Response Curve Question - Home Theater Sub

[MY99 2.5GT]

My big question is: For a good vented home theater subwoofer should I build a box that puts the response curve above the 0 line at say +.5db @ 18Hz then levels down to +-0db @ 30Hz and stays dead flat until the crossover point? Or do good vented sub boxes normally have a large boost in there somehwere say around 35Hz where the curve jumps to +3db or more?

I'm building a new vented box for my home brew subwoofer. It consists of a single Cerwin Vega VMAX 15.2 and a 4" Precision Port with Dual Flared Ends. I'm powering it with a BASH 500W Digital Sub Amp.

When I originally put all of this together I received advice from a tech guy at Cerwin Vega to make it 3.5 cubic feet internal and use a 2" X 12" X 26.6" vent.

Ive been playing around with WinISD and see that the box I originally built has a really weird response curve. That 3.5 cubic foot box with that huge vent looks more like a trunk thumper because at 33Hz its at +3.8db with a -3db of around 23Hz. It looks like a ski slope from 33Hz to 100Hz where it finally returns to 0db.

The curve I explained in the first paragraph is from WinISD mapping a 5.75 cubic foot box with a single 4" X 13.5" port. Tuning frequency is 18.63Hz.

So what do you all think? Ski slope curves or flat?

Thanks
Brad

 

[peerlesser]

My big question is: For a good vented home theater subwoofer should I build a box that puts the response curve above the 0 line at say +.5db @ 18Hz then levels down to +-0db @ 30Hz and stays dead flat until the crossover point? Or do good vented sub boxes normally have a large boost in there somehwere say around 35Hz where the curve jumps to +3db or more?

I'm building a new vented box for my home brew subwoofer. It consists of a single Cerwin Vega VMAX 15.2 and a 4" Precision Port with Dual Flared Ends. I'm powering it with a BASH 500W Digital Sub Amp.

When I originally put all of this together I received advice from a tech guy at Cerwin Vega to make it 3.5 cubic feet internal and use a 2" X 12" X 26.6" vent.

Ive been playing around with WinISD and see that the box I originally built has a really weird response curve. That 3.5 cubic foot box with that huge vent looks more like a trunk thumper because at 33Hz its at +3.8db with a -3db of around 23Hz. It looks like a ski slope from 33Hz to 100Hz where it finally returns to 0db.

The curve I explained in the first paragraph is from WinISD mapping a 5.75 cubic foot box with a single 4" X 13.5" port. Tuning frequency is 18.63Hz.

So what do you all think? Ski slope curves or flat?

Thanks
Brad

I would go for the flat response. Compared to how much the room is going to affect the overall result, +0.5dB emphasis is basically inaudible anywhere throughout the response. Too much emphasis toward the low-end can make the bass sound a bit "slow".

 

[MY99 2.5GT]

Thanks for the response.

The current 3.5 cubic foot box sounds pretty crappy except for a pretty narrow frequency range. Once it starts getting higher it sounds unresponsive and hollow. Once it get down really low it falls off and sounds airy.

I will go for the 5.75 cubic foot box with 4" port.

Do you guys have any reccomendation on accoustic dampening material for the inside of the enclosure and where it should be placed?

Brad

 

[Johnd]

Thanks for the response.

The current 3.5 cubic foot box sounds pretty crappy except for a pretty narrow frequency range. Once it starts getting higher it sounds unresponsive and hollow. Once it get down really low it falls off and sounds airy.

I will go for the 5.75 cubic foot box with 4" port.

Do you guys have any reccomendation on accoustic dampening material for the inside of the enclosure and where it should be placed?

Brad

Brad: You may do better with this thread in the diy forum. Also, if you fail to get a response in the next day or two, shoot annunaki a pm...I'm sure he'll be happy to help you out. Here's a link to some inexpensive dampeing material...Annunaki or someone else should be able to aide you with the thickness. Cheers.

http://www.partsexpress.com/webpage.cfm?webpage_id=3&CAT_ID=48&ObjectGroup_ID=137

 

[Sheep]

I would go for the flat response. Compared to how much the room is going to affect the overall result, +0.5dB emphasis is basically inaudible anywhere throughout the response. Too much emphasis toward the low-end can make the bass sound a bit "slow".

IYO.

Which do you prefer OP? Do you like a midbass bump or do you prefer it flat? Have you even heard a flat subwoofer? Is your room EQ'd flat or is it untreated and thusly, NOT flat?

SheepStar

 

[annunaki]

Go for the flat response curve. I would also do more than a single 4" vent if it is a 15" woofer. Especially if it is tuned that low. I would suggest three, 4" vents at minimum.

