subwoofer volume at various frequencies

Discussion in 'Archived Threads 2001-2004' started by Stefan A, Apr 8, 2002.

  1. Stefan A

    Stefan A Second Unit

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    I was reading another post regarding subwoofer drop off at certain frequencies. I am not completely sure what that is, but I wanted to test my subwoofer. I put on "Shake Rattle and Roll" from my Chesky test disc. What it does is play a tone starting at a frequency of 300 and goes down gradually over a period 2.5 minutes to 20. Using my sound meter, I observed the decibel level as the pitch went down. I was expecting the decibel level to stay about the same. For most of it, it stayed at around 90 Db (+/- 7 or so). At about 50, it started to drop off. It went to about 80 Db. Then at about 30 it dropped pretty fast to about 60 Db by the time it reached a frequency of 20.

    Is this normal for a Rava subwoofer?
     
  2. Wayne A. Pflughaupt

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    Stefan,

    A sub’s response will depend on the driver, the enclosure configuration and its placement in the room. If you haven’t done it already, try putting the sub in a corner, one with the longest uninterrupted wall length in both directions. There is a good chance this will improve extension.

    Regards,

    Wayne A. Pflughaupt
     
  3. Stefan A

    Stefan A Second Unit

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    Actually, that is where I have it.

    Stefan
     
  4. Geoff L

    Geoff L Screenwriter

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    Stefan,

    Are these just straight SPL meter readings, in other words, you have not used correction numbers in your spl readings?

    From your posted numbers I would belive they are (un-corrected) readings.....I wouldn't expect to see it droping off that hard till around 30hz. I don't own the Rava, hopefully others that do own it will chime in.

    Is this the case, UN-corrected numbers for your readings????

    If so the responce is close to what I would expect to see out of that design.

    Room size and layout can play a major factor in frequency responce. Just look at the many post dealing with eq-ing subs for flat responce, house curves, ect.

    On a side note as you don't mention this, are your hapy with the Rava???

    Also im not familar with the disc your useing!

    The Rava is a well built, nice high quailty sealed sub and generaly comes highly recomended for smaller rooms. It shines on music, though said to do HT well also. This drawn from the many read post regardng it.

    Geoff
     
  5. Richard Greene

    Richard Greene Stunt Coordinator

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    It's normal for a sealed 12" sub to have bass roll-off below roughly 30Hz. in an ordinary home listening room.

    A good Rule-of-Thumb is that bass roll-off begins about 1/2 octave above a driver's FS spec. -- for the 12" Shiva driver, FS is 21Hz., suggesting that bass roll-off

    of 12dB/octave will begin at about 30Hz. when that driver is used in a sealed (0.707 Qtc design) enclosure without equalization.

    With a lower Qtc enclosure design (very large sealed enclosure) bass roll-off will begin at a higher frequency but will roll-off at a slower rate (perhaps 8 to 9dB/octave).

    There are several ways to solve the problem:

    (1) Bass boost (equalization) built into the subwoofer amplifier

    (2) Add an external equalizer

    (3) Use sub in a small room (under 200 square feet)

    where room gain (bass boost) of 12dB/octave below the lowest frequency room mode (565/largest room dimension in feet = lowest frequency room mode in Hz. for a rectangular room) offsets the driver's natural bass roll-off

    (4) Convert enclosure to a ported or passive radiator design

    If you are using a sound meter with a C-weighting, you would have to add the following corrections to convert

    from C-weighting to U-weighting (unweighted actual SPL):

    20Hz. +6.2dB

    25Hz. +4.4dB

    31Hz. +3.0dB

    40Hz. +2.0dB

    50Hz. +1.3dB

    63Hz. +0.8dB

    80Hz. +0.5dB

    100Hz. +0.3dB

    C-weighting attempts to compensate for the fact that our ears have increasing difficulty hearing bass at normal SPLs as the frequency declines. In theory the average person would hear a flat bass frequency response when a sound meter measured flat using C-weighting. But in practice, everyone hears differently and has different preferences for their own bass frequency "house curve" .
     
  6. Stefan A

    Stefan A Second Unit

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    I am not sure I understand what you guys are saying about corrected numbers and weighting. My meter has a switch for a or c weighting. I have always left it on c. As far as the specific decibels that I recorded, that is just where my volume on my receiver was. I turned the volume to a comfortable level when the sweep began and then just watched the meter.

    Stefan
     
  7. Geoff L

    Geoff L Screenwriter

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    I'll try to explain and keep it simple.

    As Richard pointed out, you have to add the correction #s to your readings in order to get your (real spl) at a given freqency.

    With the your disc your useing this might take a little more time as sweeps make it a little tuffer to take readings.

    You did it correctly, you just have to add the correction numbers to your readings to get the actual SPL. So if at 31-hz your meter was reading 80db you would ~{add the correction #'s for 31-hz = +3.0db}~. So for 31hz your actual reading is 80db + 3.0db for 83db actual spl.

    So set your volume control to a comfortable spot (not to loud as sines and sweeps at loud volumes heat the driver coil fast) then *leave it there*. Rat Shack Meter set to C-weighting Slow-Responce and run your sweeps wrighting down the numbers as you go. When done, add the correction numbers to them and you have your actual readings.

    Rat Shack correction #'s...

    10hz -- +20.5

    11hz -- +18.0

    12.5 -- +16.5

    14hz -- +13.5

    16hz -- +11.5

    19hz -- +8.0

    20hz -- +7.5

    22hz -- +6.5

    25hz -- +5.0

    28hz -- +4.0

    31hz -- +3.0

    36hz -- +2.5

    40hz -- +2.5

    45hz -- +2.0

    50hz -- +1.5

    56hz -- +1.5

    63hz -- +1.5

    71hz -- +1.5

    80hz -- +1.5

    89hz -- +1.5

    100hz -- +2.0

    111hz -- +1.0

    125hz -- +0.5

    142hz -- +0.5

    160hz -- -0.5

    200hz -- -0.5

    So have at it Stefan and as Wayne pointed out, sub placement can make a big differance. Moveing the sub as little a 6 inches can change the responce allot! So don't be afraid to experiment with placement and re-measureing your response each time the sub is moved.

    Im not real good at explaining things and hope you can make head or tails of what I tried to explain. Also if I screwed up somewhere im sure Wayne or Richard will straighten me out.

    Good luck and happy measureing Stefan.
     

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