How to correctly use REW for home theater room measurements?

[obstler42]

hi,

I recently converted a bare basement room for use as a home theater (quick and dirty as the whole basement will be renovated in 1-2 years time). All the a/v equipment is from previous use in my living room, so that's all the same. the only thing I can now do in the new room is acoustic treatments (i'll leave the details for the acoustics forum once I know how to correctly use REW).

so from simply walking around the room with the generator running a sine wave at specific frequencies I can hear that the room has some serious acoustic problems (almost square, all concrete walls/floor/ceiling. rugs on floor, curtains on side walls). for example at 80 hz there are positions where it's completely inaudible, a step away it's loud again etc. I really don't know a lot about acoustics, but from what i've read so far this seems to be the result of standing waves.

Now I want to make measurements with REW to figure out what needs to be done with the room to make it acoustically better, and I'm not quite sure what the best way to measure is for that purpose.

I have my htpc connected to the receiver, and the standard analog SPL to my line in and can make measurements just fine. However last night I did all of them in "All channel stereo mode" on my receiver, thinking that since the room is for movie 5.1 use only all the speakers need to be on at the same time (that mode routes a stereo signal to all 5 speakers, not the sub). However reading the posts here seems to tell me to turn off any AV receiver processing -- using the analog multichannel input pretty much does that. but then I can only connect 1 or 2 speakers at the same time.

So should I run with just one speaker active? Or split the signal to all 6?

Regarding the measurement position: since even small changes in position make a huge difference where should I measure? At the main listening position and then x cm away in each of the four directions?

Sorry for the long first post, but I'm trying to get it done correctly ;)

[salvasol]

First of all ... Welcome

obstler42 wrote:

...I can hear that the room has some serious acoustic problems (almost square, all concrete walls/floor/ceiling. rugs on floor, curtains on side walls)...

I've read on other threads that square rooms are one of the worse room for a good frequency response ...and then the concrete surface you have won't help either.

My suggestion is to change the room dimensions and to make walls using studs and drywall, and fill all cavities with insulation

...last night I did all of them in "All channel stereo mode" on my receiver ...

Most of the time you have to take a measurement just for the sub, then add the main speakers to see how they interact with the sub; and to have a better picture you can test each speaker individually and like you did all speakers together ...

Will see what the experts say ...

Do you have everything in place (speakers, TV/screen, projector, etc.)

ANd don't forget to introduce yourself in the Members Area

[obstler42]

ok, so I did single speaker measurements, with just one channel connected to the receiver's analog multichannel input, and receiver set to direct mode. here's the sub:



Unfortunately this looks like a mountain relief instead of a nice smooth curve...

Some of the dips and peaks correlate pretty good to those frequencies where it makes a lot of difference where exactly you listen or place the mic, so I just did it in place where my head in the main listening position is.

Does this look like a valid measurement as basis for discussion about the room in the acoustics section?

[Wayne A. Pflughaupt]

obstler42 wrote:

Does this look like a valid measurement as basis for discussion about the room in the acoustics section?

Yes.

Regards
Wayne

[obstler42]

ok, good to know I'm not messing it up completely

now, regarding room measurement: what is the best procedure to measure the room? small changes in the recording position make a big difference in the result (probably be cause the room is very unoptimized so far).

[salvasol]

obstler42 wrote:

... what is the best procedure to measure the room? small changes in the recording position make a big difference in the result (probably be cause the room is very unoptimized so far).

Most of us optimize everything to our sweet spot ... and let the other seats suffer

Or you can take different measurements and average all responses

I have two rows of seats, and my receiver is capable of saving six memories ... so What I did is to save one for my sweet spot (middle seat on back row) and another for the front middle seat

[obstler42]

ok, makes sense.. can't optimize for the whole room anyway, and one tends to sit at the same position... but I guess trying to figure out if a seating position would be better a little bit to the front/back/left/right is probably something left until the room has been acoustically treated?

[salvasol]

obstler42 wrote:

... but I guess trying to figure out if a seating position would be better a little bit to the front/back/left/right is probably something left until the room has been acoustically treated?

Not necessarily ... if you'll treat your room in a couple of months; you can optimize your room without accoustic treatments, and then again after is accoustically treated

That's the fun part in this hobby ... tweek all equipment to get the best out of it