Speaker Placement or Room

Speakers are Vandersteen 3A's. I've got them set up according to the placement instructions that came with the manual. In general they sound really good.

I've just taken some readings with my Radio Shack spl meter and the Rives test disc, using the frequencies that are corrected for that device. I know no room is perfect and a flat response will not happen, but for the most part everything is "relatively flat". However, I'm getting a huge dip between 80hz & 100hz. 80hz is -23db, 100hz is -12 db and 125hz is -15db. 160hz is back to flat.

Is this an issue of speaker placement or is it the room? If it's the room, where do I begin to minimize these dips?

I meant to say, the dip is betwen 80hz & 125hz
Most likely you can correct much of this through placement. Sometimes there are rooms that do not support certain wavelengths and without analyzing the dimensions it would be impossible to know, but likely you are getting a cancelation due to placement.

Here is our resource page. On the right side you'll see an article on loudspeaker placement. Hopefully this will help you some.
While it may be obvious, what the hell!

Don't forget that where your listening seat is located can play a huge difference. Especially if you've got it sitting in a null. Play with both speakers and listening position. For example, with your test disc playing, stand at the present listening position and when a 100hz signal (or any signal) plays move the meter back and forward a foot or so and see what happens. You might be surprised. Take your time, make lots of notes and graphs as you go.

BTW, FWIW I find it easier to use a disc with pink noise such as those available thru Stereophile than the Rives disc with its 'constant tone'. I just factor in the corrections for the errors in the RS meter.

FWIW, The Cara program on Rives site is an excellent starting point, it hit my speaker locations perfectly, but missed my listening position by a foot or so.

While I did know to check my seating position, I completely forgot to. I'll have to play around some more later.

Why would pink noise be any different than the constant tone?

how do they sound ?
How does what sound?

BTW, I did some more tests, moving the db meter in front of & behind my seating position. By moving the meter less than 18 inches forward, the 80hz response increased by 16db! Thanks for reminding me to check that Newbee.
Ecruz, Glad it helped. FWIW, and perhaps its just me, but I find the pink noise in the Stereophile disc to be more constant and easier to track. The tone on the Rives disc did not seem as constant and it also seemed more beamy, especially in the higher frequencies. But, then, maybe I've just got unsteady hands and one needs to hold the SPL meter steady, like on a tripod (actually I did just that) when using the Rives disc. Anyway, what ever works best for you. The're cheap anyway, waste a acouple of bucks and see for yourself which you prefer. :-)
Oh, the other thing I liked about the Stereophile disc was the assending order of the frequencies. Makes a bit more sense to my poor brain, and I also like that they have a 1k hz reference tone at the beginning so you don't have to mush arouand in the disc to find one.
There's no set rule on where to position your speakers. With the Vandys I'd stay close to what is recommended but you can angle them or whatever. EVERY room is different so you just do what you can.

Room treatments can help alot. At least two bass traps could help your room significantly. You can make them yourself for 50 bucks a piece. Email me if you want to know how. YOu also try the Acoustics Circle at audiocircle.com.

Another option is the Behringer DEQ 2496 digital equalizer. You can use this purely in the digital domain. It's only 300 bucks with a microphone, the ECM 8000, if you google around or buy used. The learning curve is very steep. The potential for a very large improvement in sound quality is likely.

Some people actually like the onboard dac in the Behringer but you can use an outboard dac like most of us so it's completely transparent. This thing is no toy and has way more features than you'll ever use. It even has an AUTOEQ mode where you can put in a room curve and it will do it for you!

Another nice feature is the spectrum analyzer. The Vandys are a pretty good speaker but any speaker needs good acoustics to perform as it was designed. The room just changes everything so much and treatments are such a bargain for the improvement in sound it's crazy to not do it.

Hope this helps you, Cheers