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» Film-Tech Forum   » Operations   » Digital Cinema Forum   » Subwoofers and SPL measurements (Page 1)

 
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Author Topic: Subwoofers and SPL measurements
Bruce Cloutier
Expert Film Handler

Posts: 161
From: Gibsonia, PA, USA
Registered: Aug 2016


 - posted 11-25-2019 08:41 AM      Profile for Bruce Cloutier   Author's Homepage   Email Bruce Cloutier   Send New Private Message       Edit/Delete Post 
So this is a home theater question but I thought to ask you guys... This weekend I discovered (to me it was almost instantly obvious) that my subwoofer had bit the dust. Okay so it was antique and I had rebuilt it maybe a decade ago after a lighting strike fried the preamp. So I am well overdue to have upgraded that but... it was quite satisfactory up to now. I am all Klipsch by the way. That's not the result of any elaborate evaluation of other speaker manufacturers but more the result of just falling into that camp at first through some deal ages ago. I can be loyal to a point.

Anyway, what can you tell me about SPL measurements? Should I get a calibrated mic and fool with the REW package or something like that to optimize the setup of the new subwoofer(s)? I've not done that in this space. I studied some of that in considering replacing the one sub with two. Any advice on subs or measurements?

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Carsten Kurz
Film God

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From: Cologne, NRW, Germany
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 - posted 11-25-2019 09:01 AM      Profile for Carsten Kurz   Email Carsten Kurz   Send New Private Message       Edit/Delete Post 
Hi Bruce,

https://hometheaterhifi.com/editorial/the-misunderstood-01-lfe-channel-in-51-digital-surround-sound/

- Carsten

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12785
From: Annapolis, MD
Registered: Dec 1999


 - posted 11-25-2019 09:54 AM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
He gets it mostly right. He doesn't quite understand the LFE signal in 70mm (and even names it incorrectly...it was known as "baby boom.")

The other area he gets wrong (and honestly most people seem to get wrong) is the overall SPL (Sound Pressure Level). In cinema, we measure our SPL of full-range channels in a "C" weighting...which is mostly flat but rolls off the extreme highs and lows. This weighting is key to understanding the SPL number. It is also one reason why using a simple SPL meter will mislead a tech.

For stage speakers, yes, -20dBFS should measure 85dBc if tuned to ISO2969 (aka X-curve).

Surrounds, unless they have some form of bass management, rarely will have the proper bandwidth (40-16KHz) to mimic the stage channel so it is more important to get the in-band level right than the overall SPL. That said, people will tend to set them high and nobody ever complains about loud surrounds but they sure do if they aren't playing all of the time.

The subwoofer does NOT play at 115dB! Its bandwidth is typically from about 10Hz (if you can play it) on up to about 180Hz give or take. For 70mm, it went up to 250Hz, at least at first, then was lowered to 180Hz and I'm sure some stop it at 120Hz.

The key here is, it isn't wide band so you can't use an SPL meter (particularly one who's response is unknown...e.g. Radio Shack...sure you calibrated it to your reference mic when playing Pink Noise from Center but you have no idea what frequencies it is more sensitive to...it works because you calibrated it while playing to ISO2969).

The subwoofer level HAS to be set via RTA such that its in-band level is 10dB higher than the in-band level of the stage speakers (e.g. center). If you look at your RTA when things are tuned and the SPL is showing 85dBc on Center...presuming a 1/3-octave analyzer, you'll find those individual bands are hovering around 72dB. So the subwoofer bands will need to hover around 82dB. That is 20dB down so if you go up 20dB for full scale, you are at 102dB per frequency...add them up (logarithmicly) and you'll typically find that the subwoofer channel, if measured via a simple SPL meter will be in the 110-112 range...not 115dB and since the meter is set to "C" weighting...anything from below 20Hz doesn't get added in. That 3+dB difference in level is a doubling of amplifier power so it is quite significant when dealing with a power hungry channel like subwoofers.

Note too, for surrounds, in commercial cinemas, we still are backwards compatible with monaural surrounds. So the mono surround channel should represent an SPL of 85dBc. a 5.1 system has two channels of 82dBc that acoustically sum to 85dBc. For 7.1, the individual channels should drop another 3dB so again, if all four channels are playing they sum to 85dBc.

Such is not the case for home cinema. All channels are set to the same level (as we should have done for Digital Cinema...we already established that 5.1 was to be the minimum...then the levels could be handled in the mix, rather the theatre/sound processor).

Home Theatre equipment also doesn't output full bandwidth pink noise, typically but instead a band limited noise (don't want to blow anything out) so the key is to balance all of the channels with said noise.