 

[peerlesser]

IYO.

Which do you prefer OP? Do you like a midbass bump or do you prefer it flat? Have you even heard a flat subwoofer? Is your room EQ'd flat or is it untreated and thusly, NOT flat?

SheepStar

For the room I use this:
http://www.dspeaker.com/en/support/anti-mode-technology/measurements.shtml

So yes, my room-response is very flat. That is the newest hi-tech auto-EQ that just came out january 2008. I don't know if it is officially sold to US.

As for the driver, I use Peerless XLS 10" in vented enclosure tuned to 19Hz.

 

[MY99 2.5GT]

Go for the flat response curve. I would also do more than a single 4" vent if it is a 15" woofer. Especially if it is tuned that low. I would suggest three, 4" vents at minimum.

Thanks for the response but are you serious about the 3 - 4" ports?

WinISD says if using 3 - 4" ports I would need to make them 47.42" long. I think I'll stick to my single 4" - 13.5" long port. Both port ends are flared so turbulence should be kept to a minimum.

What do you guys think about firing the woofer and port downward into a pedestal made of MDF. By that I mean subwoofer and port would be mounted to the bottom of the box, 4 - 5" legs would be attached to bottom corners of the box and a sheet of painted and finished MDF would be mounted to the bottom of the legs. This would be done to simulate corner loading/placing the box.

Does that make sense or would this really be a bad idea?

Thanks
Brad

 

[TLS Guy]

Thanks for the response but are you serious about the 3 - 4" ports?

WinISD says if using 3 - 4" ports I would need to make them 47.42" long. I think I'll stick to my single 4" - 13.5" long port. Both port ends are flared so turbulence should be kept to a minimum.

What do you guys think about firing the woofer and port downward into a pedestal made of MDF. By that I mean subwoofer and port would be mounted to the bottom of the box, 4 - 5" legs would be attached to bottom corners of the box and a sheet of painted and finished MDF would be mounted to the bottom of the legs. This would be done to simulate corner loading/placing the box.

Does that make sense or would this really be a bad idea?

Thanks
Brad

I don't have the T/S parameters for that woofer. I agree with Anunaki, make the response flat, even if F3 is raised a little. Make sure the vent velocity is below 30m/sec. I know Anunaki likes the velocities about half that, and we have had this discussion before. To get the velocity in the 12 to 15 m/sec range you will almost certainly have to create a tunneled slot vent.

Don't obstruct the port. There will be a lot of air movement.

 

[MY99 2.5GT]

Here is a link to the manual for the Cerwin Vega VMAX 15.2. It lists most of the necessary parameters.

http://www.cerwinvegamobile.com/manuals/mobile/VMAX_Sub_Manual.pdf

http://www.cerwinvegamobile.com/VMAX15.php

I can't seem to find a port velocity measurement in the version of WinISD that I'm running.

I just did some quick figuring for a slot port and found that I could get away with a 2.5" X 10" X 27.8". That would keep tuning freq. @ 18.92Hz but since I can't see what port velocity will be at tuning freq I don't know if that makes it any better then the single 4" round port.

 

[annunaki]

Well, I modeled the driver in WinIsd. A single 4" port in a 5.75 cu.ft. enclosure, even with flanged ends, reaches audible vent noise at around 28hz. Going down to the tuning frequency with a single port would be a chuffing fest.

After playing around a bit, I found that a slot vent of 3.0" x 16" x 62.5" long will yield a tuning frequency of 18hz and inaudible port noise to the tuning frequency and below. (about 17m/sec vent velocity) The issue here is that you would require a crossover frequency at around 60hz or so as the 1st port resonance is at 108hz. Even at 60hz I would suggest a 36db/oct. filter.
A reasonable compromise would be a 2.5" x 16" x 51.5" port with the first port resonance at 131hz. I would still suggest a 36db/oct filter or 24hz worst case at a frequency no higher than 80hz. You would be at a vent velocity of about 22m/sec. Port compression is kept to a minimum with either choice.

Remember that the more port area you have the more air you move. The more air you move the more impressive the output.

 

[TLS Guy]

Cerwin Vega VMAX 15.4

I have looked carefully at this driver. It is not really suitable for ported alignment. Qts is 0.56, so this is a very high Qts driver. The ported enclosure is large with a big slot vent. You can not use tube ports. This driver will sound better with a sealed enclosure. At 600 watts sealed cone displacement is only 10 mm. It is only 8db down at 20 Hz. Now sealed enclosures tolerate Eq well. I would get a plate amp with Eq and add 3db at 40 Hz 6 db at 30 Hz 9db at 20 Hz. Response will be excellent. Qts of the sealed system will be 0.682, so bass will be nice and tight.

