Isophonic curves are very important graphs that help us to have a better understanding of how the human ear responds to different frequencies. They were created by processing data on a statistical sample. A subset of a population were exposed to a set of sounds produced in an anechoic room. Such a room is designed to reduce wall reflections as much as possible in order to allow only the direct signal to reach the listener's ears. The curves indicate how the human ear reacts differently to various frequencies, in relation to the intensity of the perceived sound. Let's presume that we have a sound source capable of generating sinusoidal waves that have a variable frequency and a constant amplitude. When we produce a sound with an 80 dBspl amplitude [3 ], we notice how a listener perceives the low frequencies as having a very low volume, and how, as the frequency is progressively increased, he will perceive an increase in volume (bearing in mind that the sound pressure that has actually been generated hasn't changed, remaining at 80 dBspl). This kind of behaviour can be explained 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 1 KHz, 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 phone 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 low to high frequencies. At 20 Hz we need to produce a sound pressure of 118 dBspl. This shows us how the human ear is less sensitive to low frequencies. If we then follow the curve up to its high frequencies we see that for the ear to perceive the same sound intensity a lower sound pressure level is required. At 1 KHz we come across the reference value of the isophonic curve we are referring to, 80 dBspl. Beyond this level we notice a minimum value at 3 KHz and we see that, in order for the ear to perceive the same sound pressure, the 3 KHz frequency signal must generate 70 dBspl. Comparing this value with the one at 20 Hz we notice that there is a 50 dBspl difference, which is a lot in terms of sound. This minimum value depends on the fact that the resonance frequency of the ear canal is of approximately 3 KHz [Resonance frequency of the ear canal ] and therefore this frequency is already perceived at low dBspl levels. Beyond 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. Each curve identifies a separate phone value according to the changes in the human ear's behaviour as sound pressure levels vary. We can also see how at high sound pressure levels, the isophonic curve rate is almost flat.
The loudness control in home amplifiers is regulated by the rate of these very curves. When the volume is very low, the activation of the amplifier's loudness shall result in an increase in low frequencies, aligning their amplitude with the other frequencies. This alignment takes place naturally in the ear at high volumes so, activating loudness would have almost no effect.
The lowest isophonic curve is called audibility threshold and it indicates the smallest pressure variation an ear can detect at different frequencies. It's important to bear in mind that these curves are based on the elaboration of statistical data and therefore these reference values can vary considerably from person to person. Some reference values related to this curve could be handy on a practical level:
Table 2.1. Some reference values for the threshold of hearing curve
Sound pressure with values that are above this curve are perceived as physical pain by the ears and can cause irreversable damage if exposed to them for too long.
The ideal mixdown volume is around 80-90 phons [Mixing ] . At these values frequency volumes are quite well balanced. If the mixdown were to take place at an excessively low volume, for example at 40 phons, you would perceive fewer lows and therefore feel tempted to compensate them by using the equalisers. However once we listen back to the mix at 80 phons, we'd find it inundated with lows.