Film-Tech Forum ARCHIVE: calibrating cinema sound

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Posted by George Murray (Member # 6991) on 08-31-2014, 08:41 PM:

how do most of you calibrate your cinema sound systems? do you set the speaker levels to a certain db with a spl meter and is that it? I am trying to reproduce cinema sound within my home theater. Also what are the features of cinema processor do they do more than store the channel levels or do they offer some sort of volume control like audysseys dynamic volume.

Posted by Mike Blakesley (Member # 26) on 08-31-2014, 09:58 PM:

Movie theaters (ones that care, at least) have a technician that brings in a slew of microphones connected to a computer, and he uses them to adjust the volume and EQ of each channel. Special content (with film, it was test loops; with digital, it's just an audio file) is played through the sound system to make these adjustments.

There are lots of different types of sound processors, but a typical one does indeed store the eq settings and gain (volume) levels of each channel for various formats, and it also has a volume control so the operator can adjust the volume of the entire system at once.

You probably can't duplicate the sound of a cinema on a home system unless you have a great big room. Along with the actual sound quality, the thing that always defines cinema sound for me is the sheer size of the sound space.

Posted by George Murray (Member # 6991) on 08-31-2014, 10:26 PM:

but is there an automatic volume control like audysseys dynamic eq? also what levels are the each of the speakers set to?

Posted by Mike Blakesley (Member # 26) on 08-31-2014, 10:52 PM:

I have no idea what Audyssey is but there are folks here who probably do. There is no automatic volume control on a cinema system.

As far as the individual channel levels...you got me, I'm an owner not a tech! [Smile]

Posted by Brad Miller (Member # 2) on 08-31-2014, 11:05 PM:

quote: Mike Blakesley
You probably can't duplicate the sound of a cinema on a home system unless you have a great big room. Along with the actual sound quality, the thing that always defines cinema sound for me is the sheer size of the sound space.

Come visit someday Mike and I'll give you a private show. [thumbsup]

George, the Dolby CP65 manual is a good place for you to start, as it has a pretty indepth step-by-step on calibration.

Posted by Mike Blakesley (Member # 26) on 08-31-2014, 11:36 PM:

Maybe I should have said "a big room and a very large budget"! [Smile]

Posted by Carsten Kurz (Member # 5396) on 09-01-2014, 07:15 AM:

quote: George Murray
but is there an automatic volume control like audysseys dynamic eq? also what levels are the each of the speakers set to?

Why should a cinema use that kind of feature?

No - cinema sound systems basically are set up with a static EQ and sound level. The idea is to have an identical sound space as the sound engineers did during the mixing. You don't want any automatic dynamics or EQ go inbetween.
Some of the advanced systems use realtime dynamic processors to control speaker response, but not in the sense of modifying the sound, but to achieve the best possible response from the system.

- Carsten

Posted by Dave Macaulay (Member # 813) on 09-01-2014, 08:54 AM:

Calibration naturally depends on the system. First you have to confirm that everything works, and hopefully fix what doesn't. Then confirm that the system is adequate to meet cinema standards, so it gets set up in a way that it won't be damaged in use - equalizing a system with inadequate speakers for ideal bass response may be possible at 85dB using pink noise, but playing a typical action movie with explosions will destroy the bass drivers. Same with sub bass drivers, meeting "the spec" may be possible during setup but cause severe distortion and speaker damage on real program. If the speakers are adequate but amplifiers underpowered, the speaker horn drivers can be quickly destroyed at high volume if the amp clips: most modern cinema amps clip politely, turning their gain down rather than producing true clipping which produces distortion with damaging HF. Older or non-professional amps don't have this feature.
Rattles and hums should be investigated and cured. Securing any screen frame parts and getting scrap dumped behind the screen removed is worthwhile. The system should not hum audibly - that indicates a grounding problem or failing components. Some background hiss is sometimes unavoidable but by managing the gain structure in the sound system it can be minimized. A very slight hum or hiss with no signal is tolerable but far from ideal, program sound will mask it.
Then the channel levels are set and a preliminary EQ done with pink noise and a spectrum analyzer. Using one microphone at one location is not recommended because acoustic standing wave patterns will cause problems. One can use a multiple microphone multiplexer or move a single microphone while the analyzer integrates the signal. The actual equalization procedure depends on the equipment but should start with "overall" controls like crossover levels if bi/tri amped, then bass/treble controls, then any parametric EQ available, and finally whatever "graphic" equalizing you have. This should all be done a few times with channel levels reset after each pass.
Next comes listening tests with known program material to adjust the equalization: the pink noise and spectrum analyzer setup will be "pretty good" but often causes harsh sound when you've compensated for deficiencies in the speakers - if a speaker has a characteristic output dip in the midrange (not unusual) it can sound better with that dip not "fixed" in EQ using pink noise.
With channel levels reset to spec, the final test is to run a known "challenging" cinema clip at a bit over the normal show level (the "offical" level, fader 7 with Dolby cinema processors, is much louder than cinemas actually use) and watch the amps for overloading and listen for distortion and any bass drivers bottoming. If the subs bottom, you need to turn the sub level down and should recommend a better sub system to the owner. If the screen speakers bottom you need to adjust the EQ to reduce low frequency level: you're probably trying to get LF sound out of them below their cutoff frequency, or using B6 tuning on a speaker not designed for that. Surround systems installed for optical film sound are probably incapable of meeting the demands of digital cinema: instead of the matrixed signal from the stereo optical track they now have a full range digital signal - digital movie sound tracks can put a lot of sound into the surrounds. You have to compensate for any inadequacies by reducing the LF content and making sure the amps do not clip.
This all takes a lot of time, and theatre owners are renowned for their reluctance to pay for such work or for correcting deficiencies. In reality most sound setups are less comprehensive, and the results adequate but not optimum.

Posted by Bobby Henderson (Member # 840) on 09-01-2014, 11:18 AM:

That brings up the whole "good enough" syndrome affecting so many commercial movie theaters. Some theater operators deliberately cut corners to improve profit margins while others would like to install better systems, but just can't afford it. It costs quite a bit of money to install the speakers and amplifiers really needed to do the job right in a given auditorium. In a perfect world every movie theater would have a top of the line wall of 4-way speaker systems behind the screen, a fleet of sub-bass cabinets (rather than just a couple or so) and surround speakers that were appropriately powerful for the size of the room.

Now we have the situation where the standard priced theater sound systems in many big chain multiplex operations are under powered. They're only willing to do better in the premium priced houses, but even in those rooms they're willing to go only so far. It seems like making it loud rather than doing the job right is the rule of order in a bunch of premium priced theaters.

I don't expect theater operators to go to the extremes Brad Miller and others who worked at the General Cinemas Northpark 1-2 theater did in the 1980's and 1990's. Every time they got a new movie they would re-tune the sound system. I'll admit I got spoiled to that and how consistently great the theater's audio sounded. However, if a movie theater is going to add another $2 or $3 to the ticket price just because the room is a little bigger and has some fancy acronym or brand name on it they kind of need to rise to the same standard I observed at the Northpark theater. I sure charging people extra for it (while the Northpark theater never charged extra, even when it was running something in 70mm).

Posted by Terry Lynn-Stevens (Member # 7349) on 09-01-2014, 12:40 PM:

quote: George Murray
I am trying to reproduce cinema sound within my home theater.

I am surprised there is no B.S. app for an iPhone that can do this.

Posted by Monte L Fullmer (Member # 2797) on 09-01-2014, 02:17 PM:

At least for film houses:

With C weighting on an SPL meter and standing a bit more than half of the length of the room and off to the side.

Using pink noise track on all channels.

85dB for screen speakers
82dB for surrounds.
(sub, depends on how loud you want it, but 91dB was the norm.)

Seen some home cinema users use these same settings....but as mentioned: depends on the room.

Then you do EQ settings after the pressure settings. Seen the quad mic setup, or even the single mic setup for EQ settings - depends on the technician.

But the big taboos is not to have a square room where width and length can divide into itself equally.

Posted by Louis Bornwasser (Member # 3063) on 09-02-2014, 08:30 AM:

I am in Louisville. Calibrated sound left with the CP-500s and 650s. When digital went in they installed cp-750s. Almost none ever saw a calibration of any kind. Many don't have all the channels still working. The goodnews is that they still have good speakers, acoustics, and amps, if still working.