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

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Jarod Reddig
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From: Hays, Ks
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 - posted 11-25-2019 10:28 AM      Profile for Jarod Reddig   Email Jarod Reddig   Send New Private Message       Edit/Delete Post 
These days in home cinema modern AVRs and surround processors have really good built in room correction and auto EQ, tho I have found they are very room dependent. The less they have to correct the better. Comparing REW to XT32 Pro I am able to get the graphs to look a bit better but listening is a toss up. My AVR can calibrate two separate subs at the same time which is really key in small rooms to even response. Made my 18" subs go from flabby to tight and punchy. Mine has an app where you can make unique curves. I love a big rise from 30hz on down to 12hz.

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Bruce Cloutier
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From: Gibsonia, PA, USA
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 - posted 11-25-2019 10:51 AM      Profile for Bruce Cloutier   Author's Homepage   Email Bruce Cloutier   Send New Private Message       Edit/Delete Post 
quote: Steve Guttag
BTW...Technically, this should be an "Afterlife" discussion since it is for home-theatre.
Right. I suppose that I just wanted to poll us here on the digital cinema side rather than the mix in the other forum. I've only really paid attention to us here.

I wouldn't really call my viewing arrangements at home a "home-theatre" either even though there are some bucks tied up in it. My wife actually referenced "Atmos" this weekend and, well, I'm not ready to go there. That was a surprise though. I don't think she realizes what kind of Pandora's Box opens there.

Carsten gives me some lunchtime reading. Thanks.

The low end on some of these Klipsch home-theatre subs is 24Hz. How important is the spectrum below that in the opinion here? I need to check specs on the system upstream to see what I might be capable of. I wonder how fast it falls off below 24Hz?

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Jarod Reddig
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 - posted 11-25-2019 11:25 AM      Profile for Jarod Reddig   Email Jarod Reddig   Send New Private Message       Edit/Delete Post 
Just don't go off factory spec alone. Many are painfully inaccurate. Check out Data-Bass.com. If you want true low frequency performance you need to go with an ID company that cuts out the middle man and your only buying high performance: like Seaton Sound, JTR Audio, Hsu Research, Power Sound Audio, etc. You may have never heard of these brands but they are all top performing sub ID companies that cant be touched by anything bought from a Best Buy.

On the topic of ultra low frequencies, Ive found with movies that the lower the response of the sub in the room the more weight and sense of scale there is to the sound. Some mixes are filtered heavily below 30hz. Many that arent tho.

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12785
From: Annapolis, MD
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 - posted 11-25-2019 11:55 AM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
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.

I built some subwoofers that I wanted to go down to 18Hz...because I could (JBL SUB18 clones...little larger cabinets and port tuning to get the response). They sound awesome...outside (they staid inside, I went outside). So, there will be some high passing involved when they are finally set because one isn't going to hear or feel it in the room they are in.

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Bruce Cloutier
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From: Gibsonia, PA, USA
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 - posted 11-25-2019 12:11 PM      Profile for Bruce Cloutier   Author's Homepage   Email Bruce Cloutier   Send New Private Message       Edit/Delete Post 
quote: Steve Guttag
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.
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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Jarod Reddig
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 - posted 11-25-2019 01:16 PM      Profile for Jarod Reddig   Email Jarod Reddig   Send New Private Message       Edit/Delete Post 
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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Steve Guttag
We forgot the crackers Gromit!!!

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From: Annapolis, MD
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 - posted 11-25-2019 01:35 PM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
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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Bruce Cloutier
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From: Gibsonia, PA, USA
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 - posted 11-25-2019 02:53 PM      Profile for Bruce Cloutier   Author's Homepage   Email Bruce Cloutier   Send New Private Message       Edit/Delete Post 
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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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12785
From: Annapolis, MD
Registered: Dec 1999


 - posted 11-25-2019 07:59 PM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
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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Marcel Birgelen
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From: Maastricht, Limburg, Netherlands
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 - posted 11-26-2019 12:11 AM      Profile for Marcel Birgelen   Email Marcel Birgelen   Send New Private Message       Edit/Delete Post 
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. [Razz]

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
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.
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. [Razz]

quote: Bruce Cloutier
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.
Excuse me... We're doing movies here. We DO have sound in vacuum, especially loud explosions accompanied by bombastic music! [evil]

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Bruce Cloutier
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From: Gibsonia, PA, USA
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 - posted 11-26-2019 07:02 AM      Profile for Bruce Cloutier   Author's Homepage   Email Bruce Cloutier   Send New Private Message       Edit/Delete Post 
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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Steve Guttag
We forgot the crackers Gromit!!!

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From: Annapolis, MD
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 - posted 11-26-2019 07:11 AM      Profile for Steve Guttag   Email Steve Guttag   Send New Private Message       Edit/Delete Post 
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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