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Old 15th July 2005, 08:59 PM   #1
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Default Newbie design goals: how low/loud?

I'm a newbie considering building a sub for my home theater system. I've read as much as I could in the past few days and am quite convinced this is a reasonable thing to do. (I didn't ask the wife for her opinion about this, of course.)

Before I get into design, I need to set goals for my sub: how low and how loud do I want this sucker to go? I've thought about this and just don't have the information I need to answer these questions.

I've tentatively set goals for 105dB at 20-80Hz. I need a sanity check on those numbers.

My mains will easily handle 80Hz and above.

How low does movie sound go? What will I be missing if I don't go lower than 20Hz? Let's say I'm totally irrational about this. How low would my insane alter ego want this sub to go for home theater?

How loud is loud? At 20 Hz, is 105dB enough? Too much? Ludicrous? I mean, when that T-Rex starts a-comin after the characters, I want to *KNOW* about it! Of course, I don't want to have to provide structural reinforcement to the house. (I won't be listening at insane volumes regularly of course. If I do build my own sub, though, I do want to be able to give jaw-dropping demos to guests.)

Basically, I'm just fishing for feedback from the experienced crowd here: how low and how loud are reasonable (yet aggressive) goals for a home theater subwoofer? Does 20-80Hz @ 105dB qualify as "reasonable" to this crowd?
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Old 15th July 2005, 09:51 PM   #2
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105db max at 1m may fall a little short, but at the seating position it should be enough. Having extra in reserve is always a good thing. As far as jaw dropping, usually what visitors hear in the 30hz range is what impresses them, however, as a DIYer you really want down into the low teens for HT. Down there it's felt more than heard even if it's only you who really appreciates it. If build a sub with good output only down to 20hz, then you'll just end up building another one later. There's not much info below 25hz, but when it's there and you feel it, you'll be glad you went the extra yard.

It think you should start with how much space is available and what is your budget. Then maximize your extension and output from there.
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Old 16th July 2005, 05:29 AM   #3
simon5 is offline simon5  Canada
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If you want to go at 20 Hz, you'll miss extreme low bass in some movies, like U-571, Lords of the Ring, The Matrix, Titan A.E. and some more.

105 dB at 1m anechoic is ok at 20 Hz because your wife will probably push the subwoofer in one corner, so you'll gain 6 dB and if your room is not too big, you'll gain a few dBs so near 115 dB at 1m.

105 dB is easy to achieve, you just need to choose how low do you want to go. This will be dictated by budget if you want a small box. If you can tolerate a big box, then even with low money you can go low.

Since you're in US, with about 300$ you can design something flat in-room down to about 16 Hz with a big box at around 118 dB in-room.

If budget is higher, you can get higher output or smaller box or even lower tuning.
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Old 16th July 2005, 09:33 AM   #4
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Low teens?

Holy schlamoly!

I'd like to see a sample WinISD model of that type of response. Do you have sample data that would give me an idea of what it would take to get that?
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Old 16th July 2005, 03:19 PM   #5
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First of all, very few people can even hear below 30 Hz, personally, my hearing cuts out at 40 Hz!!! For listening, you shouldn't need the sub to go any lower then your hearing capability.

For movies, a lot of soundtrack have sub-sonic sound (below 20 Hz). These sub-sonic sounds can not be heard, but felt.

I don't think that answers your questions completely, but is more something to think about.


This website has a nice comparsion of dB/SPL:

http://www.jimprice.com/prosound/db.htm
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Old 16th July 2005, 05:30 PM   #6
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Keep in mind, the lower you go, the bigger the amp will have to be to achieve your SPL. I wouldn't worry about reaching all the way to 16 Hz. That's a bit of a stretch. There are very woofers cabable of that.
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Old 16th July 2005, 08:15 PM   #7
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Noggin,

Here's an EBS ported shiva in a small room. Check out post 12
Autotuba measurements, plus shiva
107db in room at 16hz is pretty strong for a 12". You must consider room gain in the response of any HT sub, so you aren't looking for a graph that is flat from 50-60hz down to it's tuning cutoff.

