Sweat over Room Dimensions and Mode Calculations, or Just Deal With Them?

[Jeff Hedback]

some studies

Hello Caesar,

I've completed some studies on your 14'x20'x9.5' existing space. The modal density, spacing, intensity are very good. You can view the black lines in the attached graphs and see the even spacing. A worse room would have wider gaps between black lines. ...Fortunate to have this space for consideration.

Now also shown in the studies are predictive LF responses (see red curves) based on spkr/LP locations within your space. Two studies are with the room orientated with 20' as length and one with 14' as length. These studies are shown to highlight how locations of source/receiver are going to be "touchy" in this space.

I actually agree with Microstrip that even a 15' width can be a difference and as I mentioned earlier...~17' might be significant. This falls as much into speaker type (pattern control, directivity pattern, etc) as room modes.

Also Caesar, if you haven't found Bob Gold's Mode Calc...check that out (especially the good/bad curve at bottom of sheet). Your room is indeed a good one!

The net take away is that your existing space can be an excellent dedicated listening environment and (to repeat) I feel you're fortunate. A ground up (new build) offers a level of excitement and possibility. At least you have a bit more data on your current space.

 

[Jeff Hedback]

clarification...in Study #3 in above attachments the Listening Position is 2/3 in from front wall (it reads speakers in the caption).

 

[caesar]

Thank you, Jeff!!!!!

 

[DonH50]

Jeff, which scenario would you choose, and why?

 

[Jeff Hedback]

Hello Don,

Great question and I would immediately throw out Study #2 with the LP dead center. Ultimately, I'm not sure either of the other two are obvious winners.
The Study #1 with the 20' dimension as length is a good starting point and could likely be tweaked from there for a more smooth response. The issue then becomes that the speakers are pretty darn close to the sidewalls. Speaker depending, this may not provide exceptional imaging, soundstage...etc.
The study #3 has my greatest curiosity. Yes, it goes against a basic common convention to use the longest dimension as the length. But I think there is just enough (with the 14' length) to keep ears off the rear wall and have lots of flexibility to place the speakers for modal smoothness and great mid/hi response.

 

[audioguy]

Jeff:

In my previous room I used double drywall. In this room I did not. I was single at the time I built this room so was not concerned about noise traveling throughout the house. I was told (by the designer) the benefit of doing this was to REDUCE some of the bass issues from my previous room and allow some of the bass that would have increased the amplitude of bass bumps to "leak" out.

Whether for this reason or others (like room dimensions: the bass in this room is smoother than my previous room (also designed by the same company !!)

Was his recommendation on "leaking" bass a valid one?

 

[Jeff Hedback]

Audioguy,

Absolutely there is a direct relationship between transmission (through surface) and reflection (back into the room). Additionally, some of the sound energy is absorbed by the wallboard (membrane) itself. In your case, it certainly seems that the right "compromise" was a single course of drywall for this room. So yes, I would say it was valid. The construction systems are very much part of the sonic experience.

I do personally prefer a double course of drywall for fullrange/hi-res speakers. There is a consistency and punchy-ness to the first ~3 octaves that is typically not the same with single drywall. Not that this is an absolute necessity. There is also a relationship to acoustical treatments and definitely a relationship to the surfaces surrounding the room. For example, if one side was a brick exterior and the other side an interior hallway the low frequency energy pressure patterns would be affected.

 

[Tom Sidebottom]

Check out this page on room ratios

I'm studying Acoustics at Salford University and the course leader Prof Trevor Cox has done loads of work on room ratios. This page gives some ratios that are robust to changes in volume (most aren't) and then it offers an excel spreadsheet with a whole load of optimised room ratios for a volume of 100m^3.

http://www.acoustics.salford.ac.uk/acoustics_info/room_sizing/?content=best

 

[FrantzM]

I'm studying Acoustics at Salford University and the course leader Prof Trevor Cox has done loads of work on room ratios. This page gives some ratios that are robust to changes in volume (most aren't) and then it offers an excel spreadsheet with a whole load of optimised room ratios for a volume of 100m^3.

http://www.acoustics.salford.ac.uk/acoustics_info/room_sizing/?content=best

Welcome to the WBF. Great first post!
A question to you and those in Acoustics. I have thought that he distribution of modes in a room is dependent of the position of the of frequency drivers in the room. Is that correct? Also what does happen when the low frequency sources are multiplied? Does it change the distribution of modes?

 

[Tom Sidebottom]

Hi thanks!

Well every room will have some room modes. The optimised ratios provided by Cox and D'Antonio do not take source position into account, but instead assume the worst case scenario which is one source in one corner and one receiver in the opposite corner. In this way all possible modes are excited. Selecting a room based on this is just a first step. Using an image source model to ray trace the path of the sound coupled with a knowledge of where the pressure nodes and anti-nodes of your modes are can be used to avoid placing source or receiver on prominent anti-nodes, but this probably quite tricky to get right. I'd probably just try to keep both source and receiver away from walls and corners to start with - unless you want to artificially try to emphasise any frequencies.

I haven't got into sub-woofer placement too much yet, but the same theories should apply. The room optimizer program offered by RPG is based on work done by Cox and D'Antonio, so that should be sound.

 

[JackD201]

^

I like him