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#371784 - 04/01/12 02:06 PM Re: LFE test tones. [Re: JohnK]
casey01 Offline
aficionado

Registered: 07/03/08
Posts: 852
Loc: Toronto
Originally Posted By: JohnK
Mike, I suppose that's why I began the previous reply with "if you mean". I interpreted what you were saying to be the fairly common observation that after the calibration(with the sub correctly set to equal level with the speakers)the receiver had set the sub trim much lower than the speakers(e.g., speakers at +2dB, sub at -8dB). This is no big deal and running the calibration again with the level control on the back of the sub set lower brings the sub balance point closer to 0.

I see that what you really mean is that the sub level, as measured with an SPL meter, is actually 10dB too low after the calibration. This of course can be easily corrected manually, but no explanation for the difference immediately comes to mind. The Denon manuals do have a 10dB sub attenuation setting that can be applied for certain material, but this isn't the default, and doesn't accidentally get set during an auto-calibration. Generally, as you noted with your 3808, the auto calibration would be at least as accurate as a manual calibration with an SPL meter.


Ian mentioned this in a video sub tutorial he had awhile back and Outlaw has also mentioned in their sub manuals in that the SPL meter doesn't read accurately below 50HZ and as a result their is a significant attenuation in volume level as you go deeper in HZ level. I find this the same with Room EQs as well which, in the end, requires manual volume adjustment upward anyway.

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#371793 - 04/01/12 03:29 PM Re: LFE test tones. [Re: casey01]
J. B. Offline
connoisseur

Registered: 01/19/11
Posts: 1291
Loc: Quebec, Canada
my SPL meter is supposedly flat down to 20 Hz.
if you're interested, you can find the make and model in my signature.
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#371941 - 04/02/12 03:37 PM Re: LFE test tones. [Re: Murph]
Murph Offline
axiomite

Registered: 10/05/06
Posts: 6955
Loc: PEI, Canada
Good conversation but no answers to my question yet though regarding the audible variations in LFE levels when I am standing still in one place.

Nice to know I'm not the only one who experienced it though.
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#372012 - 04/03/12 10:40 AM Re: LFE test tones. [Re: Murph]
alan Offline

connoisseur

Registered: 01/29/02
Posts: 3267
Loc: Toronto/New York/parry Sound
Hi Murph,

I've been following this thread and I suspect that the audible variations you experience are all a product of wave theory--that is, stationary standing wave patterns with nodes and anti-nodes that are formed by the low-frequency test tones. By way of illustration, if you imagine a long taut rope that you set into motion, you'll see the wave patterns and standing waves vary and move along the rope as it continues to oscillate.

Another contributing factor is your stereo hearing. Inevitably there will be ongoing cancellations and reinforcements of low frequency tones as they reach your separate ears through different path lengths from reverberation and reflections in the room.

There! Now I have to rest my brain and have another late-morning coffee. . .

Regards,
Alan
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#372013 - 04/03/12 10:59 AM Re: LFE test tones. [Re: Murph]
Murph Offline
axiomite

Registered: 10/05/06
Posts: 6955
Loc: PEI, Canada
Thanks Alan,
While your explanations make some sense, I'm afraid most of my 'wave theory' comes from boats vs. audio theory. While I can visually picture it, the concept that the room nodes (an anti nodes) are actually shifting the frequency in what I perceive to be a random pattern while another part of my brain tells me that if my ears are not moving, and the room remains static, that life should be static........ will take some work. Of course, who knows what the frequencies vibrations are doing at these node points and even how much the resulting vibration in these points is enough to change the reflected results.

In any case, I get that it is much more complex than that. No further explanation necessary. I'm just stating my amazement at the complexities involved in what prior to my Axiom board days I would have considered very oversimplified.

Fun stuff.


Edited by Murph (04/03/12 11:04 AM)
Edit Reason: trying for "less" confusing.
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#372016 - 04/03/12 11:15 AM Re: LFE test tones. [Re: Murph]
alan Offline

connoisseur

Registered: 01/29/02
Posts: 3267
Loc: Toronto/New York/parry Sound
Now you've got me obsessed with wave theory!
Ok, since I've spent quite a lot of time on boats of various types (motor and sail-driven), it's a good analogy to help visualize what's happening.

Let's say your mono SPL microphone is in the middle of a pond or lake, and a big wave is generated (a large boulder is dropped into the center of the pond)--that's the low-frequency wave, and it radiates out and hits a flat dock or concrete abutment at the side of the pond. (Think of the latter as a wall in your room.) When the wave hits that, a secondary wave is generated and reflected back and it travels along the pathway of the continuing larger waves coming from the boulder and hits the microphone of the SPL meter. There will be contructive and destructive wave reinforcement and cancellation, respectively, which will cause varying energy levels striking the diaphragm of the SPL meter's microphone, hence the varying levels seen on the meter.

The dock or breakwater is the equivalent of your room's walls. The acoustic wave is generated by the test tone from your subwoofer; the wave motion in the lake could be from a passing motorboat or from the wind. The principle is the same.

(There will be a multiple-choice test following this discussion!)

Alan
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#372019 - 04/03/12 11:43 AM Re: LFE test tones. [Re: Murph]
Murph Offline
axiomite

Registered: 10/05/06
Posts: 6955
Loc: PEI, Canada
Thanks Alan,
Despite my comments on how the concepts were complex verses previous oversimplifications and my speculating even deeper into the nodes themselves vibrating into generating a new (although perhaps irrelevant) effect.... I believe I actually understood you the first time.

Your analogy is excellent though and does indeed help to focus the mental picture and now I know that I understood (that much anyways) instead of just 'hoping' that I understood.

Thanks again!!!
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#372021 - 04/03/12 11:51 AM Re: LFE test tones. [Re: Murph]
MarkSJohnson Offline
shareholder in the making

Registered: 09/27/04
Posts: 11445
Loc: Central NH
I would need to read this thread through... but are you guys talking about the "wavering" (no pun intended) of a meter with bass notes?
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#372023 - 04/03/12 12:29 PM Re: LFE test tones. [Re: Murph]
J. B. Offline
connoisseur

Registered: 01/19/11
Posts: 1291
Loc: Quebec, Canada
Alan, am i right in thinking that it's the same thing happening when i have a steady pink noise playing through the front speakers, with the spl meter at the LP, and when i move around the room a little, the meter registers the variations (in SPL) caused by varying reflections of my body movements?

when i'm finished calibrating my system with Audyssey, i check speaker levels with my SPL meter and the receivers' test tones, and i have to stand just outside the AV room while i do the measurements so i can get good readings on the meter.
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#372036 - 04/03/12 01:51 PM Re: LFE test tones. [Re: J. B.]
alan Offline

connoisseur

Registered: 01/29/02
Posts: 3267
Loc: Toronto/New York/parry Sound
J.B.,

Absolutely, your meter is reading the effects of constructive and destructive interference on the acoustical waves as you move about the room.

--Alan
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Alan Lofft,
Axiom Resident Expert (Retired)

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