Tweakers' Asylum

Re: Help measuring frequency response of my room

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pkell44

(A)

In Reply to: Help measuring frequency response of my room posted by Tony on November 29, 2004 at 06:57:07:

You might try mounting the SPL meter on a tripod at about ear height in your listening chair, with the the mic pointed up rather than toward your speakers. Then stand to one side of the meter and as far away from it as reading its' scale will allow. You might even want to stand outside the listening room and read the meter with a pair of binoculars (?!) Every room has 3 modes (standing waves). One each for height, length, and width. There are formulae for calculating the frequency of these modes, which will allow addressing them. Search the Asylum archives for the room mode formulae and also for Jon Risch's Room Lens which is a battery of 3 Helmholtz resonators (one for each of the room's modes), and also the formula for tuning a Helmholtz resonator to a specific frequency.
Happy Listening

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The room lens... - David Aiken 21:07:03 11/29/04 (0)
is a battery of 3 Helmholtz resonators, but they definitely aren't tuned to each of the room's modes. That would require different lengths for each pipe.
Each pipe is the same length and contains some absorbing material in a different location. The idea is to make the absorption response of each tube more broadband - an empty tube has a very narrow absorption bandwidth, often less than 1 or 2 Hz to either side of the frequency it is tuned to - and the different locations for placement ensure that the response is different with each tube, so producing a more broad band absorption spectrum than you would get using identical filling in each tube. Despite all that, the actual amount of absorption the room lens provides is fairly minimal. It really is more a mid to high frequency diffusor than it is a room mode (low to mid frequency) absorber.
David Aiken

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