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Speaker placement and dB differences

So I’m pretty anal about speaker placement. I use tape measures, along with laser lights and levels to assure proper and equal placement from walls. I was using my new Omni mic and testing software to run some white noise frequency graphs vs db range to see where I stand.  After overlaying the R/L graphs I noticed that there is about a -1db difference in the right channel generally across the entire frequency range and about -5 dB at about 250 hz. 
Also my room is 12’ x 13’. 

I remeasured my speaker distance from the seating position wall and I noticed the left speaker was 96” from the front the chair and the right speaker was 98.5” from the chair. By my calcs that would only account for a .22 dB difference. Not enough to account for the 1 dB drop in the right speaker and certainly not the -5 dB at 250 hz.
Question is, is a 1 dB difference between two speakers normal. My speakers are Thiel CS 2.4’s and I’m using an older ARC tube amp - which can certainly account for 1db.

the bigger questIon is can a 2.5” difference in room placement cause a 5 dB difference at 250 hz?
last_lemming

Showing 3 responses by audiokinesis

audiokinesis
2,764 posts
 
Last_lemming, you might try switching your speakers, left speaker to where the right one is, and vice-versa.  Assuming there are no issues upstream of the speakers (like a weak tube in one channel), this will help narrow down what is a speaker issue and what is a room issue. 

Duke
audiokinesis
2,764 posts
 
Unless you are using time-gated curves, windowed to avoid room reflections, what you are picking up is speakers + room.   In that case, unless your room is acoustically symmetrical across the spectrum (which never happens), some measured difference between the two speakers is virtually inevitable.   And also totally okay.  What matters is that the first-arrival sound be the same from both speakers, and you can't measure that without time gating, and even then you can't measure it down very low in frequency.   

In particular, that 5 dB difference at 250 Hz is not the result of "a 2.5 inch difference in room placement".  Your two speakers are much farther apart than that.  The room starts to dominate below about 500 Hz, and from there on down each speaker interacts with the room somewhat differently.   

None of this PRECLUDES the possibility of other factors being the cause of the issues you see, but they could just be normal room interactions. 

Duke
audiokinesis
2,764 posts
 
Time gating is a technique for measuring the speaker's output without including any room reflections.  A specialized measurement system is required. 

The microphone is switched on for just long enough to capture the direct sound from the speaker and then switched off before the first reflection arrives. So all room reflections are excluded from the data.  But the ability to collect good data at low frequencies is limited by the shortest reflection path length in the room (typically the floor bounce), because the microphone is off before it can adequately capture longer wavelengths.  

Time gating is one of the techniques John Atkinson of Stereophile uses when generating "quasi-anechoic" frequency response measurements.  He uses a different technique called "close micing" at low frequencies, which is where the microphone is placed very close to the woofer(s) and then very close to the port(s) and those curves are combined with appropriate weighting.  The result is then spliced with the time-gated curve. 

Duke
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