Where to Start with Audio Tuning and EQing, etc...

[Chris Haller]

Folks,

We had one of our lovely, held over from the Hotys era, Ashly FTX-2001 amps kick out in one of our medium houses last week. It took the center channel with it, and I was out of out town so I hit the C->LR bypass switch and came in to investigate what happened as soon as I could.

I ended up rewiring the output from our GDC AIB-3000 into a different amp's spare channel to restore functionality of the center channel and set the gain of that channel in software back to what I considered a reasonable level so we could get theater back in mostly functioning order. I didn't change the EQ settings stored for the center channel for the AIB-3000, but I know that its good form to re-EQ the room and take precise measurements to ensure proper audio playback after equipment is changed.

So I guess my question is.....how do you learn the ways of manual audio calibration? I have very little to go on in terms of what equipment is needed, and how to use it. I suppose I could just call our tech to come out and do it, but he's typically a 6 hour drive away and pretty hard to get ahold of in short order. I'd much rather start learning the skill myself, as we have something like 50 or so of these Ashly amps that are aged quite a bit. It would be a pain to have to call out our tech every single time one eventually gave up, as I do have some spare amps on hand to work with from our install.

If this is too broad a question, I understand. It does seem rather inaccessible from where I am skill-wise though.

[Harold Hallikainen]

SMPTE has recommended practices on this:

• 2096, Cinema Sound Calibration
  • RP 2096-1, Cinema Sound System Baseline Setup and Calibration
  • RP 2096-2, Cinema Sound System Maintenance Calibration

[Stefan Scholz]

So if I understand right, one of of these legacy amplifiers from another era has ceased operation. Well, normally this is not so difficult after installing a replacement amp.
In digital technology, EQ values and gain settings are quite stable, compared to analog legacy equipment used 30 years ago. Which required recalibration every once in a while.
All that has to be done here is setting the level of the affected channel(s) again. This does not even require complex measuring equipment. A simple SPL meter, and noise test sequence DCP is enough.
All you have to do is to adjust the level on the new amp to a setting that matches right and left channel.
You don't even need a sound pressure microphone calibrator calibrated to a NIST standard. Just the relative value is ok, as power amplifiers should be considered as linear within the operation frequency range.

To really learn and understand EQing a cinema hall is a process that requires knowledge, and more than an SPL meter. Just reading SMPTE standards will not really help. You should attend a professional course on the subject of audio adjustments. Unless you are responsible for a number of venues, the required measurement kit and microphones is a cost factor, as is the classroom attendance.

[Marco Giustini]

To really learn and understand EQing a cinema hall is a process that requires knowledge, and more than an SPL meter. Just reading SMPTE standards will not really help.

Stefan is right though I disagree on the SMPTE part. Reading the standard is a start. Then it's a matter of experience and knowledge.

Setting up proper DCI colours on a projector is just a matter of following a procedure. Unless you have laser light source and metamerism in the way, if the meter reads the proper values, nobody can question that.

With sound it's another story and will take some knowledge and experience. I don't see why you shouldn't begin experimenting a bit, bearing in mind that there isn't a simple, unique method to EQ a sound system. Yes, the old "X-curve" is a start but there are a gazillion variables in the equation. A big one is that your RTA is "time-blind" while our brain isn't. So the "X-Curve" will give you just part of the picture. This is why you should expect a good sound technician to run some familiar content after the sound system has been tuned to confirm it sounds as expected.

An SPL meter is a good tool to have but honestly I seldom look at those numbers, they are just a wild approximation of what your RTA traces tell you. And it cannot really used to properly set the LFE.

If you want to invest some time on sound, I'd recommend investing in Smaart and a simple measurement microphone (avoid Behringer) and then it'll take time to read the documentation and experiment and LISTEN.

In short, an SPL meter will help getting the sound system to a basic baseline but it's far from being the only tool to achieve the end result. But as Stefan said, if just the amplifier failed and your system is NOT bi/tri-amped, what you're trying to achieve is to set the new amp at the appropriate level and that can be done by ear to be honest by comparing the channel with the new amp to the existing ones.

I hope this helps a bit

[Steve Guttag]

Honestly, download some Dolby Cinema Processor manuals (including from the film era). The process is the same today. They don't discuss microphone multiplexing but aside from that decent techniques can be found there. SMPTE 202 is also a decent enough document and I believe a version of it is found in the back Dolby Technical Guidelines that is downloadable from there.

Once there was a time where "Dolby School" existed and they would have a hands-on session to allow techs learn or have their hand at tuning a theatre without any repercussions (after the class, they would put the tuning back). There is no substitute for actual hands-on when it comes to tuning and seeing/hearing its effects. Different theatres/acoustics will respond differently to various tuning techniques too.

