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66.51.146.3
In Reply to: RE: Sub integration - please evaluate (SPL graph attached) posted by VynylTap on January 12, 2009 at 19:43:26
YOU WROTE:
"The blue line is in room at listening position with no sub
The red line is 0-phase w/ 40hz crossover
The green line is 270-phase w/ 40hz crossover"RG
Did you try 180 degrees out of phase?The simple test is listening to a slow sinewave sweep from 20 to 100Hz. that lasts at least 15 seconds. You'll easily hear bass booms and nulls.
Your measurements should be made with sinewave test tones spaced no more than 1/6 octave apart, and even that would result in a rough measurements.
Your current measurements using some test tones 1/3 octave apart are almost certainly NOT capturing the actual SPL peaks and troughs that would be visible with sinewave tones for each frequency: (20Hz. 21Hz. 22Hz. ... 99Hz. 100Hz.)
The test tones should be steady sinewaves. Sinewave warble tones, pink noise and white noise will smooth the measurements by failing to excite room modes much (unlike bass guitar notes and kick drum notes)
YOU WROTE:
"All measurements have been corrected for the errors in the RatShack SPL meter."
RG
The Rat Shack meter needs no correction for the bass frequencies.Most of the so-called "error corrections" are in fact a simple conversion from SPL C-Weighting to a flat SPL weighting (unweighted SPL) that would apply to all sound meters.
Rat Shack meter samples are also not consistent enough to be sure corrections for someone else's Rat Shack meter would be useful for yours.
Most Rat Shack meters read a little low for bass frequencies, so you could add +1dB to all measurements under 100Hz. and most likely inprove the accuracy.
Measurements under 30Hz. are very likely to be too low, but those frequencies are hard to hear with music playing at typical volumes, so the low readings under 30Hz. may actually help the meter correlate better with what you hear/feel at 20Hz.
Richard BassNut Greene
"The Floyd R. Turbo of Bingham Farms Michigan"
Edits: 01/14/09Follow Ups:
it with some sweeps if they're on the Sterophile CD's I borrowed.Here are the correction tables I used in the link, it had me add +5db @25hz! This is the same table I've found all over the net when searching "Radio Shack SPL Meter Corrections":
Edits: 01/13/09
It's the treble readings over 2000Hz. that are really far from accurate, and should be ignored! Much worse than the treble error corrections shown at the link (unless the meter has been redesigned recently).The Rat Shack meter uses C-weighting.
C-weighting is very useful for average SPL's up to about 90dBA
90dBA is very loud.
Our hearing doesn't work well for bass tones unless the bass is very loud.
That's why for most listeners (who don't listen to music averaging over 90dBA), C-weighting is the best measurement to use
C-weighting assumes bass at 20Hz., for one example, will need to be +6.2dB louder than 1000Hz. to sound equally loud, because our ears have trouble hearing deep bass.
Therefore, any sound meter using C-weighting will read 6.2dB LOW at 20Hz. compared with an unweighted SPL measurement.
So, FOR ANY SOUND METER AT ANY PRICE, the conversion factor from C-weighting to an unweighted measurement at 20Hz. is +6.2dB.
It's not fair to combine a +6.2dB conversion factor that applies to ALL sound meters, with a +1.3dB error correction that applies to one sample of a Rat Shack sound meter, and then claim Rat Shack meters need a +7.5dB "error correction" at 20Hz.
That falsely makes the Rat Shack meter look bad.
In reality, a Rat Shack meter's C-weighted readings should be +/-2dB in the 30 to 100Hz. range ... and merely adding +1dB to the readings is likely to improve the accuracy to +/-1dB = very good for a meter that doesn't cost $300-$500.
Note: Measurements of sinewaves under 30Hz. may not be useful because a big portion of the sound energy may be harmonic distrortion (aka doubling) at higher frequencies which would just make a subwoofer look more powerful than it really is at 20Hz., for example.
.
.
.Richard BassNut Greene.....................................................................
The "Cliff Claven" of Audio
and the "The Floyd R. Turbo of Bingham Farms Michigan"
Edits: 01/14/09
... and I realize (and understand) it's not the most useful way of measuring, but I just switched the correction to a straight +1db below 100hz.
Just to be clear too - the sub is not corner loaded, etc. it's sitting just to the right of the left speaker. I have no choice in placement in the temporary room I'm in!
The C-weighting (+1dB for typical Rat Shack meter errors) is a decent measurement of what we hear at reasonable volumes.
At very high volumes (over 90dBA), an unweighted SPL measurement is better.
I think you said your room was 14' by 14'
The lowest room mode (standing wave) would be about 40Hz. (565/14ft)
The "modal zone" would be above 40Hz. = no standing waves below 40Hz.
Unless your room was unusually well sealed (rare), there would be no cabin gain below 40Hz., and placement in a corner would not affect the bass below 40Hz. For those huge wavelengths, any spot in the room would be "near a corner".
Your small room should provide smooth bass under 40Hz. ... but a rough bass frequency response for a few octaves above 40Hz. based only on the dimensions (aka armchair analysis).
.
.
.
Richard BassNut Greene.....................................................................
The "Cliff Claven" of Audio
and the "The Floyd R. Turbo of Bingham Farms Michigan"
All you need is a soundcard, Radio Shack meter or Mic/preamp, misc cabling, and this free program, lots of folks use it for what you are trying to do:
.
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