some room measurements.

[DrWho]

It's been a while since I've taken these measurements, but I'm just now getting around to posting them.

I recieved my Behringer mixer and 'calibrated' microphone in the mail

the other day. There was a deal at Musicians Friend where it was free

shipping for orders over $100 and the mixer was reduced to $40....which

meant I could throw in a 50 foot shure mic cable for free [] I was

anxious to try out a free RTA program I picked up a while back, but it

turns out the program doesn't work, so I decided to give Room EQ Wizard

a try (the only other free thing I was aware of).

It seems REQW isn't able to measure pink-noise and that it doesn't

filter based on frequency? I tried tricking the program a bit by

playing pink noise over the system while it thought it was measuring

its tone sweep. The program has a built in function to measure sine

waves at fixed intervals, but that's even further from the ideal than

the fast sweep []

So anyways, I took two measurements to show the difference of reversing

the polarity to all my subs. One of the things I found most interesting

is the change in output at the lowest frequencies (~20Hz). There

shouldn't be any acoustical interaction between the subs and mains

(that are set to small) in that region so I'm not sure why the output

is changing that low. Perhaps some nonlinearities with the amplifier???

Or the drivers? I did recone all the drivers being used as subs - would

this occur if I put a driver slightly off center?

The peak in the 40-50Hz region is mostly the result of the frequency

response of the JBL driver, but I do have room resonances at 40 and

50Hz which can be seen on the waterfalls. The dip at 70Hz is most

annoying. Looking at the waterfall plots, there isn't even any output

at 70Hz at t=0, so I don't think it's necessarily a room related

issue??? I have on sub in the front right of the room, one sub directly

to the left of the TV and the other sub in the front left of the room.

I pretty much have free reign over the location of the subs along the

front wall except the FR sub has gotta stay in its nook. So far this

has been the best sounding, but that dip in the 70Hz region is most

annoying. The peak at 40-50Hz isn't as bad as it seems though - even

the raw Chorus II's are tipped up in that region too. It'd be nice if I

could slide that peak into the 70Hz trough.

But all in all I'm quite satisfied with the output. I can get a clean

110dB and "flat" extension down to 24Hz. I'm building a new sub in a

subwoofer workshop so I hope to exceed this performance with a single

driver...then I'll have more stuff laying around to try and figure

something to do with...shucks []

I also just downloaded some free Praxis software which looks like I'll

be able to measure some pink noise, which I feel more comfortable with

(even though it's still not the best measurements to be making). I'll

have to dig into the program, because apparently there is a lot more

that can be done - I just gotta figure out what it is.

[DrWho]

so here's the room with normal polarity:

[DrWho]

And then here's inverted polarity:

[DrWho]

I just wanted to add that these measurements were taken with the mic positioned in the main listening position as low as I could get it - which is about 1 foot off the floor hovering just above the chair in which I sit (yes, I have a very low chair). 1 foot is a 1/4 wave situation at about 200Hz - but that also puts me 7 feet from the cieling which is a 1/4 wave reflection at about 40Hz. Niether really explains the seemingly periodic dips in the response.

It also looks like the "80Hz" crossover on the reciever is more like a 90Hz crossover based on the interference patterns with the inverted polarity.

[colterphoto1]

Sounds like you need a road trip to Indy soon buddy!

M

[rcarlton]

I just wanted to add that these measurements were taken with the mic positioned in the main listening position as low as I could get it - which is about 1 foot off the floor hovering just above the chair in which I sit (yes, I have a very low chair). 1 foot is a 1/4 wave situation at about 200Hz - but that also puts me 7 feet from the cieling which is a 1/4 wave reflection at about 40Hz. Niether really explains the seemingly periodic dips in the response.

It also looks like the "80Hz" crossover on the reciever is more like a 90Hz crossover based on the interference patterns with the inverted polarity.

Why are you measuring 1 foot from the floor? Do you listen laying on the floor? Doesn't it make sense to put the mike at ear level in your listening position? Try Home Theater Shack for more information about REW.

[mas]

Just lay the mic on the floor (assuming it is non-resonant)...and minimize the floor reflection introducing comb filtering. (Be aware that you will still have the other surface reflections to contend with - but they can be minimized a bit if you have some absorbant material handy as well. In order to do this, you might also try taking several measurements using absorbant material to obstruct different secondary reflective pathways above and to the sides of the mic to determine the impact of the various reflections on the measurement as you can effectively and selectively damp each one out.

[DrWho]

The floor IS resonant (gotta love apartments) and my mic stand can only extend down so far - the purpose of putting the mic "on the floor" (or as close as possible) is to reduce the effects of comb-filtering from early reflections.

I am mostly trying to get a rough idea of what the speakers are doing....not figure out what my ears are hearing. In fact, I already know what my ears hear []

One of these days I'll get around to measuring my speakers outside (probably next spring/summer when it's not crappy outside) - and then Room EQ Wizard will be just fine.

Btw, does anyone know if REQW filters the input signal coming in? I haven't tested yet, but I wonder if it's not just measuring the voltage of the input and assuming the tone it's playing is the same tone it's measuring. The behavior at 17Hz is exceptionally wierd considering the size of my room and the speakers being measured. Also, the basic concepts of super-position claim that EQ shouldn't be able to fix dips in the response - yet there are a lot of measurements illuding to people achieving just that...I wonder if the program just happens to be picking up the increase in distortion associated with those frequencies??? I really don't like the idea of using EQ to account for room-related issues.

[colterphoto1]

get your testing butt down here Who! I'm free after this weekend.

M

[jacksonbart]

Change your sheets to reduce early reflections

[DrWho]

lol Mike - if only I had the time!

I am booked solid until after thanksgiving, and then right after that it's final week. Give me some time to figure out what I'm doing too...I might even be able to sucker some people into letting me get the more expensive measuring software too - and then I'll definetly be all over the place []

[colterphoto1]

Christmas break buddy boy- and I don't take NO for an answer!

And bring a couple co-eds with ya, ok?

[DrWho]

lol, I'll be a snowplowing this winter - gotta make me some cash. But I'm sure I'll get bored so keep in touch.