Voyagers blew our sub!?!

[Jon Dent]

We have a dual 18" sub box in one of our theaters (I believe it's JBL). Sometime in between last Friday and last night (Thursday), it blew. We were playing Voyagers. Has anyone else notices anything really punishing in that movie, bass wise? The sub withstood a week of Godzilla Vs Kong without any issues so I'm a bit stumped.

Additional info: we usually run at 4.5 on the volume and the sub is being driven by one side of a QSC DCA1222 with the 30Hz high pass engaged.

[Sean McKinnon]

You are running a dual 18 sub on one channel of a DCA 1222? Even at the best case scenario of a 4 ohm load that is still only ~300 watts and if its an 8 ohm load its only ~200 watts. How big is this room? Are you sure the amp isn't in bridged mode? If it is in fact the speaker that is damaged it is likely because of the underpowered amplifier. If your sub is a JBL 4246A for example one channel of that amplifier is delivering less than a third of its rated power handling capability. I also would question the use of the 30hz high pass for a subwoofer that has a frequency response of 22hz because now you are eliminating part of its energy spectrum in its band pass which means you are having to push more power (which you don't have) to hit the same SPL.

[Jon Dent]

Unfortunately we're a small cinema in a small town so we have to make due with the budget we have . It is not running in bridged mode. The 30Hz filter was intended to keep the amp from burning power on sub 30Hz content. As it is underpowered (as you noticed) we wanted to have more headroom in the higher bass frequencies. Room size is about 20 x 40 give or take.

edit: I believe it is a 4246A but I'm not 100%. I've never seen it but we do have a 4246A in our other theater and I've been told they are the same model.

[Carsten Kurz]

Well, sometimes, things 'just' die. But, it is never a good idea to run speakers with underrated amps. Many people still believe this may actually save a speaker from being damaged, but, the opposite is true.

One should have a look at the speaker and diagnose what is wrong. Fortunately, drivers for LFEs can usually be exchanged at decent cost. You may of couse replace them with expensive original drivers, but, it shouldn't be complicated to find suitable drivers from other companies. But DO upgrade the amp first! It doesn't have to be an exceptionally costly amp.

[Jon Dent]

Hey Carsten, I was looking at picking up a pair of ISA280 or ISA450 amps. We use 450's for all our other channels and they've performed well. Bridged they do 800 or 1200 Watts give or take, which on paper looks sufficient.

[Mark Gulbrandsen]

Years ago (1984), just as THX systems were being installed in quantity the movie that blew the most subs was 70mm 2010!!

[Marco Giustini]

is your "clip limiter" engaged on your 1222?

Also, that 4.5 is "wrong". Either your sound system is incorrectly tuned (leve-wise or EQ-wise) or you are running it way too low! Sorry, I don't mean to sound patronising here! 7.0 may be too loud to some ears but 4.5 cannot be right! (Unless Voyagers is incorrectly mixed at a stupidly loud volume).

As others said, a single channel of a 1222 cannot be right for a 4642A. The clip limiter SHOULD keep things safe but I don't know what happens to those amps when they are constantly in "clipping mode".

Are you sure it's the subwoofer which blew and not the amplifier that died?

[Carsten Kurz]

I see many systems using Dolby processors running main features at 4.5

[Jon Dent]

Also, that 4.5 is "wrong". Either your sound system is incorrectly tuned (leve-wise or EQ-wise) or you are running it way too low! Sorry, I don't mean to sound patronising here! 7.0 may be too loud to some ears but 4.5 cannot be right! (Unless Voyagers is incorrectly mixed at a stupidly loud volume).

No worries.

Our system was professionally calibrated to 85dB, and I double checked the levels myself with a measuring mic. EQ wise it may not be perfect, but it sounds good, and dialog intelligibility is fine. 7 is just too damn loud. Occasionally we run at 5 if the movie is mixed at a low level (like the Marksman) but 95% of the time 4.5 is plenty loud while not being deafening.

I'd say Voyagers has a pretty 'typical' soundtrack: definitely loud in a few places, but well balanced overall.

On the other hand, Godzilla Vs Kong was stupid loud even at 4.5. I can't fathom how they mixed it without hearing damage.

Also after some testing (see the other thread) it's become pretty clear that the sub itself is fine, and the issue is more likely amplifier based. As everyone's pointed out (and I agree) we don't have a strong amp driving the sub.

[Marco Giustini]

Carsten,

Sure. That doesn't make that "right" though.

