Preparing for REW+BFD

**Trekari** · **01-23-08, 02:44 PM**

In seeking out the best place for my subwoofer (i.e. the one that will require the least amount of EQ with the BFD), I was planning on putting my sub onto my couch and taking REW measurements at potential locations for the subwoofer.

However, where should I be placing the sound meter while doing this? I have a 12" downfiring subwoofer, so should the sound meter be placed on the floor, or 1" off the floor where the cone of the subwoofer normally rests?

Within the next few days, this thread will be turned into my REW measurements thread, once my BFD arrives

-Jason

**Wayne A. Pflughaupt** · **01-23-08, 05:03 PM**

Getting it close to where the driver “fires” into the room makes sense to me, but I’m sure as long as the mic is within a foot or so of your proposed location you’ll be fine.

Regards,
Wayne

**Trekari** · **01-23-08, 05:29 PM**

Thanks for the reply Wayne. Keep your eyes peeled for what hopefully turns out to be decent pre-EQ waterfalls

Also, is it possible to place the BFD directly on a wooden shelf as I asked in this thread here:

http://www.hometheatershack.com/foru...html#post74968

My concern stems from the fact this is a rackmount device usually, but I've *never* used such equipment before and do not know if the chassis of the product is expected to be in a rackmount setup, or whether or not I would be posing any sort of a grounding/fire/safety hazard in placing it directly on the wooden shelf.

-Jason

**Wayne A. Pflughaupt** · **01-23-08, 05:43 PM**

The rack-mounting feature basically means the unit is designed so that it can be fully supported by its faceplate. But it doesn’t have to be used that way. You can locate the BFD anywhere that suits your needs. You might want to add some stick-on rubber feet to the underside, since it doesn’t have any, to avoid scuffing your wood shelf.

http://www.radioshack.com/product/in...entPage=search

Regards,
Wayne

**OvalNut** · **01-23-08, 06:40 PM**

Quote:

You might want to add some stick-on rubber feet to the underside, since it doesn’t have any, to avoid scuffing your wood shelf.

D'oh! Boy, do I feel like Homer right now. That is a great simple idea Wayne. Thanks!

Tim

**Trekari** · **01-25-08, 08:18 PM**

Here are my REW graphs for 4 sub positions.

I was unable to get the input volume higher than -22dB during actual measurements. The measured -3dB according to JBL of my sub is 25Hz, but my graphs are way worse than that?

How 'bad' do these actually appear? Subwoofer only was measured, the mains were completely disconnected.

**brucek** · **01-26-08, 12:13 PM**

Quote:

I was unable to get the input volume higher than -22dB during actual measurements

Use a more sensitive scale on the meter (as long as it doesn't clip during the measure - don't get so sensitive as to peg the needle).
Or you can measure at 80dB to increase the input level.

Quote:

The measured -3dB according to JBL of my sub is 25Hz, but my graphs are way worse than that?

Do a near field measurement with the sub in the middle of the room to check the actual speakers response with minimal room influence.

Quote:

How 'bad' do these actually appear?

Are they filtered responses or are they raw?
BTW, if you want your filters to be sequencial, there is a "sort" button on the filter pop-up screen.
Overall the response looks great for a sub with a cutiff around 35Hz.
It's nice to see at least one raw measure to evaluate better.

brucek

**Trekari** · **01-26-08, 01:19 PM**

Those were raw feeds directly into the AUX connection of my Onkyo TXSR705. I had disabled Audyssey entirely before running these tests.

Hopefully later today once my neighbors are gone I can run some more measurements, specifically the near-field one you asked for. (I live in a condo and as I'm sure you know, REW measuring is rather loud

)

I'm rather confused about two things however.

1) Why is it my internal measurements/calibrations are just fine as far as signal levels, yet when I go to make an actual measurement, the levels are way low, despite not changing *anything* between the internal checks and measuring.

(I understand I can change the sensitivity of the sound meter, but it was already set to 80, which I thought was appropriate?)

2) The cabling I have is mini-stereo line out to dual RCA, converted to single RCA at the receiver end and input via the Right AUX channel. From the sound meter I have a Right-only RCA connected, going to the Right-only AUX2 connection on my X-Fi Platinum (which in this case is using an RCA line-level input on the front panel). Is this correct, or should the sound meter output be converted into a dual-mono signal?

**brucek** · **01-26-08, 01:44 PM**

Quote:

1) Why is it my internal measurements/calibrations are just fine as far as signal levels, yet when I go to make an actual measurement, the levels are way low, despite not changing *anything* between the internal checks and measuring.

That's because the Check Levels routine in the Measurement screen and the Check Levels routine in the Settings screen use different values to generate the pink noise for level setting.

