> The lower frequency makes it both less loud to the human ear and subjectively less "annoying"
Exactly. The difference can be huge, even if the number of "dB" measured are the same, to the point that one would not recognize the noise of one running HD, whereas another could be extremely irritating.
There is an exact reason why: the curve of perception limits of our ears is not a horizontal line but a curve, where the higher power of the tone is needed when moving to the lower frequencies.
See: https://en.wikipedia.org/wiki/Equal-loudness_contour
Knowing that: 5400RPM == 90Hz; 7200RPM == 120Hz
The proper reporting of the noise levels would therefore be a noise histogram, or at least should include the frequency of the peak level and not be just a single number if one'd want to compare how irritating the noise is. And even a single histogram can miss the time component, as the noise with the main peak level at some frequency can also come in the waves, making it even more irritating, basically, the histogram of the histogram could also be needed to tell you what's going on.
Moreover, the said "equal-loudness contour" is not exactly the same for different listeners, and as a result different listeners could hear the same noise sources differently. E.g. my girlfriend and I hear the same noise sources (emitting different levels of noise over different frequencies) completely different.
In short, not having precise histograms and analysis simple "is it 5400 RPM" was a really useful information to know the expected noise characteristic of a HD. It's not an accident that these two became two different speeds with the expected use in different environments.
I've intentionally bought 5400RPM disks whenever it wasn't for some servers locked in the server rooms.
Edit (answering "floatingatoll"'s comment below): All the above should be an argument that: "“are they essentially silent from a few feet away”, which is often simple to answer given a drive to test" is not something that has the same answer for everybody. If I'm hearing the lower frequencies better than you, what is "essentially silent" for you can be very annoying for me, and that's why I'd need 5400RPM where you could tolerate 7200RPM. It's also an argument that the proper measurement could indeed be made to know what will be irritating for which person, but it's not common to be present even in the reviews, even less in the specs of the manufacturer. Which is again not surprising, as the frequency histograms sadly aren't common even in the reviews of the music equipment. Common consumers indeed prefer some simple "yes/no" verdicts ("is it quiet" enough), like the one
"floatingatoll" below proposes. But such statements just aren't the answer usable for everyone -- that's anyway why the number for noise level is reported. But it's not enough. I know it firsthand, as I personally do hear better low level sounds than many people around me. What's "quiet" for them is "irritating noise" for me.