# Don’t know room dimensions? Use LF modes!



## basscleaner (Aug 22, 2011)

Recently I did acoustical measurements at new recordings studio in Astana (town – capital of Kazakhstan). Unfortunately, there were not any architectural documents, but it was need to know the height of the room. There was not any stairway, the height was too high to use a chair or table. After some minute thinking, I’ve carried measurements of LF resonances in the room by ETF 5.8 and got this values: 31; 39; 43; 49; 52; 59 Hz. Next step was to calculate frequencies by well known formulas and to compare. I’ve got the same magnitudes at dimensions 9.9x8,7x5,5 m. So, the height 5,5 meters was determined, however, I was not sure, that it’s truth.
I know, that sound velocity depends on temperature, humidity and atmosphere pressure level, but was surprised, when AutoCAD plans had showed practically the same results (9920x8650x5500 mm concrete walls and floor-ceiling). Maybe this is a simple method for hided hard walls determination?


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## Zeitgeist (Apr 4, 2009)

basscleaner said:


> Recently I did acoustical measurements at new recordings studio in Astana (town – capital of Kazakhstan). Unfortunately, there were not any architectural documents, but it was need to know the height of the room. There was not any stairway, the height was too high to use a chair or table. After some minute thinking, I’ve carried measurements of LF resonances in the room by ETF 5.8 and got this values: 31; 39; 43; 49; 52; 59 Hz. Next step was to calculate frequencies by well known formulas and to compare. I’ve got the same magnitudes at dimensions 9.9x8,7x5,5 m. So, the height 5,5 meters was determined, however, I was not sure, that it’s truth.
> I know, that sound velocity depends on temperature, humidity and atmosphere pressure level, but was surprised, when AutoCAD plans had showed practically the same results (9920x8650x5500 mm concrete walls and floor-ceiling). Maybe this is a simple method for hided hard walls determination?


That's an interesting idea to use acoustical measurements to derive dimensions. In my experience the modes have been close to the dimensions, but are never 100% accurate. Rare to have a true box. 

I've used an Ultrasonic tape measure for some measurements on hard surfaces that has worked pretty well - for spaces that are difficult to get to. 

Interesting to hear your experience.


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## basscleaner (Aug 22, 2011)

Real dimensions determinations by FURDD (Frequencies Use for Real Dimensions Determination) cannot be accurate method, no doubts. Obviously, we can discuss approximation, based on sufficient amount of measured eigenvalues (or eigenfrequencies), which corresponds calculations with good accuracy. Anyway, this is a subject of science work with not evident perspectives of practical use. Unfortunately, I’ve got no any opportunity to continue this interesting investigation right now.:huh:


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