[Marco Giustini]

good idea.

I had forgotten Dolby processors have an integrated RTA which can be useful for a basic EQ. However, I had a quick look at the GDC manual though and I it seems that it doesn't feature an RTA. Too bad.

[Leo Enticknap]

Once there was a time where "Dolby School" existed and they would have a hands-on session to allow techs learn or have their hand at tuning a theatre without any repercussions (after the class, they would put the tuning back).

I did it in 2018, and so it still existed then. However, that was in the acoustically perfect Dolby screening room in Burbank. It was a great foundation for learning the basic concepts, procedures, functions of equipment, and so on, but if you go straight from that to a 1920s picture palace to do your first "for real" tune, you're going to be in for a shock. A lot of what I have learned since from techs who have been doing this for decades has been just as valuable as the class itself.

[Bruce Cloutier]

It is amazing how well everything works in the controlled environment wherein it was designed. And then there is the real world.

They could have easily had one or two challenging halls for you. You would think their engineers would appreciate that too.

[Marcel Birgelen]

The GDC thingy doesn't have a built-in RTA and also doesn't seem to have a "room auto-calibration feature". It does have a few test/calibration signal features though, like pink noise. Their manual also doesn't explain how to correctly calibrate it, so I guess that's something you only get told when you're buying a training from them.

Like Steve indicated, those Dolby manuals always came with a large section regarding correct room calibration, at least those from the film-era did, I haven't checked the CP950 or IMS3000 manual in a while...

[Leo Enticknap]

They could have easily had one or two challenging halls for you. You would think their engineers would appreciate that too.

I think that starting your training in an environment that isn't going to throw you any curveballs is a good idea. It's the same principle whereby pilots begin their training in a Cessna 172 in perfect weather, not trying to fly an A380 through a hurricane. The important thing in basic training is to understand the fundamentals of gain structure, mic positioning and calibrating, what an RTA does, what the X-curve is and why it's important, etc. etc. Once you have that foundation, then you can start to get to grips with big and/or asymmetric reverb, finding a blown bass bin in one of the stage stacks, HVAC systems causing a spike at 120-150 Hz that you'll never be able to tune out, and that sort of stuff. About my only criticism of the Dolby class was that they didn't flag up that the first time we tried to tune an actual theater, if it wasn't a cookie cutter multiplex room, to expect challenges. But ultimately, that's all that a one-week training class can realistically do: give you the foundations and a starting point from which you then build on with experience in the field. It'll never be able to turn you in to Ray Dolby.

[Harold Hallikainen]

Recall also that the USL JSD-80, JSD-100, and JSD-60 include an RTA. The JSD-60 includes automatic tuning, which some people like, and others don't.

There is also a great discussion of all this at https://www.smpte.org/hubfs/er0994-2014.pdf . I was part of the study group that produced it and participated in the tests at Venue F.

[Hong Sung Hyeok]

After completing the b-chain with the CP950's auto-calibration feature, I was surprised at how well the calibrated sound was.
I figured it wouldn't be much different from what I'd spent 2-3 hours on. I hate to admit it, though.

The world has changed.

Ask GPT and it'll tell you almost anything you want. It might be a bit nonsensical, but that's okay.

Gather all the sound processor manuals, amplifier manuals, and b-chain documentation from various manufacturers.
Gather Smaart and SMPTE documentation,

upload them to GPT, and just ask.

[Hong Sung Hyeok]

Download 'RTA Analyzer' (or 'RTA Pro' if you're paying) and 'Audio Tool' from your smartphone app store right now and give them a try.
It'll be more than you expect.


You'll definitely need a measurement microphone, a microphone stand, a dB calibrator, and an audio interface.

While there are professional paid software like Smaart and EASE,
REW - Room EQ Wizard Room Acoustics Software is free.

Just try.

[Chris Haller]

Thank you for all the suggestions - I do have an entry level measurement microphone. I bought a Umik-1 just to get the ball rolling and take some basic RTA data and SPL measurements. I plan on using a tool like friture or something along those lines for MacOS that will handle that.

I will dig into some of the older manuals for tips and ideas. It seems like the right thing to do if I'm going to continue to maintain the two theaters under my jurisdiction properly. One theater is a former Hotys turned Regal location with stadium box-style theater builds, and the other is a former General Cinemas that had its big THX screen split into 4 for a total of 9 very bizarre theaters. Lots of variables and interesting spaces to figure out.

[Marco Giustini]

The umik-1 will be perfectly fine, don't forget the calibration file. REW can also be a good starting point indeed.