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 30dB–that'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 at 100Hz.

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 what’s 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!

-updated 12.2001