Your slot vent is given for the ported enclosure 2.5 X 6 X 41.72 inches. Vent velocity is 19 m/sec.

Here is your ported alignment.

Name: VMAX 15.4
Type: Standard one-way driver
Company: Cerwin Vega Inc.
No. of Drivers = 1
Fs = 23.4 Hz
Qms = 4.2
Vas = 82.32 liters
Cms = 0.0572 mm/N
Mms = 808.7 g
Rms = 28.31 kg/s
Xmax = 17 mm
Xmech = 25.5 mm
P-Dia = 358 mm
Sd = 1007 sq.cm
P-Vd = 1.711 liters
Qes = 0.63
Re = 1.61 ohms
Z = 2 ohms
BL = 17.43 Tm
Pe = 600 watts
Qts = 0.56
no = 0.161 %
1-W SPL = 84.23 dB
2.83-V SPL = 87.8 dB
-----------------------------------------
Box Properties
Name:
Type: Vented Box
Shape: Prism, square
Vb = 5.823 cu.ft
Fb = 17.93 Hz
QL = 5.952
F3 = 16.67 Hz
Fill = minimal
No. of Vents = 2
Vent shape = rectangle
Vent ends = two flared
Hv = 2.5 in
Wv = 6 in
Lv = 41.72 in

Here is your sealed alignment.

Name: VMAX 15.4
Type: Standard one-way driver
Company: Cerwin Vega Inc.
No. of Drivers = 1
Fs = 23.4 Hz
Qms = 4.2
Vas = 82.32 liters
Cms = 0.0572 mm/N
Mms = 808.7 g
Rms = 28.31 kg/s
Xmax = 17 mm
Xmech = 25.5 mm
P-Dia = 358 mm
Sd = 1007 sq.cm
P-Vd = 1.711 liters
Qes = 0.63
Re = 1.61 ohms
Z = 2 ohms
BL = 17.43 Tm
Pe = 600 watts
Qts = 0.56
no = 0.161 %
1-W SPL = 84.23 dB
2.83-V SPL = 87.8 dB
-----------------------------------------
Box Properties
Name:
Type: Closed Box
Shape: Prism, square
Vb = 2.051 cu.ft
Qtc = 0.682
QL = 17.79
F3 = 36.62 Hz
Fill = heavy

I strongly recommend the sealed box for this driver.

Don't forget to add the diver volume, volume of braces and the amp to the total volume. Good luck with the project and have fun!

 

[TLS Guy]

Just a few other points.
First the vent should say ONE port and not TWO and flush not flared ends.
This is the tuning. Vent air speed is 15 m/sec. First port resonance around 130 Hz. I think this is too low.

Type: Vented Box
Shape: Prism, square
Vb = 5.823 cu.ft
Fb = 17.93 Hz
QL = 5.952
F3 = 16.67 Hz Fill = minimal
No. of Vents = 1
Vent shape = rectangle
Vent ends = two flush
Hv = 2.5 in
Wv = 16 in
Lv = 54.65 in

If you change the port dimension to 2.5 X 12 X 40 inches you get.

Type: Vented Box
Shape: Prism, square
Vb = 5.823 cu.ft
Fb = 17.93 Hz
QL = 5.952
F3 = 16.67 Hz
Fill = minimal
No. of Vents = 1
Vent shape = rectangle
Vent ends = two flush
Hv = 2.5 in
Wv = 12 in
Lv = 40.05 in

The first port resonance is around 160 HZ. The driver needs to be 24 db down at that point to avoid exciting the resonance. That means using a fourth order crossover and not setting the crossover frequency above 80 Hz.
For that reason I think the above is the maximum practical port dimension. The vent air speed velocity is 19 m/sec

Now a word about F3. Remember a ported box rolls off at 12 db/octave below F3, a ported box at 24 db/ octave blow F3. So the two alignments have the same output at 10 Hz, that is were the response curves cross. That is before adding Eq to the sealed box.

For all of the above reasons, I think a sealed design with Eq is more practical for a driver with these T/S parameters. I think it will sound better also. If you use Eq, I would recommend an amp in the 300 to 600 watt range. Max spl with 600 watts is 112 db. Eq will reduce this a little depending on frequency range of content below 40 Hz.

My apologies for having the two vent and flared checked with the first post. I was trying to do the job with round ports, but it was not practical.

 

[Sheep]

Damn TLS Guy, you sure get a lot of information. Kudos!

SheepStar

 

[TLS Guy]

Damn TLS Guy, you sure get a lot of information. Kudos!

