In the figure, the abscissa represents pure tone signals of different frequencies, and the unit is Hertz (Hertz); the ordinate represents the amplitude of the corresponding sound wave (sound pressure level); Different units on the same iso-loudness curve are pure tone signals with decibel (dB) frequency and different sound pressure levels, giving people the same sense of loudness. For example: 50dB / 100Hz pure tone and 40dB / 1kHz pure tone are equal, because they are on the same contour curve. That is to say, you want to make a 100Hz bass and 40dB / 1kHz The midrange sounds as loud as it is necessary to make a signal of 100 Hz 10 dB higher than 1 kHz. From the figure we can draw the following simple conclusions:

1. The sensitivity of the human ear to different frequencies of sound is different. Specifically, the sensitivity for 3 ~ 4 kHz sound is high. As the frequency increases and decreases towards both ends of 3 ~ 4 kHz, the overall trend is that the sensitivity decreases.

 

2. The sensitivity of the human ear to sounds of different frequencies is also related to the size of the sound pressure. As the sound pressure decreases, the sensitivity of the human ear to low and high frequencies is reduced, especially for low frequency sounds. This is why when we turn the volume down (ie, at a low sound pressure level), even if there are more bass components in the program, we still feel that the bass is insufficient. (Above 80 decibels and above), you will feel the richness of the bass.

It can be seen from the isometric curve that if the sound is reproduced at a sound pressure level lower than that of the original sound (during recording), the bass and treble need to be boosted by the equalizer to ensure the original tone balance. For example, if a band is playing, if both low frequency and high frequency are recorded at about 100 dB, because the equal loudness curve is almost flat at this time, the bass and treble sounds similarly loud. If the sound pressure level during playback is low, such as 50 decibels, the 50 Hz sound can be heard just now, while the sound of 1 kHz sounds 50 squares, and other sounds of different frequencies have different loudness Level, so it sounds like both low frequency and high frequency sounds are lost, that is, the original tone has changed. At this time, if you want a 50 Hz sound to sound about the same loudness as a 1 kHz sound, you must increase it by about 20 dB. It can be seen that the equal loudness curve is one of the important basis for us to use the equalizer.