[JoshK]

I think some of my flagellations have been sub 20hz.  

[Shamrock Audio]

Assuming the speed of sound (lots of variables here, but let's pick a number), to be around 1130 feet per second (fps), we can find the wavelength in feet by dividing the speed of sound by the frequency. So, 1130fps / 20Hz = 56.5 feet. Close enough for our discussion.

We know that the average room isn't large enough for a wavelength of 56.5 feet to fully develop. What happens is as the wavelength being reproduced becomes longer than the dimensions of the room, the pressure in the room increases. Higher frequencies allow the pressure wave being reproduced (say, 100Hz at 11.3 feet long), to dominate what we hear. But as we decrease the frequency (80Hz = 14.3ft., 60Hz = 18.83ft., 30Hz = 37.67ft. etc.), the pressure wave is overcome by the pressurization of the room itself 80, 60,or 30 times each second. This is commonly refered to as room gain, or room lift.

Typically, upstairs rooms present the worst case scenario as sound pressure is lost in all directions. That's why upstairs rooms have trouble supporting pressurization below a certain frequency. Yet, go down stairs and the bass will often seem deeper and stronger than upstairs.

When you're looking for deep and powerful bass, realize that at some point depending on your room, you're no longer dealing with a wavelength, but a pressurization of the room so many times each second. Getting the amplitude of those room pressurizations equal at all frequencies, and proportionate to the rest of the frequencies being reproduced at your listening position is often an exercise in frustration.

Headphones work on the same principal. Pressurize the ear canal 20 times per second and we percieve that we're hearing 20Hz.

Hope this helps.

[JohnR]

Here's a more intuitive explanation

Imagine you were small enough to fit into a (sealed) subwoofer. Suppose you were put inside the box and the system turned on. What do you think you would hear? Nothing?

[nathanm]

I think you mean "flatulations" Josh!  Heh!

56.5 - Ummm, I guess I flipped the 6 over the wrong way!  So if this bass response is a matter of pressure, would you get more bass from making your room as "air-tight" as possible?  Sometimes I wonder if this is a factor because my speakers seem to sound better in the bedroom, which can has two doors which can be shut whereas my living room area has openings to a hallway and the kitchen.  I also notice that with headphones the bass improves if you have a good seal against your head.

Getting the amplitude of those room pressurizations equal at all frequencies, and proportionate to the rest of the frequencies being reproduced at your listening position is often an exercise in frustration.

Well that's good - that means I'm getting plenty of exercise after all!

[JohnR]

openings to a hallway and the kitchen

My understanding is that these kinds of things can form a kind of Helmholtz resonator, thus absorbing bass energy.

A giant bass-trap, if you will

[MaxCast]

Has anyone ever listened to speakers out in the middle of a huge field with nothing around to block the sound aside from the ground? On a calm, windless day it would be interesting to try.  Probably would need a generator, or battery power of some sort.  Hmmm.

I believe that is the goal of an anechoic chamber.  To have space to test a speaker with no influence of side walls.  That includes the ground, so you'd have to elevate the speaker outside.

[Shamrock Audio]

So if this bass response is a matter of pressure, would you get more bass from making your room as "air-tight" as possible?

It's not that you get "better bass", rather, it becomes easier to pressurize the room by making it less lossy. This means that you would tend to be able to generate higher pressure levels at those frequencies affected by the losses otherwise found in the room. This is because the pressure in the room is retained to a greater degree.

Sometimes I wonder if this is a factor because my speakers seem to sound better in the bedroom, which can has two doors which can be shut whereas my living room area has openings to a hallway and the kitchen.  I also notice that with headphones the bass improves if you have a good seal against your head.

First, I'm betting that while your bedroom is quite lossy due to things like space under the doors and things like that, it is also smaller than your living room in total area that your speakers have to pressurize. Therefore, the effects of room pressurization will begin at higher frequencies than in the larger livingroom area. Also, because it's a smaller area than your livingroom the same speaker will be able to generate higher pressures at lower frequencies.

Second, headphones work well because their able to more accurately predict and control the environment they're working in. Very small "bud" type headphones work because they only have to pressurize the ear canal itself. IMO, they are also the most dangerous. Headphones that completely cover the ear and seal around it still have a known quantity in terms of the air volume they are pressurizing. In one sense, a controlled environment.