The Check Levels routine in the the Settings screen uses the fact that you have the Check/Set levels with Subwoofer pull-down selected to generate its test noise. It uses a pink noise low cut of 30Hz and a high cut of 80Hz. This is the energy band that you use to set the Check Levels in the settings screen. See the first attached pic below.

But then to double check and verify that the end frequency of the sweep that you have selected in the Measurement panel matches that subwoofer setting, its Check Level routine uses the end frequency as the test pink noise hi-limit cutoff. So if you had 20KHz as the end frequency to measure for your sweep, you can see how the pink noise would be different (between the two Check Levels) and as such you would experience quite a different level. See the second attached pic where 200Hz is the end frequency and the hi-cut of the pink noise is 200Hz. In this case there will be a small difference in the check level result.

Set the End Frequency in the Measurement panel to 200Hz.................. it should be fine.....

check_levels_settings.jpg

check_levels_measure.jpg

Quote:

I understand I can change the sensitivity of the sound meter, but it was already set to 80, which I thought was appropriate?

It is. I thought you might be using a higher one...

Quote:

2) The cabling I have is mini-stereo line out to dual RCA, converted to single RCA at the receiver end and input via the Right AUX channel. From the sound meter I have a Right-only RCA connected, going to the Right-only AUX2 connection on my X-Fi Platinum (which in this case is using an RCA line-level input on the front panel). Is this correct, or should the sound meter output be converted into a dual-mono signal?

That all seems fine. The jacks on the soundcard are stereo, and we need to break them out into the two left and right channels as you've done.
Then you use one of those broken out channels for the line-in from the mic and the line-out to the receiver. I use a Y-splitter at the receiver to feed left and right AUX in so I can have both mains playing when the time comes - but whatever....

brucek

**Trekari** · **01-26-08, 05:31 PM**

Ok, here is a near-field measurement from the middle of my living room. Below the first graph (near-field) is a measurement from the left of the fireplace next to the dart throw-line. The purple dotted line is what I'd like to correct it to if that will not damage the subwoofer.

Below the purple graph is the waterfall from the current default position near my television. I appear to have the dreaded 'hum' problem? However it's only noticeable at high volumes so...hrm. **EDIT** Nevermind about the hum...it was my coax (television) cable being routed into the surge protector that was causing it.

**brucek** · **01-26-08, 06:27 PM**

Quote:

The purple dotted line is what I'd like to correct it to if that will not damage the subwoofer.

You cannot add +10dB at 28Hz and expect good results. That much gain isn't a good idea. An EQ is better used to remove peaks. If your sub begins to drop off at 35Hz, then that's what it does. Throwing a bunch of gain at it won't solve the problem I'm afraid.

brucek

**Trekari** · **01-26-08, 11:40 PM**

I guess I should hold off on purchasing a duplicate PB12 to get better sub response

. The product literature doesn't specify a -3dB frequency, it simply states the frequency response is 25Hz-crossover setting.

After all this excitement with BFDs and REWs and HCFRs and $$$$ I come to find out my subwoofer, which I used to think was fairly good, is just crap at frequency response :'(

At least I have Comcast coming out tomorrow to fix their grounding problem for free

-Jason

**JohnM** · **01-27-08, 04:08 AM**

Where is the sub placed? Sub response specs are often "in-room" assuming some room gain, putting the sub by a wall or in a corner will extend the LF output.

**brucek** · **01-27-08, 07:28 AM**

Quote:

I guess I should hold off on purchasing a duplicate PB12 to get better sub response

Adding a like or similar sub only adds headroom, not response extension...........

**Trekari** · **01-27-08, 01:09 PM**

Wouldn't it add volume at the lower frequencies? Basically I would've been able to make more filters (cuts) on the range that is already reproduced appropriately, but the second sub would've added a few dB to the lower frequency range that the single sub currently drops off at.

Anyway...I'm impulsive and am looking into a much improved sub anyway

**brucek** · **01-27-08, 01:20 PM**

If you had ten subs that dropped off at 35Hz, the response would still drop off at 35Hz......

Buy a sub that extends to 15Hz and you'll be happy...

brucek

**HClarkx** · **01-27-08, 03:17 PM**

Quote:

brucek wrote:

Adding a like or similar sub only adds headroom, not response extension...........

Bruce, I'm not sure I agree fully with this statement. If the response at the listening position is the same wherever you put the sub, then I agree. But, if the potential sub locations vary in the response they provide at the listening position, then there's an opportunity to have the two subs complement each other. I was able to do that. The result wasn't magical, but I did pretty much eliminate one peak and one dip and pick up a few db below 30 Hz from the combination. And, as you or others have mentioned, I got some headroom that allowed the low end to be EQ'd with less risk of exceeding Xmax (using Audyssey in my case).

