Recently, members of several online audio forums have reported hearing high-frequency "ringing" on many of their CD recordings - a sound they say is similar to the high-frequency pitch often heard from CRT televisions and monitors. And they've got frequency spectrum plots to prove it.
Indeed, posted frequency plots of some of the offending tracks show spikes right at 15 to 16 kHz - the same frequency as the "whine" that occurs frequently from CRT TV flyback transformers. Here's an example from the intro to Seal's "My Vision" that someone posted over at the AVS Forum:
Speculation is that these tones were caused by TVs or monitors in the recording studio during the performance. However, not all the "mystery" tones appear to be so easily explained. Check out this plot from Dire Straits' "Brothers in Arms":
In addition to a spike at 15 to 16 kHz, there appears to be an even more pronounced one between 18 and 19 kHz. Speculation varies as to what it might be: Is it acoustic noise from some other equipment in the studio? Electrical noise added during mixing? An equipment malfunction? Maybe even a signal processing artifact?
My guess is it's probably acoustic noise/feedback or electrical noise from interaction between some electronic equipment. With all manner of electronics likely to be present at any given recording event - everything from the recording electronics, the performers' equipment, and even broadcast equipment - both analog and digital, tube and solid state, the presence of electronic "noise pollution" seems inevitable.
In any case, according to forum posters, the above recordings are not isolated cases. Other recordings reported to contain spurious high-frequency tones include the following:
B-52s - "Cosmic Thing" (from Cosmic Thing) @ ~16 kHz
Beastie Boys - "Egg Man" (from Paul's Boutique) @ at 19.25 kHz
Eric Clapton - "Layla" (from Unplugged) @ at 15-16 kHz
Fiona Apple - "Sleep to Dream" (from Tidal) @ 19 and 20 kHz
Norah Jones - "Sinkin Soon" (from Norah Jones) @ 15-16 kHz
Sting - "Moon Over Bourbon Street" (from Bring On the Night) @ ~16 kHz
Talking Heads - "Take me to the River" (from Sand in the Vaseline) @ ~20 kHz
Tears for Fears - "Everybody Wants to Rule the World" (from Tears Roll Down - Greatest Hits) @ ~16 kHz
I own several of these, so I thought I'd check them out to see for myself. Sure enough, a plot from Tears for Fears' "Everybody Wants to Rule the World" shows an unmistakable spike at around 16 kHz:
I was especially curious about the recordings with claimed frequency spikes at 18 kHz and above. "Take me to the River" by the Talking Heads didn't disappoint - it shows a clear spike just above 20 kHz:
However, I had to search to find the reported 19- and 20-kHz spikes on Fiona Apple's "Sleep to Dream," which aren't nearly as obvious:
So what does it all mean? Obviously many - if not most - listeners won't be bothered by or even notice these high-frequency anomalies. Still, it's reasonable to expect that the recordings we buy not contain extraneous noise.
With the capabilities of today's sophisticated recording hardware and software, it should be easier than ever to identify and eliminate such issues before they ever get recorded in the first place. Many of the examples listed above appear to be from somewhat older recordings - perhaps we can expect more recent (and future) recordings to be much better in this regard.
Comments, questions or suggestions? Email me at firstname.lastname@example.org.