OK, but the subs are located in a home theater. It's a small room. A few different things are at play like room modes and psychoacoustic perception of low frequencies over time.
A 20Hz soundwave will propagate around the room many times before your brain recognizes it, which means you'll be exposed to the frequency for a longer time inside a room compared to an outdoor location. Longer frequency exposure translates into perceived loudness. You don't need as much volume level at 20Hz in small rooms to hear the sound. That's why the F3 rolloff doesn't matter as much. You get lower SPL at 20Hz but you don't need high SPL at 20Hz when you have 6 typical room boundaries.
By typical room boundary I mean a regular house, a house without three foot thick bass treatments on opposing walls. If you absorbed the bass frequencies with outrageously think room treatments then you'd need to start boosting low frequency volume because you'd reduce modal behavior as well as psychoacoustic time exposure.
A 20Hz soundwave will propagate around the room many times before your brain recognizes it, which means you'll be exposed to the frequency for a longer time inside a room compared to an outdoor location. Longer frequency exposure translates into perceived loudness. You don't need as much volume level at 20Hz in small rooms to hear the sound. That's why the F3 rolloff doesn't matter as much. You get lower SPL at 20Hz but you don't need high SPL at 20Hz when you have 6 typical room boundaries.
By typical room boundary I mean a regular house, a house without three foot thick bass treatments on opposing walls. If you absorbed the bass frequencies with outrageously think room treatments then you'd need to start boosting low frequency volume because you'd reduce modal behavior as well as psychoacoustic time exposure.
Magnavox, back in the 1960's, built their stereo consoles with different woofers, to eliminate "booming" at their resonant frequency.
Thus, the bass response of those massive coffins was smoother.
Other manufacturers used a similar idea of using identical woofers, but each in a differently sized/tuned chamber.
I did that with my RCA Victor console "redesign" and I like it.
You can stuff that "one note bass" crap, that's for the annoying juvenile car enthusiasts.
I’m not sure room gain is gonna be enough to save his 40hz subs from bottoming, hard.
Yeah, a good rule-of-thumb is to tune the cab to the lowest frequency likely to be reproduced [AKA EBS alignment]. No way room gain outside a vehicle will pump up a vented's ~24 dB octave roll-off below tuning enough to protect the driver.
GM
GM
Small rooms and most home theaters don't have room gain because they are too leaky. You have vents, open walls, windows, whatever.
What you do have is low frequency modal behavior and psychoacoustics. He's not going to blow out his 40Hz subs in a home theater environment.
What you do have is low frequency modal behavior and psychoacoustics. He's not going to blow out his 40Hz subs in a home theater environment.
I’m confused why you think his subs won’t be over driven when encountering loud passages well below tuning frequency?
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Very few people in the world use Geddes/Welti multi-sub optimization methods to equalize their low frequencies in small rooms. Most people buy sub woofers from the big box stores and don't know what they're doing so they use their subs to make their house rattle then call it good bass. People who use their subs to rattle their walls blow up their subs. Not only did the poor guys have terrible bass but they have to buy new subs to keep producing terrible bass.
Use enough of the cheap subs and no problems, my beloved did however put her petite little foot down when it got to 7 of them in the room. 7@ $99- each was a lot cheaper than a big powerful woofer in an WAF unacceptable size
7 mixed 10" & 12" woofers at a 100 watts nominal each is a lot of bass
The OP safest route is to tune the 2 40hz subs to 30hz. All subs are equal then. He can fill part of the slot port to make the effective dia smaller. Then EQ in the room.
Using subs below the ported Fb is a recipe for bad performance, the cone is totally uncontrolled.
Using subs below the ported Fb is a recipe for bad performance, the cone is totally uncontrolled.
This does not answer why you think having a sub tuned to 40hz is gonna survive a loud passage well below this frequency.
The op states these are home theater subs. Movies are not music, they have all kinds of crazy low frequencies and can really damage a sub tuned too high and over driven.
Well the cheap and easy, simple solution would be to seal off the ports on the 40Hz tuned boxes and use them sealed and then listen. It might work out. Obviously the box can be simulated as a sealed enclosure to give an idea/approximation of the F10.
@Ben
I must be having a very slow day here but I simply don't understand what you are trying to say. HT subwoofers ? Do they actually have a "Passband" They get a signal from the soundtrack that only contains bass effects information and should have no or very little HF information. In the context of a full speaker system which drivers pass band are you referring to? The sub driver or the woofer?
As I said my thinking must be fuzzy today
I must be having a very slow day here but I simply don't understand what you are trying to say. HT subwoofers ? Do they actually have a "Passband" They get a signal from the soundtrack that only contains bass effects information and should have no or very little HF information. In the context of a full speaker system which drivers pass band are you referring to? The sub driver or the woofer?
