LFE test tones.

Posted by: Murph

LFE test tones. - 03/30/12 10:36 AM

I was playing with speaker levels last night for no good reason and then re-ran the db calibration in my Denon to adjust them back to proper levels. I noticed that when the sub test tone was playing, the sound level constantly seemed to fluctuate. It wasn't in a sin wave type pattern either, it seem to randomly sound louder/quieter every second or two.

I wondered if maybe this was just my hearing or the fact that maybe I wasn't keeping still enough or ?? So I brought out my db meter and had it measuring while I listened. I know the average db meter is not very accurate at LFE frequencies but the changes on the digital reading by +/- up to 2 db did seem to match the fluctuations that my ear was hearing.

Is this a normal effect for subwoofer test tones? I don't recall hearing it before but maybe I just never noticed. Am I just paying more attention or is something a bit wonky?

I have a calibration DVD I can try tones from as well to see if I hear the same thing but I thought I'd ask the experts before I get too far into it.
Posted by: CatBrat

Re: LFE test tones. - 03/30/12 10:38 AM

That could be a case of comb filtering. I've noticed this when playing around with sounds with the M22 on wall speakers. Walking across the room would cause the sound to get louder and softer, very rapidly. Probably just an anomaly with any on-wall speakers.
Posted by: michael_d

Re: LFE test tones. - 03/30/12 12:51 PM

I've always wondered why the spl wanders all over the place as well. Even with the meter set to a slow response, the db just bounces up and down, although the test tone does not seam to get louder - by ear.

I've also discovered that the Denon 4311 sets LFE about 10 db lower than the rest of the speakers. The 3808 always got the sub pretty close. I'm totally clueless why the 4311 is setting it so damn low.
Posted by: fredk

Re: LFE test tones. - 03/30/12 08:33 PM

I noticed the same thing when I set up my sub. I just figured it had something to do with that other dimension that sucks socks out of your washer.
Posted by: St_PatGuy

Re: LFE test tones. - 03/30/12 08:39 PM

Originally Posted By: fredk
I noticed the same thing when I set up my sub. I just figured it had something to do with that other dimension that sucks socks out of your washer.


With all those low frequencies that dimension has sucked up it needs some kind of damping.
Posted by: fredk

Re: LFE test tones. - 03/30/12 09:32 PM

Originally Posted By: St_PatGuy
Originally Posted By: fredk
I noticed the same thing when I set up my sub. I just figured it had something to do with that other dimension that sucks socks out of your washer.


With all those low frequencies that dimension has sucked up it needs some kind of damping.

grin Thanks. I knew I was on the right track. I just couldn't quite put it all together.
Posted by: JohnK

Re: LFE test tones. - 03/30/12 10:47 PM

Mike, if you mean that all your settings before you calibrated were the same with the 4311 as with the 3808, it would seem to indicate that the sub test tone in the 4311 is at a higher level. If so, the calibration is still correct when the 4311 sets it at -10dB, but lowering the sub level control before calibration would give an end result closer to 0dB.
Posted by: michael_d

Re: LFE test tones. - 03/31/12 12:51 PM

Uh....I don't quite understand what you wrote there John.

My standard MO is to run the Audyssey auto set up first. I let it do it's thing, then I go into the manual set up and change the crossover (as generally, the mains are set to large), then I verify SPL with my Radio Shack digital meter. The 3808 would get the channel levels very close, including the LFE channel. The 4311 however, consistently sets the LFE channel about 10 db too low. I'm not sure why it is doing this.
Posted by: JohnK

Re: LFE test tones. - 03/31/12 09:53 PM

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.
Posted by: michael_d

Re: LFE test tones. - 04/01/12 01:36 PM

Ya, this is just weird. All the Audyssey and DD volume nannies are turned off too, so I have no idea why it's reading LFE as high as it is. It does have a pretty neat feature for running two subs. At the beginning of the calibration, it makes you adjust the sub gain if it thinks it is too high. It sends a test tone to each sub, and you just dial it down while the display tells you what the DB is. This is quite helpful for balancing the two subs. Very cool feature....
Posted by: casey01

Re: LFE test tones. - 04/01/12 02:06 PM

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.
Posted by: J. B.

Re: LFE test tones. - 04/01/12 03:29 PM

my SPL meter is supposedly flat down to 20 Hz.
if you're interested, you can find the make and model in my signature.
Posted by: Murph

Re: LFE test tones. - 04/02/12 03:37 PM

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.
Posted by: alan

Re: LFE test tones. - 04/03/12 10:40 AM

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
Posted by: Murph

Re: LFE test tones. - 04/03/12 10:59 AM

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.
Posted by: alan

Re: LFE test tones. - 04/03/12 11:15 AM

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
Posted by: Murph

Re: LFE test tones. - 04/03/12 11:43 AM

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!!!
Posted by: MarkSJohnson

Re: LFE test tones. - 04/03/12 11:51 AM

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?
Posted by: J. B.

Re: LFE test tones. - 04/03/12 12:29 PM

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.
Posted by: alan

Re: LFE test tones. - 04/03/12 01:51 PM

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
Posted by: alan

Re: LFE test tones. - 04/03/12 01:51 PM

Yes!
Posted by: fredk

Re: LFE test tones. - 04/03/12 06:40 PM

Originally Posted By: Murph
.... I believe I actually understood you [for the] the first time.

Persistence is a wonderful thing. How many years has it been? grin

As a Canadian, I believe Alan will appreciate this take on wave theory:


Posted by: alan

Re: LFE test tones. - 04/04/12 08:23 AM

Ha, ha!

When she was younger, the Queen used to have a sort of screwing-in-a-lightbulb wave action. Over the years, that's changed.

Fred, I'll leave it up to you to describe the Queen's current wave. . .

-- Alan
Posted by: Murph

Re: LFE test tones. - 04/04/12 10:23 AM

I can't decide Fred if you insulted me or Alan. I'm going to go with you insulting the Queen instead. We can all be amused at that. Unless of course your currently a loyal Britt living in the Falklands.
Posted by: fredk

Re: LFE test tones. - 04/04/12 09:30 PM

Funny how the mind works. The moment I read wave theory that image of the Queen with her unique wave came to mind.

Shucks Murph, I wouldn't dream of insulting you. I was just re-interpreting that sentence the way I saw it. I think I missed my calling as an editor.
Posted by: BobKay

Re: LFE test tones. - 04/04/12 10:38 PM

Originally Posted By: Murph
I can't decide Fred if you insulted me or Alan.


Well, if he were any good at it, there wouldn't be a question, would there?
Posted by: Murph

Re: LFE test tones. - 04/05/12 10:21 AM

My point exactly Bob, if this bunch ever stopped having good natured fun by abusing me as much as I pick on them, I'd begin to think I did something really wrong. The joust of the jest is one reason I enjoy this forum so much.
Posted by: fredk

Re: LFE test tones. - 04/05/12 12:58 PM

Originally Posted By: BobKay
Originally Posted By: Murph
I can't decide Fred if you insulted me or Alan.


Well, if he were any good at it, there wouldn't be a question, would there?

Hey! That was a three-fer. You're just jealous.