Raimund Specht wrote:
> Of course, the hearing threshold is not the same for all
> frequencies. The minimum threshold of about 0dB is restricted to the
> frequency range of our speech between about 500 Hz and 6 kHz.
> Nevertheless, the sound level units are based on the average hearing
> threshold (0dB corresponds to 20=B5Pa). It is obvious, that our
> hearing thresholds may also vary in the short term. A very loud rock
> concert or an extreme bang may also shift the hearing threshold
> temporarily. Surprisingly, the self-noise level of many microphones
> is frequency-depended too. The MKH series microphones match the
> human hearing threshold curve very well (there is an increase of the
> self-noise outside the speech range).
Our hearing thresholds vary both short term and long term. They get
worse with age, particularly with higher frequencies. Cronic exposure to
relatively loud noises (which are all too common) will gradually erode
hearing. It's a error to think it will recover. My father, for instance,
has fairly extreme high frequency hearing loss. Which was a result of
flying lots of bomber runs in WWII as a radar officer. It has never
recovered.
From my tests it was extremely rare for hearing response to be flat
even in the 500 Hz to 6 kHz range. Typical falloff by 4 kHz for people
with very good hearing was 15-20 dB in our tests.
It is a historical fact that the dB scale was chosen based on the
knowledge of the time as to human hearing. As far as best human hearing
it's off by at least 10dB, as there were a fair number of folks who
tested to -10dB. And the equipment used back then did not test a very
wide frequency range.
My point was exactly that human hearing threshold is very frequency
dependent. I see far too many who think we have to worry about something
up at the upper end of human hearing (20kHz) that's only 10 dB or such.
Maybe with a newborn baby, but not with a adult.
And modern sound exposure levels are making this all worse.
> Sure, a single ear is not an ideal omnidirectional 'microphone'.
> Perhaps, the directional characteristic of a single ear is similar
> to something like a cardioid. But both ears together should be more
> or less omnidirectional. This is of course simplified, but it would
> be impossible always to cite the associated textbooks when
> discussing a certain issue ;-). At least the ears are not comparable
> to shotgun microphones (which have the lowest available self-noise
> of around 5 dBA). Even the omnidirectional MKH 20 is not really
> omnidirectional for all frequencies. Its getting more directional at
> higher frequencies. Our ears seem to behave in the same way. It is
> this frequency-dependency which also helps us to locate sounds
> (despite of the time-of-arrival or phase differences between the two
> ears and other effects as defraction around our head).
Just like the housing of a microphone capsule is part of determining
it's pickup pattern so the head, if not the whole upper body is the
"housing" for our ears. Taken as a whole it's hardly a omni. Probably
best not to think of it as one of the standard mic patterns. Were the
head a omni, then duplicating it's response would not require the
elaborate head like mics that are used.
This is not a scientific group at a seminar, so textbook citations
should be minimized. It's really a group working on practical field
recording, which is only partially amenable to theory. It's best to try
and keep things so the maximum number of members of the group can
understand.
Walt
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