Left in and out on the rca  plugs. It also show Right and Left in for the powered ins.

Where is the line out to the speakers?

It also seems that most pics show the amps close to the subs. How far can you run the wires to the speakers from the amp?

thanks again
Marty

Danny Richie

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Re: sub woofer questions
« Reply #5 on: 17 Nov 2017, 03:29 pm »
Thanks everyone for thew replies. A couple of more questions.

For a 2 channel system, one amp per channel? From what I've seen in other posts that is what it looks like people are doing. The amp show Right and Left in and out on the rca  plugs. It also show Right and Left in for the powered ins.

Where is the line out to the speakers?

It also seems that most pics show the amps close to the subs. How far can you run the wires to the speakers from the amp?

thanks again
Marty

To do two channels you need two amps.

You can use RCA inputs or speaker level inputs. And the amp can sum and left and right input on either if used in a mono configuration.

The way the servo control system works requires that the amp be close to the speaker. Long runs of speaker wire will cause a delay that takes away from the performance of the system.

emailtim

Re: sub woofer questions
« Reply #6 on: 23 Feb 2018, 03:59 am »
...
Making the box deeper will allow for slightly more output down low as the front to back wave separation is increased. However, this will also reduce the upper frequencies ranges that it can play up to. Increases the space between the sides will allow it to play higher up again. And it will also reduce some of the loading on the woofers keeping them from playing higher SPL's down low.

Hi Danny,

Am looking to optimize low frequency output.

  • How much deeper can you make the H-Frame for 12" speakers without incurring ill effects (current is 6.5" deep on both front and back) ?
  • How much lower will it go ?
  • What will the high cutoff be ?

TIA

Danny Richie

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Re: sub woofer questions
« Reply #7 on: 23 Feb 2018, 05:53 am »
Hi Danny,

Am looking to optimize low frequency output.

  • How much deeper can you make the H-Frame for 12" speakers without incurring ill effects (current is 6.5" deep on both front and back) ?
  • How much lower will it go ?
  • What will the high cutoff be ?

TIA

You can add an inch to each side with no ill effect.

It is servo controlled. So the control system will maintain a flat response regardless, and will play as low as the extension filter is set.

The upper range will be reduced as depth increases. There isn't a specific cut off.

emailtim

Re: sub woofer questions
« Reply #8 on: 25 Feb 2018, 08:13 am »
You can add an inch to each side with no ill effect.

It is servo controlled. So the control system will maintain a flat response regardless, and will play as low as the extension filter is set.

The upper range will be reduced as depth increases. There isn't a specific cut off.

Hi Danny,

Got another question.  What would the tradeoffs be if the rectangular cavity 13"x13"x6.5" in each side of the H-Frame was replaced by a 13" diameter x 6.5" deep cylinder in each side of the H-Frame ?   

The 16"x16"x14" cubes would be heavier, have thicker "walls" and should have less resonances.  The stress around the perimeter of the center driver plate should be consistent because it is evenly supported @ the entire circumference of the driver (circle compared to square).

The cylinder has less volume than the rectangular cavity.  Should the volume remain the same (e.g. to keep driver loading consistent) or would the cylinder have less turbulence/better efficiency ? 

The cylinder does not have parallel walls and does not have a stark round to square transition.

Would it be better for the cylinder to be a cone like the bell of a bass horn ? 

TIA

P.S.
Looking to play with the router.

Danny Richie

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Re: sub woofer questions
« Reply #9 on: 25 Feb 2018, 06:00 pm »
Hi Danny,

Got another question.  What would the tradeoffs be if the rectangular cavity 13"x13"x6.5" in each side of the H-Frame was replaced by a 13" diameter x 6.5" deep cylinder in each side of the H-Frame ?   

The 16"x16"x14" cubes would be heavier, have thicker "walls" and should have less resonances.  The stress around the perimeter of the center driver plate should be consistent because it is evenly supported @ the entire circumference of the driver (circle compared to square).

The cylinder has less volume than the rectangular cavity.  Should the volume remain the same (e.g. to keep driver loading consistent) or would the cylinder have less turbulence/better efficiency ? 

The cylinder does not have parallel walls and does not have a stark round to square transition.

Would it be better for the cylinder to be a cone like the bell of a bass horn ? 

TIA

P.S.
Looking to play with the router.

The cylinder shape will load the driver more and limit how high it can play. To keep the loading (or tuning) the same the cylinder would have to be shortened to about 4.5" on each side. Shortening it would shorten the front to back wave separation as well. So the room loading would also be different.

