quote: Steve Guttag

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BTW...Technically, this should be an "Afterlife" discussion since it is for home-theatre.

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quote: Steve Guttag

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When it comes to the ultra low-frequency, you can't ignore the physics of it...what is the wavelength of a 24Hz signal? What are we at? 47-feet or thereabouts? How big is your room? There will come a point where the wave is just going to go through the walls.

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Hmm. A standing wave in a 20 foot room is like just below 60Hz. We aren't trying to setup resonances though. There is some value to the impact of the pressure wave smashing into your face.
 

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Posted by Jarod Reddig (Member # 6363) on 11-25-2019, 01:16 PM:
 
Those waves are very long yes but can still be utilized in a room especially if you got room gain. There comes a point around 20hz and below where you no longer hear it and it just comes across as pressure. This sensation adds a lot to a helicopter fly over or a diesel engine starting or even a heavy door slam. Will shake the hell out of your seats too like you have actuaters. ULF is all around us. Drive down the interstate then roll down one of the back windows in a 4 door car and you will hear the modulation of ultra low hz. Ive been in a few home theaters with rotary subs that go down to 5hz at 120db. Its a sensation lol. This is the one area I think exhibition cinemas truly lack.
 

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Posted by Steve Guttag (Member # 268) on 11-25-2019, 01:35 PM:
 
Subwoofers in commercial cinemas can be tricky.

First off, most people, seemingly don't know how to size them (quantity or power). Next, there remains the physics of it all and how low you want to go. There are the real aspects of containing the low frequency sound so that it does not disturb the other parts of a complex...which likely means that the theatre had to be built right to begin with and just double-staggered stud drywall with installation isn't going to do squat to that LF signal.

Tell your architect that you need STC-75 rated rooms and see what design comes back. It is going to have very thick, multi-layered walls, no common anything (walls, ceilings, floors or another means to use the solid as a transmission medium). It is going to get very expensive.

There are some pretty decent subwoofers available out there though most stop around 22-24Hz (JBL loves to tune their cabinets at 25Hz). The JBL 5628 can be a very impressive speaker though it would be more so if the cabinet was larger and tuned lower. Then again, you need thousands of watts to get it to really perform.
 

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Posted by Bruce Cloutier (Member # 9615) on 11-25-2019, 02:53 PM:
 
Good comments. It helps to not feel like you are thinking in a vacuum. Although if I were really in a vacuum I wouldn't have to worry about audio... or much else really.

Then there are (obviously critical) concerns like the appearance of spun copper cones.

There is a video of a Klipsch SW extinguishing candles 3 or 4 feet away which is pretty interesting.

I am of the thinking now that I want to replace the now dead single SW with two subwoofers to at least eliminate destructive interference effects (more physics). It might be interesting and fun to grab the $100 calibrated USB mic and the freebie REW software and sweep the frequency spectrum. But yeah... It's just my approximation of a home theatre.

Thanks guys.
 

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Posted by Steve Guttag (Member # 268) on 11-25-2019, 07:59 PM:
 
The extinguishing the candles is really nothing. Play a tone at the port tuning frequency and you'll find that the air movement on the ports to be incredible and, at the same time, the cone motion to be nearly non-existent! It's almost as if one is getting free sound!
 

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Posted by Marcel Birgelen (Member # 6801) on 11-26-2019, 12:11 AM:
 
At port tuning frequency, the whole thing acts like a Helmholtz resonator, most of the power of the speaker will go through the ports.

Not always easy to roll your own sub design and getting the ports right. First you really need to know the performance of your drivers. Make the ports too small and they're effectively doing nothing, make them too big and you essentially just punched a nasty hole in the cabinet. Put them in the wrong place and you got yourself some nice destructive interference.

But nowadays, we can "fix" most bad speaker design in software. Just like we fix bad aerodynamics on planes with some software.

