Question

One pure tone at 2000 Hz has an SIL of 55 dB. In order for a second pure tone at 125 Hz to sound like the same loudness, what SIL would that lower frequency sound need to have?

Answer 1

The Fletcher - Munson curve for the equal loudness for pure tones is given below (the image is collected from the internet )


If we look at the 2000 Hz and 55 dB point in the graph, then, we see that there is no graph for this dB. But, we will eye estimate from the graph. Let us approximately look at a middle point between the 50 dB and 60 dB and assume this is 55 dB and similarly the 2000 Hz is at the middle between the 1000 Hz and 3000 Hz.
  Now if we consider this as the curve of equal loudness then, from the plot this implies that at the 125 Hz this loudness curve intersects at the 60 dB.
   So, we can approximate this as the 60 dB. So, the 125 Hz pure tone of equal loudness as that of 2000 Hz with SIL 55 dB, has the SIL approximately 60 dB.