What in the world is a Fletcher-Munson
equal loudness curve, and why should I care?! I truly hope that's
not your initial response, but if it is, please read on--you're
about to learn something very interesting!
Humans don't hear all frequencies of sound at the same level.
That is, our ears are more sensitive to some frequencies and
less sensitive to other frequencies. Not only that, but the
sensitivity changes with the sound pressure level (SPL), too.
Take a look at the chart below. You'll notice it's marked horizontally
with a scale denoting the frequency of sound. Vertically it's
marked in SPL. On the chart are a number of curved lines, each
with a number (loudness level) marked. Let's begin by looking
at the lowest solid line marked with a loudness level of 10
phons. (The loudness level in phons is a subjective sensation--this
is the level we perceive the sound to be at.) From about 500Hz
to roughly 1,500Hz the line is flat on the 10dB scale. This
means that for us to perceive the sound being a loudness level
(LL) of 10 phons, (the overall curved line), frequencies from
500Hz to 1,500 Hz must be 10dB. Make sense so far? OK, now look
further into the higher frequencies, say 5,000Hz. Notice the
line dips here--this says we perceive 5,000Hz to be 10 phons
when the source is actually only 6dB. To perceive 10,000Hz at
the same level (10 phons), it would need to be about 20dB. From
this we can clearly see the ear is more sensitive in the 2,000Hz
to 5,000Hz range, yet not nearly as sensitive in the 6,000Hz
and up range.
Lets take a look down at the lower frequencies now, say 100Hz.
For us to perceive 100Hz as loud as we do 1,000Hz (when the
source is at 10dB), the 100Hz source must be at 30dBthat's
20dB higher than the 1,000Hz signal! Looking even farther down,
a 20Hz signal must be nearly 75dB (65dB higher than the 1,000Hz
signal)! We can clearly see our ears are not very sensitive
to the lower frequencies, even more so at lower SPL levels.
Why is this? A simply physical explanation is that resonance
in the ear and ear-canal amplifies frequencies typically between
2,500Hz and 4,000Hz. Why didn't God design our ears to hear
every frequency at the same level? One reason could be this--because
most intelligibility is found in the 2,000Hz to 5,000Hz range,
He designed our ears to be more sensitive here. While our ears
are capable of hearing the lower frequencies, our bodies feel
them more than we actually hear them. This is the reason why
many people who are nearly or completely deaf can still enjoy
music--they can still feel the low frequency content in their
bodies. (This assumes the level is sufficient that they can
feel it. Often such people will actually sit on a speaker so
they're in direct contact with it and the vibrations of the
speaker are conducted right into their body.)
Notice how as the overall loudness level increases that the
low frequency curved lines flatten out. This is because at higher
SPL's we're more sensitive to those lower frequencies. Also
notice that as the SPL increases we're less and less sensitive
to the frequencies above 6,000Hz. This explains why soft music
seems to sound less rich and full than louder music--the louder
the music is, the more we perceive the lower frequencies, thus
it sounds more full and rich. This is why many stereo systems
have a loudness switch--when you're listening to the stereo
at low volumes, you activate this switch which boosts the low
and some of the high frequencies of the sound.
Typically people become uncomfortable with levels above 100dB.
You'll notice 100dB is needed to perceive a loudness level of
100 phons at 1,000Hz--only 90dB is required to give a perceived
loudness level of 100 phons at 4,000Hz. Again, about 104dB is
required to produce a perceived loudness level of 100 phons
Why is all of this so important? Simply put, it helps us understand
why many subwoofers are required to produce a loudness level
equal to those attained at higher frequencies. It shows us how
much more sensitive our ears are to the higher frequencies which
can become very piercing if too loud.
Many times it helps to use an equalizer to cut some of the
frequencies around 2,000Hz to 5,000Hz a little if music is being
played loudly. This action keeps the sound crisp sounding, but
not distorted and piercing at higher SPL levels.
A decibel meter (or SPL meter) measures the amplitude of sound.
Inexpensive meters react to all frequencies equally, resulting
in whats called "flat response". More expensive
SPL meters allow measurements to be taken with both "C-weighting"
and "A-weighting". A-weighting is more close to resembling
the frequency response of our ears (the low end of the measurement
device is rolled off, downward to simulate our lesser sensitivity
to the low frequencies). C-weighting takes more of the low frequencies
into account, even though our ears don't hear them at the same
level. Thus, it's best to make measurements with an A-weighting
setting to know how our ears are responding to the sound. At
the same time, it's interesting to flip the switch to look at
the C-weighted response as well--this factors in the low frequencies
we don't hear, but feel. During heavy rock music or a Fourth-of-July
fireworks celebration, the difference between the A-weighted
measurement and C-weighted can be 10dB or more!