Previously we looked at the impact of the rear chamber >
Now we'll take that one step further and see how that impacts extension. Here is our 40 Hz horn with Peerless XLS.
This trick relies on some room gain. Here is a measurement of the in-room bass in my room:
Green: left sub
Red: right sub
Black: both combined
Horizontal divisions are 8db
You can see there is a lot of gain here. So if we use this gain, we can make a small horn get down a bit lower. Many would design a horn flat down to 40 or 50 Hz. That gives better results in the simulation, but in the room you get a big peak in the midbass that requires EQ to remove.
So instead of a horn that will go flat down to 40 Hz, we might instead try extending it lower with low tuning. We'll need a fairly small rear chamber with a sharp knee and we will see a less appealing response with midbass dip, but this will be deliberate. The room will fill in this dip. The ideal response would be close to the inverse of the room transfer function, but we won't get that. We are now making a design that will work best for one room.
Now let's combine the two:
The grey line shows combined room response of corner subs that are flat from 20 - 80 Hz. The red line is simulated response of a 30 Hz undersized horn. It isn't really big enough to get down that low (more like 40 Hz) but it's intended to work with room gain. You can see the midbass response dips down where room gain will bring it back up and there is a knee in the bottom end. In-room response should get down to about 28 Hz. Unfortunately the 70 Hz dip will be present and so we need another bass source there to fill it in.
Don't make the box too big. It will smooth the knee and it won't get down quite as low, and excursion won't be as well controlled.
They grey version is what you would do if you didn't know room response. The black version has a smaller chamber and lower tuning.
No guess work
You need to measure your room to benefit from this. Otherwise it's a game of pin the tail on the donkey - you have no idea what you are dealing with.