Comments on my first measurements? - Page 7 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #61 of 73 Old 07-02-09, 03:23 PM
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Re: Comments on my first measurements?

Quote:
brucek wrote: View Post
Why is the 128K sweep so poor?
Definitely the $64,000 question! Could the sub have some sort of auto standby that is not turning it on in time for the faster sweep? Certainly very odd.
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post #62 of 73 Old 07-02-09, 03:29 PM
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Re: Comments on my first measurements?

My initial thought was that the subs servo-mechanism was acting up whenever a fast attack signal was applied.

Then I wondered if it was perhaps his computer that was a bit challenged and couldn't handle the start of the faster sweep.

It's the first time we've ever seen this though.

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post #63 of 73 Old 07-02-09, 08:01 PM
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Re: Comments on my first measurements?

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weverb wrote: View Post
Have you tried doing any nearfield plots of the driver also?
I wasn't positive about how to do a nearfield measurement, but I took a shot...
  • I couldn't get the sub outside, so I just left it where it was in the room.
  • I put the SPL meter mic about 3" from the center of the sub cone.
  • I ran Calibrate and set my pre-pro until I got a reading of about 75db. This led to my pre-pro volume being set at about 8.0 (max setting is 80.0). I took a reading using 128k and 256k sweep lengths. Just for the of it, I turned the volume control up to 15 and took the same two readings again. Here's what I got...

Green = volume 8.0, 128k
Red = volume 8.0, 256k
Gold = volume 15.0, 128k
Aqua = volume 15.0, 256k

FYI, the sub is set to the Always On setting.

Also, can someone confirm... I've been setting my SPL meter for C Weighting, Slow Response. Is this correct?

Thanks.
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post #64 of 73 Old 07-03-09, 07:11 AM
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Re: Comments on my first measurements?

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I've been setting my SPL meter for C Weighting, Slow Response. Is this correct?
Yes....
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post #65 of 73 Old 07-03-09, 08:42 AM
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Re: Comments on my first measurements?

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brucek wrote: View Post
Yes....
Thanks. Were my nearfield readings done correctly? Do they offer any insight?
post #66 of 73 Old 07-03-09, 09:03 AM
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Re: Comments on my first measurements?

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Were my nearfield readings done correctly? Do they offer any insight?
It looks like you did everything correctly, although I am a bit confused by the statement:I ran Calibrate and set my pre-pro until I got a reading of about 75db.

Generally, you first run the Check Levels routine (during which time the pre-pro volume is adjusted to get a reading of 75dBSPL on the RadioShack meter).

Then the Calibrate routine is run to match the REW SPL meter to the 75dB reading of the Radio Shack meter.

Anyway, there's no way to establish what's wrong without substituting one of the possibilities. You have to substitute either the measuring computer or the subwoofer.

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post #67 of 73 Old 07-03-09, 06:10 PM
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Re: Comments on my first measurements?

Quote:
brucek wrote: View Post
It looks like you did everything correctly, although I am a bit confused by the statement:I ran Calibrate and set my pre-pro until I got a reading of about 75db.

Generally, you first run the Check Levels routine (during which time the pre-pro volume is adjusted to get a reading of 75dBSPL on the RadioShack meter).
Yeah, I meant to say Check Levels not Calibrate.

I do have a second computer that I can give this a try with. Will try to do so over the weekend.

Thanks!
post #68 of 73 Old 07-04-09, 01:22 PM
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Re: Comments on my first measurements?

I gave this a try on another computer... 0 to 200Hz sweep using 128, 256k, 512k, and 1M sweep lengths (graph 1, seperated traces is turned on). Graph 2 shows 0 to 20,000Hz sweeps at 128k and 1M.

Computer 1 is my HTPC... 2.5ghz dual core, Gigabyte GA-E7AUM-DS2H motherboard using the onboard Realtek Digitial Input and Output.

Computer 2 is a Lenovo ThinkPad X61T, Core2Duo 1.6Ghz, using USB Soundblaster X-Fi.

As you recall, my initial question surfaced when my results from a 0 to 200Hz sweep were different from the results from a 0 to 20,000Hz sweep between 0 and 20Hz (graph 1 below for reference).

Anyway, comparing these with the earlier readings, it looks the computer was the source of the difference and not the sub or pre-pro. Is my understanding correct? (I suppose if I was really motivated, I'd connect the X-Fi to Computer 1... just don't have the time or energy right now.). Any thoughts on why this might be the case?

Thanks!


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Comments on my first measurements?-20000.jpg  

post #69 of 73 Old 07-04-09, 01:32 PM
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Re: Comments on my first measurements?

Quote:
Any thoughts on why this might be the case?
It does look like the computer is the source of the problem. I made the suggestion to check that because before I got my fancy new laptop, I use to have a second computer that I would use for REW, and it was certainly challenged by REW and would always stumble at the start of a sweep and I would have to try quite a few times before it didn't hitch at the beginning.

I'm surprised that the computer giving you the problem is the faster one of the two you've tried, but who knows what other resources you have tying up the bus besides running REW.

Anyway, looks like you should use the thinkpad for your testing from now on....

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post #70 of 73 Old 07-12-09, 03:22 PM
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Re: Comments on my first measurements?

I did some tweaking today using the ThinkPad for my readings. Not surprisingly, I now have more questions. :-)

1. The first graph below shows a major dip at around 150Hz from my right front speaker. It seems to even out when both speakers are on. Is this kind of thing to be expected?

Blue = Sub + Both mains
Gold = Sub + RF
Red = Sub + LF
Green = Sub only

2. I also played around with moving my seats and front speakers. The green line in graph 2 shows my seats moved forward about 12" and my front speakers moved about 24" closer to the front wall. Better?

3. All of my readings thus far were taken with my sub set at 28Hz/Low Damping. The purple line in graph 2 shows my sub setting changed to 20Hz/Low Damping. Looking better?

4. Is there something that I can do to address those dips at 71Hz and 172Hz? Should I just keep trying different speaker/seating/sub locactions? Is there any method to narrowing in on the seemingly infinte number of combinations of these three factors?

5. Graph 3 shows a reading from 200Hz to 2,000Hz. This is clearly way choppier. Is this normal?

6. How does the waterfall graph look? Are my bass traps paying off?

Thanks!!
Attached Thumbnails
Comments on my first measurements?-dips.jpg  

Comments on my first measurements?-dips2.jpg  

Comments on my first measurements?-upper-end.jpg  

Comments on my first measurements?-waterfall.jpg  

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