Loudspeakers have no such luxury. We can generalize about a "typical" room, but not much more than that. That's also why I think it's pointless to audition loudspeakers anywhere but in your own home and on your own gear.

[ZooDog]

This is slightly off topic but I was at a club here in Boston last night called Avalon which has the most amazing PA system I have ever heard.  The bass is like nothing I have ever experienced.  It just goes right through you and is totally clean and unstrained.   I was drinking a beer out on the dance floor and I was literally having trouble swallowing because of the bass pressure.  I had a long coversation with the owner one night about the system and he says it is capable of 145db  

[JohnR]

Did it have four or eight 18" JBLs embedded in the bottom of the stage? I had a similar experience recently.

[ZooDog]

They use a series of speakers designed by EAW specifically for dance clubs.  They are actually named the "Avalon" series because they were created in a joint venture between the club and EAW.  You probably wouldn't want to listen to Diana Krall over them but man can they pound.  Here's a link:  http://www.eaw.com/products/DC/

[Jack Gilvey]

I'd heartily second John's recommendation of a sealed Adire DPL12.  I've got one in 5ft^3 for Qtc=~.577 and it's about as good as it gets, second only to the same driver in a dipole configuration, lacking only in ultimate output (not a priority for you, anyway). Only needs a good 150 watts in this box, and it should be flat in-room right down to 20Hz (yes, it can "exist" in small rooms ). Smaller boxes, down to 2.5ft^3 or so, are excellent also.

 I'm also playing with a Stryke AV12 driver in just over 1 ft^3 sealed. With 250 watts and a Linkwitz transform applied, this is also an excellent config. in a much smaller package. Capable of considerably more output than the DPL12, but I really need more amp for it in this small a box.

[dayneger]

Hmmm, a dipole sub.  What are the pros and cons?  I don't know anything about them.

What's the difference between the DPL12 and the Shiva III in a sealed box?

As per your original question, JohnR, I definitely meant in-room response (and I have a fairly "average" room size, whatever that means ).

What's the relationship between how deep a sub can play and how loud it can play at a given frequency, anyway?  Are there subs out there that can't play at true reference levels down to 16-20hz, but that still play cleanly down there?

[MaxCast]

What's the relationship between how deep a sub can play and how loud it can play at a given frequency, anyway?  Are there subs out there that can't play at true reference levels down to 16-20hz, but that still play cleanly down there?

The idea is to move air.  Especially for output.  Given the same dirver the larger the box the lower the frequency extension.  Once you hit that mark you need a lot of x max in the driver to move some air to reach reference level output.  Many drivers would explode trying to hit 16Hz (or 20Hz for that matter) at reference levels.  The more a driver has to move the more distortion it will produce.  Then there are eq tricks too.

If you are looking for a sub you must define your goals/use and design or buy a sub taking care of those parameters.  It is quite possible to get to -3dB, 20Hz with out asking it to reach reference levels.

[JohnR]

DPL12 compared to Shiva? The best answer is to show the graph . This is the only way you can properly answer these kinds of questions.

[dayneger]

Admit, John, you just didn't feel like writing a thousand words. . .     How's the Berkely life treating you?

How does the amplifier fit into the equation--does more power with a given driver and box volume mean higher SPL or that it'll play deeper, assuming the driver doesn't blow up?  Same SPL und depth but in a smaller box?

[Xi-Trum]

Can anyone explain how in-room bass response can be lowered by a couple of Herts?  For example, a 20Hz sub may actually extend to, say, 18-19Hz in a room.  What's the physics behind it?  Thanks.

[Jack Gilvey]

Thanks for the graphs, John.

 If you normalize John's FR graphs so the level is matched at 80Hz or so (just as you'd do when calibrating/blending with the mains) the extra extension of the DPL12 becomes more obvious. You do give up a bit of efficiency for it, though.

[JohnR]

How does the amplifier fit into the equation--does more power with a given driver and box volume mean higher SPL or that it'll play deeper, assuming the driver doesn't blow up?  Same SPL und depth but in a smaller box?

I missed this one

More power means it will play louder, but the driver is also more likely to be pushed to its excursion limits at low frequencies.