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