Isophonic Curves are very significant graphs that allow us to have a better understanding of how the human ear responds to different frequencies. They have been created by elaborating data on a statistical sample that has undergone a series of sounds produced in an anechoic room. Such a room is designed to reduce wall-reflections as much as possible so as to solely allow the direct signal to reach the listener's ear. The curves indicate how the human ear reacts differently to various frequencies, and measures them in terms of the intensity of the perceived sound. Let's presume that we have a sound source that is able to generate sinusoidal waves that have a variable frequency and a constant amplitude. If we say that the amplitude is 80 dBspl [2 ], we can notice how a listener perceives the low frequencies as having a very low volume, which, as the frequency is progressively increased, he will perceive as an increase in volume (whereas the sound pressure that has actually been generated has remained as it was: 80 dBspl). This kind of behaviour is explainable by the fact that the human ear perceives sound intensity differently when frequency is varied. Isophonic curves indicate the dBspl value necessary to perceive a sound that remains at the same volume along every curve. The reference frequency value for each curve is 1KHz, and at this frequency, the dBspl value is equal to the value that identifies a particular curve, and which is called phone. For example, a 40 phon isophonic curve is a curve that at 1 KHz has an amplitude of 40 dBspl. Let's begin to take a look at these (rather hostile looking!) graphs, and try to understand them a little better:
Let's take one of the curves, for example the 80 phon curve, and follow it from the low to the high frequency. At 20 Hz it is necessary to produce a sound pressure of 118 dBspl. This shows us how the human ear is less sensitive to low frequencies. If we follow the curves up to its high frequencies we see that in order for the ear to perceive the same sound intensity it requires lower sound pressure levels. At 1 KHz we encounter the reference value of the isophonic curve we are referring to, 80 dBspl. Beyond this level we can notice a minimum value at 3KHz and we see that, in order for the ear to perceive the same sound pressure, the 3KHz frequency must generate 70 dBspl. Comparing this value with that at 20 Hz we notice a difference of about 50 dBspl less, an enormous difference. This minimum value depends on the fact that resonance frequency of the ear canal is of approximately 3 KHz [Resonance frequency of an ear canal ] and therefore this frequency is already perceived at low dBspl levels. Over 3 KHz the curve goes back up, showing us the necessary dBspl level for the same perception of volume to take place at high frequencies. The curves are shown in phon units in that the behaviour of the human ear varies with each variation in sound pressure. We can also observe how at high sound pressure levels, the isophonic curve rate is almost flat.
Tip
The loudness control in home amplifiers is regulated by the rate of these very curves. When the volume is very low, the insertion of loudness in the circuit shall result in an increase in low frequencies, aligning their amplitude with the others. For high volumes, this alignment takes place naturally in the ear and therefore to action loudness at such volumes would have almost no effect.
The lowest isophonic curve of all is labelled audibility threshold and it indicates the smallest pressure variation an ear can detect in different frequencies. It's important to remember that these curves were obtained by elaborating statistical data and therefore the values we are using can vary greatly from person to person. Some reference values connected to this curve could be handy on a practical level:
Table 2.1. Some reference values for the frequencies
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Sound pressure whose values are higher than this curve are perceived as physical pain in the ears, which can cause irreversable damage if exposed to such levels for too long.
The ideal volume for mixdown to take place is around 80-90 phons [Mixing ] . At these values the balancing of frequency volumes is quite even. If the mix down were to take place at an excessively low volume, for example at 40 phons, one would perceive fewer lows and therefore feel tempted to compensate by using the equalisers. However once we listen to the mix, at 80 phons it would result as being inundated with lows.
Related topics on Wikipedia
Isophonic curves