Securing the center panel that the driver mounts to is a non-issue. It is little more than a frame. There is nothing there to resonant and it is well supported.

The wavelengths that they play are way to long for there to be any effect of the shape being round or square, so there is no transition.

mlundy57

Re: sub woofer questions
« Reply #10 on: 25 Feb 2018, 07:44 pm »
At what frequency would cavity resonances become an issue if the 13”x13” cube was 8-1/4” deep instead of 6-1/2” deep?

gab

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Re: sub woofer questions
« Reply #11 on: 25 Feb 2018, 07:59 pm »
from pg 3 of this white paper: http://www.quarter-wave.com/OBs/U_and_H_Frames.pdf

The U and H frame designs used a common cross-sectional area and cavity
depth. The internal cross-sectional area is 16” wide and 16” tall. The depth of the cavity
was defined as 7.5”. The depth was selected to push the first quarter wavelength
resonance above the desired crossover frequency of approximately 200 Hz.

Leffective = 7.5” + 0.6 x reffective
Leffective = 7.5” + 0.6 x 9.0”
Leffective = 12.9” = 0.328 m
f1/4 = c / (4 x Leffective)
f1/4 = 344 m/sec / (4 x 0.328 m)
f1/4 = 262 Hz

Adding additional length to either the U or H frame drops the quarter wavelength
resonant frequency resulting in a peaking SPL response around the crossover
frequency. Efficiency of the bass output would not be increased with this additional
length. The efficiency of a U or H frame could be increased by using a larger crosssectional
area. The depth of U frame or either side of H frame enclosures should be set
to place the quarter wavelength resonance above the selected low pass crossover
frequency.

mlundy57

Re: sub woofer questions
« Reply #12 on: 25 Feb 2018, 09:19 pm »
Ok so if I wanted a -6dB crossover point (the sub is -6dB @80Hz), what would the depth of the cavity be?

I’m not sure what you mean by leffeffectice or reffective. I assume the 7.5” is the depth of the cavity but what is the 0.6 and the 9.0”?

gab

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Re: sub woofer questions
« Reply #13 on: 25 Feb 2018, 10:17 pm »
Ok so if I wanted a -6dB crossover point (the sub is -6dB @80Hz), what would the depth of the cavity be?

I’m not sure what you mean by leffeffectice or reffective. I assume the 7.5” is the depth of the cavity but what is the 0.6 and the 9.0”?

Mike - This is more of a question for Martin as he is the author of the paper and the designer of the Mathcad spreadsheet for H-frame calculations. But I think the .6 refers to this portion of the paper on pg 5:

Impedance :
 The electrical impedance of the OB, the U frame, and the H frame are shown in
Figure 3. Plotted in each curve is the infinite baffle impedance as a reference. Moving
from top to bottom in Figure 3, the OB driver resonance is equal to the infinite baffle
value of 41 Hz while the U and H frame resonances drop to 35 Hz and 31 Hz
respectively. If we attribute these drops in resonant frequency solely to a portion of the
air in each cavity adding parasitic moving mass to the driver cone, the mass of added air
can be calculated to be approximately 22 gm. This represents about 60% of the air in
each 7.5” deep cavity.


The r effective is not defined in the paper so I don't know how he arrives at 9. Assuming r is radius in inches, the diameter would be 18" which doesn't make sense if the internal height of his H-frame cavity is 16". So I'm clueless.

I think the take away is his conclusions:

The maximum length of the U and H frame geometry should be sized so that the
first quarter wavelength resonance occurs above the low pass crossover frequency.

Lmaximum < c / (4 x fcrossover)

If the length is set correctly, there is no need for fiber stuffing in the cavities used to form
the U or H frame. Adding length will not increase low frequency efficiency, it will only
produce peaks in the SPL response. Efficiency of a U or H frame can be adjusted up or
down by increasing or decreasing the cross-sectional area of the cavities.


So if your selected crossover frequency is 80 Hz, then you want the maximum length of the H-frame to be less than 17.2".

I'm going to bow out of this as I am certainly no expert. For some further confusion, read Linkwitz's site here about H-frames: 

http://www.linkwitzlab.com/frontiers.htm#L

Perhaps others will chime in.

gab


Danny Richie

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Re: sub woofer questions
« Reply #14 on: 26 Feb 2018, 03:38 pm »
Thanks for posting that gab. I really didn't wan to have to dig into the math to create an answer for every application one can come up with. That involves a lot of time. 

As designed our H and W frames will push the cavity to just above 400Hz. That's well above the crossover points used with them and allows for really good extension and response.