Adjust the calibration tone to be just audible
Count the number of each staircase you can hear
Although sounds with a greater sound pressure level usually sound louder, this is not always the case. The sensitivity of the ear varies with the frequency and the
quality of the sound. Many years ago Fletcher and Munson (1933) determined Curves
of equal loudness for pure tones (that is, tones of a single frequency). The curves shown
below, recommended by the International Standards Organization, are similar to those of Fletcher and Munson. These curves demonstrate the relative insensitivity of the
ear to sounds of low frequency at moderate to low intensity levels. Hearing sensitivity
reaches a maximum around 4000 Hz, which is near the first resonance frequency of the
outer ear canal, and again peaks around 13 kHz, the frequency of the second resonance.
The contours of equal loudness are labeled in units called phons, the level in phones being numerically equal to the sound pressure level in decibels at f = 1000 Hz. T phon is a rather arbitrary unit, however, and it is not widely used in measuring sound.
In this demonstration, we compare the thresholds of audibility (in a room) of tones having frequencies of 125, 250, 500, 1000, 2000, 4000, and 8000 Hz. The tones are 100 ms in length and decrease in 10 steps of -5 dB each.
Naturally, the threshold of audibility in a room depends very much on the character of the background noise. Nevertheless, in most rooms the threshold should increase measurably at low frequency. The listener should be reminded that pure tones cause standing waves in a room, especially at the higher frequencies, in which the maxim and minimum levels may differ by 10 dB or more.