Trial and Error Bass Tuning
|
|
Purpose: To find
the perfect location and setting for my speakers to provide an
undistorted clean frequency response without electronic equalization.
Hypothesis: (Educated Guess) I predict there will be
Response dips at 44-51hz, and 70hz. There will be peaks at 80-113hz.
These guesses are based on the calculated standing axial resonances of
my room.
Test: Here is a diagram of the theater room (Pics see
bottom of page). Testing is done with a Radio Shack SPL meter (Slow
C-weighted on a tripod). Graphs and compensation can be downloaded
below as well as the calculators/formulas I used. Email me if you want
to "real" detailed files as I would like to conserve space and keep this
article brief.
|
 |
First lets see
what happens when you use dual subwoofers... Here we see that the
average output increased by 4-5 decibels and the subwoofer's locations
weren't even in the same corner. (Left front and right front corner).
We also see the "width" modes being canceled out as predicted which
isolates the other modes so they can easily be identified. The
explanation for the 2 different 80hz and 100hz peaks is that the room is
13.5 feet wide on the left subwoofer's side, and 11 feet in length on
the right subwoofers side. The standing wave calculator was right on
the mark in predicting those "single" subwoofer peaks. (Calculator
estimated 87hz and 103hz)
|
 |
Here we see how
little difference in SPL the changes in corner spacing made. Closest to
the corner represents 1-5" space from the box and walls. "Middle"
represents 8 inches spacing from each wall, and "Spaced from Corner"
represents a 12-15" distance from corner. Looks like I'll keep the sub
close to the corner for that small edge in SPL for now.
|
 |
This graph shows
how the different frequencies respond to a small turn on the LFE level
of my Pre/pro. As I moved the volume indicator in 2 click increments,
it seemed to effect all frequencies fairly even. (As it should)
|
 |
The response
wasn't really getting anywhere so I decided to "Move" the listening
position by using a higher chair and moving it forward a foot (Now the
mic is at 3.5 feet high as opposed to the 2.5 feet. This resulted in a
dramatic elimination of the 80hz peak. Anyways there's still the 70hz
"height resonance" causing a dip in the middle of the room that I can't
really adjust. So I guess I could start tweaking the crossover point.
Here I can see that from 40-80-120hz it clearly effects the SPL level of
frequencies near the crossover region. Deeper bass signals were
untouched. Too bad this still didn't solve any problems only helped me
decide that I want to use the 80hz crossover point. (What a shock..)
|
 |
Well, Finally
After realizing that what I was dealing with was mostly "length" axial
room modes I decided to bring the speakers forward 20 inches from the
front wall and see what happens. Sure enough, the SPL level dropped but
so did the peaks which resulted in a more even response that's about +/-
4.5 decibels. So moving the subwoofers away from walls really DOES
minimize the effects of standing waves, well worth it if the SPL doesn't
drop too much in my opinion. Overall, Not too shabby for such a small
room and no equalizer I can't wait to see what the Bass trap/ HH
resonators have yet to contribute. However, I do like the extra 2
decibels or so that corner loading adds so I think I might run stereo
corner loaded subs for the time being.
|
 |
Conclusion:
Here we see all of the graphs overlapped to get a general Idea of where
"most" of the immobile peaks and dips are. The calculations proved
fairly accurate to the actual measured in-room
frequencies. The 40hz artifact is questionable as the room's design
shows that if anything there should be a 44hz "dip" and not a peak. The
fact is that the theory involved in predicting standing waves holds more
truth in walls made of stone. The walls in my house are thin
drywall... These walls resonate and have their own acoustical effects
on the response. An assumption is that the walls boom back and forth
(or windows) that amplify 40hz. However, when I have taken measurements
and measured what happens when I change variables around, I can then
hand pick the response to use.
Update: I failed to mention that the crossover
point of my receiver (sound card) is an extremely high 540hz. This
could very well be affecting my "bass tuning". The sub crossover
on the plate amp is now set to 40hz to compensate for the unusually high
receiver crossover point.
|
Guess Dips: 44-51hz, and
70hz.
Actual Dips: 55-70hz
Guess Peaks: 80-113hz.
Actual Peaks: 40hz and 80-125 |
