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Author Topic: Subwoofers and SPL measurements Jarod Reddig Jedi Master Film Handler Posts: 513 From: Hays, Ks Registered: Jun 2011 posted 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.  |  IP: Logged Sean McKinnon Phenomenal Film Handler Posts: 1712 From: Peabody Massachusetts Registered: Sep 2000 posted 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?  |  IP: Logged Marcel Birgelen Film God Posts: 3357 From: Maastricht, Limburg, Netherlands Registered: Feb 2012 posted 11-28-2019 02:41 PM                     quote: Sean McKinnon So Steve, if you were setting a sub on a surround channel (non Atmos) would you set it for +10db in band gain? 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.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 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.  |  IP: Logged Sean McKinnon Phenomenal Film Handler Posts: 1712 From: Peabody Massachusetts Registered: Sep 2000 posted 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.  |  IP: Logged Mike Babb Master Film Handler Posts: 250 From: Norwich UK Registered: Jul 2002 posted 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.  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 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.  |  IP: Logged Brad Miller Administrator Posts: 17775 From: Plano, TX (36.2 miles NW of Rockwall) Registered: May 99 posted 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.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 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.  |  IP: Logged Mike Babb Master Film Handler Posts: 250 From: Norwich UK Registered: Jul 2002 posted 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.  |  IP: Logged Frank Angel Film God Posts: 5305 From: Brooklyn NY USA Registered: Dec 1999 posted 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?  |  IP: Logged Marcel Birgelen Film God Posts: 3357 From: Maastricht, Limburg, Netherlands Registered: Feb 2012 posted 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.  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 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.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 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.  |  IP: Logged Marcel Birgelen Film God Posts: 3357 From: Maastricht, Limburg, Netherlands Registered: Feb 2012 posted 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.  |  IP: Logged

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