How to diagnose the need for room treatment?

SPL meter readings could be a LOT worse - a few observations.....

No kidding. The bass response is superb - almost to the point of being suspicious - hardly any room is ever that good without being treated.The broad dip in the midrange is probably just "midrange scoop" - most speakers are weak off axis in the upper midrange and your ears and SPL meter will sense this.

Since you have positioned your speakers in the middle of the room (15 feet out) they will not excite the room length mode nor its harmonics - as the speakers sit at a pressure null. I think this is the principal reason you are getting such good bass - as usual "acoustic physics" dictates how it will sound...

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

most speakers display the sharp roll off in the highs above 10K. I think mostly because you are comparing your 'in room specs' with anechoic measurements made by the manufacturer either 1 or 2 meters from the speakers.

I wonder if treble beaming could be a factor as well, assuming the tweeters are pointed straight ahead. Should be easy enough to check, by toeing the speakers in a little, and re-measuring.

I had looked at the specs for the speakers, and they specify a horizontal dispersion covering a 60 degree arc (meaning +/- 30 degrees from straight ahead). Which would make toe-in especially important were they to be moved further apart.

It's unfortunate, though, that it's impractical to move them further apart; I agree with Newbee that it would be likely to improve imaging considerably.

Regards,
-- Al

I wonder if treble beaming could be a factor as well, assuming the tweeters are pointed straight ahead. Should be easy enough to check, by toeing the speakers in a little, and re-measuring.

Only very high up (at 10 Khz and above), however,the 804S is a typical B&W with the "midrange scoop" - they run the excessively large midrange up to 4 Khz and the tweeter has no waveguide. This means you get beaming on the midrange and which causes the mids to lose energy between 1khz and 4Khz when in a room. Above 4Khz the energy returns as the tweeter takes over with its typical wide dispersion pattern in the absence of a waveguide.The power response would show a "scoop" in the midrange and a stereophile horizontal dispersion plot would show this hole at 60 degrees off axis. Perceptively this sounds laid back in the mids but with plenty of presence detail - it creates a very nice sound with emphasis on articulation that B&W is known for.

The only B&W's I know that do not exhibit this "midrange scoop" are the B&W silver signature, the B&W 805N (Nautilus and not the "S" version) and their top of the line 4 way Nautilus. It is actually not uncommon for most two ways with large 6 inch midranges and a tweeter crossed over high to sound this way.

The in room HF roll off is quite typical anyway - that would be completely normal for a room that size.

Here is a useful chart to explain how the speaker will sound. You can see why B&W would choose to voice it this particular way: bass string pluck, drum and guitar attack and vocal presence will all be increased- giving the impression of more detail compared to a flat power response.

Of course some people believe that measurements are like the zodiac and reading the position of planets ....i.e. "completely meaningless" ....just don't tell that to the engineers mixing/mastering your music. They use many rules of thumb based on measurements - a good engineer will know exactly what to do to create a desired effect and so will a speaker designer....