Lots of bass at walls, lack of bass in center of room/listening position


I guess this is relatively common in listening system. Is there any way to smooth this out so I get more bass energy at my listening position? This happens with our without my 2x 18 inch subs. Room is 12 x 16 x 8 ft, speakers 4.5 ft apart on long axis and I am sitting 4.5 feet away. I tried moving back and forward but the entire middle center of the room except near the walls has decreased bass.
Is this a boundary effect or could it be due to bass cancellation effects?
smodtactical

Showing 7 responses by millercarbon

That is so fascinating using all different kinds of subwoofers. Is there any downsides to this? Do they have different speed or timbre? 
Speed or timbre. With a sub. Good one. Guess you missed the part above where I said:

human beings cannot even hear low bass at all, at less than a full wave.

So for proof, my first sub was a ported isobaric Talon Roc. Isobaric is two drivers mounted back to back so one removes the air pressure from the back of the other one helping it move faster. Because the Talon Khorus uses the same isobaric design and so of course the bass has to be "fast" to match. This is the way they think. 

This way of thinking is baloney. My DBA with sealed and ported, and none of the drivers as massively powerful as the one in the Roc, is way better. It sounds faster, more tuneful and articulate, in spite of what looks on paper as if it would have to be much worse.  

You have to get out of the trap of thinking of low bass the way we think of midrange and treble. They are both waves and the physics is the same but the way we hear them is quite a bit different. 

Like, midrange we can pinpoint the source in 3D space. Low bass we cannot localize at all. Treble and midrange we can hear a millisecond blip. Low bass we cannot hear less than a full wave. At all. That's like 50ms. Midrange we are very sensitive to volume and hear volume changes accurately. Low bass we hardly hear at all until it gets quite loud (hence the Loudness switch), and then once it does we are overly sensitive to volume.  

What all this adds up to is with midrange and up we need two and only two perfectly matched speakers placed precisely symmetrical to create a 3D soundstage- but we can use a whole bunch of subs of all kinds and put them just about anywhere and yet they will integrate perfectly into that 3D sound stage.
Absolutely. Notice my system https://systems.audiogon.com/systems/8367 There's a 12" isobaric Talon Roc sub in the front left corner, two sealed 10" subs towards the front on the left and right, and two 10" ported subs in the back along the L and R walls. Each sub is near the wall but they are all different distances from the corners.  

The response of each of these subs is different, and the locations are different, and they even are adjusted somewhat differently. The Talon is powered and so its independent. The left two are on one Dayton amp, the right two on another. Left and right are close but not identical in level. Or phase. All this asymmetry would be a nightmare for the mains, but is actually what you want with subs. All the slightly different responses produce different modes, in different places, and at different levels.  

Placement becomes a whole lot easier with more subs. With just one you have one or two nasty humps to figure out. With four each one plays at a lot lower level so the humps are much smaller, and you can put them just about anywhere you want. I tried a lot of different things because it mattered so much with just the one. But with five I find it hardly matters at all.  

Hook em all up, you will see. 
It would be great though if I could solve this without subs. I wonder if bass traps would allow me to turn my subs up more while still retaining cleaner bass.


So here's how that works. In order to get even bass the conventional way you have to turn the subs up loud enough to bring the drop-offs up to where they sound good. This means having way too much energy at other frequencies. So you try and damp that out with traps.  

But the traps and loud spots are only in certain places, while the excess energy is everywhere in the room. This physical energy excites the walls, the whole room, and physically dissipates over time. Its the dissipation of this energy that smears and muddies bass response. There really is nothing you can do about it. More and more traps leads to more and more EQ and you just never get enough. 

What's funny is everyone knows the last thing you want is to have an overdamped room. A certain amount of acoustic reverb is nice and helps create a sense of spaciousness. Too dead and the room sounds... dead. Yet that is exactly what a lot of these bass trap people are having you do, only with bass instead of midrange and treble. So you don't notice it when you walk in the room the way you would with panels all over the walls that kill the sound. But its the same thing, only lower in frequency. 

