"Room gain" is one of those misnomers that is true in theory but never found in practice. It comes from the fact that any sealed enclosure will have a mode at DC, which is to say that it can maintain a static pressure. But no room is perfectly sealed. My room, being sound proof, has sealed doors and as many leaks as possible are sealed, but it still has to have HVAC, so its always leaky. In theory the DC mode causes a 1/f pressure rise, but when there is leakage, this levels off below some frequency. The frequency and level depends on the rooms volume and the leakage.
Very small rooms like cars can see this rise, but larger rooms like home listening rooms will always have too much leakage to have any real gain. I don't see it in my room and I've never seen it in any listening room, but I have seen it in cars. By the time you get to a van or SUV size its gone.
We will have to disagree since there are posted measured difference that shows it for more rooms then you think.
Many Custom HT room have gain, MKTheater over on AVS has some of the best room gain I have ever seen, he has +18dB down around 10Hz. Others with high end systems that have great output down below 10Hz also see the room increasing the output.
One just needs to do a measurement of a sealed system @ 10Hz or @ 20Hz to see how the room interacts and see that the room can increase the ouptut buy 10 to 20dB.
Im not sure you have enough output low enough in your room to see the gain though so maybe its a combination of room design/size and bass output. Do you have the nearfield measurements of your ULF sub vs LP measurements in your room down to 10Hz??
Many Custom HT room have gain, MKTheater over on AVS has some of the best room gain I have ever seen, he has +18dB down around 10Hz. Others with high end systems that have great output down below 10Hz also see the room increasing the output.
One just needs to do a measurement of a sealed system @ 10Hz or @ 20Hz to see how the room interacts and see that the room can increase the ouptut buy 10 to 20dB.
Im not sure you have enough output low enough in your room to see the gain though so maybe its a combination of room design/size and bass output. Do you have the nearfield measurements of your ULF sub vs LP measurements in your room down to 10Hz??
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Then we will have to disagree. My system is flat to 20 Hz. but to me, thats not what I call "room gain", thats just LF modes. It all depends on what you call "room gain".
Then there are those of us living in homes built in the early 1900's (mine built in 1920) that has 2" wide wood slats attached horizontally on the interior side of the frame that then had plaster applied by hand to form the interior walls. The ceiling above the main room looks similar, not sure how it was done. Above that is the upstairs as it's a 2-story home, so there's a wide variety of room conditions, not just current construction techniques.
I listen in my basement (small floor plan, no room for a full system upstairs) that has a concrete floor, cinder block walls that on two sides are increasingly buried underground toward the upward slope with one interior wall added that is practically acoustically transparent at low LFs. The (low) ceiling is hardwood flooring on top of the beams.
Not an easy situation in which to do anything, but it's still a very leaky system with a set of stairs to the main floor and many other leaks to the upstairs area (chimney, etc.). This probably is a large reason why my current dipole woofers are a significant improvement in perceived response. Dipole bass in my situation appears to be the better form. Extreme LF is supplied by a closed box (Velodyne) crossed first order at 45Hz. I have yet to try multiple subs.
Dave
Room gain is the difference between your subs natural curve and the in room response. If you posted both we would know for sure.
Also, most of the gain discussions are happening around and below 20Hz so a measurement down to 10Hz is needed to really show the most gain rooms will have.
That definition has a lot of problems. Room gain is commonly defined as an increasing sound pressure level due to pressurization of a sealed enclosure. As Earl already pointed out this doesn't happen in normal listening rooms. You might see it in cars.
The difference between free field response and in-room response is caused by the room's modal behavior and not by pressurization.
Earl, don't you also have ported woofers? You yourself wrote a paper on how ported boxes behave like dipoles at low frequency. They can not pressurize a room even if the room is perfectly sealed. There is no next volume displacement as the frequency drops. Volume velocity of the port and driver have the same magnitude and inverse phase at the frequency drops below the cut off frequency. So, assuming I am correct that you are using your bandpass woofer no conclusion can be drawn from what you see in your room.
That is the definition of gain though, you can have all the problems in the world about trying to define but its just there.
You either have increasing the dB level of output because of the room or you do not. No one ever cared if its from pressurization or from modal behavoir. Its all just part of "room gain".
There is a reason many subwoofer designers market sealed subwoofers as flat in room to X dB. They use "Room Gain" (Call it what you want) as part of their strategy.
I know arguing semantics is what Diyaudio does more then anything else so please feel free to give it a new name. Everyone else will consider it "room gain", how about from this moment just define it as such because its a useful definition.
It happens below the room fundamental. To have a fundamental below 20 Hz the room dimensions woull have to exceed 28' or so.
Such an obvious fact now you point it out, but one I'd somehow never really thought about in the context of room gain.
