UMIK/REW/Equalizer APO and room modes...

[dtvmcdonald]

I tried something new for analysis. The room treatment remained as in my last post with graph.
I got the cardiod mike, and made up a correction curve for it from the published response curve and
an absolute sensitivity measure.


I then made up an impulse track with a 6 dB down point of 525 Hz. This has impulses from left (first)
and right (second) speakers. The equalizer is in use. I then took time responses with the cardiod mike,
top being the front facing halfway between the speakers, at the usual listening position, This is the top
response line. The bottom line is with the front of the mike facing exactly backwards from the
same position.

View attachment 3726104


Its interesting that despite the rather different gains in the equalizer below 500 Hz, the overall impression
is that the two sides are roughly equal. This is a very interesting finding and was worth buying a mike just for it.
Its an Audio-Technica ATR2500x-USB. One requirement was that it has a published response curve and published
rudimentary (few frequencies) polar curves. Its a lot higher noise floor than the omni one I got. The frequency response of the mike is
corrected by the curve I installed.

I also did the same with clicks made with a center frequence at about 800Hz, in the range of the midrange speaker.
Note that this one does not reproduce the same reflection timings as the one above, as the midrange and
woofer are at very different heights.

View attachment 3726134

I didn't do the tweeter since I can barely hear it.

These sort of tests provide more info than possible with an omni mike.

Doug McDonald

 

[dtvmcdonald]

While all the treatment I described made a big improvement, something still felt wrong with the bass.
It didn't sound as "rich" as in my car, or even with a small system in my workshop
with speakers that start falling off at 70 Hz. But then I realized that it did sound
good when listening to movies off a DVD. So I hooked an antenna to my receiver
and compared FM from it with that from my computer Airspy. The receiver sounded much better
(both in mono .... its too noisey to use in stereo.)

I also noticed that a test track played through REW sounded different from the same track played
with any other program on the computer. So I started trying to find every possible setting in every possible
"system" place and all the Dell and Realtek drivers. Its all so confusing. It turns out that computer
soundsdifferent in the bass if you turn on "effects" even if each and every effect on the list
if individually turned off. And doing so make the computer sound like the FM tuner in the receiver when
playing the same station. IT had zero effect on anything happening in REW. All the effects were below 200 Hz.
This did not effect imaging at all. The system still sounds much better with the equalization turned on.

I hate. I hate computers that do things that you don't tell them to.

 

[dtvmcdonald]

I hate computers that do things that you don't tell them to.

But today I tried something new and it worked. I was looking at the various
gain settings and overload for the umpteenth time., using an oscilloscope.
It appeared that below 45 Hz was being rolled of by thecomputer when
not using REW. And, oddly ... that some sort of "soft clipping" was going on.
The latter per se would not effect any prior tests because they were some
25 dB down from the effect. To avoid overload I had set all the program sliders
in volume mixer to 20%.

So I took a deep breath and tried do a "factory reset"on any setting involving audio: Dell Audio,
Realtek, MAXX, plain control panel, and Microsoft effects. The soft clipping and the 20%
necessity went away, as did the low rolloff. I tried turning on and off "effects" in control panel,
as I did before, and now it had no effect (unlike before, even with all effects turned off).

There had to have been some effect that was invisible turned on.

The need to have the left channel about 3-4 dB louder to keep everything centered when
in mono remains. This is a room effect ... it varies a bit as different doors are opened and closed.

 

[dtvmcdonald]

I've continuerd to tweek. I discovered a bad buzz in my right main speaker at 75 Hz. I got these
speakers cheap back when new because it had been hit with a fork lift. At that time there was
no audible problem. But apparently over 4 decades of sitting one developed. I drilled a tiny hole
through the veneer and injected Elmer's furniture glue. Then I put several fat but short wood
screws in to hold it together. This fixed the buzz.
I've also made minor changes to a couple of my large TV's positions, moved the left speaker
three inches farther from the walls and moved a couple of large cushions a bit. I redid the
equalization with a final pass with my ears with emphasis on keeping mono sound centered.
The surround speakers remain unchanged.

The measured response is attached. This is with a calibrated frequency response cardioid mike.
The main speakers have 20dB added. It sounds real good, the bass problems are gone.

 

[dtvmcdonald]

Here is the current room response, averaged (RMS) over both speakers, three measurements for each, one 8 inches in front of my
face in the usual listening position, and one a foot from the center of my head, right and left, 6 measurements total.
Note the scale. 1/12 octave smoothing.