Dip in FR of Centre Speakers - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 7 Old 10-21-2017, 11:03 PM - Thread Starter
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Dip in FR of Centre Speakers

For some of the centres a distinct dip is seen at ~900-1000Hz

KEF Q900 Speaker System HT Labs Measures | Sound & Vision
Polk LSiM707 Surround Speaker System HT Labs Measures | Sound & Vision
Revel Ultima2 Salon2 Speaker System HT Labs Measures | Sound & Vision
KEF R900 Home Theater Speaker System HT Labs Measures | Sound & Vision (slight one here)
Emotiva Airmotiv 5CH Speaker System Review Test Bench | Sound & Vision
https://www.soundandvision.com/conte...-labs-measures

Now it may not be always there , eg .
https://www.soundandvision.com/conte...iew-test-bench

And this has more in addition too
https://www.soundandvision.com/conte...iew-test-bench

But in the cases above where its present, the fronts/L&R are quite flat (in some case extremely)
So find it hard to believe that its bad design/mistake by the company.
Is it intentional when designing a good horizontal centre channel ? Or given the way it placed ? Or some measurement issue when measuring a centre ?

An opposite example is that of KEF X300a
http://www.connect.de/testbericht/ke...t-1506924.html
Now whats that rise at ~900-1000Hz ? It can't be that big a mistake by KEF ? (Even the desk mode hardly reduces that spike, in case its related)

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post #2 of 7 Old 10-22-2017, 04:56 AM
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Dip in FR of Centre Speakers

Interesting discussion, it doesn’t appear that it’s centered at 900-1000 hz on any of them, but varies quite a lot. The center from the 900c system has a dip centered at 2.8khz, which just so happens to be at a crossover frequency, where the r900 systems center appears to have a bump at the 3khz crossover point and a dip in total power above that, which to me, indicates poor attenuation of the midrange/bass units to the tweeter. I’m having a hard time reading what frequency the divisions are in the graphs.

I hate to say it, but I’ve also sometimes questioned SV’s measurements, since they sometimes produce measurements that don’t match up with others measurements, including my own. For example here is the Klipsch rp 150m measurements from SV

And here is the measurement I took of the individual drivers

The bump at 14khz and 800hz has never been replicated by me no matter how I measure it. In addition, the speaker achieves a response outdoors of 47hz -3dB vs their measured 66hz. The bump and subsequent dip at 14khz indicates a breakup mode at 14khz for the tweeter, as this is often how drivers behave at breakup, I don’t have the photo on my phone at the moment, but according to my measurements, the actual breakup of the titanium tweeters is at about 24khz, with a big spike and crash above that.

I also have a measurement of the center floating around somewhere and it certainly doesn’t display the 5dB shelf at 2khz seen in their measurements. If there truly was a +5dB shelf on the center at the high frequencies, rising as it does in the graph, it would be near unbearable due to extreme brightness.

Some of their sensitivity ratings seem off as well. I own two speakers measured by them, the rp 150m and the Polk owm3. They rated the owm 3 at 89dB, and the rp 150m at 88dB. Out of curiosity, I hooked both to my receiver , one to the left channel, and one to the right channel, placed them side by side and proceeded to level match them in the Avr setup menu using a c weighted spl meter. The OWM3s required +3dB of gain in order to match the Klipsch, which begs the question of how SV achieved a higher sensitivity rating for the Polk, especially considering the Polk is a small satellite with a direct radiating tweeter, while the Klipsch is a moderately sized bookshelf fit with a horn.


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post #3 of 7 Old 10-22-2017, 10:41 AM - Thread Starter
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The reason I said 900-1000Hz (for the first dips)

The green line dips at 900Hz ?


The green line dips at 900Hz again ?


Yaa, this is slightly more than 1kHz


The lowest part of dip is 1kHz and and then extends further


~900Hz again ?

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post #4 of 7 Old 10-22-2017, 12:47 PM
 
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Quote:
Originally Posted by Hifisound View Post
For some of the centres a distinct dip is seen at ~900-1000Hz
So many centers are still the stupid design where there are two spaced drivers, distance = d, reproducing the same range. BAD IDEA. It causes comb filtering and one of the teeth of that comb may be in the range you've noticed.



IIRC, Sean Olive has a better post about this but for now here's a link to the topic from another source.

The FR graphs from S&V are windows of several measurements from a tight window of say up to 15% off axis averaged together, IIRC.
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post #5 of 7 Old 10-22-2017, 01:46 PM
 
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Quote:
Originally Posted by m. zillch View Post
So many centers are still the stupid design where there are two spaced drivers, distance = d, reproducing the same range. BAD IDEA. It causes comb filtering and one of the teeth of that comb may be in the range you've noticed.

Yup, AKA speaker lobing.
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post #6 of 7 Old 10-22-2017, 10:07 PM - Thread Starter
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Lobing ? The ones with dip are all 3-way centres
Though thats interesting as the one above which doesn't have the dip, PSB Imagine XC, is a 2-way !

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post #7 of 7 Old 10-22-2017, 11:26 PM
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Dip in FR of Centre Speakers

Quote:
Originally Posted by Hifisound View Post
The reason I said 900-1000Hz (for the first dips)



The green line dips at 900Hz ?





The green line dips at 900Hz again ?





Yaa, this is slightly more than 1kHz





The lowest part of dip is 1kHz and and then extends further





~900Hz again ?



Oops, I apparently read the surround graph as the center!

Mills generally are centered around the crossover frequency in two way mtm center speakers. The cause of this is because there is differing distances between the drivers when seated off axis. One way to minimize this problem is to design the speaker to have a crossover point that has a ka=2, so that the directivity of the woofers matches the tweeter, and to ensure that the woofers are less than 1 wavelength of the xover frequency from the tweeter.

The center channel I am using is a Klipsch RP-250c. The speaker is a two way mtm with 5.25” woofers. The two woofers are 7” center to center to the tweeter, crossed over at 1500hz. To achieve a Ka=2, a 5.25” woofer should not be crossed over any higher than about 1700hz.

Measurements of the RP-250c reveal minimal off axis dips at the xover frequency because of this.

I’m going to guess the 900hz dip may be due to their technique. It would be interesting if someone who owned one of the centers you listed took a measurement with rew to see if that’s the case. It’s not likely comb filtering, as that would, as I said, be the most severe at the xover points.




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