I use an SPM meter on my boat to test any changes in SPL levels due to modifications in the engine room (i.e. gasketing, shft alignment, valve adjustment, insulation) or deckhouse (i.e. carpeting) that impact sound propogation. The vessel is a downeast flybridge cruiser with the deckhouse situated immediately above the engine room.
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Somehow the other I think that your ears are much better than you the credit you are giving them!
The RS SPL meter is not very sensitive & it is quite flawed in that the freq. response is much tapered off owing to the cheap components & "incorrect" bandwidths used in the design. I'm sure that you know that there is a correction factor to be used with the RS *analog* SPL meter. Here is the link:
Your room is definitely part of the overall equation (That you absolutely cannot discount!) when you use the test CD to figure out subwoofer placement.
>>When I heard fairly dramatic volume drops in certain >>middle freq. ranges I assumed it was the speakers but >>when I set up the meter and recorded the db levels there >>was no drops of significance during those parts .
Did you measure at the same point as the point you were sitting/standing when you heard these drops in volume???
Before you figure out subwoofer placement, figure out your room response. That will tell you a heck of a lot + will set the baseline to which you need to add the subwoofer to.
Also use C-weighting & "slow" response settings on the meter. IMHO. FWIW.
I found that when using test signals standing waves are much more noticeable. In my case in the mid-band moving my head from side to side made a dramatic change in the level. It's quite likely that the meter is reading correctly and your ears are hearing correctly, it's just that they are in different locations! With respect to the RS meter being inaccurate, yes it is but more so in the lower regions. In the mid-band its within a dB or so, close enough for what you're using it for. Rives does make a test CD that corrects for the meter if you don't want to do the math yourself.
Your ears are how you hear live sounds. Your ears frequency sensitivity would probably look terrible on a plot. However, not to worry, because your brain has learned the appropriate "equalization".
If it sounds good, it is good. There is no extra credit awarded for having your tone controls set flat.
Pmotz got it right. The difference in location of your ears vs the location of the meter is the most likely reason for what you heard that the meter didn't measure. As little as one foot is sufficient to make a noticeable difference. This is because of room modes and the way sound waves propagate in air...your ears may be at the location of a peak at the same moment that the meter is at the location of a trough. This is the reason it is imperative that the meter be placed as near the listening position (ie where your head would be) as possible.