[BobM]

The old JBL Room mode calculator doesn't seem to be available anymore online. I really liked the visual aspect of it. Here's the link I had:
http://www.harman.com/wp/index.jsp?articleId=131

I did find a downloadable spreadsheet calculator here, that seems pretty comprehensive:
http://www.ultimateavmag.com/news/10388/

and a standing wave calculator here:
http://www.marktaw.com/recording/Acoustics/RoomModeStandingWaveCalcu.html

There was also one I saw once that let you visually position your speakers and listening chair within a room / box that you could set the dimensions on. Then it would show you nulls and peaks in a grid and first reflection points too, I believe. Anyone know what I'm talking about and wher I might find it?

Any others?

Thanks,
Bob

[Ethan Winer]

Bob,

> Any others? <

I wrote a simple mode calculator for my company's web site HERE. It's a stand-alone .exe program (only 56 KB) that runs on any PC so it does not require Excel etc.

> let you visually position your speakers and listening chair within a room / box that you could set the dimensions on. Then it would show you nulls and peaks in a grid and first reflection points too <

I'm skeptical of most such "prediction" programs because they rarely give the same results you'd get from actually measuring. Maybe they can be useful as a starting point, but the only 100 percent reliable way to find the best speaker and listener placements is to measure.

--Ethan

[BobM]

One general question for everyone. My listening room has just migrated to the basement where I have a drop ceiling. Should I measure height to the bottom of the drop, to the support beam bottoms, or to the floorboards above?

Thanks,
Bob

[Ethan Winer]

Bob,

> Should I measure height to the bottom of the drop, to the support beam bottoms, or to the floorboards above? <

For mode calculations you need to measure to the rigid boundary above. Modes are mainly about bass frequencies, and those pass right through typical office ceiling tiles.

--Ethan

[BobM]

Thanks Ethan. I'll take some measurements tonight, run some calculations and see just how bad things look (in theory).

One quick question. My setup is in a basement. One section of the room is my stereo and the other section is exercise equipment (an open floorplan width wise). So the length of my "listening space" is fixed front to back, but the side to side measurement is quite a bit wider since its spanning both of these sections. I assume I need the true wall to wall width for the calculators and not just my "listening space" width.

I'm full open on the left side with a wall to my right.

Thanks,
Bob

[Ethan Winer]

Bob,

> I assume I need the true wall to wall width for the calculators <

Yes. But I have to tell you that a mode calculator is of little use for an existing room. The main purpose is when designing a new room, with new walls, to determine the optimal dimensions. For treating an existing room that you don't plan to change the correct approach is to work on placement of the speakers and listening position, and then add acoustic treatment.

> I'm full open on the left side with a wall to my right. <

That's not great for two channel (or surround either, for that matter). Left/right symmetry is very important, especially the portion of the room from your ears forward. Any chance you can post a floor plan or a photo of that part of the space?

--Ethan

[BobM]

Here's a BMP schematic of my current basement room. The space on the left is our exercise area. The stairway is behind the piano. The space on the right is my "dedicated audio area". There is a 8" x 8" soffit on the right and bottom cieling walls below the drop ceiling. The total room dimensions are 16 ft long, 22.5 ft wide, 7.5 ft high (2700 sq ft).



I hope this image is large enough to see the measurement details for speaker placement and seating location. If not, the speakers are 48.75" off the back wall (measured to the speaker front), the righ speaker is 26" off the side wall (to tweeter center). I am sitting 7.5 feet from the speakers, 36 inches from the back wall and 62 inches from the side wall. (I have records racked to the ceiling on my right.) The speakers are MTM's, with two 8" Scanspeak Kevlar drivers and 9300 tweeter, firing almost straight forward (perhaps a 5 degree toe in).

The mode calculators show problems at the following frequencies:
75.3   -   150.5   -   176.4   -   225.8   (less so at 250, 282, 301, 352, 276)
Yes, I do notice that these are esentially multiples of 75, the height frequency.

These other frequencies show tangential problems:
 83   -   103   -   228   -   303

Obviously bass damping is in order here, especially since my speakers are rear ported. If I plug the ports the reinforced frequencies definitely quiet down, but the dynamic is lost and it just doesn't sound balanced anymore. I have the CD cases at the 1st reflection point acting as diffusers (which actually seems to be working fairly well - the imaging is good).

So, anything else that I should worry about or that you would recommend?

Thanks for all your help.
Bob

[Ethan Winer]

Bob,

> I am sitting 9 feet from the speakers, 20 inches from the back wall and 62 inches from the side wall. <

One problem I see is a lack of left/right symmetry. This is very important for good imaging. Ideally you'd rotate the setup 90 degrees clockwise so the speakers are against the 16 foot wall, facing toward the left in the drawing. Then slide the couch up to be centered left and right when facing toward the right (which is then the front).

> The mode calculators show problems at the following frequencies <

The particular mode frequencies predicted may not match what's really happening in the room, which you can best determine by measuring. But it doesn't really matter much anyway because the solution is always the same: Broadband absorption that works well to as low a frequency as possible.

>  I have the CD cases at the 1st reflection point acting as diffusers (which actually seems to be working fairly well - the imaging is good). <

I'm always skeptical of the value of CDs and LP and bookshelves etc as diffusors. As an experiment you could hang heavy blankets at both the left and right side-wall reflection points - I bet that will make things even better. But really, you need to address the symmetry before anything else, as that will have a profound affect on imaging.

--Ethan