Need help understanding room effects on bass response (Wooch?)

[kexodusc]

I'm looking at building 2 subwoofers, one will replace the PW-2200 I have the other will supplement my 2-channel system in a separate room.
I'm not so much looking for suggestions at this point, as I've already decided to go the DIY route, but instead, I'd like to understand what limitations my rooms dimensions and shapes might have on the responses. I want to make sure I don't buy more subwoofer than I really need (as if that's possible), or get stuck with not-enough subwoofer (even worse).

I'm guessing Woochifer, Sir Terrence, Geoffcin, etc (the established members) will have some of the info I need but anyone feel free to chime in...I can hold my own when it comes to speakers, but subwoofers are still rather new to me...

Room A) 24' X 20' the TV is placed along the shorter (20 foot) wall. Optimal sub placement is on the front wall, about 9 feet away from listening position, in between TV and left speaker. Currently, my -3 dB is at 24 Hz in this room, -7 dB is at about 21 Hz (I won't talk about the annoying peaks at higher frequencies, a BFD is on the way).

Room B) 12' X 19', the stereo is placed along the short wall, I listen farfield, about 9 feet way from the speakers (which are 2 ft out from the wall), so seat is about 11-12 ft out from front wall. When I had a 2nd subwoofer, I placed it in betweent the two main speakers with good results. The sub was limited to about 23 Hz at -9 dB as I recall...could be wrong though.

Both rooms have 8 ft ceilings. Both are carpeted, furnished, typical rooms.

I have little understanding of room gain/loss effects, particularly below 40 Hz, and just want to make sure I'm not wasting my time aiming for a 20 Hz goal within +/- 3 dB.

Both drivers I have are about 91 dB sensitive at 1 meter. I rarely listen to HT or music where I'd need an in room response above 103 dB...maybe the odd peak, but I'm usually quite a bit lower, 80-86 dB average volume, maybe some peaks taking it above 100 dB. I'm more about quality than quantity I guess, though I love cranking it for some good progressive rock, or classical performances.
I think realistically, a good 500 watts and 250 watts is all I need for each respective room to make things shake, both drivers can handle more than this though. The PW-2200 I have now is more than enough for me at this point, but I'm trading it for a PW-2100 and then selling that PW-2100 as soon as the first sub is built.

Any comments, or can I just ignore my room and dive right in?

[Richard Greene]

YOU WROTE:

"I have little understanding of room gain/loss effects, particularly below 40 Hz, and just want to make sure I'm not wasting my time aiming for a 20 Hz goal within +/- 3 dB."

RG
Most subwoofers produce bass within+/-3dB down to at least 30Hz., and often 25Hz. (rarely 20Hz.) measured an inch from the cone / port.

By the time the bass reaches your ears, even if you sit only six feet away, the frequency response is rarely within +/-10dB.

The frequency response changes are caused by room reflections that are in or out of phase with the bass still coming from the subwoofer.

The bass frequency response is so rough at the listening position that it's very difficult to specify what the -3dB frequency really is.

With a parametric equalizer to cut bass peaks, and some seat position flexibility to move your ears away from bass troughs, the frequency response can usually be improved to flatten bass peaks, but you're still going to have at least one or two bass troughs ("nulls") of at least -6dB.

In addition to the effects of standing waves (aka "room modes") there is a bass ramp up of 12dB/octave ("Cabin Gain") in a well sealed room starting below the lowest frequency standing wave (565/longest room dimension in feet = approximate lowest frequency standing wave center frequency in Hz.).

This Cabin Gain effect can cause excessive bass output at 20-25Hz. in small rooms when ported subwoofers are used.

Sealed enclosure subwoofers have a natural bass roll-off of 12dB/octave that can be a mirror image of the 12dB/octave bass ramp up caused by the room's Cabin Gain.
They are ideal for small rooms (under 200 square feet) and for medium-sized rooms (under 400 square feet) if you use two.

Large rooms (over 400 square feet) often need ported subwoofers for sufficient low frequency output.

[Woochifer]

Doc Greene's prescription is correct.

I think that your biggest decision right off the bat is whether you want to go with a ported or sealed sub. With room A, it might be a large enough such that you'd want a ported sub. Of course, ported subs are trickier to build because you need that precise match between the interior volume and the port opening. And of course, everything has to correspond to the driver characteristics. With that larger room, another alternative would be to go to a 15" sealed sub.

My room is about 13'x18' and that takes the -3 db point on my Rava down to about 22-23 Hz. It's at -7 db at 20 Hz. Adire rates the anecholic -3 db point at about 27 Hz.

Regardless of what you do, how well the sub measures is but one part of the challenge. How you set it up, and how you correct for the room effects are every bit as crucial as the parts that you choose for your DIY project.

Concurrent to that though, I think you should consider hooking up the BFD parametric EQ FIRST and see how that grabs you. If the difference that it makes with your system is anywhere near what it did for the Adire Rava that I use, you'd think that you already bought a brand new sub after hooking the thing up. The difference is that dramatic. You know that I don't go around talking about "night and day" differences very often. Plugging in the BFD qualifies as a definite "night and day" difference maker.

