Help analysing REW measurements

[aps]

Folks

My introduction to REW occurred a couple of years ago. Now, however, I’ve had to move my 2-channel set-up into a smaller room and am looking for council on how to optimise bass performance in this room. The back-ground is as follows:

• Room is 12ft x 16ft x 10ft (W x L x H) with speaker firing down the length of the room. My speakers comprise are 3-way active with low-pass to woofers, band-pass to mid and high-pass to high. The cross-over points are fixed – with x/o point to woofers set at 80Hz at 3rd order – but it’s possible to change the relative gain via volume-pots on the cross-over.
  
• Setting of the gain on the woofers was, initially, done by ear but then validated the setting by checking the woofers and mid-range were balanced at the 80Hz cross-over point. This approach involved measuring SPL at the speakers with just mid-range active and, then, with just the woofers active – with the gain adjusted until the second reading matched the first.

This set-up has, now, been in place for a couple of months with incremental tweaking of speaker locations and, then, addition of some large foam bass traps in the front left / right corners. A number of REW sweeps measurements have been taken with the main points as follows:

• Posted measurement of the listening position shows a single large peak at 35Hz and, then, a few dips at 40Hz, 140, 175, 200 and 300Hz which, I guess, all correspond to room modes. That said, on average the result seems quite smooth with smoothed readings being +/- 3 dB from the target.
  
• No comprehensive view on the frequency response at other room locations, however, a simple measurement of SPL for 20-250Hz pink noise confirms that the SPL in the listening position is 2 to 4dB less than that at the front / back wall and the corners.

My initial interpretation is that equalisation isn’t going to be too much value – in that its usefulness is limited to addressing the 3.5dB peak at 35Hz. I’m guessing, then, that the real issue is addressing the dips seen in the 100 to 300Hz range via the use of additional bass traps. Is this the correct interpretation?

Regards
APS

[brucek]

Posted measurement of the listening position shows a single large peak at 35Hz and, then, a few dips at 40Hz

These are the main problem for sure. I would start by moving the sub around until this was corrected to the point where EQ can smooth the result.
The overall trend (which could be seen by adding smoothing) appears fairly good from 100-300Hz with typical comb filtering from reflections. Treatment can help this area as you said. For specifics on treatment, you're best to go to our acoustics section where they're better equipped to answer those kinds of questions.

brucek

[chadcummings]

What do you have as far as room treatments currently?

Can you do a quick sketch to show your speakers, seating and room treatments.

[MikeD]

I don't think OP has a sub. I think he just has large mains, which he could still move around to get the best response.

[Wayne A. Pflughaupt]

MikeD wrote:

I don't think OP has a sub. I think he just has large mains, which he could still move around to get the best response.

The problem with main speakers is that you also want the position with the best imaging. That can limit your options for best-response positioning (or vice-versa).

aps wrote:

My initial interpretation is that equalisation isn’t going to be too much value – in that its usefulness is limited to addressing the 3.5dB peak at 35Hz.

EQ could also address the 40 Hz depression. You just have to keep in mind that EQ taxes headroom, both of the amplifier and the driver. This may be more of an issue in your situation than it would for say, large-driver subwoofer with a high-powered amplifier.

Regards,
Wayne

[aps]

Wayne A. Pflughaupt wrote:

The problem with main speakers is that you also want the position with the best imaging. That can limit your options for best-response positioning (or vice-versa).

Thanks for the responses. The sketch below shows the room and speaker layout. Set-up doesn’t involve subwoofers but a 3-way active speaker in which woofers are driven via their own amplifier. The speaker location has been set-up to optimise imaging so there is little room for movement. Some basic foam bass traps are located in the corners and some 2" acoustic panels at the sidewall first reflection points.

Wayne A. Pflughaupt wrote:

EQ could also address the 40 Hz depression. You just have to keep in mind that EQ taxes headroom, both of the amplifier and the driver. This may be more of an issue in your situation than it would for say, large-driver subwoofer with a high-powered amplifier.

Interesting. The amplifier driving the woofers is a large PA amplifier (600W RMS mosfet amplifier) while the woofers are just 10" drivers (4 per channel). How does one equalisation for a dip within REW? (My set-up involves using REW to create filters which are then exported as a Filter Impulse Response and used with Convolver / Windows Media Center.

Regards
APS

[brucek]

How does one equalisation for a dip within REW?

Once REW has recommended its filters you simply enter your own filters manually until the response is flat. Pull down the EQ button and enter. The filters REW recommends aren't struck in gold - you may modify or enter as many as you like on your own while watching the corrected response curve until it suits you. Then you download the filters.

Be advised that a gain filter may do absolutely nothing to the response of a dip. You'll just be using power for no return. It has to be tested to check its effect. If the dip is a direct result of a reflection from a signal 180 degrees out of phase, you'll be increasing the direct signal that the mic receives, while at the same time increasing the level of the opposing 180 degree signal. The result is that the dip remains.

brucek

[aps]

brucek wrote:

Once REW has recommended its filters you simply enter your own filters manually until the response is flat. Pull down the EQ button and enter. The filters REW recommends aren't struck in gold - you may modify or enter as many as you like on your own while watching the corrected response curve until it suits you. Then you download the filters.

Be advised that a gain filter may do absolutely nothing to the response of a dip. You'll just be using power for no return. It has to be tested to check its effect. If the dip is a direct result of a reflection from a signal 180 degrees out of phase, you'll be increasing the direct signal that the mic receives, while at the same time increasing the level of the opposing 180 degree signal. The result is that the dip remains.

brucek

Bruce

Thanks. I'll give this an attempt Is it possible to do a measurements with REW with the filters in place to check the actual response? The reason for the question is that standard set-up uses Convolver / WMP to playback with filters rather than a BFD.

Regards
APS

[brucek]

Is it possible to do a measurements with REW with the filters in place to check the actual response? The reason for the question is that standard set-up uses Convolver / WMP to playback with filters rather than a BFD.

REW's output sweep is not altered by the filters. You can't apply them to the sweep - no. You can download them to a BFD or export them for use in a convolver setup.

brucek

[Wayne A. Pflughaupt]

aps wrote:

The amplifier driving the woofers is a large PA amplifier (600W RMS mosfet amplifier) while the woofers are just 10" drivers (4 per channel).

Given the amount of drivers and amplifier power, and the size of your room, it appears you should have no problem in the headroom department.

How does one equalisation for a dip within REW?

Typically a dip that can’t be equalized is narrow and deep. The one you have at 40 Hz – I dunno, tough call, could go either way. Try a 1/6-octave filter for starters and see what happens.

By the way, when modeling your filters in REW, it might be better to use some other equalizer option, rather than the BFD. For a given filter bandwidth (say, 1/3-octave for instance) the BFD’s filters tend to be wider than other equalizers.

Thanks. I'll give this an attempt Is it possible to do a measurements with REW with the filters in place to check the actual response?

REW’s “filters” only exist on a computer screen. It is not an equalization device. It only predicts what equalization will accomplish.

Regards,
Wayne