Where:
N=level in dB
f=frequency
t1=treble time constant, 75uS
t2=medium time constant, 318uS
t3=bass time constant, 3180uS
For the same sound level, a low frequency requires a larger groove which
gives two drawbacks:
- Less recording time
- Difficulties for the cartridge to follow it and thus, higher distorsion
At the other end of the spectrum, the contact between the stylus and the groove makes noise, a high frequency noise. By increasing the high frequencies level during recording we can obtain a better signal/noise ratio as the noise is reduced by the playback curve.
Several other curves did exist before the RIAA but it replaced them completely during the 60'.
Here is the formula to obtain the original RIAA curve:
Where:
N=level in dB
f=frequency
t1=treble time
constant, 75uS
t2=medium time constant, 318uS
t3=bass time constant, 3180uS
In 76', IEC introduced a modification of this curve by introducing a
new time constant that modifies only the extreme bass. It is known as the
RIAA/IEC curve.
This correction has never obtained a big success, the original RIAA
curve is still the most widely used.
For information, here is the formula:
Where: t4=extreme bass time constant,
7950uS
The RIAA reproduction curve:
 |
| Hz | RIAA | IEC |
| 20 | 19.36 | 16.35 |
| 22 | 19.24 | 16.62 |
| 25 | 19.04 | 16.89 |
| 28 | 18.83 | 17.04 |
| 31 | 18.61 | 17.09 |
| 35 | 18.29 | 17.06 |
| 39 | 17.96 | 16.95 |
| 44 | 17.54 | 16.73 |
| 49 | 17.12 | 16.45 |
| 55 | 16.61 | 16.07 |
| 62 | 16.02 | 15.59 |
| 70 | 15.37 | 15.03 |
| 79 | 14.67 | 14.40 |
| 89 | 13.93 | 13.72 |
| 100 | 13.18 | 13.01 |
| 110 | 12.54 | 12.39 |
| 120 | 11.94 | 11.82 |
| 130 | 11.38 | 11.27 |
| 150 | 10.36 | 10.28 |
| 170 | 9.46 | 9.40 |
| 190 | 8.67 | 8.62 |
| 210 | 7.97 | 7.93 |
| 240 | 7.04 | 7.01 |
| 270 | 6.25 | 6.23 |
| 300 | 5.57 | 5.55 |
| 340 | 4.80 | 4.79 |
| 380 | 4.16 | 4.15 |
| 430 | 3.49 | 3.48 |
| 480 | 2.93 | 2.92 |
| 540 | 2.38 | 2.38 |
| 610 | 1.86 | 1.86 |
| 680 | 1.43 | 1.43 |
| 760 | 1.02 | 1.02 |
| 850 | 0.63 | 0.63 |
| 950 | 0.26 | 0.26 |
| 1100 | -0.23 | -0.23 |
| 1200 | -0.52 | -0.52 |
| 1300 | -0.79 | -0.79 |
| 1500 | -1.31 | -1.31 |
| 1700 | -1.80 | -1.80 |
| 1900 | -2.27 | -2.27 |
| 2100 | -2.73 | -2.73 |
| 2400 | -3.39 | -3.39 |
| 2700 | -4.04 | -4.04 |
| 3000 | -4.65 | -4.65 |
| 3400 | -5.43 | -5.43 |
| 3800 | -6.17 | -6.17 |
| 4300 | -7.02 | -7.02 |
| 4800 | -7.82 | -7.82 |
| 5400 | -8.70 | -8.70 |
| 6100 | -9.64 | -9.64 |
| 6800 | -10.50 | -10.50 |
| 7600 | -11.39 | -11.39 |
| 8500 | -12.30 | -12.30 |
| 9500 | -13.22 | -13.22 |
| 11000 | -14.44 | -14.44 |
| 12000 | -15.17 | -15.17 |
| 13000 | -15.85 | -15.85 |
| 15000 | -17.07 | -17.07 |
| 17000 | -18.14 | -18.14 |
| 19000 | -19.09 | -19.09 |
| 21000 | -19.95 | -19.95 |