Below are some measurements I made in my home theater using my ETF/ECM8000/MobilePre setup. Each time I do a set of measurements I first do a new baseline set (what is shown below) before making changes and measuring the impact of the changes. I also measure at least the six marked positions along the main sofa for the baseline and each change. This takes a little bit of time, but isn’t too bad and provides a lot more information than measuring at a single point only.

The first measurement shows the frequency response at each of the six points along the main sofa starting toward the center of the room and working toward the wall (see www.bradjudy.com/audioblog/2005/08/07/the-room/ for the room diagram).

As you can see, the response holds mostly steady at some frequencies as the measurements move and I suspect this is because the measurements are in a straight line parallel to one set of walls. However, at other frequencies, the response changes significantly along the sofa. The extreme of this change is highlighted in the graph with exact values on the right-hand side.

This is a waterfall graph for one of the middle (sub3) positions fromt he same set of measurements:

And here is a time slice view of the same sub3 measurement. The author of ETF suggests using the time slice graph method over the waterfall for displaying information on decay.

As you can see from these two graphs, there is a significant ‘ringing’ of a room mode around 23.5Hz as well as a smaller one around 32Hz. Fortunately, most of the 10-100Hz frequency range doesn’t exhibit much ringing.

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Subwoofer phase is a pretty simple concept in speaker/sub integration, but it is often overlooked by people starting out. Typically a subwoofer has either a phase switch (often labeled with either 0/180 or normal/reverse settings) or a phase knob (which allows more granular control of the phase).

This is sometimes misunderstood because the idea of a crossover point is viewed as a ‘brickwall’ or an immediate hand-off from the speakers to the subwoofer at that frequency. In reality, there is a fairly wide range where the speakers and sub overlap.

The basic concept is for the speakers and subwoofer(s) to be working together in this overlap area instead of fighting each other (constructive versus destructive interference to use physics terms). This is accomplished by adjusting the phase (and possibly position) of the subwoofer to get it in sync with the main speakers (in a purely home theater setup, an argument can be made that coordination with the center speaker is more important).

Below are two graphs showing the frequency response in my home theater with my Hsu VTF-2 sub and Ascend Acoustics CBM-170 mains using the 0 degree and 180 degree phase settings. In this example, the 180 degree setting results in a pretty flat response (close to +/-2db from 20Hz to 130Hz), but the 0 degree setting has a very large drop (greater than 15db) in the crossover range.

180 degree phase setting (click to enlarge):

0 degree phase setting (click to enlarge):

Measurements using standard setup at position 3 in room diagram.

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Acoustisoft (aka Doug Plumb), makers of ETF, have released R+D (Resonance & Distortion) 1.0. It’s similar to ETF, but designed to do averaging of multiple curves for working with room problems. It also does DSP/EQ modeling of the Behringer DSP1124 Feedback Destroyer (BFD) in a way that is supposed to be much more accurate that typical EQ result modeling. The advanced version also has features to automatically pick EQ points for a give measurement or average of measurements. Here is an AVS thread on R+D

I have gone through Doug’s videos and started poking around with the application. I will post more when I have a chance to try it and compare to ETF. Doug says it will eventually have all of the capability of ETF and could be considered ETF v6.

It’s an interesting deveopment of the product geared toward addressing room issues via EQ, which is a popular topic at the moment. Given the low cost of a the BFD ($100) and the relatively low cost of the software compared to some options, it seems like a useful tool for many people.

Of course, Room EQ Wizard is still free and overlaps in function, but lacks the important averaging of multiple measurements. Unless you listen alone in one place, this is very important.

Since the BFD isn’t designed with home entertainment use in mind, it does lack some function of its more expensive counterparts (Onix R-DES $400 and Velodyne SMS-1 was $500 now $700), particularly their ability to store mulitple EQ curves and easily switch between them. On the other hand, the BFD is designed for full-range EQ and could be used on main speakers as well as subwoofers.

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I have started reading the Master Handbook of Acoustics to learn more about acoutics and room behavior. So far it seems like a pretty good resource, although the author makes some assumptions about the reader’s understanding of mathematics and ability to pick up on things quickly. To pick up on some of the topics hands-on, having a copy of ETF (the author uses ETF in examples, so having this particular program is useful, but other software could accomplish the same thing) and appropriate hardware to to basic measurements is helpful.

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Before I get around to posting measurements, here’s a quick breakdown of the equipment that I am measuring with. It’s nothing too fancy, but it seems to do a pretty good job:

Behringer ECM8000 mic
M-Audio MobilePre as both the mic pre-amp and the soundcard
Dell Inspiron D600 laptop on battery power
ETF 5.98x software

RadioShack SPL meter for setting the SPL calibration level in ETF

I have some additional software installed to play with like the free Room EQ Wizard and trial versions of various programs to try out (TruRTA, Spectra Plus, etc.).

I highly recommend the Room EQ Wizard if you have a soundcard with a stereo line-in (many notebooks only have a ‘mic’ in and not a stereo line in) and want to measure your in-room response. It’s an excellent program and can be used with just a Radio Shack SPL meter as the mic.

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I’m going to spend some time talking about working on low frequency response in room. I’m just learning this stuff, so feel free to point out information of note (references would be great, I’ll be picking up some books on the topic soon).

The room itself in in a fully finished basement with pad and carpet on concrete and 7.5′ ceilings. Here is a diagram of the room, click to enlarge (pink - speakers, purple - sub, green - treatments):

The numbers along the sofa are reference points for the measurements I’ll be posting later. Each time I do measurements, I measure at all six points for each test. It takes some time, but gives a much more complete picture of how things sound for various positions. The sofa back and mic stand are also marked to try and keep measurement positions consistent between runs. I also run a new baseline for each test and the baselines have remained consistent.

The room dimensions noted here aren’t exactly correct. I remeasured and the length is just under 22′. That would give an axial mode of about 25Hz (I tend to use this page for calculating modes - >http://www.marktaw.com/recording/Acoustics/RoomModeStandingWaveCalcu.html ). Since the main sofa is right about the half way point in the room, that places it in prime first axial mode location.

Since the room is open to another area and a staircase, it’s more difficult to predict behavior. My measurements show that the expected 25Hz mode is closer to 22Hz, but that’s for another post. I’m hoping to pick up CARA sometime and model the full space.

Measurements to come soon.

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