SheepStar

Well as you can tell, I'm getting old and careless. It does help that it is 53 years since I built my first speaker.

 

[annunaki]

He actually has the Vmax 15.2 and the Qts is lower.

I have looked carefully at this driver. It is not really suitable for ported alignment. Qts is 0.56, so this is a very high Qts driver. The ported enclosure is large with a big slot vent. You can not use tube ports. This driver will sound better with a sealed enclosure. At 600 watts sealed cone displacement is only 10 mm. It is only 8db down at 20 Hz. Now sealed enclosures tolerate Eq well. I would get a plate amp with Eq and add 3db at 40 Hz 6 db at 30 Hz 9db at 20 Hz. Response will be excellent. Qts of the sealed system will be 0.682, so bass will be nice and tight.

Your slot vent is given for the ported enclosure 2.5 X 6 X 41.72 inches. Vent velocity is 19 m/sec.

Here is your ported alignment.

Name: VMAX 15.4
Type: Standard one-way driver
Company: Cerwin Vega Inc.
No. of Drivers = 1
Fs = 23.4 Hz
Qms = 4.2
Vas = 82.32 liters
Cms = 0.0572 mm/N
Mms = 808.7 g
Rms = 28.31 kg/s
Xmax = 17 mm
Xmech = 25.5 mm
P-Dia = 358 mm
Sd = 1007 sq.cm
P-Vd = 1.711 liters
Qes = 0.63
Re = 1.61 ohms
Z = 2 ohms
BL = 17.43 Tm
Pe = 600 watts
Qts = 0.56
no = 0.161 %
1-W SPL = 84.23 dB
2.83-V SPL = 87.8 dB
-----------------------------------------
Box Properties
Name:
Type: Vented Box
Shape: Prism, square
Vb = 5.823 cu.ft
Fb = 17.93 Hz
QL = 5.952
F3 = 16.67 Hz
Fill = minimal
No. of Vents = 2
Vent shape = rectangle
Vent ends = two flared
Hv = 2.5 in
Wv = 6 in
Lv = 41.72 in

Here is your sealed alignment.

Name: VMAX 15.4
Type: Standard one-way driver
Company: Cerwin Vega Inc.
No. of Drivers = 1
Fs = 23.4 Hz
Qms = 4.2
Vas = 82.32 liters
Cms = 0.0572 mm/N
Mms = 808.7 g
Rms = 28.31 kg/s
Xmax = 17 mm
Xmech = 25.5 mm
P-Dia = 358 mm
Sd = 1007 sq.cm
P-Vd = 1.711 liters
Qes = 0.63
Re = 1.61 ohms
Z = 2 ohms
BL = 17.43 Tm
Pe = 600 watts
Qts = 0.56
no = 0.161 %
1-W SPL = 84.23 dB
2.83-V SPL = 87.8 dB
-----------------------------------------
Box Properties
Name:
Type: Closed Box
Shape: Prism, square
Vb = 2.051 cu.ft
Qtc = 0.682
QL = 17.79
F3 = 36.62 Hz
Fill = heavy

I strongly recommend the sealed box for this driver.

Don't forget to add the diver volume, volume of braces and the amp to the total volume. Good luck with the project and have fun!

 

[TLS Guy]

Here is the VMAX 15.2 You now have two for one. The parameters only change this adversely. There is now ripple from the ported alignment. I have played with it for minimal ripple and the best is +5db at 45 Hz The vent is now of unsatisfactory dimensions.

I have put the coils in parallel this time as the impedance of each coill is 2 ohm.

The good news is that the closed box alignment is even better. There is no ripple and F3 is 28 Hz. It is only 8db down at 20 Hz. Cone displacement is only 8mm so there is plenty of room to EQ flat to 20 Hz. The Qtc is 0.589 which is on the low side, but the bass will be tight.

Here is the ported, but in my view it should not be built.

Name: VMAX 15.2
Type: Standard one-way driver
Company: Cerwin Vega Inc.
No. of Drivers = 1
Fs = 23.4 Hz
Qms = 4
Vas = 82.32 liters
Cms = 0.0572 mm/N
Mms = 808.7 g
Rms = 28.31 kg/s
Xmax = 17 mm
Xmech = 25.5 mm
P-Dia = 358 mm
Sd = 1007 sq.cm
P-Vd = 1.711 liters
Qes = 0.53
Re = 2.64 ohms
Z = 4 ohms
BL = 17.43 Tm
Pe = 600 watts
Qts = 0.47
no = 0.161 %
1-W SPL = 84.23 dB
2.83-V SPL = 87.8 dB
-----------------------------------------
Box Properties
Name:
Type: Vented Box
Shape: Prism, square
Vb = 3.972 cu.ft
Fb = 19.01 Hz
QL = 6.356
F3 = 18 Hz
Fill = minimal
No. of Vents = 1
Vent shape = rectangle
Vent ends = two flush
Hv = 2.5 in
Wv = 12 in
Lv = 53.75 in

Here is the closed/sealed box, which is the way to go with this driver.

