How seriously do you take room gain?

[Brian Hinther]

Having read Steve's LLT explanation, I find myself wondering about room gain for the first time. Steve explains that the average room contributes 4-8 db of low bass gain per octave. His Subwoofer-DIY reference suggests that the ideal response curve would be down one db at 30 Hz, and then fall off a cliff down to -9 db at 20 Hz.

However, I don't see any subs that model this behavior. In fact, when I look at Steve's graphs, the one he describes as "too low" is closer to the above than the one he calls "good," so I'm confused.

I'll be the first to admit that designing a sub to be down 9 db at 20 Hz on purpose seems sick and wrong, so what gives? Am I misinterpreting the data? Is the data flawed? Or is this a case where you just do what sounds right and forget about the data? I'd sure appreciate someone straightening me out!

[arande2]

Well, actually it all depends on the room. I think smaller rooms have more gain, but don't hold me to that statement.

My personal design that I'm working on has 1dB down at 30hz, 3 down at 20hz, 6 down at 11.5hz, and 10 down at 10hz. Considering MY personal room, based on testing, that should be about flat to 10hz, maybe with a slight curve up going down, then falling off below that.

For you, it would be quite different I bet, but I don't know that and neither do you.

[Geoff St. Germain]

Room gain is dependent on the size of your room, which makes it hard to write a description of the ideal anechoic response that will create a flat response in room to some desired frequency. The smaller your room, the more room gain you will have (and the higher the frequency it will begin at). Cars have cabin (room) gain that can start near 100 Hz and on the other side of that we have outdoors, which offers basically no room gain.

Of course, I would prefer to have a frequency response coupled with room gain that makes the in room frequency response rise at the low end and have to cut low frequencies than have the reverse situation and have to boost low frequencies to achieve the same response (and deal with decreased headroom at low frequencies).

[SteveCallas]

Where do I suggest being down 9db at 20hz?

[Brian Hinther]

Steve, the 9 db comes from the article you referenced on Subwoofer-DIY about EBS . It said the average room is down 1 db at 30 Hz and down 9 db at 20 Hz, leaving me to wonder if that's really true, in the first place, and--if so--whether that was something to shoot for. From the responses I'm getting, it seems not.

By the way, I posted first in your LLT thread without attracting much attention, then noticed you started it in 2006 and thought maybe you weren't paying much attention anymore, so I started the other thread. Anyway, thanks for responding. I really think you're onto something and I would like to learn as much about LLT as I can.

Brian

[BoomieMCT]

I think room gain is very important. As a minimum you need some sort of control over how your sub rolls off. If you are really fancy you should design a sub whose roll-off matches the gain of the room. Failure to pay attention to room gain will make a weak sub with no depth or a boomy sub.

[SteveCallas]

I guess I'm just not following were you are saying I stated that at Here is what I wrote on the matter in the LLT Explained thread:

By fine adjusting the enclosure volume and tune, you can mold an anechoic FR that is flat throughout the bulk of the bass range with a shallow roll off starting at ~25-30hz, extending down to the tuning point, with the 4th order roll off occurring somewhere below that. This shallow roll off area is key, as most rooms typically yield about 4-8db of room gain/octave.

- FR with a ~4db/octave rolloff between the tuning frequency and the lowest room node

In "average" sized rooms, gain starts to kick in ~25-30hz, with it starting even lower in larger rooms. So unless you have ~5db/octave gain in a really small room where it would kick in starting at 40hz, I definitely would not aim to be 9db down at 20hz.

[cixelsid]

The massive variability in room architecture, the location of the sub, and the physical characterizes of the room construction, mean generalized statements regarding the amount of room gain obtained at a given frequency are fundamentally useless.

As a example, a new house with an open floor plan, with lots of windows and a vaulted ceiling, will have significantly different room gain characteristics compared to a older house with rectangular rooms, 8' ceilings, and minimal doors and windows.

This means there's no such thing as a "average" room, so generic recommendations to design a sub based on a expected amount of room gain isn't the best idea.

There are room acoustic modeling programs, CARA 2.0 and RPG Acoustics Room Optimizer are two such programs. Using a program like these is as integral to understanding how the room will operate as Unibox or WinISD are to designing the enclosure.

In the end there are usually two options, forget about room gain, design a sub based on the performance of the driver and live with whatever room gain you get. Or design the sub specifically for the driver, the room, and the location of the sub in the room.

[OvalNut]

Or too, use an external EQ to bring room gain back down and level out the frequency response.


Tim

[Brian Hinther]

Thanks to everyone who has responded to this question so far. I'm just a new guy trying to learn, and beginning to see what a can of worms I've opened.

Steve, this is what confused me:

I've been asked what separates a LLT from a standard extended bass shelf design explained here. A LLT matches that description and also meets the following minimum requirements.

And when you follow that link to DIY-Subwoofers, it says:

One of the main reasons given for using an EBS system instead of the usual "maximally-flat" systems is to take advantage of "room-gain," the increase of bass response at lower frequencies, starting at around 1dB at 30 Hz and increasing to 9dB at 20 Hz, that you get in most rooms. This "room-gain" can tend to produce a boomy response with speakers designed to be maximally flat to frequencies below 30 Hz.

So, no, you never said anything about being down 9 db at 20 Hz--the article did, and I wasn't sure if you meant that, too. It seems pretty clear now that you don't.

Anyway, thanks to all for trying to help me get my head around this room-gain thing, and I welcome any further insights you may have. And I've just now discovered the REW and BFD forums which may help, too (I always thought BFD stood for something else--but oh, well...)

[warnerwh]

The sound quality of the bass that is properly done compared to just putting a sub in the system is large. With these subs a BFD is a necessity. Considering that someone has been nice enough to write REW for everyone it would be a shame to not use it.