How to diagnose the need for room treatment?

The human hearing mechanism tends to some degree to "latch on" to the leading edges of transient waveforms, and give them greater emphasis than what may follow a few milliseconds later.

Re my earlier comments on this issue, in a post in a different current thread Shadorne referenced a Wikipedia writeup on the "Haas Effect," which is what I was referring to without knowing its name:

http://en.wikipedia.org/wiki/Haas_effect

A brief excerpt from that article:

When two identical sounds (i.e., identical sound waves of the same perceived intensity) originate from two sources at different distances from the listener, the sound created at the closest location is heard (arrives) first. To the listener, this creates the impression that the sound comes from that location alone due to a phenomenon that might be described as "involuntary sensory inhibition" in that one's perception of later arrivals is suppressed.

The Haas effect occurs when arrival times of the sounds differ by up to 30–40 ms. As the arrival time (in respect to the listener) of the two audio sources increasingly differ beyond 40 ms, the sounds will begin to be heard as distinct.

Thanks, Shadorne!

Regards,
-- Al

Good point about the Haas effect. One should also note that there is a lower limit to time interval needed for distinguishing direct from reflected/delayed signals. If the arrival interval is on the order of 5msec +/-, the signals are not distinguished and the perceived source location is intermediate between the original source and the phantom source related to the reflection.

Kal

Good info, Kal, thanks!

To put all this in perspective, we should keep in mind that as a rough approximation sound propagates through air at around 1 foot per millisecond, varying somewhat with temperature, altitude, and other factors.

Regards,
-- Al

Thank you all again for the useful input.

I got my Radio Shack SPL meter (analog), a Stereophile Test CD 2, and got the RS SPL correction curves from Rives. All set! Today I had fun measuring and would very much appreciate your help in making sense out of the readings. For the record, I followed Rives' instructions and set up the meter in a tripod exactly where I sit, mic facing ahead and right in between the speakers, and adjusted the volume so that a 1kHz signal would be read at 0 dB. BTW speakers are 6.6 feet apart and I sit at 11.6 ft from either one of them. Both speakers are 5 feet away from the side walls and about 15 feet away from the back wall (yes, 15 ft; not a typo).

I can't find a way to post a graph here, so I'm posting a table of the measurements after the correction had been made (sorry it's so painful to read):
Hz -------- dB
<20.........no test tracks available
20..........-6.5
25..........-1.0
31.5........ 5.0
40.......... 2.5
50..........-2.5
63.......... 2.0
80.......... 3.5
100.........-0.8
125.........-2.5
160.........-1.5
200.........-2.7
250.........-4.5
315.........-3.0
400.........-3.5
500.........-2.0
630.........-1.0
800......... 1.0
1000........-0.5
1250........-1.0
1600........-1.5
2000........-4.5
2500........-5.5
3150........-6.5
4000........-5.0
5000........-0.5
6300........ 1.0
8000........-2.0
10000.......-9.0
12500.......-17.5
16000.......-20.0
20000.......-21.0

First thing that caught my attention is the huge drop above 10kHz. Maybe the system can't deliver? BTW system is made of B&W 804s + McIntosh MC275 + Rotel pre + Rotel CD player.

The low point at 20 Hz isn't surprising, as the 804s can't deliver that kind of bass. I'm looking at getting a sub, but that's a different story.

The 804s have their crossover frequencies at 350 Hz and 4 kHz, so I guess the troughs near those points are not to worry much about...I guess. However the one on the higher frequencies starts around 2 kHz and extends all the way to 4 kHz, so I'm second guessing if I should worry about it or not.

It seems to me I should focus on the peaks at 31.5 and 80 Hz. Should I worry about what's going on at 800 Hz or 6.3 kHz?

What am I missing? How should I move forward?

This is fun, even though now I’m officially a freak (my wife came by today when I was playing the warble tones and said ”yeap…you ARE an audio freak”. Oh well…it’s fun nonetheless!

Thank you!!
Horacio

Nice work, freak!

The peaks @ 31.5hz & 80hz are probably the biggest concerns I see. In room high frequency plots tend to drop off steeply as you see, so not as surprising(at highest frequencies) as you might expect. However your overall picture of bass all + and treble all - doesn't seem like it would sound very natural. Do you get the sense that your "bass is turned up, and treble down?" Speaker placement & seating changes could affect this greatly. Since you are already far out from back wall, I'm guessing that current placement is exciting some room nodes. Experiment in small increments(i.e. 6in).
Lastly, not a big problem, but by definition, shouldn't 1000hz = 0? Cheers,
Spencer