For cinematic experiences, the 18-36 Hz range is still pretty important, although most subs won't make it down all the way to 18 Hz. The problem with most home cinema setups though is, that those frequencies do not only rumble your room, but also anything adjacent, at least if you want to play it at refference...

quote: Bruce Cloutier

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I am of the thinking now that I want to replace the now dead single SW with two subwoofers to at least eliminate destructive interference effects (more physics). It might be interesting and fun to grab the $100 calibrated USB mic and the freebie REW software and sweep the frequency spectrum. But yeah... It's just my approximation of a home theatre.

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Wait, you're going to solve destructive interference with two subs instead of one? That sounds (pun somewhat intended) like trying to solve traffic jams by introducing more cars.

quote: Bruce Cloutier

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Good comments. It helps to not feel like you are thinking in a vacuum. Although if I were really in a vacuum I wouldn't have to worry about audio... or much else really.

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Excuse me... We're doing movies here. We DO have sound in vacuum, especially loud explosions accompanied by bombastic music!
 

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Posted by Bruce Cloutier (Member # 9615) on 11-26-2019, 07:02 AM:
 
Yeah. I've noticed the creative sound effects laid over the action in space. It does bother me more when ships and fighters exhibit aerodynamic effects, or are otherwise piloted as if in atmosphere, in scenes supposed to be in the vacuum of space. Some show did it right lately. THAT I did take notice of. It was either Dark Matter or The Expanse. I don't recall which at the moment. Explosions and whatnot were without audio impact unless experienced from onboard the ship.

I'd like to setup some digital cinema equipment at my place since this is a huge market for us. I have a JNIOR on my furnace but not involved in the TV room. But at the moment it is more like I just need to patch the holes and keep her running. I bet some of you can relate.

I was looking for a set of VU meters. Eight of them in a 1U so I can see the relative energy being sent to each of the 7.1 channels that I have. Getting tired of walking around checking for active channels. Any suggestions? I mean I can build something obviously. It doesn't have to be calibrated.

The article re LFE was interesting. Basically you don't know what you are getting there. In my system it is the subwoofer channel and I can configure whether or not bass is copied from front channels. I also have an RTA and the system can balance channel levels and timing for me. But... getting a plotted spectrum is still intriguing.
 

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Posted by Steve Guttag (Member # 268) on 11-26-2019, 07:11 AM:
 
If you really want to do some experiments on multiple subwoofers...get the EASE data and appropriate software and you can tune the placement to get the responses you want and pattern control. Heck, get three and point one backwards to get a cardioid pattern from a subwoofer.

Here's an idea...put your subs on casters and with an analyzer going with Pink Noise...roll them around to see the response.
 

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Posted by Jarod Reddig (Member # 6363) on 11-26-2019, 02:55 PM:
 
I agree Steve its very hard to contain that low bass. Not impossible but ya takes lots of money and know how during the planning and building process. The last THX certified cinema is was at still had bass bleed thru from the theater beside it. I think one would have better luck with a DIY solution for ULF in a cinema. Its usually down to the box being too small and the driver specs.
 

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Posted by Sean McKinnon (Member # 612) on 11-28-2019, 01:29 PM:
 
So Steve, if you were setting a sub on a surround channel (non Atmos) would you set it for +10db in band gain?
 

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Posted by Marcel Birgelen (Member # 6801) on 11-28-2019, 02:41 PM:
 

quote: Sean McKinnon

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So Steve, if you were setting a sub on a surround channel (non Atmos) would you set it for +10db in band gain?

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I'm not Steve, but why would you do that? The LFE channel is recorded with the 10dB headroom in mind, the sub channels aren't.

When you do bass management, where you "summarize" the bass from all channels, including the LFE channel into one or more new channels, you account for the +10dB on the LFE channel.
 

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Posted by Steve Guttag (Member # 268) on 11-28-2019, 11:40 PM:
 
Sean, of course not.

