Standing waves

Well, on the one hand, standing waves cause the largest measurable deviation from an ideal response in sound quality. On the other hand, problems get worse as you go down in frequency so as a result smaller speakers, with higher cut-offs tend to sound better.

If you want your listening room to be close to flat from 16 Hz to 30kHz they must be dealt with.

But this is all general theory. Better question is what is your specific problem and how to deal with it. :)

The first question needing to be answered is the dimensions of the room. Generally, the smaller the room, the worse the standing waves (eigenmodes).

They're about as detrimental as your ears think that they are...really.

Nevile Thiel (rip) gave me great advice once to counter standing wave problems. To use odd multiples rule.
Position your speakers so the bass driver for best standing wave countering is at 1/3rd of the room length or if this can’t be done 1/5th or 1/7th. Same goes for the width.
Unfortunately floor to ceiling is pot luck.
I have mine which can go down to 20hz -3db at 1/5th length and 1/7th width, and it worked a treat.

Cheers George

@g11657 - here's an example....

My previous listening room was 15 x 12 x 8(high)

My speakers are floor standers with a LARGE rear port

I also opted for the   1/3rd 1/5th or 1/7th rule, but at certain low frequencies the bass was very "boomy" and resonated with something in the room - probably the walls.

Some tracks had an extremely bloated bass, whilst most other tracks sounded OK

I've since moved to a new house and the listening room is much bigger 40 x 16 x 7.5 - so I now have approx 8ft behind my speakers (1/5th)

That bloated bass is no longer a problem and the music is sounding extremely detailed and rich

Low frequency standing waves of this nature are extremely difficult to deal with - I had placed baffles inside the port, which made the effect less noticeable, but it was still there on a couple of albums.

You might find, like I did, a partial solution, but a complete solution can cost a lot and detract from the room aesthetics

You migt find a front vented port is better, but low frequency standing waves are difficult to remedy

Regards.... 

Standing waves are everywhere, well almost. Get a hold of an SPL meter and a test CD with test tones. You will find all sorts of standing waves, echoes, peaks and reflections that interfere with the pure unadulterated signal from the speakers. Everywhere you find a sound pressure level at least say, 4 or 5 dB above the average sound pressure in the room you got a problem. Don’t be surprised to find some peaks greater than 6 dB above average. Think of it like a lot of speakers playing simultaneously, with some playing 10 times louder than your speakers when playing at moderately loud levels. Every room is different and every speaker produces different radiation patterns.

Geoff is right. Have you ever set up your speakers outside? They sound excellent, but just have no volume, or bass. That is what the room provides. It's way more than most people think.

Conversely, I HATE music played and heard outside. My drumset sounds thin, flat, and dead, with no resonance, no low frequency fullness, no sustain. Music reproduction is a different matter, added room resonance, fullness, and sustain (all boom) very undesirable.