As I said my thinking must be fuzzy today
That's when you break out the drywall saw and start putting them in the ceiling and walls. I see no reason to stop at seven if you can add a few more she can't see.
good vented subs should have a hipass filter to protect them from frequencies below the tuning frequency. Further to that with four or more subs you are significantly reducing the individual sub playback level and any one of those subs would be exposed to very much harder play alone in a system which is the norm out there.
According to Geddes, you'd dedicate one of the 30Hz subs to the first modal region (1st, 2nd, 3rd modes) and place it in a corner. Then you'd take the other three subs (two 40Hz, and the remaining 30Hz) and place them in statistically independent locations in the room to handle the upper modal/bass region up to the transition frequency. Another way to say statistically independent is "asymmetrical locations." Yet another way to say it: the locations shouldn't look like they're balanced in any way -- floor to ceiling, front to back, side to side.
The second and third modes in the first modal region are usually around 40Hz and 60Hz depending on the room.
Equalize the sub in the first modal region. Use multi-sub optimization in the upper modal region.
I've never heard Todd Welti discuss the first modal region so I don't know what he does about it. Maybe his software automatically takes care of it. Maybe Welti's software reaches the same conclusions Geddes reaches through the brute force calculations. Unfortunately, we don't have access to that software because Harman owns it.
Frequencies in the modal region are easy to excite. In addition, the lowest of the low frequencies are physically long. They bounce around the room at the speed of sound and excite our ear drums over many cycles before we perceive them. The long exposure is perceived as loudness. That's why F3 is less relevant in small rooms.
Music is the same thing as movie soundtracks. The reason people blow up their subs is because they play them too loud. They play them too loud because they don't understand they're dealing with modal behavior in small rooms.
Someone linked to movies and games that contain infrasonics (<20Hz) earlier in the thread. Humans can't hear infrasonics. If you want to reproduce infrasonics don't use sub woofers because they're the wrong device for the job.
Floyd Toole explains it better than I did. Chapter 8.1 Sound Reproduction 3rd Edition:
"Figure 8.1 shows for a rectangular room: (a) the orientations of the three axial modes, (b) the three tangential modes, and (c) one of many possible oblique modes. Because some energy is lost at every boundary interaction, modes that complete their “cycle” of the room with the fewest reflections are the most energetic. The axial modes are therefore the most energetic, followed by the tangential modes and the oblique modes. It is rare for an oblique mode to be identifiable as a problem in a room. Tangential modes can be found in rooms having significantly reflective boundaries, or when multiple sources are appropriately located. Axial modes are omnipresent, and they are the usual culprits in bass problems in small rooms. Fortunately, they are easily calculated, as described in the box.
Calculating all of the modes in a rectangular room is more difficult, as the equation in Figure 8.2 shows. The method of mode identification is helpful if one is trying to track down the origin of an irritating bass boom and from that to decide on a remedy (see Figure 6.2)."
(The lowest frequencies are the longest and reach out to the axial boundaries. Their energy is least absorbed by the room boundaries = less energy required to excite them. Most rooms don't pass the axial modes until after 40-60Hz.)
"Figure 8.1 shows for a rectangular room: (a) the orientations of the three axial modes, (b) the three tangential modes, and (c) one of many possible oblique modes. Because some energy is lost at every boundary interaction, modes that complete their “cycle” of the room with the fewest reflections are the most energetic. The axial modes are therefore the most energetic, followed by the tangential modes and the oblique modes. It is rare for an oblique mode to be identifiable as a problem in a room. Tangential modes can be found in rooms having significantly reflective boundaries, or when multiple sources are appropriately located. Axial modes are omnipresent, and they are the usual culprits in bass problems in small rooms. Fortunately, they are easily calculated, as described in the box.
Calculating all of the modes in a rectangular room is more difficult, as the equation in Figure 8.2 shows. The method of mode identification is helpful if one is trying to track down the origin of an irritating bass boom and from that to decide on a remedy (see Figure 6.2)."
(The lowest frequencies are the longest and reach out to the axial boundaries. Their energy is least absorbed by the room boundaries = less energy required to excite them. Most rooms don't pass the axial modes until after 40-60Hz.)
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Seems to me you're all right. It all depends on how loud he intends to crank these up. Ideally they would all be tuned to 16hz and have limitless excursion, but the question is will the addition of the 40hz subs help or hurt. Above 40hz, it will even out the bass across the listening area if they're placed properly, which is often incredibly valuable. The total output will drop below 40hz unless you use bass management, which your receiver almost certainly does, correct? BUT, if you turn it up too far, you risk woofer damage without a hp filter. True of any sub, but especially one tuned so high. I would use all the subs you have and see if it meets your volume requirements without destroying one. If you destroy one, then you know the limits. If your receiver has 2 sub outs, all the better. Mine tests them both independently during the room correction.
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