You're dead right, a bass reflex enclosure can't pressurize a room at all below it's box tuning frequency, so not only do you have a 24dB/oct ultimate roll off compared to 12dB/oct for a sealed box, you also lose any pressurization gain the room might offer below the box cut-off frequency which would be experienced by a sealed box regardless of it's cut-off frequency...
Indeed, and it happens rather quickly below port tuning as well. I just opened some nearfield measurements I recently took - 8" driver in a 45 litre cabinet tuned to 43Hz, and looked at the phase response of both driver near-field and port near-field.
It's a fairly gradual roll off alignment and yet the relative phase shift between the two has reached 180 degrees by 37Hz and tracks almost perfectly below that. The two are in phase above about 48Hz, leaving a relatively narrow transition region from 37Hz to 48Hz where the driver phase response has a 180 degree "kink". (Yellow in the graph is speaker phase, red is port phase)
This also poses another interesting question - if a bass reflex system is essentially a dipole once you get about 5-10Hz below the box tuning frequency, does it then make a difference (to the response below the box tuning frequency, measured at the listening position) whether the port is on the front or back ?
On large cabinets I've always measured noticeable increases in bass response with rear mounted ports compared to front mounted ports with identical tuning, especially at the low end of the bass.
I always assumed it was additional boundary gain from the front wall proximity in the rear port scenario, but I wonder whether some of it is that putting the port on the back increases the acoustic short circuit path length for the frequencies below box tuning where the port is 180 degrees out of phase - essentially making it like a conventional dipole in this region where the positive phase radiation is coming from the front of the cabinet, and the negative phase radiation is coming from the back, with a baffle in between.
With a port on the front next to the woofer the short circuit path is very short indeed and doesn't have to navigate the perimeter of the baffle/box, therefore there is no baffle effect on the out of phase radiation...(In other words the 180 degree phase cancellation is almost complete with the port on the front, but less than complete when it's on the back, just as it is with a baffled dipole)
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Another Mechanism
DBMandrake said:
On large cabinets I've always measured noticeable increases in bass response with rear mounted ports compared to front mounted ports with identical tuning, especially at the low end of the bass.
I always assumed it was additional boundary gain from the front wall proximity in the rear port scenario, but I wonder whether some of it is that putting the port on the back increases the acoustic short circuit path length for the frequencies below box tuning where the port is 180 degrees out of phase - essentially making it like a conventional dipole in this region where the positive phase radiation is coming from the front of the cabinet, and the negative phase radiation is coming from the back, with a baffle in between.
With a port on the front next to the woofer the short circuit path is very short indeed and doesn't have to navigate the perimeter of the baffle/box, therefore there is no baffle effect on the out of phase radiation...(In other words the 180 degree phase cancellation is almost complete with the port on the front, but less than complete when it's on the back, just as it is with a baffled dipole)
My Thoughts:
Another mechanism at play when you're down at the frequencies that make the rear ported woofer act like a dipole, is that to some extent the energies would cancel at the sides, as they do in any dipole, and therefore stimulate room modes differently. 😎
DBMandrake said:
On large cabinets I've always measured noticeable increases in bass response with rear mounted ports compared to front mounted ports with identical tuning, especially at the low end of the bass.
I always assumed it was additional boundary gain from the front wall proximity in the rear port scenario, but I wonder whether some of it is that putting the port on the back increases the acoustic short circuit path length for the frequencies below box tuning where the port is 180 degrees out of phase - essentially making it like a conventional dipole in this region where the positive phase radiation is coming from the front of the cabinet, and the negative phase radiation is coming from the back, with a baffle in between.
With a port on the front next to the woofer the short circuit path is very short indeed and doesn't have to navigate the perimeter of the baffle/box, therefore there is no baffle effect on the out of phase radiation...(In other words the 180 degree phase cancellation is almost complete with the port on the front, but less than complete when it's on the back, just as it is with a baffled dipole)
My Thoughts:
Another mechanism at play when you're down at the frequencies that make the rear ported woofer act like a dipole, is that to some extent the energies would cancel at the sides, as they do in any dipole, and therefore stimulate room modes differently. 😎
In my dipole setup, this null at 120Hz corresponds pretty much exactly with the distance between the front wall and the speakers and the effect in my system is that the reflection cancels the sound at that frequency quite significantly. No amount of EQ can fix it and based on previous statements this probably can not be fixed with a line array either.
I might try the idea of putting a woofer against the wall behind the speaker.
I wonder if, rather than trying to mask this effect and potentially smearing the impulse response, perhaps we can try to cancel (or "catch") the rear wave by;
- running a front speaker at a very close proximity to the front wall
- 180 degrees of phase with rear response of the front speaker
- delayed by 3ms to match the arrival time of the rear wave from the dipole
- at a reduced in level
- only at the specific frequencies (or perhaps a range) where the cancellation problems are most prominent? (i.e. ~120 & ~240)
Is this a completely crazy idea?