You don't need 12" or 14" subs. Those are for when you haven't figured out the answer is more subs and still think its more sub. That "s" makes all the difference in the world. You can easily use 10" subs. I have four of em. Its not how big. Its how many. And where. Dispersed asymmetrically around the room. 

This will cost less, take up less space, and work better than all other options. I really cannot think of a single objection, other than the mental effort required of any new idea.
You can play with them till the end of time, no amount of moving them around will have the least effect on the underlying physics. Been there. Done that. Found a better way. Don't do that no mo.
I did play with the delay of the subs based on what Tom wrote at PSA where he said you should add about 1msec delay for every foot away the sub is from you. So I have 1 sub that is 3 feet away facing me and 1 sub in the back left corner that is 9 feet away so I did adjust that. Can't really tell if that made a big difference though.

Of course not. Because he's full of it. The one foot per millisecond is based on being approximately the speed of sound in air. One foot per millisecond.  

There's a couple times this matters. One is side wall reflections. Side wall reflections that arrive within about a 3 to 5 ms window of the direct sound have a very bad effect on imaging. That is why we want our speakers a good 3 feet away from walls. The other time is with movies. If the screen is several feet closer or farther away from you than the speakers then the people talking on screen will be out of synch. That's it. 

This rule of his is not applicable to subs for the simple reason human beings cannot even hear low bass at all, at less than a full wave. Since a 20Hz wave is 1/20th of a second that is 50ms before we even hear it. So when it comes to subwoofer bass you can forget about timing. Its simply not a factor.
Miller so in a smaller room you cannot get good bass without subs ?

Correct. And small means small relative to wavelength. Since the the wavelengths of the lowest bass we want are 40 to 50 feet or more (depends how low you want to go) then small is anything less than that. Which means pretty much every room in every house is small. 

Look, its pure physics. Even at much shorter wavelengths we have to be clever with speaker placement and use things like diffusers and absorbers to get a nice smooth response. Exactly the same is true of bass, only the methods used have to be physically bigger on account of the physically longer wave lengths involved. 

Notice when you go to a concert in a large dome or hall the bass is wonderfully deep and uniform no matter where you go in the hall. That's because these spaces are large even relative to low bass waves. The waves have room to run and so when they do reflect off the walls its in different directions and times relative to the wave. 

In a normal room, say 20 ft, the leading edge of the wave bounces off the floor, ceiling, and all four walls even before one complete cycle! The darn thing is coming back and canceling itself sometimes right at the woofer! Think about it. 

But the flip side is if you take all this into account and do it right then you can actually have really fantastic bass even in a small room. You just have to abandon the old false paradigm of one or two subs and embrace the physical realities of long waves and use lots of subs. 

Because there's lots of subs they don't have to be big and powerful. Four or five ten inch subs is way better than one 20" even if the one is ten times as powerful. Because the one will create one powerful set of room modes. Bass will be thunderous but only at certain locations and frequencies. You will fight and struggle to tame this lumpy boomy awful bass. Which will not even go that low, by the way. But the four small subs will create four times as many different modes, each much smaller, so the bass will be smooth, powerful, and go really deep. 

DBA subs don't even have to match. Mine are a combination of three different subs. The bass is to die for.  https://systems.audiogon.com/systems/8367
Diffusers work based on wavelength. That's why you see those panels with wood blocks of different sizes at different heights. Each frequency has a wavelength. The higher the frequency the shorter the wavelength. Little blocks of wood can diffuse high treble because the wavelengths are on the order of inches. The low bass you're concerned with is at a wavelength on the order of some tens of feet. Ten to forty or more feet. In order to work your diffuser needs to be at least that big. This is why we have the low bass problem in the first place: even an entire room is small compared to low bass waves!

This is also why bass traps are no solution. They can do something, just not much. The only real solution is a room that is multiples of the lowest bass frequency. A room a hundred or more feet on a side. Think about the phenomenally good bass you have experienced at a rock concert. Because: big room! The next best solution is lots of subs. Each sub still has the same mode problem. But with so many each one puts out less bass, so its mode is small, they are in different places, and together they average out to exceptionally smooth powerful bass.