Greetings from lockdown in Wales, how are we all doing?
I've been keeping busy and decided to build a new sub for my surround sound in the living room.
Now I am no magician and can't just cast a spell to remember why I bought two lab-12 drivers 7-8 years ago for subwoofers that were definitely not a labhorn... but I did. I have been using one of them in a 100l reflex in the lounge for a while.
Knowing I had the spare driver knocking about gathering dust I decided I would try to build an isobaric box and reduce the volume.
The original 100l reflex (as Suggested byt user THINK on here) had 4 65mm diameter ports. I got large corks I could use to plug them up. Use 4 ports for 32Hz tuning with 3db bump at 40Hz, 3 ports open for 28Hz tuning with a little rise near 35Hz. 2 ports for 23Hz tuning that starts to dip then roll off near 30Hz. 1 port for a long slow roll off at 17Hz tuning. Most of the time I used it with 2 ports for home theater.
It was big and cuboid and didn't really fit anywhere nicely in the new house.
I didn't do a lot of reading before building the new one but I did read this:
Isobaric subwoofer box design - Advantages and disadvantages
After reading that and the comments I was expecting this box to be as "loud" or 3db "quieter" than my current sub. The disadvantages didn't really apply because the amp is fine at 3 Ohms for two Labs in parallel, I already owned the drivers, and I would build a thick single baffle for an easy build.
I modelled it in WinISD (Pro Alpha 0.5a7, I like being able to choose port length and have it adjust tuning, is it really flawed?) and compared to the old box' transfer function magnitude.
Red, yellow, green are three of the 100l tunings, purple is the isobaric.
As you can see from that, I'm getting a slight increase in extension from a box that's 67L net, or 2/3 the size. I'm putting it into an alcove on the side of the chimney breast and it's sized to suit, including some areas around the sides and back to hide wiring and internet hubs that sort of junk. It's also a shelf, so far no rattling items hah!
I'm very pleased with the build, it was going perfectly until I had a big woops with the router on one of the counterbores on the removable baffle 🙁 Not sure what to do about the squashed pics there.
Ok so now for the confusion. this sub is louder! I use a microphone to set the levels on my surround sound so they're all similar in my spot on the sofa 😀
and when I wired this up and switched it on for the first time I had to dial in 6db of attenuation to get it to the same level as the old sub.
To make me more confused I then looked at the modelled SPL in WinISD and it's showing it should be at least 6dB quieter than the old subwoofer. what the hell is going on? how does WinISD model isobaric drivers? I can't see an option for series or parallel.
Ok, help me understand this?
The impedance being half effectively means it's drawing more power from the amp if I haven't touched the volume knob? as it happens it has an indented reference level on the knob and I set the attenuation on the receiver.
If anyone fancies taking a stab at helping me get to grips with this I would appreciate it 🙂 I presume I'm just ignorant of some pretty important things happening with the science of it all.
Thanks,
Robyn
I've been keeping busy and decided to build a new sub for my surround sound in the living room.
Now I am no magician and can't just cast a spell to remember why I bought two lab-12 drivers 7-8 years ago for subwoofers that were definitely not a labhorn... but I did. I have been using one of them in a 100l reflex in the lounge for a while.
Knowing I had the spare driver knocking about gathering dust I decided I would try to build an isobaric box and reduce the volume.
The original 100l reflex (as Suggested byt user THINK on here) had 4 65mm diameter ports. I got large corks I could use to plug them up. Use 4 ports for 32Hz tuning with 3db bump at 40Hz, 3 ports open for 28Hz tuning with a little rise near 35Hz. 2 ports for 23Hz tuning that starts to dip then roll off near 30Hz. 1 port for a long slow roll off at 17Hz tuning. Most of the time I used it with 2 ports for home theater.
It was big and cuboid and didn't really fit anywhere nicely in the new house.
I didn't do a lot of reading before building the new one but I did read this:
Isobaric subwoofer box design - Advantages and disadvantages
After reading that and the comments I was expecting this box to be as "loud" or 3db "quieter" than my current sub. The disadvantages didn't really apply because the amp is fine at 3 Ohms for two Labs in parallel, I already owned the drivers, and I would build a thick single baffle for an easy build.
