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only 10dB decay at 36.7Hz: defective speaker?

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Posted on December 31, 2016 at 13:15:13
dave789
Audiophile

Posts: 474
Joined: September 21, 2001









I did a measurement in someone's living room. The walls are mostly cement walls.

Mic placed on a typical listening position on sofa (about 30cm from wall behind the listener). Another measurement with mic positioned 170cm from that wall.

An interesting thing is that the decay at 36.7Hz over 300 milli seconds is only about 10dB. This is the same with both mic positions.

The measurement was done with both channels playing REW's length 1M sweep sound.

Can a room mode be so bad? Or is the speaker defective?

To ears, the speaker sounds honky on music an voice (news). At the moment music contains 36.7Hz, it sounds boomy.

 

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room and location IN room IMO., posted on December 31, 2016 at 13:53:32
3+4=5
Audiophile

Posts: 542
Location: Midwest
Joined: December 24, 2016
Have you moved the speaker around much?
If no matter where in the room the same single frequency problem.. It most likely would be the speaker also has that peak.
If it changes a lot with speaker placement. Then it is just the room.

 

RE: only 10dB decay at 36.7Hz: defective speaker?, posted on December 31, 2016 at 22:13:51
Inmate51
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Posts: 8948
Joined: July 6, 2005
You cannot accurately measure speaker output below about 200-300 Hz in a typical living room, even with a gated system.

You're measuring the room. It doesn't surprise me that you're down 10 dB after 300 ms.

My question to you is: What were you trying to measure, and to what end?

:)

 

RE: only 10dB decay at 36.7Hz: defective speaker?, posted on January 2, 2017 at 08:03:52
Dave_K
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Posts: 1031
Joined: September 30, 2014
Contributor
  Since:
December 0, 0000
Hi Dave.

It would be helpful if you included the room dimensions.

If one of the primary room dimensions is around 15-16 feet, then you are looking at the first order axial mode associated with that dimension. The reason for the long delay time at 36Hz is that there is nothing in the room to absorb such low frequencies. The typical sheet rock over stud walls are flexible enough to dissipate low bass energy, but cement walls are not.

I'm going to make a guess. In the 170cm waterfall, the reason why the peak around 70Hz is taller than the peak at 36 Hz is because the ceiling height is ~8 feet, so the first order floor to ceiling mode (0-0-1) at ~70Hz roughly lines up with the second order mode associated with the 15-16 foot dimension (2-0-0 or 0-2-0). If this is the case, it also helps explain why the overall response is so peaky; whenever one room dimension is close to the multiple of another, their modes will reinforce each other.

If I had to guess a third dimension, it would be 11 or 12 feet, or maybe 23-24 feet. That would explain some of the minor waterfall peaks. Although if it's the shorter of the two, the modes should be stronger unless that dimension is broken up by an entryway into another room or an L.

The honky sound may be due to the voicing of the speakers, or it may be the room. At the listening position 30cm from the wall, the 400-500 Hz band is 10-12dB higher than the rest of the midrange. That would certainly be an audible coloration. The waterfall at 170cm looks smoother in the midrange. If the honky sound is significantly less noticeable at 170cm, then I would blame the room.

Another thing that concerns me about the measurement is the treble. Aside from that peak around 10-11 KHz, the tweeter seems to be MIA. Or it's horn loaded and the speakers aren't toed in so the mic is outside of their main lobe.

Happy measuring.

 

RE: room and location IN room IMO., posted on January 2, 2017 at 08:17:38
Dave_K
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Contributor
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December 0, 0000
Note the frequency of a room mode does not change with speaker placement. When you move the speakers around you are affecting the degree to which you excite the room mode, but not the frequency of the mode.

 

I lost the opportunity to measure again, posted on January 14, 2017 at 19:27:02
dave789
Audiophile

Posts: 474
Joined: September 21, 2001




I could not take additional measurements as his wife came - she does not like his hi-fi hobby -.

By the way, I had made a measurement with the mic placed very close to the speaker front.

 

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