learn from my mistakes.

[Hakka]

Playing around with REW over the weekend I noticed mistake I've made in the past that messed up all my results.

I have my USB soundcard constantly connected to the back of my AVR, when I use REW I just plug the USB plug into the laptop. The signal runs through the bass management of the receiver. To test the mains and sub integration I have both the mains and the subs on, to test the LFE channel I was switching off the power amp for the mains, instead of connecting the soundcard output directly to the BFD(can't get behind my rack).

The thing I realised after about 3 hours of tweaking my curve is, the 80hz crossover is still in the signal chain, but the LFE channel rolls off at 120hz. The crossover in the AVR needs to be reset to 120hz to measure the LFE channel. When I did this and re measured everything above 80hz was now way up on the previous measurements, 110hz was equal level to 20hz and I have a mean house curve in place.

I did my setup again from scratch and the result is a HUGE improvement over what I had previously.

Another change I made was to use a 100hz crossover instead of 80hz, it gave a lot smoother response than the 80hz, localisation won't be a problem as I have one sub behind each main speaker.

While I was on a roll I thought I'd play around with positioning of the mains, I ended up bringing them forward about 10 inches and the imaging and soundstage depth improved immensely. I've actually been sitting down listening to 2ch music for hours on end since I reposioned the speakers and made a music preset for the BFD, something I never did previously.

All in all a quite productive sunday arvo.

Hakka.

[brucek]

I was switching off the power amp for the mains, instead of connecting the soundcard output directly to the BFD

Switching off the mains power amp is the correct way of doing it actually.

The LFE does indeed extend out to 120Hz in the Dolby spec, but my understanding is that the sound engineers roll that off themselves to integrate it more smoothly into the standard 80Hz LPF (low pass filter) crossover provided by all home processors.

The actual spec'd information when given to the sound engineer goes all the way to 120Hz and then is digitally cut off. This would be horrible to listen to if sent to a sub, so I guess it's their creativity to roll it off (and a smart move).

Since the 'small setting' redirected bass is normally rolled off from 80Hz (when that's the crossover you choose) at 24dB per octave, I understand that the LFE channel is given similar roll off characteristics.

At 24dB per octave, the amount of signal left at 120hZ is quite weak. Myself, I wouldn't be concerned about any differences in setup because of the LFE channel.

brucek

[Chrisbee]

Unfortunately not all rooms respond to speaker movement. I moved mine a foot in each direction today with no obvious change in my REW graphs. They ended up right back where they started.

[Hakka]

brucek wrote:

At 24dB per octave, the amount of signal left at 120hZ is quite weak. Myself, I wouldn't be concerned about any differences in setup because of the LFE channel.

brucek

I had a large hump around 100hz , there must be a bit of signal getting through at those frequencies because after I added a filter to that peak I could hear the difference when watching DVDs and it wasn't subtle. I suppose it could depend on what the source material is and how it was mixed.
I don't have my graphs with me at the moment to show the difference in the curves.

Chris, I have moved my speakers around before and used REW and I generally didn't see much difference in the response curve. I just positioned them by ear for the best imaging/sioundstage.

Hakka.

[Chrisbee]

Aha! I too have a 100Hz peak from my speakers. My attempts to shift the speakers around had no real effect on this peak. Each speaker was producing a different response in this area but the effects were additive. I finally managed to improve things just a little by equ-ing the subwoofer after making it "hotter" and increasing the slope of the housecurve.

May I add a few of my bloopers to your thread as a warning to others?

I should have worn ear defenders or ear plugs yesterday. I woke up this morning with a really fierce whistle in my left ear! I sit about 1 foot from the left speaker at my computer. First time I've ever bothered sweeping the speakers seriously. I usually concentrate on my subs. I must have done at least 50 fullrange sweeps above 80dB while sitting right next to my speaker. Never though about ear damage at the time. Daft really, because I usually wear my radio ear defenders without hesitation when there are high noise levels.

My latest BFD filter set must be reducing sub output by around 20dB. I did a sweep after running over to adjust a filter and had mistakenly left the BFD on another filter set. The resulting sweep nearly blew the IB into the garden! Woops!

I also did a loud sweep after quickly checking the results of my filtering efforts with organ music. Then forgot to turn the volume down to REW calibration level afterwards!

Not really a bad day for me! My wife calls me Mr Pastry after such events. (An old slapstick comic)

[Hakka]

Just came across some spectra labs plots by Ikka in another thread. The movie is Flight of the Phoenix LFE channel (only took me 15 minutes to work out what FOTP was). There's some high spl 100hz stuff in there.



In the same thread the plot for War of the worlds shows that theres not much over 70hz in the LFE, all the higher bass frequencies are in the full range channels.



It looks like the upper limit of the LFE is dependant on the mix, when I watched Flight of the Phoenix I do remember having to dive for the remote in the barrel roll scene because the bass was unbearable, looking at the plot the red 100hz stuff lines up with one of my 12 db peaks. This movie might be an exception though as I have not had to do that with any others, although I could hear a difference when switching presets while watching the AVS LFE demo the other night.

I would like to see more of these plots but can't find them online.

edit: just found some more, LOTR LFE only





Titan AE LFE only












Chris, I changed from an 80hz xover to 100hz and it really helped out with the integration between mains and subs, it smoothed out some peaks that were caused by the sum of the mains and sub. It might be worth trying some different xovers if your AVR will do that.

Those full range sweeps are pretty painful at the top end, I'm gonna get some earplugs for my next session.



Hakka.

[Chrisbee]

I have an active, dual mono crossover. No amount of crossover frequency adjustment has helped that 100Hz peak so far. Though it has just occurred to me that I could try compensating for the individual speaker response variations by adjusting the roll-off frequency independently in each channel. My wife insists I enjoy REW more than listening to the programme material! I can't think why.