The sub on a surround channel merely allows a surround channel to have a frequency response below 50Hz (and more commonly below 80Hz or 100Hz. It is "bass management." It is NOT an additional LFE type "effect" channel. If your surrounds were full range (can at least go down to 40Hz like a stage channel), the surround sub, even in an Atmos house would be completely superfluous and actually inferior. The realities are though, nobody (anymore) is going to put up (except maybe IMAX) surround speakers the size of the stage speakers (I remember when...Altec 604s, A8s (not my favorite) and even A7s were used as surrounds and had every bit as much bottom end as their stage speaker counterparts. When you are using a typical surround, they just don't have the drivers and/or cabinet volume to play anything really deep (and would be horribly inefficient if they did).

That said, most of the surrounds I deal with can get an honest 50-60Hz so you are really only picking up an additional ⅓-⅔ octave by having the surround LFE channel. To that, you are having to be careful about time artifacts since your LFE compared to the surround speaker will be significantly different for each listener, particularly if you do just two surround LFE channels. I have opted to use four smaller LFE channels so the LFE speaker is physically much closer to the speakers it supports.
 

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Posted by Sean McKinnon (Member # 612) on 12-02-2019, 07:48 AM:
 
Thats what I thought. I only ask because i've had this discussion with people who assume otherwise. I treat it like a low frequency of a screen channel speaker and "extend" the frequency response trace from the crossover point. While the surround speaker may have a listed frequency response of 60-50 hz where is it's 6 or 10 db down point? I find that I can use the main surround speaker in its most efficient bandpass and I think it sounds better overall but that subjective and ymmv.
 

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Posted by Mike Babb (Member # 1379) on 12-02-2019, 02:33 PM:
 
I'm absolutely convinced adding low end capability to the surrounds is money well spent. We did this years ago in Hawaii before Atmos arrived, using arrays with 15" drivers, and everyone was impressed. That cinema was later converted to Atmos. But you definitely need well built walls as Steve says.
 

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Posted by Sam D. Chavez (Member # 1841) on 12-02-2019, 08:44 PM:
 
Lonny Jennings and I added 2 custom rear corner subs at the Gold Coast Cinemas in Vegas for demos of SR and SR.D in early '90's. The THX surrounds at that time tended to be Boston Acoustics A70's and we were interested in crossing them over around 100 to 125 Hz. to keep them on the wall. Worked well as expected.

Bi-amped surrounds go back to Cinerama. Uptown in DC had bi-amped A7's in a Grass Valley Group based system.
 

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Posted by Brad Miller (Member # 2) on 12-02-2019, 10:43 PM:
 
Mike, I'm going to assume if this was in the film days that you guys were running SRD or SDDS.
 

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Posted by Steve Guttag (Member # 268) on 12-03-2019, 06:11 AM:
 
Surround LFE will certainly extend the bass down (depending on your speakers and placement).

However, I think people are over simplifying it. You introduce time artifacts since the LF section is now physically separated (possibly by 10s of feet) from the speaker the person hears and as you move about the room, this time relationship is different. You are definitely introducing problems while trying to correct for others (isn't that always the way of things?).

So you can pick your golden seat as your time reference (or even a golden rectangle...we'll call it the Central Listening Area®) and set up your time delays from there...knowing you may have to introduce delay on some surround speakers/arrays depending on the relationship...so you introduce other artifacts (unless you are into comb filters and such). Or, you'll find that, at the crossover point, you've got things adding in an unflattering way for some/many seats and you are WORSE off at the crossover frequencies for those seats. As I said, it is a tricky business.

Then, you have to decide if there is enough low frequency information on the surround tracks that you are missing and beneficial to the effect/movie to make it a worthwhile venture.

Play a movie you like (really play several because if you like Woody Allen, no amount of surround LFE will alter his soundtracks!). Turn the LFE(s) on/off and see if it adds anything significant. I suspect that some movies it adds a little kick whereas others (and I suspect the majority) they do not.

