Sunday, February 27, 2011

Living Room Dimensions and Room Modes

Had friends over for my monthly discussion party tonight, so I was able to make measurements of living room (which requires second person on opposite wall, since there are no straight paths on the floor).

Length: 187.5 inches (15.63 ft)
Width: 157 inches (13.08 ft)
Height: 98 inches (8ft, back), 96 inches (7'10" front), 110.5 (9.21ft, center)

I'm running the program ModeCalc from RealTraps, which produced the plot above.  I used 8.6ft average height.

Length has modes which are multiples of 36.15 Hz (36, 72, 108, 145, 181, 217, 253)
Width has modes which are multiples of 43.20 Hz (43, 86, 129, 172, 215, 259)
Heigth has modes which are multiples of 65.70 Hz (66, 131, 197, 263)

Total volume: 1758 cu ft
Ratios: 1: 1.52 : 1.90

Note that the height is an average one, so the 65 Hz sequence, and the ratios, are fuzzy.

If I chose to use max height, for example, the ratios would be:

and the height modes would start at 61.35 Hz.

Given that all rooms have to have modes, it doesn't look bad, the modes are reasonably well spaced and don't tend to pile up much.  You notice in all such plots that as you get to higher frequencies, you get more mutual node re-inforcement because on a log frequency scale the frequencies get closer together.  So there's a gradual increase up to the maximum calculated 500Hz.  Below 125Hz, there is only one area where two node-multiples come together.  That is the 2nd harmonic of 36hz (72 Hz) and the primary height mode of 66 Hz, and the height mode should be somewhat soft because of the center-peaked ceiling.  In fact, I have noticed a mode in that area.  Though RoomCalc doesn't show it as a pileup, you could also argue there is a pile up because of the two lowest principal modes at 36Hz and 43hz (caused by room length and width).  That's actually a 1.2 ratio, which is about the smallest acceptible such ratio.  Then there is a more pronounced pile-up at 130Hz (2nd harmonic of room width by third harmonic of height).  Then the next  next pile up is with 215 and 217 Hz.  Even "good" ratios have a few pile-ups like that.  But in my system, dipolar speakers play above 85 Hz and their output seems to stimulate room modes less.

So, not that I could change the dimensions anyway, they don't look too bad.  It looks good enough that you might believe the house developer (low cost San Antonio builder Rayco, which sold out to national developer K&B about 10 years ago) actually considered the acoustic properties in the floorplan, which has no doubt been used countless times.  No, it's not one of the "optimal" ratio sets, but it's not bad either.

Anyway, I now agree with what Real Traps says.  Regardless of how the room physical or acoustic measurements work out, if you are starting from a pre-existing room, about all you can do is add as much bass trapping as you possibly can.  So that is one of the next projects.  It will also be useful to run before-and after measurements to see the affect of different materials and different placements.

1 comment:

  1. A friend sent the following comments:

    Dimensions look pretty good.

    When I look at them, I discount the height because it is a range thanks to peaking, and one can't do much about how high or low one sits or stands in relation to vertical nodes, and suspending damping material from the ceiling may not be very practical. We move about in a room in relation to the length and width, but not much in relation to height.

    Still, if a ceiling were flat and doubled or tripled another dimension, it would be bad news and have to be dealt with.

    But beware: if the ceiling beam goes "athwart length", that is, across the width of the room, then there are two gigantic reflecting surfaces that can produce the acoustic equivalent of FM multipath. Secondary delayed arrivals of transients. Very bad. Better is if the ceiling beam goes along the length of the room.

    So looking more critically at the length and width, ....

    Not much overlap, really pretty good. I don't much like the twin modes at 215/217, it's in the voice area, but that's life. You may be able to attack that pair.

    I think there may be a good argument for smaller listening rooms. Their lower modes don't happen as much at mid-bass frequencies because they start higher up. The resonances that occur higher up are easier to damp... I don't know if anyone has mentioned this before, but a small room may sound less coloured and blurred overall, especially since the delays in multi-path arrival times would be smaller.

    It would be interesting to look at a log plot (assuming a Q of around 1.4) of the length modes, the width modes and the superposition of the two.

    D and I found we could perceive very small changes in amplitude response when it occurred over a range, sort of as if what counted was area under the curve rather than amplitude at a frequency. Particularly for female voice.