John Atkinson wrote:

Regarding Mr. Krueger's complaint that I did not offer any comment on
his comment that 6Hz tones are audible, I first would like him to
clarify what spound pressure levels are required for 6Hz to become
audible.

No spound levels were involved.

On the chance that this was just another one of Atkinson's many careless
mistakes...

The sound levels involved were not extreme in any way. I did most of the
listening with Sony 7506 headphones and FSI IEMs. The signal source clips at
about 1.5 volts RMS, which sets a reasonably low natural limit.

It's also true that SPLs of many natural infrasonic sounds look pretty crazy
on paper.

For example, most people are pretty amazed the first time they measure SPLs
in a car traveling down an interstate at legal speeds, if they turn the
normal weighting curve off.

When people talk about 70 dB SPL noise levels in a car, that usually means
70 dB A-weighted. A-weighting is something like 50 dB down at 20 Hz, and
falling off at the rate of about 40 dB/decade or 12 dB/octave. So, at 2 Hz,
its more like 90 dB down, 78 dB down at 4Hz, 66 dB down at 8 Hz and so on.

70 dB A-weighted SPL could translate into something like 136 dB at 8 Hz.
That's extreme, but it shows the trend. More than 100 dB are not unusual
IME.

This in turn begs the question of how an audio system can possibly reproduce
low frequency sound, that loud or louder. There's a tendency for a closed
room to transition to a bass-boost mode of operation below some low
frequency, depending on the size and construction of the room.

This is one reason why car audio tends to be so bassy - the extreme levels
of bass common in car audio are not all that impractical to generate,
because the *room* is so small. A living room act like a bigger closed
space with similar bass boost. There will be a lower transition frequency.

The bass boost due to the room's size and the fact that it is a closed space
tends to naturally be about 12 dB/octave. If you have a subwoofer that is
flat down to your room's transition frequency, and falls off below that at
12 dB/octave, then the merger of the room response and the speaker response
can approximate flat response, at least down to the next lower cut-off
frequency.

My findings about the audibility of subsonic cut-offs can relate to this
next lower cut-off frequency, when the room and the subwoofer are
well-matched. Or, it can relate to the rapidly-expanding population of
people who are listening using certain common kinds of personal listening
devices. I've seen it have audible effects with both IEMs and normal
headphones.

This makes the point that a subwoofer with an extremely low cut-off point
can easily sound boomy in a too-small room. The room's transition frequency
can overlap the roll-off of the subwoofer leading to a range with boosted
response. There can be substantial boominess due to this overlap.