And, to jump in on the bandwagon...back in the day (80s and 90s) we too have done screening rooms with surround subs in a satellite/subwoofer arrangement...used JBL Control 1s as the surrounds and the Control SB-1 as the subwoofers. It came out really well. It got better when we could zone off the areas of what each sub handled relative to the main speakers...but DSP had to catch up to the idea.
 

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Posted by Mike Babb (Member # 1379) on 12-03-2019, 03:45 PM:
 
Brad, the cinema with 15" driver surrounds was about a year before Atmos. They had gotten some idea what Atmos config would be and had ceiling speakers as well as 5 stage channels. All surrounds had 15" drivers including the ceiling.

Since then, more in the last two years including yesterday I have heard cinemas using a sub as the LF and the array as HF, time aligned it sounds really good. As Steve says, there can be a discussion about listening position time alignment but I think that applies to all cinemas.
 

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Posted by Frank Angel (Member # 248) on 12-03-2019, 06:21 PM:
 
Somewhat related, is there an easy (or even a not so easy) way...possibly a formula that will tell you what rating a subwoofer or subwoofers would need to be rated at in terms of watts and/or sensitivity to be able to reach the required SPL spec in a room of a given square footage?
 

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Posted by Marcel Birgelen (Member # 6801) on 12-03-2019, 10:15 PM:
 
There are so many factors in this, that it's pretty hopeless to ask for a simple formula.

If you're using something like EASE, the question could be answered mostly scientifically and not just by gut feeling.
 

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Posted by Sam D. Chavez (Member # 1841) on 12-04-2019, 06:30 PM:
 
Calculating to come up with the appropriate subwoofer(s) and associated power isn't that hard, but most customers won't accept the cost of a truly adequate system.

You start with the sensitivity of the speaker, as in 100 dB/1 meter/1 Watt. Every time You double the distance, you double to power. 2 Watts @ 2 Meters, etc., until you get to the back of the room or 2/3rds the way back depending on who you ask. You can do this calculation a number of ways including counting on your fingers.

Now you have a crude minimum number for Watts needed. This will tell you if you have a powerful enough speaker as in amount of power it will take thermally. Also an idea of a minimum amplifier size to drive the speaker.

This method does not take into account reverb or room gain. We are only interested in the direct sound pressure level, not what the room contributes. We do have to consider how loud the soundtrack will drive the speaker as in headroom. I'll leave this to Guttag to cover and correct.

There are a number of other complications as Marcel states. You'll find one speaker box is often not sufficient, so two boxes or more. There is a also the mutual coupling aspect of additional boxes. You ideally get 3dB if the boxes are up against each other.

Anyway, this is a start.
 

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Posted by Steve Guttag (Member # 268) on 12-04-2019, 07:05 PM:
 
Frank. It isn't square footage, it is distance, as Sam points out.

The math, in the big picture, is pretty simple. I refer you to the Dolby Stereo Technical Guidelines in the warehouse for a good reference that has a nomograph as well as some formulas for calculating power requirements.

Things to consider, particularly with subwoofers:

1) Is the subwoofer located in a baffle wall or is it just sitting in the wide open. If a bass cabinet is in a baffle wall, it moves from 4Pi space to 2Pi space. Check out the SPL response on most manufacturers products at the difference between 4Pi (360 degree radiation in both vertical and horizontal) and 2Pi. Other than the construction of the wall...it is free power that accentuates the LF response...just what we need for subwoofers.

Do you located your subs on the floor or are they up with the main speakers? you can cut that space down for the subs to have a baffle wall and a floor as a boundary and increase their mechanical efficiency even more. These should be factored into subwoofer design.

Almost always, your biggest bang for the buck is adding the second subwoofer to a system. If your subwoofer has a 97dB 1W/1m, adding the second one moves you up to 100dB, which cuts your power requirements in half (which on subwoofers, can be significant). Often this pair of subs can be put on a single amplifier channel and for most amplifiers with an AB design, that makes their transfer function at or nearly twice as efficient so you have more power to work with too. Note, you need to have the subwoofers tight together to get the benefit of mutual coupling (so that act like one big subwoofer rather than two independent subs that might actually set up nodes and counteract each other in certain listening positions at certain frequencies.
 