It wasn't clear to me how Trekari put his sub in the listening position. I would think it would need to be upside down (it's down firing) and sitting on something to put the sound power out about where the head is positioned. Was that the case? Sitting lower, the sofa/chair would soak up some SPL and change the pattern.

Harrison

**Trekari** · **01-27-08, 07:31 PM**

Actually I resorted to simply moving the subwoofer with the sound meter on a tripod at the couch.

However, having my delusions destroyed by seeing a frequency response graph of my subwoofer has made me decide I needed a better one anyways.

http://www.av123.com/products_produc...s&product=15.1

Calling tomorrow to see about ordering one of those.

Other than that, my horrible hum has no solution. The cable company was here today and checked the outside ground (with me present) and even took a file and cleaned up all the connections from the ground block down the copper cable and into the earth stake.

My hum still exists without cheater plugs if the coax cable is plugged in. It makes no difference if I have the coax going into the surge supressor first and then to the tv or not.

I was thinking that one of these http://www.cs1.net/cables/products/j...rs/VRD-1FF.htm put on the main coax line into my condo (right before the cable splitter sends the signal to all the rooms), would solve the problem in all rooms?

Any other advice to offer since the hum ONLY exists when coax is connected?

**brucek** · **01-27-08, 07:55 PM**

Quote:

I was thinking that one of these

Give it a try and tell us how it works...............

**Trekari** · **01-27-08, 08:05 PM**

I was more hoping for some sort of confirmation from those who know more than I do that the problem is 'definitely' a ground loop in my coax cable system before I spend $50.

**HClarkx** · **01-27-08, 09:11 PM**

Quote:

Trekari wrote:

I was more hoping for some sort of confirmation from those who know more than I do that the problem is 'definitely' a ground loop in my coax cable system before I spend $50.

Trekari, this is probably not the correct thread for this subject, but I'm not adept at linking to other threads and such so maybe the staff can move this post to where it should be. Sorry.

I'm assuming that your electrical panel stake (or plumbing ground) and your CATV ground stake are some distance apart. That means that any stray earth currents (e.g., from nearby power lines) is causing a potential difference between the two grounds. The result is current from your electrical panel out the circuit to your HT equipment, through audio interconnect shields, and then through the TV coax shield to the CATV ground stake.

If this is the case, most of the usual treatments such as putting all HT equipment on one dedicated circuit and such aren't going to help. One that might is linking all of the cases in your HT with an external ground wire might help. That will route the stray currents away from the interconnect shields. If that helps but does not fully solve the problem, then you at least know you are on the right track.

Interconnects that have the shield lifted at one end and the ground wire twisted with the "hot" wire can help. But, there will still be stray currents through your system.

I can only think of two ways to avoid those currents. One is to isolate the CATV coax at dc and low frequencies (which the device you are looking at presumably does). Another is to bring the potential of the CATV ground stake and your electrical panel to the same potential by running a wire between them. That should help but may not be a full solution because it will have impedance and won't bypass all of the current between the two ground points (some will still flow through your HT equipment).

Long story short, the CATV isolator does seem to have the best shot at solving the problem. I'd look for one for a couple bucks at Home Depot or Ace Hardware. If you have plenty of CATV signal strength, a cheap lossy one should work fine.

Good Luck,

Harrison

**Wayne A. Pflughaupt** · **01-28-08, 12:53 PM**

Quote:

Trekari wrote:

I guess I should hold off on purchasing a duplicate PB12 to get better sub response

I suggest a consultation call with SVS. Something’s drastically wrong if your response is falling like a brick below 40 Hz with that sub. It got below 20 Hz in my cavernous room with no problem.

Regards,
Wayne

**HClarkx** · **01-28-08, 04:42 PM**

Quote:

Wayne A. Pflughaupt wrote:

I suggest a consultation call with SVS. Something’s drastically wrong if your response is falling like a brick below 40 Hz with that sub. It got below 20 Hz in my cavernous room with no problem.

I was thinking the same thing as Wayne. Before you invest in another sub, have you looked for room problems that could explain the absent low end? Is it missing all throughout the room? Is the room so large that you don't have any room gain to help out? (that would be a very large room). Are there any "cavities" or "traps" such as an opening to an adjacent room that would reflect back an out-of-phase wave at the problem frequency? Usually such reflections would create a narrower notch, but there are exceptions.

The test at the center of the room is a start, but isn't conclusive. You will have reflections from all four walls (depending on where you put the mic) that will still affect the apparent response.

The call to the Mfgr is an easy place to start, but unless the unit has a subsonic filter or a plugged port or something very wrong, it's hard to conceive of a problem that would

Advertisement