Room EQing

[James R. Skinner]

I have some questions about EQing:

• How often do most cinemas EQ their rooms? We are a small 160 cinema that has been running for 20 years but embarrassing I think its only been EQed twice. Once when setup and once by myself and a local sound tech last year. I'm thinking we should be EQing at least once per year?
• I'd like to make the room sound as good as possible. When doing the recent EQ I read the Dolby manuals (its an old CP650) and we did the best we could following that. Is there any online traning we could watch to learn how to do a better job?
• We didn't have a mic array, or even really know what that was, so we just used a single mic (AKG C414EBP48 in omni mode). What is a mic array, where do we find one, and how much of a diffrence to the room EQ will using one make

Any other tips on cinema EQing you might have would be helpful.
Thanks, James

[Leo Enticknap]

Opinions vary as to how often a room should be "tuned." I think it would be uncontroversial to opine that it should be done whenever a component of the B-chain (audio processor, amplifiers, and/or loudspeakers) is replaced, and if you still play 35mm, the analog A-chain as well. The same applies if there are any significant architectural changes to the room that could affect its acoustics, e.g. new seats or carpets. If none of that changes, I would argue that in an ideal world, an annual "touch up" tune is advisable. The performance of amplifiers and speakers does change over time.

Tuning with a single mic at the reference listening position (RLP) is definitely preferable to not tuning at all. As a general rule, the more quirky the acoustics in your room are, the more you will benefit from a multi-mic array tune. An auditorium in a multiplex that was designed by architects to have movie sound friendly acoustics will be kinder to a single mic tune than, say, a 1930s theater with a balcony, a steel roof, and lots of wood.

We use a D-2 kit for regular 5.1 and 7.1 tunes (e.g. CP950 or Q-Sys), the recommended gear for Atmos tunes (Roland Octacapture and Beyer reference mics, which the Dolby software is designed to work well with), and Trinnov's own kit for Trinnov tunes. When the D-2 kit takes a reading, it samples all four mics in short bursts over a 20-second sequence (or longer if you'd prefer), and then displays an average of all of them on the RTA EQ readout. You then make changes to the graphic or parametric equalizers for that channel, take another reading, tweak again, rinse and repeat until you get as close as you can to the X-curve (discounting spikes not coming from the pink noise being played, e.g. the rumble of HVAC equipment). The idea is to achieve a frequency response that sounds as consistent as possible across the entire auditorium, rather than perfect in one spot and a lot worse everywhere else.

[Tony Bandiera Jr]

Leo nailed it, and knows of what he speaks.

An annual tune up would be a good idea. I will also add that when using a single mic setup (Like with the RTA I have listed in the Equipment For Sale forum here) I would normally tune first to the RLP, then do one of two things: move the mic to a few other locations (three more, to be exact) and tune to the average of all the changes (Takes time and you have to note each channel's settings at each location, then figure out the average.)

I did that until I got a mic multiplexer. USL made an excellent one at a good price point that you can find used (But be sure to get ALL 4 of the original mics with it.) My second alternative method after using the RLP was to run my standard test reel so get an idea of how the room sounds in different spots, then I would make MINOR tweaks to the Eq by ear on the affected channels. After my hearing started getting worse I'd always use the mic mux or move the single mic.

The HIB 100 room I ran in the pictures Warehouse was a prime example of a very difficult room to tune. It was way too live, and wouldn't settle down until at least 200 butts were in seats. (Even then it sucked.) I ended up tuning to the best possible curve when the room was empty with the mic mux, then I modified the known live EQ reflections by ear on the fly based on educated guesses. Whatever I did, it must have worked because of all the directors to play films there, no one ever complained, and I got a few who actually thought it sounded great.

[Marin Zorica]

Akg 414 is great mic, but not suited for calibration as it is not flat mike, plus, if you chose omni characteristic you could even get more bad results from comnined response in room, plus some standing waves etc.
Except for that, great care should be taken while eq room than in case any high +/- deviation in room, you should check system, not just trying to "fix" it with eq. For example, this summer i was in the room where hf was so bad from center. I have check eq and all bands above 2khz as well treble was on full! So, before doing anything i reset eq and went to inspect speakers, when horn on center channel was aimed on roof instead on room......so, somebody probably either did forget to aim horn or something, but person who did eq wanted to fix it with eq

I am using octa capture as sound device, not only for atmos, but also for test of measurement. As it is very good audio device and has great microphone preamplifier. Also beyerdynamics mm1 are good because there are correction curves for each microphone which can be obtained from beyerdynamic factory. Besides that i am using latest Smaart suite, which can give you all kind of measurement, including multi microphone averaging as D2 does.

[Mark Gulbrandsen]

Since the mid 1990's I used an R-2 and then up till I retired, a D-2, but I kept the R2 as a backup. Always got excellent sounding results with it. There are even better, more expensive multi-mic analyzer options out there. However the THX analyzers are intended for cinema use and an inverse X curve is built into the analyzer.

