Why do you guys pursue a flat frequency response when you buy a subwoofer?

Deep_333 wrote:  "Why do you guys pursue a flat response when you buy a subwoofer?... The minute you put that flat sub in your room and take some measurements, it is anything but flat (it's a rollercoaster with all kinds of peaks/nulls etc, EQ to the rescue)..." 

Deep, I come at this situation from the other side of the fence; I make subwoofers. 

At the risk of overgeneralizing, I see the room as typically doing two things to the response of a subwoofer: 

First, the usually room imparts a gently rising trend to the subwoofer's output as we go down in frequency.  This is because as we go lower and lower in frequency, the room's surfaces become closer and closer to the sub in terms of wavelengths.  So we get progressively more and more approximately in-phase reinforcement from the room surfaces as we go down in frequency.  This trend has been called "room gain" (which may or may not be technically correct), and 3 dB per octave below about 80 Hz is an approximation which has been suggested by a couple of different researchers (Martin Colloms and a woofer designer whose name slips my mind at the moment)... the exact figure of course depending on the room's acoustics as well as the subwoofer and listener locations within the room. 

The second thing that happens is, room interaction imposes that roller-coaster peak-and-dip pattern you mentioned.  The specifics of how the subwoofer's output is altered by the room once again depend on the room's acoustics, as well as the subwoofer and listener locations within the room. 

Imo the first issue can be addressed by designing the subwoofer to have a native frequency response which slopes gently downwards by the approximate inverse of "typical" room gain, or 3 about dB per octave.  I'm not saying this is the only way of addressing this issue, but imo it results in a reasonably good starting point. 

The second issue (rollercoaster in-room response) presents an interesting challenge.  Opinions vary on how to address it. 

I think "flat frequency response" is a reasonable target for a subwoofer system, as long as we're talking about the ACTUAL IN-ROOM response.  I do not think a "flat" frequency response which ignores the room's effects is the ideal starting point. 

Duke

Why do you ask?

Why buy a sub from a manufacturer who can't even get a flat frequency response when so many can?  Of course there are room issues that need attention but you might as well start with a good fundamental frequency response.  

The minute you put that flat sub in your room and take some measurements, it is anything but flat (it's a rollercoaster with all kinds of peaks/nulls etc, EQ to the rescue).....So, why do you dudes continue to look for the flat line?

Good question. This dude used to think like that, until he started reading the other guy, aka Duke, aka Audiokinesis, and that guy led the dude to reading some other guys, none of whom think like most dudes but instead actually studied the problem. All this reading led to actually for the first time understanding the problem, which yes most dudes do get wrong but this guy now does not.

What the real sub guys know that the dudes do not is what Duke alludes to above as "an interesting challenge".

You are right deep, put one sub in a room and you get peaks and valleys all over the place. Move the sub, peaks and valleys move but are all still there. You move the one sub around endlessly. Been there. Done that.

So, to review, one sub no matter where you put it produces lumpy bass. Moving the sub only moves the lumps, but never gets rid of them. The solution, which you can only shake your head at the brilliant simplicity of it all, is more subs.

With more subs each sub has to put out much less volume. So the lumps get smaller- and there are more of them. Which when you get enough small lumps turns out the result is pretty smooth. This brilliant idea of using lots of bass sources distributed around the room is called a distributed bass array or DBA.

Duke the speaker guy makes em. He calls his version the Swarm. Mine is based on his. https://systems.audiogon.com/systems/8367  Notice there's three different types of subs- 2 ported, 2 sealed, 1 isobaric. There's other guys here with DIY DBA, and other guys using the Swarm. The one thing all us guys have in common is we love our smooth, articulate, deep and totally awesome bass.

That and, we wish more guys knew about it, and we could all be less like the other dudes.

" Imo the first issue can be addressed by designing the subwoofer to have a native frequency response which slopes gently downwards by the approximate inverse of "typical" room gain, or 3 about dB per octave. I'm not saying this is the only way of addressing this issue, but imo it results in a reasonably good starting point.

The second issue (rollercoaster in-room response) presents an interesting challenge. Opinions vary on how to address it."

Duke, 
Can you name the popular opinions among sub manufacturers on how they think it should be addressed? Of course, more subs a.k.a an array of 4, 8 or 16 is great for business. But, are there other opinions out there?

I currently have Rythmiks (room1) and HSUs (room2) at home and a BIC PL 200 II in my shop. The BIC is not flat and i saw some clowns arguing in a forum that the BIC is very very bad because of the peak in its spec! However, this budget BIC actually sounds pretty freaking good to me and i use it largely for music in my shop. As far as the manufacturers coming up with a flat response (Rythmiks, HSUs, SVS, etc), Is it just a "statement" that they are competent enough to come up with a flat response? (though it means jack sht after it goes in the room)