Room correction setup guide for ML’s

[JonFo]

This thread will cover the topic of how to accurately measure and setup a MartinLogan based system using modern room correction systems, such as Audyssey, found on many new processors and receivers.

The thread will primarily be about the measurement process, but it will also cover certain setup and room-related issues that have a strong bearing on the how well the room correction will ultimately perform.

The first thing to say is that one of the most important aspects here is the actual measurement process itself. I can’t stress enough that how the mic is handed, and where it’s placed during the sequence has a huge impact in the resulting outcomes.

The analogy is that you can take a top-notch D-SLR camera, and if you try and shoot straight into a light, or with sun streaming in at an angle into the lens (lens flare), the pictures will have unnatural anomalies in them. It’s not that the camera can’t take good pictures; it’s that the operator exceeded the reasonable limits of the system. Proper care in how it’s used will result in great pictures.

Same with these modern ‘automatic’ room correction devices, it’s all in how you provide them input.

Before we get started, If you are wondering what a ‘room corrector’ is, here’s a post I did on Audyssey. http://www.martinloganowners.com/~td...363&#post94363

[JonFo]

There are two basic tricky parts to measuring a MartinLogan set vs. ‘normal’ speakers:

1. They are dipoles – Which means that there is much more happening in terms of reflected sounds in the room, and this adds substantial complexity regarding how to setup and measure
2. They are line-sources – Which is both good from a dispersion and depth-of-field perspective, but also means height needs to be considered as well.

Center Channels can be very tricky to set up and measure. More so with ML as well see.

There is a great set-up guide already written for the Audyssey measurement process that we will be referencing as a basis for many of the topics. This thread will expand or override the guidelines as necessary to reflect the unique properties of dipole, line source electrostats.

For those interested in more detail about the tech and theory behind these advanced room correction systems, there is a wealth of information in the Official Audyssey thread over on AVS. Warning, there is a pretty bad signal-to-noise ratio in that thread, but posts most worth reading are by: Pepar, LarryChannin, batpig, FilmMixer, David Aiken, (+ others I’m forgetting now) and of course, Audyssey. Oh, and that real nut-case: JonFo

[JonFo]

How and where you place the mic during calibration has a LOT to do with whether you get a good result or not.

If the results do not seem right, then re-do the measurements.

It generally takes new users three to four tries before they learn enough to get this right.

Secondly, do not tweak the distance settings Audyssey auto-detects. Unless off by >5ft, it is probably correct, as it factors in any additional delays introduced by downstream processing (typically in powered subs).
Levels are OK to adjust a bit if you must, but frankly, should be left alone if you picked a good measurement grid.

I’d recommend doing as many positions as the product supports, most can do at least six.

The set-up guide goes into a lot of detail about the measurement process, read it, re-read it, then apply the modifications outlined here for our ML’s.

I’d recommend specifying the last three measurements in a tight little cluster +/- 2ft from the prime position (#1) and varying the height of the mic by six inches or so.

This will allow the process to ‘see’ more of the room, and to better correct for the fact that ML’s are big line sources, as well as get more of the vertical modes of the room.

The need for a mic boom based mount for the mic is critical, I documented a good bit of it on the Audyssey thread over on AVSForum.

http://www.avsforum.com/avs-vb/showt...0#post16221900
http://www.avsforum.com/avs-vb/showt...8#post16222398
http://www.avsforum.com/avs-vb/showt...2#post16224812

And for those who want a quick path to the mic stand and adapter, here are the two products you will need from a vendor I trust:

Stand: http://www.sweetwater.com/store/detail/MicStdBoomLt/
Adapter: http://www.sweetwater.com/store/detail/CameraAdapt/

[JonFo]

Ensure you have the quietest noise levels possible, so turn off the HVAC, fridges, etc. that could be heard in the room where you are measuring. One has to optimize for some baseline, and since the fridge, HVAC, etc. are not always on, it is recommended to EQ against the quietest background one might consistently have.

This means that there won’t be undue EQ for things like a projector fan noise when the PJ is off.

Although conversely, it does mean that the fan noise will impinge on the quality of the sound if it’s on.
But then, any extraneous noise does.

So the idea is to EQ without any *variable* noise source present when measuring.

Create a cheat-sheet guide for yourself. Hand-draw (or use Visio) an outline of the room, the speakers and here you will be placing the mic. Create and review this diagram against the recommendations in the set-up guide and this thread.
This will make it easy to just focus on the process of moving the mic around, and not having to wonder where it goes next during the measurement process.
It will also help you keep track of which patterns work and which ones don’t. Trying to remember where one put the mic six or eight times for each measurement run is impossible. Document.

Make sure you can actually move the mic and mic stand around to the mapped out locations.

[JonFo]

During measurement of dipoles, it’s common to see the measurement process report a ‘Phase error’.
Phase error reports are driven by either of these possibilities:

1. Your speaker is really out of phase and reversing the leads to it is the fix
2. Your big dipole radiator energizes reflections that fool Audyssey into thinking the speaker is out of phase when it's not. Re-measure.

To confirm it's one or the other, do a 'test', re-measure with the mic placed in the near-field of the speaker in question. That is, within six or eight feet of the panel, and as directly aligned with its center as possible

Always take the phase error with a bit of skepticism, look into it but often, it’s just fooled by reflections.

Remember, our MartinLogans are large, linesource dipole radiators, and the rear wave from the panels is out of phase. If that rear wave energy has a short enough reflected path to the measurement mic (typically six or so feet relative to the front direct energy), then Audyssey can miss-read the phase on that channel.

Again, that can be verified by placing the mic closer to the speaker in question, and re-doing a measurement and checking phase on that speaker.
If that speaker is good, then leave it alone, and re-do the entire Audyssey measurement for the rest of the speakers.

Addendum 11/2009: Note that several ML models feature out of phase woofers realtive to their panels. This is a design decision that normally improves panel to woofer integration. But it can be seen as an error by Audyssey. Ignore it if your model is designed this way.

Audyssey totally depends on a well executed measurement run for its accuracy. Practice makes perfect, so don’t be afraid to redo these several times.

[JonFo]

The preamp / receiver will set the crossovers automatically. If you have a sub, I’d recommend leaving (or setting) all speakers to small. As a for instance, my Monoliths are set to small in my Denon AVP-A1HDci.
Please read the set-up guide and the Audyssey CTO’s blog post about large vs small for further details and rationale.

The system will set the correct crossovers between each speaker and sub based on in-room measurement not the ‘specs’.

See more detail on this topic in this Blog posting by the Audyssey CTO on Large Vs. Small.