Newbie design goals: how low/loud?

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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?
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.
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.
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:

Here's an EBS ported shiva in a small room. Check out post 12
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.
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.
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.
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.

Paul I agree with most of your points except that 90db peaks aren't that loud.

You get an F for comprehension there!

I said:

Peaks around 90db are also very loud.

I believe it is actually 10db that corresponds to perceived doubling, although 6db is certainly significant

I'd like to go below 25 Hz, however considering my room and the room gain you have here in Australia, I don't think in my case it's worth it. I believe in the US the construction is different and you guys have worse room modes than us. It's necessary to balance everything - too much extension costs you output, it's a balance thing.

I would call 25 Hz a lot more than marginal! Not that in the US this is going to be an issue, since with typical room gain it is so easy to get down lower.

As far as SPL goes, I'd say that's a lot more of a priority for me than most audio enthusiasts. My current predominant interest is high efficiency/power handling with low distortion from 40Hz - 20k from a compact package with subs for the bottom octave. I'd like to achieve 110db ... or 105 easily.

We must not be on the same wavelength this morning. I still think peaks at 90db aren't that loud. Let's take music for example with 10db peaks, which is fairly typical. That's listening at 80db with peaks to 90db. I would consider that a moderate level, not loud.

Back to the topic, while a 25hz cutoff is probably just fine for the vast majority of music, it really is marginal for HT and the original poster wants an HT sub. If it doesn't at least meet the THX standard, I'd be hard pressed to call it anything better than "marginal".

The industry standard for music reproduction is that a high quality system should be able to play at 90dbm. with 20db. headroom.
The mixing studio for film sountracks to the THX standard calls for the average level to be 85db. at the mixers consol, note however that the .1 output on systems with music or home theater modes boost this level by 6db. when in the home theater mode.
For a high quality system especially if you play orchestral music you really should have a clean peak capability of 120db. as this is the live peak level that has been measured for an fff.
On the subject of room gain the construction methods in the hotter parts of the USA is probably simlar to that in Australia, i.e. brick veneer timber framed, with plasterboard walls and ceiling picture windows and open plan partition walls etc. with large natural ventilation grills and so on.
This type of building is virtually impossible to pressurise to any extent and you do have to design a worthwhile subwoofer on the basis of no room gain, and as already pointed out this is a major consideration when choosing one.

I design based on Dolby Digital reference which requires 105 dB peaks/20-20 kHz at the listening position for each screen, surround channel and 115 dB/20-80 Hz for the LFE channel. Since the dynamic range is limited to +20 dB, this equates to an 85 dB/listening position reference for all channels and the processor should automatically increase the LFE gain +10 dB.

If the screen/surround channels are set to small, then the peak output capability of the LFE channel needs to be increased to ~122 dB/listening position to make up for all the bass management rerouting.

Even in a small room, to meet this requires a serious sound system so the designers I've talked to plus serious DIYers on the forums that 'have their say' typically fall back to the 98-102 dB, 108-112 dB (LFE)/channel reference/listening position used for broadcast TV movies, music; and depending on the 'client', may design around as little as 75 dB, the highest average most folks watch primetime TV at.

WRT how low, as I alluded to in your previous thread you're not going to hear/feel everything on some of the more recent movies unless you can maintain a flat response to near DC, so something like the '12 Dancing Shivas IB' would be required.

Really, it comes down to how much $$/space available and how much abuse your home's construction can handle. For instance I used two Servo Drive Contrabasses at DD reference for awhile which allowed for somewhat >120 dB/14 Hz plus any room gain I may have had (probably none below ~20 Hz). For my efforts, ~all the drywall joints in the entire (small) house split, popped loose ~ a couple of hundred drywall nails/screws, cracked a concrete block foundation pillar and the outside concrete block foundation wall they backed up against. And of course vibrated the @#$% out of everything that could rattle.

Admittedly, my house is of rather poor stick built construction, certainly nowhere near as stout as one built north of the Mason-Dixon Line, but the point is that pumping high SPL numbers down to near/at the construction's Fs isn't a good plan IMO, so went back to my decades old 16 Hz EBS alignment, which yields measured 109 dB peaks/16 Hz in-room, ramping up to >126 dB peaks/32 Hz (pegs the meter so don't know the actual peak).

Anyway, once a LFE channel peak/listening position is chosen, then scale the other channel's output to suit.


Thanks! Your posts had exactly the info I need as the wife leaves Wednesday and I have 6 weeks to convert my fairly open office into an Office/HT/Listening room which is detached from the house.

I have excess equipment and drivers, so it's really a matter of how much I should bridle back to prevent disturbing the neighbors or causing structural damage.