I really like the way that Trinnov handled the EQ on their cinema processor the best. And a Cinema owner can also purchase their mic rig and do regular EQ checks themselves. The ones I know of that have been installed have also been trouble free.

[Harold Hallikainen]

With digital circuitry, I don't think there should be changes in equalization unless there is a failure in the system (such as losing an amplifier, crossover, or loudspeaker driver). As such, routine checks could be "gross" checks instead of fine tuned (multiple microphones, etc.). SMPTE RP 2096-2-2017 maintenance calibration says to use a single microphone at the reference position and "Play back [ST2095-1] pink noise through each playback channel and verify the frequency response traces match the reference traces from the Baseline Calibration, and the traces from the most recent maintenance calibration within. +/- 1 dB." This is similar to the old field strength requirements on directional AM broadcast stations. Those stations had to do a large number of measurements to establish their directional pattern. Then they would do a "monitor point" measurement in a few selected points (normally in the null of the pattern) weekly and compare that with the measurement at that point in the extensive full "proof of performance." As long as the monitor point did not change, it was assumed the directional pattern of the antenna had not changed.

One trick in such routine measurements is that if there is a failure in the system, it will go undetected until the routine measurements are conducted (or your audience complains). That was the idea behind the daily automated tests run with the LSS-200.

[Steve Guttag]

One of the things that the LSS-200 will show is how the EQ changes throughout the year. I was rather surprised when noting that the HF response is pretty significantly different in the summer/humid months versus the winter/dry months. This seems to be particularly true when using just a single mic reference, like the LSS-200.

Using my preferred analyzer, the D2, I can tell when the person used the D2 because the response will almost always be the same, within +/- 3dB. I find that analyzer/mic system to be the most consistent of them all.

When THX was a thing, I had a few rooms and would do the annual "recertifications." In one screening room, in particular, I could track the decay of the HF response over time. That is, it went down a couple of dB on the very high end (16KHz) over 15-years.

[Gordon McLeod]

We do ours twice a year with the change of seasons as we have noted changes based on the relative humidity and temperature

[Marco Giustini]

An annual check is recommended mostly to make sure all is well and not because things needs to be adjusted. yes, speakers will degrade over time a bit but nothing massive happens in 12 months. If someone has to do big adjustments after 12 months, something's happened.

Now, there is one major issue with the single microphone way: try doing the EQ on one seat - then move your microphone a few seats away and check again. Particularly in a small room (and with surrounds and LFE), you'll find a wildly different result. So I don't see the point in someone showing up every 12 months to say "the EQ has changed" and "re-do" it - unless the microphone is places in the same exact spot, which is also kind of pointless since if the EQ has a different response a few meters away, a single measurement is not the best.

But the other, main IMHO, point is that the final touch is done by listening known material (NOT random trailers) and adjust where needed. I have spent hours tweaking the sound in difficult (acoustically) rooms to get to the result I liked (that is: it should as it should, not as I like). If someone else shows up 12 months later and "does the X-curve", they're probably removing that "tweak" from the equation.

Finally: acoustic. It's as (or more) important than EQ and speakers. The best EQ is "no EQ" if the room acoustic and the speakers would be perfect.

So my advice is:
- Get the room EQd by a proper professional(*) ONCE
- Run pink noise every week to check. If something major happens, you should be able to hear it
- Get the SAME tech every now and then to check things professionally. If you can afford once a year, why not. The same professional will remember the room and will probably know its shortcomings. If you call a different (good) tech, they will probably have to go through everything again to make sure all is good.

(*) tips on how to recognise a good sound engineer
- An EQ does not take 30 minutes. It really depends on many factors but a "first time" EQ for me takes many hours. The more complex the sound system is, the longer it takes.
- A proper EQ includes lots of listening tests
- The engineer carries their own listening material
- A proper engineer will check the amplifiers, the crossovers, the speakers. The Equalisers is the last thing to touch in a sound EQ
- A proper EQ **must** be done with some sort of spacial averaging. Either a multiplexer or by making an average of some sort. Sticking a single microphone in the "Dolby" position is what old manuals recommend but it's not the best course of action

[Tony Bandiera Jr]

We do ours twice a year with the change of seasons as we have noted changes based on the relative humidity and temperature

Like the tuning of pipe organs, temperature and humidity have radical effects on sound quality. Thankfully most cinema auditoria are climate controlled so temperatures and humidity stays in a relatively narrow range, most of the time.

[Gordon McLeod]

Like the tuning of pipe organs, temperature and humidity have radical effects on sound quality. Thankfully most cinema auditoria are climate controlled so temperatures and humidity stays in a relatively narrow range, most of the time.