I modelled it in WinISD (Pro Alpha 0.5a7, I like being able to choose port length and have it adjust tuning, is it really flawed?) and compared to the old box' transfer function magnitude.
Red, yellow, green are three of the 100l tunings, purple is the isobaric.
As you can see from that, I'm getting a slight increase in extension from a box that's 67L net, or 2/3 the size. I'm putting it into an alcove on the side of the chimney breast and it's sized to suit, including some areas around the sides and back to hide wiring and internet hubs that sort of junk. It's also a shelf, so far no rattling items hah!
I'm very pleased with the build, it was going perfectly until I had a big woops with the router on one of the counterbores on the removable baffle 🙁 Not sure what to do about the squashed pics there.
Ok so now for the confusion. this sub is louder! I use a microphone to set the levels on my surround sound so they're all similar in my spot on the sofa 😀
and when I wired this up and switched it on for the first time I had to dial in 6db of attenuation to get it to the same level as the old sub.
To make me more confused I then looked at the modelled SPL in WinISD and it's showing it should be at least 6dB quieter than the old subwoofer. what the hell is going on? how does WinISD model isobaric drivers? I can't see an option for series or parallel.
Ok, help me understand this?
The impedance being half effectively means it's drawing more power from the amp if I haven't touched the volume knob? as it happens it has an indented reference level on the knob and I set the attenuation on the receiver.
If anyone fancies taking a stab at helping me get to grips with this I would appreciate it 🙂 I presume I'm just ignorant of some pretty important things happening with the science of it all.
Thanks,
Robyn
Nice build. Is it located at a different position in the room?
What do you mean exactly by 'isobaric'?
What do you mean exactly by 'isobaric'?
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You get 3 dB by corner placement vs straight wall. For bass it’s often more than 3 dB. The rest could be the new room, different kind of wall structure or a combination.
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thanks for your replys. Yes it is a different location, now that you mention it corner loading makes sense, but I didn't think it would be so pronounced. This was the before:
This is the new:
Isobaric meaning I have two drivers mounted on either side of the baffle facing each other, one wired in reverse polarity, drivers wired in parallel.
This is the new:
Isobaric meaning I have two drivers mounted on either side of the baffle facing each other, one wired in reverse polarity, drivers wired in parallel.
Robyn,
I'm confused by winISD showing -6db in isobaric too. I *think* its like this:
WinISD must assume series connection for isobaric, if so total power is halved (-3db). You can/should/would double the input power to regain this 3db.
In addition, you now have that halved power split between two drivers, but only one contributing a radiating surface, so there's your other -3db.
In other words, Isobaric is a way of getting the output of a single sub in a box of size 2x - from two subs in a box of size x - by converting the entire output of one driver into sort of pre-pressurizing the box. But the inside driver still uses the same amount of power to not create any output as the outside one.
Connecting in parallel by default would make a lot more sense, IMO. I think it might be easier to just model it as one driver in double the volume, and check your final impedances and power ratings yourself, than to use the built-in isobaric option with it stuck at serial connection?
Hope it helps and someone corrects this if it's wrong.
-Ben
I'm confused by winISD showing -6db in isobaric too. I *think* its like this:
WinISD must assume series connection for isobaric, if so total power is halved (-3db). You can/should/would double the input power to regain this 3db.
In addition, you now have that halved power split between two drivers, but only one contributing a radiating surface, so there's your other -3db.
In other words, Isobaric is a way of getting the output of a single sub in a box of size 2x - from two subs in a box of size x - by converting the entire output of one driver into sort of pre-pressurizing the box. But the inside driver still uses the same amount of power to not create any output as the outside one.
Connecting in parallel by default would make a lot more sense, IMO. I think it might be easier to just model it as one driver in double the volume, and check your final impedances and power ratings yourself, than to use the built-in isobaric option with it stuck at serial connection?
Hope it helps and someone corrects this if it's wrong.
-Ben
this seems like a fair assumption to make, I wanted to snip some comparison graphs but they were so different I was asking myself why I bothered making it! haha. Cheers