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Posted by Marcel Birgelen (Member # 6801) on 12-05-2019, 02:42 AM:
 
Like Sam and Steve indicated, the "basic ingredient" for SPL is distance rather than square footage or volume of the room.

Keep in mind that, when sound (or any other "wave source") radiates the energy around (here the 4π v.s. 2π come in play), the energy gets dispersed over a bigger and bigger area the further you are from the source.

Where the volume or rather the dimensions of the room play a big part in, is the room gain, which is much more significant in a smaller room than a big room. It's pretty pointless to speculate about room gain though, without doing at least a simulation or even better, an actual frequency response measurement in said room.

Furthermore, I agree with Steve that, if you're going to deploy multiple subwoofers, you should put them as close together as possible, to avoid combing effects, which can be very severe at those powers.

That's also the problem I have with adding subs + bass management to surround channels. I think it's mostly fine to add subs to discrete surround channels, but there should be no bass information of any other channel in there and especially not the LFE channel.

Otherwise, I think it's completely fine to summarize all the bass information of all the surrounds to one central subwoofer array. There is the issue with timing and how it influences the way we humans resolve the supposed origin/location of a particular sound, but keep in mind that practically all research indicates that this "timing information" (especially that of the wave front) is mostly useless for lower frequencies, as long as it's being kept in a reasonable margin.
 

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Posted by Sam D. Chavez (Member # 1841) on 12-05-2019, 09:39 AM:
 
It is hard to determine the location of the Lfe subs in most systems but I had one instance some years back where we made use of leftover subs from Sensurround located in the left back and right back of the large house. Every time a low freq. effect occurred it was clear the subs were behind you and the audio reached your ear before the main audio. It wasn't subtle. An extreme case with W horns but...
 

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Posted by Frank Angel (Member # 248) on 12-05-2019, 10:29 AM:
 
As always, thanks guys. The Dolby Stereo Technical Guidelines is a big help. In these particular college Screening Rooms, the subs are going to be free standing in the front corner of the room, no other location possible -- that's a good thing. Adding a second unit next to the one is a possibility if need be. The coupling effect when putting them in close proximity is indeed quite dramatic. We did this with two JBL 18in subs in Community fiberglass enclosures and the output rose nearly 25% when they were moved next to each other, if memory serves.
 

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Posted by Marcel Birgelen (Member # 6801) on 12-05-2019, 11:18 AM:
 

quote: Sam D. Chavez

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It is hard to determine the location of the Lfe subs in most systems but I had one instance some years back where we made use of leftover subs from Sensurround located in the left back and right back of the large house. Every time a low freq. effect occurred it was clear the subs were behind you and the audio reached your ear before the main audio. It wasn't subtle. An extreme case with W horns but...

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Yeah, but what you're describing, while obviously related, I'd personally call another issue.

I'd say it's a general audio sync issue (a "lip-sync issue" but now for the low frequency domain only), which will be much more evident in larger venues. But even the timing of the stage speakers can become challenging in such venues.

Generally speaking though, ranges between -100 ms to +25 ms should be pretty much undetectable for human beings. If you move beyond that domain, the issue will eventually become so irritating to many people, the localization of the actual sound will play second fiddle to that irritation.

Still, I think that in that same venue you're describing, you would've a hard time giving the subs an exact location (without obviously knowing their location beforehand).

While the capability to localize a sound drastically reduces at the lower end of the audible frequency domain, I don't think the frequency-range of a particular sound has much impact on the way we perceive "async audio".

Another thing to account for though (audio is a never ending accumulation of interesting challenges): If there is a large delay between the stage speakers and the subs, is the potential combing / destructive interference you can cause on the overlapping spectrum, especially if you do bass management
 

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