The Midnight Effect - Who-How?

Electromagnetic radiation or ambient electrical noise may play some role, but we are also getting quiet at the evening.  Our internal noise is reduced and focus increased (and the booze of course).

@cleeds   They have to operate at reduced power,  but I misunderstood definition of critical hours.  Critical hours is 2  hours before sunset to sunset and sunrise to 2 hours after sunrise.  It has something to do with greatly increased propagation of the signals during critical hours ("skywave propagation").  I don't understand if critical hours are in addition to reduced power at night or power has to be reduced only during critical hours.  In previous quote from FCC they mentioned "reduced power at night" (inconsistent).   They mention daytime and nighttime power.  Do they require nighttime power in addition at "critical hours".   Few years ago I was listening to interesting AM program and always around 6-7PM signal was dropping rapidly resulting in a lot of noise.

"For AM broadcast stations, the term critical hours refers to the time periods of sunrise to two hours after sunrise, and two hours before sunset to sunset.  During these periods, the ionosphere has commenced its transition from daytime to nighttime conditions (or vice versa), resulting in greater coverage than would be expected from a daytime-only analysis.  But because the transmitting station operates with its daytime power between sunrise and sunset, the extended skywave signal can be strong enough to interfere with other stations.  This daytime skywave phenomenon was the focus of a protracted rulemaking proceeding that commenced in 1947 and terminated in 1959 (see Docket 8333 for a limited number of decisions in this docket) with the adoption of Section 73.187, Limitation on daytime radiation (also called the "critical hours" rule), and Figures 9, 10, and 11 in Section 73.190.  The rule provides for operation at a lower power during critical hours time periods to limit interference from new or changed Class B or D stations (where the changes were made after 1959) operating on frequencies specified in Section 73.25, to Class A AM stations on the same frequency."

kijanki

They have to operate at reduced power ...

You are misreading the FCC. In the U.S., broadcast radio stations operate at authorized power output at all times although, as noted, some AM stations have lower authorized power at night and others ("daytimers") sign off at night. (There is such a thing as "pre-sunrise authorization" that some stations obtain.)

FCC regs call for the transmitter to be within 90 percent to 105 percent of authorized power output at all times. Special authorization is required otherwise and stations can be fined for failing to follow the reg.

@kijanki I believe the text you're referring to is used to calculate power (so as to avoid interference) during the licensing/permitting process. Once power (which is really ERP) is authorized, no further adjustments are allowed without application.

@cleeds  Thank you for the info.  Do I understand correctly, that some AM stations have to operate at lower power at night (and critical hours), while "some" means stations that operate at frequencies of other (older?) stations?

kijanki

Do I understand correctly, that some AM stations have to operate at lower power at night ...

Yes, and it doesn’t stop there. Some AM stations may also be required to change the antenna "pattern" so as to prevent interference (it’s called "protection") with other stations. If it’s a particularly directional pattern, it’s possible that has a greater negative net result on the signal than the reduced power.

Echoing what others have said, power-line issues could explain what you’re hearing.

In my case, my house had been powered by a rooftop PV system (just panels, no batteries at the time). During the day, my stereo (kinda mid-audiophile, costing a total of maybe $25K at the time) sounded pretty good, albeit a little too bright & Class D-ish for my taste. But at night, it generally sounded better. A lot better, in terms of soundstage, imaging, and transparency. And recordings that I found hard to listen to during the day lost much of their D-type graininess and harshness after the sun went down. Huh?

It didn’t occur to me at the time that at night, the house switched from solar power to the grid. In case you’re not aware, the inverters integrated into most rooftop-PV systems produce far more noise than a typical electric utility.  Was that a contributing factor?

Then, a few years ago, I sunk 8 grand into an Audioquest Niagara power-conditioner + power cord upgrade. Using a consumer-grade line-noise meter I confirmed that the Niagara reduced my line noise by at least one order of magnitude. (Thanks, Garth Powell & Mikey Fremer!)

And you know what? Not only did the Niagara make a night-and-day improvement to my SQ overall, but now there was now no difference between "night and day" sonics (sorry!) I found that I could even shut off the entire PV system & run off the grid without a change in sonics.

Confirmation bias? Nah. I didn’t expect this to happen, so there was no belief to confirm. It was only afterwards, after consulting with some solar-power & line-conditioning engineers & contractors, did I finally figure out what was going on.

YMMV, but the differences I heard were dramatic, and were later corroborated by objective noise measurements. Nothing else changed in my system at the time, but it had never -- never -- before produced such a palpable, three-dimensional presentation or so convincintly reproduced the acoustic environment of a live performance. For a few weeks, I just couldn’t stop listening to my favorite, most familiar recordings.

So whenever someone asks about time-of-day variations in the sound quality of an audiophile-grade system, I always think first about time-dependent, cyclical factors that can affect AC line noise.

What I’ve learned: If I had the choice between spending $10K on either an amp upgrade or a mains upgrade, I might well choose the latter.

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Oh, and to address the OP’s original question (yeah, it’s not all about me!), dramatic changes can occur in the quality of a utility’s delivered power as a function of time of day.

E.g., many utilities adhere to a daily network-switching schedule that accommodates the technical limitations of alternate power sources (such as solar plants v. coal-burning), or that routes power differently depending upon average time-of-day-related demands. If the OP thinks that the change in sound quality happens at about the same time every non-holiday weeknight, it might be worth a call to his or her utility to ask whether any change in topology or other delivery details occurs at a regularly scheduled time.