One thing to remember is in northern areas tempeture and humidity even with climate control have very large seasonal swings

[Mark Gulbrandsen]

One thing to remember is in northern areas temperature and humidity even with climate control have very large seasonal swings

I didn't have really big swings in the Mountain States because the humidity is a lot lower there, and more consistent... Even the snow was dry... and that made it some of the best snow for skiing.

Tony, Most of the locations I serviced turned the heat way down at night. 55, or 60 degrees was common. Warm enough so sprinkler pipes don't burst. But they still never had major changes in the Audio response.

[Leo Enticknap]

I would normally tune first to the RLP, then do one of two things: move the mic to a few other locations (three more, to be exact) and tune to the average of all the changes (Takes time and you have to note each channel's settings at each location, then figure out the average.)

A cool feature of the Trinnov is that it gives you the opportunity to automate that process if you only have one mic (the example Mike Babb gave when training me is if you have to travel to a service call by air, and therefore don't have the luggage capacity to take the full kit in its big Pelican case with you). You can position the mic at point 1, have it do its autotune thing, move the mic to position 2, autotune there, and then rinse and repeat through all eight positions. When all is done, it calculates the averages as if it had been listening to eight mics simultaneously as the test signal was played.

- A proper engineer will check the amplifiers, the crossovers, the speakers. The Equalisers is the last thing to touch in a sound EQ

Agreed completely, and especially if you're tuning a significant time after the initial install. An amp channel or speaker driver could have been blown for years without anyone noticing. My first step is always to check that all the amp channels and speaker drivers are actually intact and working. If the stage channels are bi- or tri-amped, adjusting the power amp gains until the RTA shows you a roughly even response across the entire curve will usually get me most of the way there without having to touch individual frequency bands on a graphic or parametric equalizer.

Like the tuning of pipe organs, temperature and humidity have radical effects on sound quality.

Have you ever heard the Spreckels organ in Balboa Park, San Diego, being played? If you listen carefully, the stops are never all in perfect tune with each other, and I'm convinced that the big temperature and humidity changes it is subjected to must be a major part of the reason why. The higher pitched 8' and 4' ranks seem to be affected the most, especially the reeds. I've heard the great and swell sound to me a quarter tone or so higher than the pedal. As an organist myself (admittedly, a very amateur one!), it's about the only organ that I find it positively unpleasant to listen to, and the only one I've ever heard that isn't within a fully enclosed structure.

[Jay Wyatt]

How often do most cinemas EQ their rooms?
I think that could depend wildly based on how much QC for presentation is valued at a particular theater, how large and how technically staffed they are. Based on my experience working at three different audio equipment manufacturers, I'd say that once per 12-18mo is comfortably sufficient for resetting a baseline and tracking transducer and electronics drift. You could probably push interval a bit further by incorporating a weekly or monthly audio test routine. CJ has some interesting DCPs, and checklists for managers at cinematesttools.com. Maybe worth reaching out and seeing what works best for your theater/circuit.

Is there any online training we could watch to learn how to do a better job?
Not that I'm aware of. My suggestion would be to hire someone who knows the tricks of the trade and has the software and hardware tools to pull it off. Have them do sound alignment for your rooms and see if they'd be willing to pass on their knowledge during the process. Make judgement call if investing the time and money for equipment and software is worth it for your operation or just hire them to on whatever interval you choose.

We didn't have a mic array, or even really know what that was, so we just used a single mic.
As mentioned single mic positions can be particularly misleading, especially in the lower octaves. If I was forced to use a single mic (without measurement analysis software), I'd take multiple measurement points and try to stitch it all this together in excel and manually create an average. This of course would take a lot of time. If I was forced to use a single mic, limited to the GUI on a CP650, and under the gun on time, I'd probably only apply EQ above 500Hz. Data below 500Hz can swing wildly throughout the room the "confidence factor" I'd have in making EQ decisions below that point would be very low.

What is a mic array, where do we find one, and how much of a diffrence to the room EQ will using one make.
A mic array is a simple way of saying multiple microphones arranged in chosen pattern. A common example in cinema would be five mics in offset diamond pattern.
Capture Signal Diagram_simple.jpg

[Marco Giustini]

I'd like to add something here.

As much as I appreciate the OP's intentions, sound is not like image where if the colour meter reads 0.314x0.351 you're all set (* ok, Laser have messed that up too!).
Sticking one - or multiple - mikes in a room and dial the "X curve" is only part of the game.

Yes, it will probably give you a baseline where to start from but as I said above, the Equaliser is only a player in the outcome - and a bad one, that is, you'd rather not have it!

I don't mean to discourage you from learning your way - just be aware that when it comes to sound things are a bit more complicated - the X curve was designed as an approximation to give cinema technicians around the world a baseline but the outcome is dependant on the acoustic. An RTA is "time blind". There are more tools today but none is the ultimate one. Probably - never worked with one - the Ovation is the closest to that, I'd imagine!

In short: be aware that sticking multiple mikes in the auditorium and dial the X-curve is not the ultimate goal.