In my dipole setup...
Is this a completely crazy idea?
adding even more delayed, out-of-phase sound to what's already "blurring/smearing" the front wave;
instead of just containing the rear wave?
it sure sounds like a complicated way around one of the inherent problems with dipoles...
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Joined 2003
How about evaluating the audibility of our notches with headphones run through parametric EQ?
Good idea. You might also try facing it towards the front wall instead of the listener, because (a) it's possible to get the cone closer to the wall than if it's facing forward (may or may not be beneficial depending on circumstances) and (b) it reduces localization problems due to midrange leakage.
You still want to low pass filter it at or below 250Hz of course, but facing the driver away from the listener, perhaps angled a bit towards the nearest side wall, will help cut down spurious midrange which might otherwise give away the position of the woofer despite low pass filtering.
Adding a woofer low pass filtered at 250Hz or below a couple of feet behind your main speakers is not going to "smear" the overall impulse response in any significant way, as the rise time of the woofers "impulse" is extremely slow due to the low pass filter, making the time delay of 2 feet or so insignificant.
Also consider that although the helper woofers direct signal will arrive slightly later than the main, it's still arriving before the front wall reflection of the main speaker. The order of arrivals at the listener will be (1) Main direct (2) Helper woofer direct (3) Helper woofer front wall reflection (4) Main front wall reflection.
Whether you like it or not, the front wall reflection from the main speaker that you can't get rid of already has a longer time delay than the both the direct and reflected signal from the helper woofer located nearer the front wall...and the main speakers response goes much higher in frequency - even towards the front wall, so there will be far more smearing of the impulse response from the main speakers own front wall reflection than anything the helper woofer, suitably low passed, will add.
Might work, but sounds like a lot of work, and I'm not sure how well it would work with a dipole. (Since the predominate phase of the bass at some distance from the rear is opposite to what it would be some distance in front, while with a box woofer it stays the same..)perhaps we can try to cancel (or "catch") the rear wave by;
- running a front speaker at a very close proximity to the front wall
- 180 degrees of phase with rear response of the front speaker
- delayed by 3ms to match the arrival time of the rear wave from the dipole
- at a reduced in level
- only at the specific frequencies (or perhaps a range) where the cancellation problems are most prominent? (i.e. ~120 & ~240)
Is this a completely crazy idea?
Personally I don't think it's necessary to play with time delays to solve room modal and boundary cancellation effects - spatial diversity is the answer, have two sources of bass at the troublesome frequencies where modal/boundary notches occur (helper woofer electronically band-passed 100-250Hz say) whose location is offset by the right amount and direction to deal with as many boundary/modal cancellation effects as possible, (floor level, closer to the front wall, and either towards or away from nearest side wall) then if necessary apply a small amount of negative EQ to this region (before the signal split point between main and helper woofer) to restore balance.
Gain of the helper woofer relative to main should only be as high as necessary to mostly fill in the holes and no higher, helping to minimize localization, also if you get the level right you may not need any overall EQ correction as it will tend to only fill in the holes and leave the rest alone.
Also measure at a number of secondary listening positions to make sure you don't make things worse at a different locations, although this is unlikely.
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I didn't think I needed to make it explicit, but it's clear to me that the problem described is also very significant in the case of most every monopole I've seen because they radiate nearly 360degree pattern at low frequencies. This problem is also problematic with ported speakers with rear facing port as mentioned earlier in this thread. It is even less likely that my theoretical solution could practically be applied to a monopole however.
Maybe you have a perfect box speaker or room and therefor you don't see this effect. Or perhaps you haven't bothered to measure?
So, while the implementation of this idea may be somewhat complicated... it is not necessarily too difficult to try given an entirely active system. Theoretically at least, the premise of my suggestion could do a lot to contain/eliminate the out-of-phase sound emanating from the rear of a dipole. Whether this is practical is certainly subject to debate.
uh, i don't believe the "perfect" speaker or room exist...
didn't know i said anything like that.
just that it seems a lot easier to physically contain the rear wave; job done-- no out-of-phase, delayed wave combining with the front wave.
much clear sound; more detail.
but yes, i have measured, and no, i don't have that problem.
five-way horns, no "boxes" down to ~60Hz, tapped horns in the corners below that.
and if anyone wants to experiment with anything, more power to them!
i sure enjoy experimenting...
5 way horns? Very neat! I suppose that is another way of eliminating this particular problem.
.... but that also sounds like quite a lot of work to eliminate the problem.Where can I read about your horns?
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