How to diagnose the need for room treatment?

Guys,

Thank you very much. Really. I have a fair amount of reading ahead of me + getting acquainted with test CDs & SPL meters. I'm not looking for quick fixes, so getting a good understanding of how my room/system behave together and how to improve the sound sounds like fun.

I hope you don't mind me asking back again for further guidance in a while. Will check out the room accoustics forum too.

A lot of good info/advice. Thank you!
Horacio

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

Good stuff!

Looking at the room dimensions and its modes I found f1 for length is 34 Hz and f0 for width is 36 Hz. Close enough to 31.5 Hz?
Also f3 for length is 69 Hz and f4 is 86 Hz, plus f1 for width is 72 Hz and f0 for height is 78 Hz. Might be a good shot at the 63 & 80 Hz bump?

Further comments following yur comments:
- room is not overdamped. If anything, it's underdamped. Hard stone floors with no rugs (for now). Brick walls. No drapes (for now).
- Cannot spread the speakers further apart. Room circulaion happens at the side of each speaker and it would be unpractical to change that (unless I change the whole setup, that is).
- I swear, no treatments are in place other than a bookcase and a sofa on the side first reflection points.
- to me it doesn't sound dull...but maybe it's me being used to it.

Will try playing around with positioning again next weekend.
Keep the good ideas flowing!

Cheers,
Horacio

Nice.

Newbee: thanks for sharing your experience in your room. Your freq response does resemble indeed the shape of mine!BTW, how far is the listening position from the speakers?

Al: the speakers are indeed facing straight ahead. I gather beaming refers to high freq, right? Shadorne's [very interesting] explanation seems to suggest beaming in the mids. I have no clue what the dispersion of the mids driver is. Anyway, as Newbee said, at 11 ft away it would seem dispersion would be wide enough, right?

Bottom line, what I'm taking away is:
a) frequency response looks "good enough". No need to get into bass traps, resonators, diffusors, etc.
b) imaging is likely to be improved by moving speakers further apart (will try it for fun)
c) playing with speaker location might improve things some, particularly if they are exciting a room mode

Are these fair takeaways?

Thank you!!

Al, Shadorne,

Very informative answers. I wasn't aware of beaming.

Thanks all three for your input. If you don't mind checking back on Sunday, I will try to make time and play with all of the above and post back results. I trust your input will help me make sense of what I measure/hear.

Thanks again for all your valuable input.

Regards,
Horacio

Sandhya,

Thanks for bringing up this thread of mine again. A year and a half has gone by and I've done a fair amount of further reading and experimentation and things have improved, and I'm still working on improving things further. This might be the second most interesting aspect to being an oddiophile (ja!), following the enjoyment of music itself, of course.

I'm surprised you cant't find fiberglass panels in India. Where are you looking for them, may I ask? I say this because I'm in Argentina and I can find them at construction supply stores. Both the panel and the fiberglass wool are used as insulation for construction and industrial applications.

Horacio

Sandhya,

Thanks for the compliments. It's a big deal in this part of the World!

To me it has been worthwhile learning about this. The two books I have are Alton Everest's master Handbook of Acoustics and Floyd Toole's Sound Reproduction. Both very good and they complement each other nicely.
I've been reading a lot over the internet too. Examples: Real Traps website, as well as the other websites mentioned obove on this thread.

As far as your question about fiberglass thickness and how much and placement. It depends!!
Treating some of the first reflection points is important, and for that you can use 2" fibeglass panels of 50 kg/m3 placed against the wall.
For bass absorption you'll be better off with a fair amount of space between the panel and the wall, at least 4" space, and a 4"-thick panel.
Check out Audio Asylum for Jon Risch's panel recepies. same for bass tube traps.
For covering the fiberglass I use polyester batting (cheap insulator used for mild-cold weather jackets).

My way of experimenting has been listening and taking measurements with a Radio Shack SPL meter when changing speaker and listening positions, building 2 large bass tube traps, building 2 3"-thick panels for front wall first reflection points, and building two 4"-thick self-standing panels, and adding a thick wool carpet on the floor first reflection points. I'm now building a larger panel for a specific place where I suspect it will be very beneficial.

This takes time and effort, though, and is oftentimes frustrating. So be aware of it before going in. It pays off, though!!

Sorry for the long post.

Horacio