You have me more concerned than ever about structural damage. It leads me more to believe that my quasi dipole approach may be best. I'm thinking 1 sub at the front of the room centered under the screen and one in large coffee table at the listening position (about 2/3rds the way back about 4m from each other). Then I would wire the front sub out of phase. This should give me the potential of extreme output at the listening position, but having a net Zero for the room should go a long way to keeping the bass in the room. The sub will be HT only as my dipole music system will go down to 28hz where my longitudal mode exists. I'm really only concerned about a 2 person listening position directly behind the coffee table. Does this plan make sense? Will it go a long way to preventing structural stress?
On a more practical note, sometimes the output you can design for is as much as you can get away with! It's not just what the house can withstand, but also what neighbours or other people in the house can live with! And of course cost must be considered. There is no point in spending a lot on 120db output if you get complaints when it goes above 90db! So if you plan on going loud, consider also the cost of upgrading the room - removing rattles as well as sound attenuation of the envelope.

That said, there are always those moments when you get the house to yourself and you get to indulge the urge to crank it up loud!

My ultimate system would include a purpose built detached room with "room within a room" construction and a massive basement 10 Hz horn with 140db capability.

Coming back down to earth ... it's a bit hard for us to say how loud will be loud enough for you. Also the output you choose will impact your choice of speaker types. If you find that you like the sound of conventional hifi floorstanders with typical midbass and dome tweeter, then its difficult to get past 100db ...

I have built over 10 subs along the way. My opinion is that your goals should be 18-23Hz al 110 dB. If the box you want is small the only ways to reproduce those freq is by using one or two passive radiators (preferably of the same size or even bigger then the subwoofer); the other way is to build a bass-reflex box but with a very long port and with a big port surface.
You shouldn't wory about the house (for this avoid playing subsonic freq for long periods times: the walls may resonate and start to oscilate).
If the box is very good you may have hi-fi bass and THX specs.
re loudness

The point is paul that a symphony orchestra has a dynamic range of 80db, and since the advent of cd many recordings are recorded with this type of dynamic range.
For a peak output of 120db. that lasts for a few hundreths of a second in a finale to around 40db. for solo instrument, at this later level you can hardly hear it as it is not very much above the ambiant noise, even if as I do you live in one of the quieter suburbs of a city.
If your main speakers are only capable of 100db. peaks then they are woefully inadequate for anything than has pretensions toward high quality reproduction.
My system can reproduce clean 120db. peaks although I perhaps have done so only once, but the point is that at normal listening levels it is completely withought effort and you tend to play it loud withought noticing until you try to talk to someone for sitting next to you for instance , and you have to shout to be heard.
If your main speakers are only capable of 100db. peaks then they are woefully inadequate for anything than has pretensions toward high quality reproduction.

If only I could get my family to agree that 100db quiet is just wimpy!

On a more serious note, I don't appreciate my system being criticised as "woefully inadequate." You may have a better system than anyone else on the forum, but it's just plain rude.

My system can reproduce clean 120db. peaks

Now I'm curious. What does your system consist of to achieve this goal? And in what size room, and at what distance from the mains?

It's quite a challenge. Lets say you have a pro driver like the Beyma 12G40 12" as a midbass. 95db eff, 400w

Now at 3m, the eff will be ~85db

10w > 95
100w > 105db
200w > 108db
400w > 111db

Now, at 400w there is 4db of power compression, so the output now comes down to 107db. This is not even half as loud as 120db!

A second of this driver yields 113db, then another two of them 119db, which just makes it, but now we need 1.6kw!

Since the power is RMS and not peak, I suppose a fair argument is that much higher outputs can be reached for a brief transient peak. How you would determine this, I'm not entirely sure.
re spl

I am not being rude about your system paul it is just as I suspected and that is that you are not taking into account critical distance but using an anechoic figure.
Small showed, that in a typical room up to around 2500 cu.ft. you can take the driver nominal efficiency, i.e. spl at one meter, as to how loud it will sound in the room, since 1metre is the "critical distance" in such a room, the critical distance being where the direct and reverbrant fields are equal.
Thus if your system is 90db. at 1 metre it will be able to produce peaks of 110db. with a peak input of 100Watts, this is the standard hi fi criterion , and I doubt if your system is different to it.
My system has a nominal efficiency above 300Hz. of 94db. and it is bi amped with 2x100Watt amplifiers per chanel. There is an apparent increase in amplfier power of an amount that varies with the input spectrum, but if we split the music signal between the two ranges we get an approximate 91db. n.o. and a conservative +3db. increase in effective power, a peak input of equivalent to 800Watts. With both channels driven this is easily a peak of 120db.
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