Cut and pasted from AVS:
Guide to Subwoofer Calibration and Bass Preferences.
Created by Mike Thomas of AVS.
Section VII-C: The Equal Loudness Contours:
I have wanted to include a layman's description of the Equal Loudness Contours to the Guide in order to make some of the discussion of Reference, and of bass boosts, more understandable. The section which follows is intended to briefly explain how our hearing works, and the relevance of that to the previous sections.
Human hearing is not equally sensitive at every frequency. As noted in the section on DEQ, our hearing is most sensitive between about 2,000Hz and 4,000Hz, and is roughly equal in sensitivity in a range from about 500Hz to 5,000Hz. Frequencies above and below that 500Hz to 5,000Hz range require more loudness to be heard at the same level as frequencies within that average range. The Equal Loudness Contours graphically chart the SPL required to hear other frequencies at the same loudness that we hear 1,000Hz. That 1,000Hz standard is the basis of the Equal Loudness Contours.
People sometimes ask why we hear the frequencies we do, in the way we do? Why did our hearing evolve in the way it did? Anyone who is really interested can do some independent research on the subject. But, the explanation that I have read, which seems to be accepted by the scientific community, involves correspondence to the human vocal range. The fundamental tones of a female voice typically range from about a low of 350Hz to about 3KHz, with Harmonics up to 17KHz. The fundamental tones of a male voice typically range from about a low of 100Hz up to about 900Hz, with Harmonics up to 8KHz. (It has been reported that James Earl Jones' speaking voice could hit ~85Hz. And, "Basso Profundos" can
sing even lower than that.)
It makes sense that human hearing would have evolved to correspond somewhat to the range of the human voice, in order to facilitate communication, and to respond to warnings or calls for help. And, the correspondence of the human hearing range, and the human vocal range, are awfully close to be merely coincidental.
The Equal Loudness Contours which chart the way we hear different frequencies within our hearing range were empirically developed, in a number of studies, using young test subjects with "normal" undamaged hearing. Pure test tones were played through headphones in order to maintain a controlled environment without room influences. The Contours are, therefore, based on an average of healthy, normal hearing. It should be understood as we apply the Contours to discussions of our own audio systems, that room factors, and our own individual deviations from average, healthy, normal hearing will affect what we actually hear.
The Contours demonstrate several things. First, the Contours demonstrate that at 1000Hz, a +10dB increase in SPL equals a doubling in perceived loudness. (The Contours are based on that 1000Hz standard.) Second, the Contours demonstrate that our perception of loudness changes as frequencies change. For instance, as frequencies go down from about 500Hz, it takes more SPL for us to hear those frequencies at an equivalent volume.
"SPL" is a measure of sound pressure produced at a certain point in space. "Loudness" is a perceptual number--what we
perceive when that sound pressure level reaches our ears. So, to restate this, as frequencies drop below about 500Hz, it takes more SPL for us to "hear" those frequencies at an equal loudness to 1,000Hz. Again, 1,000Hz is always the starting point for an Equal Loudness Contour.
If we examine the image below, which illustrates the Equal Loudness Contours, we can see the phon lines, each representing a doubling in perceived loudness. And, we can see that the center of the X axis is 1000Hz, where an increase of +10dB of SPL represents a doubling in perceived loudness. Frequencies are shown on the X axis and decibels are shown on the Y axis. Using 80dB as a starting point, we can see how much the SPL curves upward on the Y axis, as the frequency level on the X axis moves toward 16Hz.
So, for instance, at 125Hz (just about the start of what I earlier chose to call the mid-bass range) it would take 90dB to hear a 125Hz test tone at the same volume as we can hear the 1,000Hz test tone which was played at 80dB. And, as noted above, an increase of +10dB is twice as loud. By the time we get down to about 50Hz, it would take 100dB to hear the same loudness as the 80dB test tone at 1,000Hz. Another doubling in loudness, compared to 90dB. And, by the time that we get down to about 31.5Hz, it would take 110dB. Still another another doubling in loudness compared to 100dB.
Remember that we are comparing the SPL required in order to hear frequencies in equilibrium with those in our normal hearing range of about 500Hz to 5000Hz. So, in the example above, that's a +30dB difference in the way that we hear equal loudness between the starting point of 1,000Hz at 80dB, and 31.5Hz at 110dB. This goes a long way toward explaining why we may need to add bass boosts to restore acoustic equilibrium as master volumes drop.
[It should be noted that something interesting happens to our perception of loudness, for bass frequencies, as frequencies go lower. The phon lines get closer together in a perceptible way, starting at about 250Hz. Below about 30Hz, the phon lines stop getting much closer together in a way that affects our perception of loudness. If you examine the Equal Loudness Contours, illustrated in the table below, you will see that the space between the phon lines is shrinking from 1000Hz down to about 16Hz. You will also notice that the space between the phon lines stays the same as frequencies go above the 1000Hz mark, which is the standard for how we hear increases and decreases in volume. (Again, at 1000Hz, a +10dB increase in SPL is defined as a doubling in loudness). The change in the spacing of the phon lines for bass frequencies affects our perception of loudness in a surprising way. That phenomenon is briefly explained at the bottom of this section.]
If we go up in frequency (starting above 5,000Hz) we see a similar pattern occurring with respect to our perception of loudness, but it occurs to a much lesser extent. Using that same 80dB at 1,000Hz, we observe that by the time we get up to about 10,000Hz, we require about 92dB to hear that 10KHz tone at an equal loudness to a 1,000Hz tone. Unlike what we see with the low-frequencies, however, as we go even higher in frequency, no further volume is required. In fact, the Contour starts to curve back down above 10KHz, indicating that a healthy young person, with normal hearing, can actually hear a 15KHz test tone at less volume than is the case with a 10KHz tone.
Of course, above about age 30 (or less) few of us have the same healthy, normal hearing than we would have had when we were much younger. Both damage and age take a toll on our hearing, and males typically experience more age-related hearing loss than females do. But, sort of irrespective of our general hearing, I think we can start to understand two reasons why we add bass boosts far more frequently than we do treble boosts.
First, as noted earlier, we may like the combined sound and feel of bass frequencies in a little different way than we do the sound of high-frequencies. And, we may want more bass simply for that reason. Second, we require
much more SPL to hear bass frequencies, in equilibrium with frequencies in our normal hearing range, than we do with treble frequencies. For music, most low bass would only go down to about 50Hz. But, even there, we would need an increase of +20dB, compared to just 12dB at 10KHz, to hear everything in our audio recording in perfect equilibrium.
For movies, and only considering frequencies down to 31.5Hz (which is just the start of the very low-bass range) we would need an additional + 30dB, compared to +12dB for frequencies of up to 10KHz. That is quite a difference. I believe it is also worth noting that the very-low frequency tactile effects, that were previously discussed, begin at about 30 or 35Hz. And, some movie fans may really want to feel those tactile effects in blockbuster/action movies.
To sum-up the brief discussion of the Equal Loudness Contours and their relevance to the way that we adjust our settings after calibrating our systems to Reference, I want to return to the issue of bass boosts. I think that there is still a third reason, besides the two listed just above for why we tend to boost our bass and not our treble frequencies. There are two parts to the explanation, and the second part is fairly complicated.