My EBS Maelstrom will significantly outperform that and neither requires a ton of power to perform great in HT. EBS ported boxes are ideal for HT because their shelved response matches pretty well with room gain. They do have 2 drawbacks. 1. The boxes are big and 2. They are pretty sloppy for music, but stuffing the ports makes them great for music due to the low Q.

If you can't handle a big box, you really need a Linkwitz transform. Then you are talking about an appropriate driver(s) and significant power to get you the extension and output to a level where you won't later get the urge to upgrade.

Another great alternative if you own your home is an IB subwoofer. They're the easiest to construct and the install can make them invisible. They're great for music and HT too (with some bottom end boost). You just need an attic, basement, or storeroom adjacent to your HT. Although 2 15"s might be sufficient, I'd go with 4 or more for prodigious output and extension. You could always start with 2 and plan to add more later. Here's a subwoofer made for this purpose. One will give you 105db at 20hz with 140 watts and they are only $120ea. If you have the right setup then IB is ideal and your friends won't know all of that great bass isn't coming from your mains.
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Old 24th July 2005, 02:26 AM   #8
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Default re subwoofer

If I remember the THX standard is for 105dbm. at 20Hz.
This can be easy or hard to achieve depending upon your room, here in Australia the typical house has no room gain to speak of so a subwoofer must go down to 20Hz. flat at 105dbm. This is hard and or expensive or large and some combination of both of the others.
With lots of room gain the Linkwitz transform is the smallest, but if done properly is expensive, since it needs drivers such as the Peerless XXLS sort with flux modulation control and dc offset compensation, and a high power amplifier, but these have a comendably high, "faf", (female acceptance factor).
The best bang for your buck if you have low room gain, is given by the highest efficiency reflex alignments, these provide maximum bass extension for a given input power, but they are large, and electronic assistance of various sorts, (such as cancelling the voice coil resistance with positive feedback), greatly enhances the performance.
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Old 24th July 2005, 11:26 AM   #9
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I consider 105db @ 25 Hz to be a good point to design for with a subwoofer and 100db for the mains, both at the listening position. This is based on my experience including measurements at my listening position ~2.5m from the mains in a 1600 cu ft room.

I have measured 115db peaks from the subs in the listening position, but my mains can only reach about 100db, and this is due in part to the use of a 4th order highpass filter to reduce cone excursions and the fact that the speakers are active so I get a fair bit more clean output than I could with a passive xo.

When you hear a 100db peak you will find that it is VERY loud, and the average level with a movie will typically be more like 75db.

Peaks around 90db are also very loud.

I find that a typical MTM floorstander won't reach the same level of dynamic range as a comercial cinema, although if they are active like mine, and paired with capable subwoofers, you can come very close.

I think there would be few among non audio enthusiasts who would actually require more than 100db peaks.

Keep in mind that what you get at your listening position, and what you get at 1m are very different. You must consider the size of your room, but even then there are room modes which can't really be predicted as their actual impact relates to the construction of the building envelope. In my room I have a prominent resonance peak at 35 Hz, which effectively extends the response of my sealed subs down below 25 Hz without any eq boost. In fact I use eq to tame the room mode peak and once that peak is removed the response extends down to about 23 Hz. If I want to get deeper, I have to then use a lot of eq (+15db @ 20 Hz) so at this point I choose to accept a slightly higher in room F3 but have extra dynamic range.

I'll put this forward as a general starting point:

If your goal is around 105db @ 25 Hz at your listening position, you should be able to achieve this under these conditions:

* <2000 cu ft room volume
* 12" high excursion subwoofer with >15mm xmax one way in a vented box of reasonable size (normally 3 cu ft / 85L or more) tuned low (Fb<23 Hz)
* (alternative) 2 smaller sealed boxes with eq, each with a driver comparable to the vented sub

Note: starting point only! Proper simulation / design required to pull this off.
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Old 24th July 2005, 03:52 PM   #10
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Paul I agree with most of your points except that 90db peaks aren't that loud. A live orchestra has much higher peaks, well over 100db, and I seem to recall that 6db more SPL is double the perceived loudness. Also, remember that he wants this for HT and wants to "wow" his friends. While 25hz is a marginally acceptable cutoff for an HT sub, it will only leave him wanting to build a better one, so you might as well just do it once.
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