Wierd Null - Page 2 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 04-07-08, 09:02 AM
rafaelsmith
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Re: Wierd Null

Quote:
JohnM wrote: View Post
Dips are generally caused by phase cancellations, which means the sound reaching the measurement point is a combination of sounds which have a phase difference of half a wavelength at the frequency the dip occurs. Adding two signals at the same frequency which are half a wavelength out of phase results in cancellation at that frequency and a sharp null or dip, the closer the signals are in level the more severe the cancellation.

With a sub and main speaker this can be caused by the sub phase adjustment being out compared to the main speaker. It most often occurs at or close to the crossover frequency.

Another cause of dips is sound reaching the listening position by 2 different paths, a direct path from the loudspeaker and a delayed path reflecting from one of the surfaces of the room. How sharp and deep the dip is depends on how close the level of the reflection is to the level of the direct signal, the lower the level of the reflection the shallower the dip. The frequency at the lowest point of the dip, 130Hz in the case of your plot, gives a clue to the other path. At 130Hz the wavelength of sound is about 2.6m or 8.7 feet (wavelength = 343m/frequency or 1125 feet/frequency). To have a half wavelength phase difference the reflected path would be travelling half that distance further than the direct path, so 1.3m or 4.35 feet. That rules out reflection from a very nearby surface (e.g. a speaker very close to a sidewell) but is fairly typical of the differences that occur from a floor reflection or a sidewall a few feet from the speaker. It could also occur if your measuring position is a couple of feet from the rear wall. To work out which surface the reflection is coming from, move the measuring position and see what happens to the dip. If the dip gets lower in frequency you have moved closer to the surface the reflection is coming from, if it gets higher you have moved further away. When you know what surface is the cause of the problem you can try some acoustic treatment on it (e.g. a thick rug on the floor if the problem is a floor bounce) or move your listening position or speakers to somewhere the effect is lessened.

Good info...I am glad I read this..

I finally got around to trying out a BFD to EQ my sub...which turned out pretty well...but when I do a measurement with my main I have a huge dip at around 130 that I had no clue what was causing it. This info probably explains it. I experimented with speaker distances and phase but the dip was always there.

Old 04-07-08, 09:49 AM Thread Starter
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mdrake

Join Date: Jan 2008
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Re: Wierd Null

John I can not thank you enough for that information!!!! It was VERY helpfu. I had forgotten the formula for calculating the wavelength, THANKS. The informaiton on how to track down the NULL was very well explained. THANKS AGAIN!!!

Matt
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Old 04-07-08, 01:14 PM
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Tony

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Re: Wierd Null

Quote:
mdrake wrote: View Post
Does anyone know what mic comes with the Onkyo 805 and how accurate it is?

Matt
Thats a very good question Mike, I wonder if a person can use it with REW?

Home theater:
Onkyo 805, Yamaha YDP2006EQ, Samson Servo 600 amp
3 EV Sentry 500 monitors across the front, 4 Mission 762i's Surrounds, SVS PB13U sub, Panasonic BDT220, Harmony 1100, Nintendo WiiU
Panasonic PT-AE8000 on a 120" 2,35:1 fixed screen

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Sherwood/Newcastle R972, Mission 765's, SVS SBS02's, A/D/S MS3u sub, Yamaha YDG2030EQ
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Old 04-07-08, 01:34 PM
HTS Senior Moderator

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Re: Wierd Null

Quote:
Does anyone know what mic comes with the Onkyo 805 and how accurate it is?
No cheap mic is accurate. As such any response-measuring device will have built-in calibration to compensate for the mic’s inadequacies, be it an expensive stand-alone RTA or a home theater receiver. REW has essentially the same thing with the calibration files used for the RS SPL meter.

That said, the Onkyo mic could be used, if you could come up with a graph of its frequency response. From that you could create a calibration file. But then you also have the issue that the mic probably has a low level signal instead of the line-level signal the RS meter’s mic element has; if that’s the case a separate pre-amp is needed, as is used with the Behringer ECM 8000 mic.

Regards,
Wayne

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Old 04-07-08, 06:08 PM
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stephen

Join Date: Feb 2008
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Re: Wierd Null

Could you use the mic8000 through a preamp instead of the receivers mic to run Odyssey....if you can/could would the mic8000 be more accurate.
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Old 04-08-08, 01:04 PM Thread Starter
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mdrake

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Re: Wierd Null

Thanks for respones on the mic. It is funny how the Behringer ECM 8000 is the recommended mic. Behringer is not exactly a high-end brand.

Matt
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Old 04-08-08, 01:41 PM
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Jerome

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Re: Wierd Null

Quote:
mdrake wrote: View Post
Thanks for respones on the mic. It is funny how the Behringer ECM 8000 is the recommended mic. Behringer is not exactly a high-end brand.

Matt
You're correct. But this mic doesn't cost much and has a very good frequency response for sub measurements. I use it too for full range measurements but not everybody recommends it...

Most of us are only a bunch of amateurs, I guess that the pros use other (better) mics to calibrate their systems.

Klipsch (THX2) | Crown | SVS | SlimDevices | Sony | Pioneer | Yamaha
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Old 04-08-08, 03:06 PM
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Re: Wierd Null

Quote:
mdrake wrote: View Post
Thanks for respones on the mic. It is funny how the Behringer ECM 8000 is the recommended mic. Behringer is not exactly a high-end brand.

Matt
This may surprise you…

Regards,
Wayne

Wayne A. Pflughaupt is offline
Old 04-09-08, 12:41 PM Thread Starter
Elite Shackster

mdrake

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Re: Wierd Null

WOW, that is surprising. I figured there had to be a good explanation due to expertise of the people using these microphones but to see a professional using them in a mission critical situation is really remarkable. Thanks for the link!!

Jerome why is it not recommended for full range measurement?

Matt
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Old 04-09-08, 01:47 PM
HTS Senior Moderator

Join Date: Apr 2006
Location: Katy, Texas
Posts: 8,809
Re: Wierd Null

Matt,
The mic in the link is a different model from the one Jerome's talking about, that we usually use with REW. Not sure it is who "doesn't recommend" the ECM - it's pretty popular around here.

Regards,
Wayne

Wayne A. Pflughaupt is offline

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