Considering sealed vs vented for my AV12 subs - diyAudio
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Old 9th December 2004, 07:58 AM   #1
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Default Considering sealed vs vented for my AV12 subs

My subs are currently in test boxes - 70L, one vented, the other sealed. I was going to go with a sealed box due to size issues, however, I have managed to sim a pretty good result for a 60L box, smaller than I thought feasible, and have managed to get it looking not too big.

Now I'm trying to decide on a small sealed 30L LT vs 60L vented, tuned @ 22 Hz. Both will be curved and I intend to place them either side of the couch, although I may at some point put them next to the mains, so they need to not be too large and chunky.
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Old 9th December 2004, 07:59 AM   #2
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60L vented version, shown with wood veneer as its easier to render, however it would be piano gloss black like the other.
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Old 9th December 2004, 08:00 AM   #3
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Another view
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Old 9th December 2004, 08:08 AM   #4
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Now for some WinISD sims. The amp will be the Behringer Europower EP2500 which puts out 650w x 2 into 4 ohms. With a suitable rumble filter for each, the subs can take that much power within their excursion and thermal limits, but the vented sub gets as much as 6db more output with less excursion. The vented version actually requires less power, as shown byt the amplifier load chart and is likely to have lower distortion and be more linear. The main challenge is designing the vent. It's a 4" vent, the biggest I can manage, very generously flared on both ends with a 60mm flare radius.

Now for some charts ...

SPL
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Old 9th December 2004, 08:17 AM   #5
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SPL shown for one sub so output will in fact be +6db + 4db room gain @20 Hz so there will be approx 118 db @ 20 Hz with the vented version. One thing I can't really allow for here is power compression. The sealed may not work as well as anticipated in sims as it uses more of the drivers excursion.

Now for room response. Note how the simulated response for the subs placed in the "direct field" either side of the couch is far smoother than that for the mains?

The response is pretty much flat +/- 3db, not bad really.
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Old 9th December 2004, 08:20 AM   #6
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Now for excursion. I keep telling myself "less is more" here, although I think I might miss the ability to get the cone moving 46 mm p-p with the vented version! It's not possible to bottom the driver or even get to xmax with the rumble filter included, which has been placed as low as possible to avoid rolloff.
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Old 9th December 2004, 08:21 AM   #7
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Now for amplifier load. Note the vented box has an input power of 650w and the sealed 600w. Even then, the apparent load to the amp due to eq is higher for the sealed box due to the LT.

Forgot to mention - sealed box is blue, vented is green
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Old 9th December 2004, 08:29 AM   #8
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Group delay. This gets quite high down low due to the rumble filters used. Without these, the GD is much lower. I may in fact implement these as dynamic eq's with Ultracurve later so the performance may be better under normal use where the rumble filters are not needed.

Some issues I'm grappling with:

* is it worth having so much 20 Hz output?

The noise from all the room rattles at this point is more than the signal itself!

* is there really any audible difference between sealed and vented for music?

I think the differences are probably in favour of vented if both are done right.

* can I live with the bigger box?

* will vent noise be an issue? (don't have the amp yet to test properly)

All comments, suggestions, advice, challenges to the sims/ideas etc welcome
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Old 9th December 2004, 09:16 AM   #9
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My sub is currently a 12 inch driver in a 60 litre box tuned to either 24 or 27 Hz (can't remember which now off the top of my head). I use a 100 mm vent. The driver is nothing fancy and is only about 120 W, 4 mm xmax. I designed it so that the limit is thermal, but only just.

There is significant (>12 dB) low boost and a 12 dB/oct Butterworth rumble filter just below the tuning frequency.

In my 17 x 12 ft room with 6 inch floorstanding loudspeakers, it plays to pretty loud levels with music (easy party levels), but runs out of excursion fairly easily on HT.

The box is fairly easy to accomodate but still the average person that visits thinks it's fairly big.

I'm upgrading to 2x 12 inch, quality, high power, long throw drivers in sealed 30 litre boxes with LT to 20 Hz. Amps will be ZAPpulse 2.2SE @ 400 W each. These will be placed either side of the sofa and be barely noticeable visually (one will be hidden under a coffee table).

I'm going to sealed because I think even with a flared 100 mm vent there will be too much wind with these high excursion drivers. It's undetectable at present with the cheap driver, but these new drivers have 4 times the excursion and putting your hand in front of the vent, there's a fair amount of movement as it is. Also, Rod Elliot seems to think that a vented box cannot pressurise a room properly and a sealed box has more impact.

I will be interested to see how you get on with this project -- I've been keeping an eye on your posts in general. I'll probably post some stuff when I get further on with mine.
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Old 9th December 2004, 09:39 AM   #10
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Default Love the style!

Frankly I like the woodgrain as opposed to black myself. Unfortunately, I'd say you know more than me as far as answering all the questions you posed. Is it worth it having 118 dB at 20Hz? I'm sure many people would think so, and is one reason the high excursion woofers from Adire and others have become so popular, along with super high output commercial units. For my own room, 110 dB is plenty, although at 20 Hz I'm closer to about 114 dB at max. But it's rare that I run the volume that high except to just 'wow' somebody. When the rattles in the room are actually louder than the signal at those kind of high output levels, I personally feel that 1. it's loud enough, or 2. you live in a lot of clutter.

I can't answer the vent noise question, as I'm using a relatively low excursion driver (would have been high 15 years ago!), and even with non-flared ports I have no noise. However, that's also because I am tuning to 11 Hz, where there is a lot less information than at 22 Hz. And further, it's my understanding that for equal sound pressure levels, when you lower the tuning frequency, port air speed goes down. This makes sense, because after all the port is resonating/moving air in and out at it's frequency. So, if you halve the frequency, you have half as much air moving back and forth, yes? Someone correct me if I'm wrong.

The group delay looks rather high to me, though whether it's audible is of course the question of the day. Many of us have heard the recommendations from Adire and True Audio (think that's the name), but lately the numbers seem to be creeping up. When I first started hearing Dan Wiggins talk about it, I seem to recall mention of 20 ms at 20 Hz. Now I'm hearing that anything under 30 ms is ok ? Of course it's easy to suggest that the lower it is, the better. Here's the problem as I see it: If you try to build two subwoofers to evaluate the effect of group delay, say the second having double the group delay of the first, you will end up with subwoofers having different frequency responses. And so how do you know that what you're hearing is the group delay, rather than the different response? If you model two subwoofers, one sealed and one vented, and adjust the parameters of the vented box such that it's frequency response curve matches that of the sealed, you'll see that the group delay is identical. Anyhow, I like to err on the side of caution, which is one reason I love to tune vented boxes very low. Group delay of my box is only 11 ms at 20 Hz, and from 30 Hz and up, mimics the group delay of a sealed box with a Qtc = 0.6. I cannot tell any difference in sound quality when I plug the ports, only extension. But, I have sacrificed a fair amount of power handling and maximum output capability. It's all compromise, isn't it.


Aaron Gilbert
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