Level adjusting is very serious business. Back when I first got Sonos in 2006 and tried "random play" the difference in levels made many things unlistenable, often because too loud. (And yet, I'm a person that generally likes "realistic loudness." But in a playlist of music, something that is too loud sticks out like a sore thumb.) I believe that virtually all the inexplicable differences that audiophiles claim to hear due to tweaks that have no explanation that is reasonable to audio engineering are actually level mismatches in disguise, since we can hear differences as small as 0.1dB (though it's not easy).
Now I rely on Roon's "Album" Volume Leveling, which works great in most cases.
Sometimes, however, it doesn't. Typically when I am playing back programs recorded from FM radio, for example, which are highly compressed. Or my own compositions. Roon fails to dial back the maximum level to make up for the high degree of compression or high level of deep bass/extreme highs. It does no good if I make the recordings at -6dB, Room simply adds +6dB gain. Depending on various factors, Roon either amplifies up to peak level of 0dB or it reduces below that level if it thinks it's going to sound too loud. But sometimes if fails to reduce, or even increases the level when it's not needed.
Two commercial albums that exhibit the same problem are Beautiful Vol. VII by Andean Fusion and Respighi Ancient Airs and Dances Suites Nos. 1-3 by Rico Saccani. The latter is actually what inspired the present experiment. Roon album leveling increases the level by 2.2dB. It sounds much better without that boost. Beautiful is already recorded at maximum peak level and Roon reduces it by a whopping 4.4dB but that's still not enough. It doesn't help when in addition to being highly compressed, albums are very dry (little reverb or ambience). This can make recordings sound painful even in the absence of distortion or clipping. "In Your Face." A slight distance is more comfortable.
I discovered I could stop this by appending a "needle drop" click (excerpted from recording vinyl) amplified to maximum level. But needle drops at maximum level are a bit annoying.
A single cycle at 1kHz was just as bad or worse. I didn't even want to try a single peak cycle at 20kHz, it would exercise the maximum peak level on a 150W amplifier that is powering my supertweeters.
But my subwoofers have internal digital EQ which steeply rolls off bass below the tolerable limits of the woofer.
So I tried a single peak cycle at 1 Hz (1 sec). It works well, and I hear hardly any sound at all (just a very soft click). This can be easily added to the end of the last track of an album, or any digital recording, using Audacity which can generate any tone, even 1 Hz, of any duration down to 1msec. While playing a single peak cycle at 1 Hz, I see the digital meters on my EQ going to 0dB slowly and back. With a peak level signal, Roon will not add any gain. When the 1 Hz peaks at only 0.999 in the Audacity generator level control, Roon adds +0.1dB gain, so I decided might as well go all the way to 1.0 then Roon won't mess with it (Roon can't, other than lower the level).
(The first time I did this, I was scared out of my chair by a temporary shadow caused by a plane flying overhead just at that moment. I didn't intend to invent the black hole speaker.)
I'm not sure if this would work on all systems. Most do reject deep subsonics quite well however.
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