Name: VMAX 15.2
Type: Standard one-way driver
Company: Cerwin Vega Inc.
No. of Drivers = 1
Fs = 23.4 Hz
Qms = 4
Vas = 82.32 liters
Cms = 0.0572 mm/N
Mms = 808.7 g
Rms = 28.31 kg/s
Xmax = 17 mm
Xmech = 25.5 mm
P-Dia = 358 mm
Sd = 1007 sq.cm
P-Vd = 1.711 liters
Qes = 0.53
Re = 2.64 ohms
Z = 4 ohms
BL = 17.43 Tm
Pe = 600 watts
Qts = 0.47
no = 0.161 %
1-W SPL = 84.23 dB
2.83-V SPL = 87.8 dB
-----------------------------------------
Box Properties
Name:
Type: Closed Box
Shape: Prism, square
Vb = 2.051 cu.ft
Qtc = 0.589
QL = 17.79
F3 = 28.14 Hz
Fill = heavy

 

[annunaki]

The response for the Vmax15.2 in a 5.75 cu.ft. enclosure tuned to 18hz is actually quite good from my model. The 1st. port resonance is at 131hz using a 2.5" x 16" x 51.5" vent with flanged ends.

Here is a shot of the modeled response:

 

[MY99 2.5GT]

I already have a 500W BASH plate amp from Parts Express. Unfortunately it has only one setting for Bass Boost so the thought of a sealed box with progressive boosting as the frequency steps down is out of the question.

I'm interested in the slot port idea but I just don't see any way of making a straight rectangular port that is 51.5" long in a 5.75 cu ft enclosure.

You said with the single 4" port @ 5.75 ft3 it would have audible port noise at tuning frequency. Since tuning freq is so low what are the chances of hitting that band during movie/music playback. Also what if I corner load the box and have the port firing towards the wall and the sub firing into the room. Or I fire both the port and woofer downward into a flat wood plate. Wouldn't that cover up some of the port noise? For comparison I will remeasure the slot port the current box has as well as the internal volume and map it in WinISD to check out how much port turbulence I have and where it occurrs for comparison.

By the way is there any calculation needed for downfiring subs? What I mean is there a way of determining how far off the ground they should sit?

Thanks
Brad

 

[TLS Guy]

I already have a 500W BASH plate amp from Parts Express. Unfortunately it has only one setting for Bass Boost so the thought of a sealed box with progressive boosting as the frequency steps down is out of the question.

I'm interested in the slot port idea but I just don't see any way of making a straight rectangular port that is 51.5" long in a 5.75 cu ft enclosure.

You said with the single 4" port @ 5.75 ft3 it would have audible port noise at tuning frequency. Since tuning freq is so low what are the chances of hitting that band during movie/music playback. Also what if I corner load the box and have the port firing towards the wall and the sub firing into the room. Or I fire both the port and woofer downward into a flat wood plate. Wouldn't that cover up some of the port noise? For comparison I will remeasure the slot port the current box has as well as the internal volume and map it in WinISD to check out how much port turbulence I have and where it occurrs for comparison.

By the way is there any calculation needed for downfiring subs? What I mean is there a way of determining how far off the ground they should sit?

Thanks
Brad

You absolutely can not build that sub with a four inch port. There will be far too much port compression. The vent velocity is 45 m/sec. The length for tuning of a four inch port is 21.55 inches.

In Box Pro there is significant ripple for all ported enclosures with that driver. The 4 cu ft box has the least and an F3 of 18.

Now the sealed box has an F3 of 28. Roll off is 12 db/octave, so it is still a really good sub. Truthfully you can tell the designers who conceived that driver had closed box in mind for its application.

In Box Pro designed by Bob Bullock, who has been a pioneer in speaker design software, there is a bar meter after you load the TS parameters that steers you to closed or vented. For your driver the bar is as far to the closed side as it can get.

I have found over the years his software has correlated very well with what really happens.

I would build a closed box for that driver. That will be simpler and you won't have these difficult port issues. If you go ported you are gong to have to limit the sub to 65 Hz with fourth order crossover roll off. I can't be more emphatic with my advice not to reflex load that driver.