Getting rid of the biggest peaks lets you more accurately set your overall level because it's no longer the boominess that dictates what you hear. It also helps improve the integration with the mains because you no longer have specific notes calling attention to themselves.

I suggest you try the BFD first. Accounting for the room effects will probably have the biggest effect over some of the other considerations that you're looking at right now with your DIY project. The sub that I have at home is more satisfying that the vast majority of subwoofers I've heard in demo rooms. The EQ is the primary reason for this, and that's even before I've added bass traps or a processor that can vary the delay timing.

[Sir Terrence the Terrible]

Doc Greene's prescription is correct.

I think that your biggest decision right off the bat is whether you want to go with a ported or sealed sub. With room A, it might be a large enough such that you'd want a ported sub. Of course, ported subs are trickier to build because you need that precise match between the interior volume and the port opening. And of course, everything has to correspond to the driver characteristics. With that larger room, another alternative would be to go to a 15" sealed sub.

My room is about 13'x18' and that takes the -3 db point on my Rava down to about 22-23 Hz. It's at -7 db at 20 Hz. Adire rates the anecholic -3 db point at about 27 Hz.

Regardless of what you do, how well the sub measures is but one part of the challenge. How you set it up, and how you correct for the room effects are every bit as crucial as the parts that you choose for your DIY project.

Concurrent to that though, I think you should consider hooking up the BFD parametric EQ FIRST and see how that grabs you. If the difference that it makes with your system is anywhere near what it did for the Adire Rava that I use, you'd think that you already bought a brand new sub after hooking the thing up. The difference is that dramatic. You know that I don't go around talking about "night and day" differences very often. Plugging in the BFD qualifies as a definite "night and day" difference maker.

Getting rid of the biggest peaks lets you more accurately set your overall level because it's no longer the boominess that dictates what you hear. It also helps improve the integration with the mains because you no longer have specific notes calling attention to themselves.

I suggest you try the BFD first. Accounting for the room effects will probably have the biggest effect over some of the other considerations that you're looking at right now with your DIY project. The sub that I have at home is more satisfying that the vast majority of subwoofers I've heard in demo rooms. The EQ is the primary reason for this, and that's even before I've added bass traps or a processor that can vary the delay timing.

I fully agree with Doc and Wooch, and definately with Wooch about the BFD. The BFD does IMO signify a "night and day" perspective when it comes to improvement.

About two weeks ago I received my first two of four custom 15" subs in 30x17x17 cabinets. These were sealed subs which is my preference because of their roll off charcateristics in medium sized rooms. Before BFD I had one peak at 6db, and another at about 7.5db. I dialed in the corresponding reduction into the BFD, and my response ended up being around +/-2.5 db at the listening position from 18-80hz. The quality, resolution and the true frequency response of the subs was more apparent after the necessary adjustments at the offending frequencies. My subs are of very high quality, but the BFD's effect within the B-chain of the system cannot be underscored. A rooms effect below 200hz should not be ignored if you want a good balanced sound with most listening material, in most rooms.

Be very careful of use of bass traps in very small rooms.(do not overdo) What you can sometimes tame in the low frequencies, you can also smother(or reflect) high frequencies. IMO a balanced approach between bass traps and parametric EQ has produced better results than either by themselves.

[kexodusc]

Thank you, gentlemen...
The BFD is on the way.

Wooch, yes, my RX-V1400 has flexible bass management settings, delay, xo, etc...should come in handy. I am ordering the BFD today finally, and I do know the significant impact it can have by itself, having heard its effects at a friend's place before.

I should explain, however, that I'm not so concerned with building these subs because I need them, or desire upgrading and improved performance...This PW-2200 is enough for me in all honesty, but rather I'm building them for the fun of it...love building speakers, working with wood, and learning about all things audio is fun, hence the hobby.

The challenging part is building two subwoofers for around the cash I'm getting for selling my PW-2200 that will outperform the incumbent subwoofer...After modeling a few designs, I don't think it's going to be that hard. One will cost $250, the other about $350 in parts...I have lots of MDF and veneer lying around...Net result should be a 2nd sub built for about $100.
The nice thing about the Tempest driver is it's reasonably efficient, not requiring a ton of power. I've conversed with a few people that are reporting excellent results with 240 and 250 watt amps. The other driver I'm getting is even better...this is allowing me to get away with some smaller amplification while saving a ton of money (though I might just buy a larger amp for now anyway)/ One thing I learned buying my last reciever, is not to get too carried away with numbers...more is not always better, especially if I won't call upon it.

My initial modeling in unibox (software) tell me that a sealed sub is still ideal for both subs in their respective rooms, though I'm not sure what to expect in the larger room as far as cabin gain below 25 Hz...We'll see...

Live and learn.

I expect you folks to be around when I hit you up for some help with bass traps and setting up the BFD.

Thanks again!