Jon,
If your system was calibrated for 85dbC, then I politely insist that 4.5 is pretty low. I appreciate it depends on EQ and speakers and acoustic but if you feel the need of running everything at 4.5, then I would look into why that is necessary.

I remember having a similar conversation with a gentleman and he told me the same thing: 7 is too loud (which I don't question). But that night a good sounding movie was being played and I took him to the auditorium and I had the volume set to 6.5 or 7.0 (can't remember) without telling him. I then asked him if that sounded too loud and he said "no, it sounds ok".

One thing I found over the years is that projectionists/managers used to evaluate the sound level from walking in the auditorium for 2 minutes and standing by the door/wall/corner of the room. Clearly if you walk in an auditorium from a quiet foyer while the Avengers are fighting for the world, that WILL sound loud.

The 1222 should be able to "mask" any heavy clipping IF the "clip limiter" is engaged. You haven't told us whether that is the case or not! Good to hear the JBL is ok though!

[Carsten Kurz]

'Right' is probably not the proper term, Marco. A while ago I posted this fader number/dB relation table for three major CP manufacturers. As one can see, a 4.5 on a Dolby relates to around 5.5 on a Datasat. 5.5 is our standard feature level on our Datasat. Yes, sometimes I do increase this to something like 5.7 or 5.8, but sometimes (rarely) I also decrease this to 5.2 or so.
Not many movies are actually mixed to 7 nowadays.

As I wrote before, I created this table because I noticed from many conversation and forum posts that many Dolby CP (650/750) installations seem to use surprisingly low fader settings (I have seen Dolbys set as low as 3.5 during preshow). Whereas I noticed that many colleagues using AP20/AP25 and USLs have theirs set to around 5.5.

[Jon Dent]

I usually screen the entire movie before deciding what volume it'll be run at. So when I say most movies we screen are quite acceptable, volume wise, at 4.5 (on a Dolby 750) it's taking the entire movie into account. I'm not a noise baby, and I like my movies at a good, impactful level. I feel that, by and large, 4.5 accomplishes that.

I agree with Carsten that it doesn't seem like movies are mixed at reference anymore (exceptions aside), as mixers are trying to compensate for theaters lowering their volumes by making 'louder' soundtracks, which leads to a feedback loop that'll never end. If we had run GvK at 7, I have no doubt that the average SPL would have been well over 90dB for the 2 hour movie, which I feel is too loud for that long of a time frame. Filmmakers are welcome to mix their movies like a metal concert, but I'm under no obligation to subject our audience to the same . It might not be 'right' in the technical term, but I don't lose sleep over it.

Carsten, those figures are interesting. For some reason I thought all sound processors followed the same volume curve. Clearly not the case...

No, the clip limiters on the 1222 are not on, as I was told by our installer that they weren't necessary. The more I learn about all this, the less I believe him, and I will be changing that... And we will be saving up for dedicated amps.

I'm also glad the sub itself seems fine. After seeing the thing in person I would not want to try to move it.

[Carsten Kurz]

The trouble is, all processors are calibrated at 7, but there is no official standard for the fader curve below (or beyond) 7. If movies would always be properly mixed to 7, more cinemas would play at 7, and these differences would be smaller. USL with the JSD60 in later firmware revisions moved towards emulating the Dolby curve. I guess it would hurt nobody if all cinema processor manufacturers would have used the Dolby curve, even if only 'silently'. Whenever I hear someone explaining they are usually playing around 4.5, I am confident that they are using a Dolby processor.


Of course, some cinemas treat preshow very different from others. It seems to be common in some cinemas to play preshow at full or half light and at very low volume with the idea that not all patrons have entered the auditorium yet, and those already inside should have a chance for a chat. So, in those locations I wouldn't be surprised to see fader levels as low as 3 or 3.5 during preshow. Whenever I tell film makers to qc their DCP in a cinema, I add that they need to make sure the processor is set to the fader level typically used for the kind of content they are qc'ing - be it advertising, trailer, shorts, or long features.

[Jon Dent]

If movies would always be properly mixed to 7, more cinemas would play at 7, and these differences would be smaller.

I've heard one of the reasons for this is that there is no Dolby print master stage anymore, so filmmakers don't have to sit through the entire movie at reference level. I've also heard that some sound teams mix with ear protection. In my opinion, if that's the case then you're too loud.

[Carsten Kurz]

Yes, there's a few different reasons for it. http://www.associationdesmixeurs.fr/...AGE-CINEMA.pdf