The human hearing range is said to be from 20 Hz to 20 Khz. It is called the Audio Spectrum.

The range of human hearing is generally considered to be 20 Hz to 20 kHz, but it is far more sensitive to sounds between 1 kHz and 4 kHz. For example, listeners can detect sounds as low as 0 dB SPL at 3 kHz, but require 40 dB SPL at 100 hertz (an amplitude increase of 100).

Subjective vs. Objective Sound Levels

SPL is an objective measurement of sound pressure, or power in watts, and is independent of frequency.

Subjective sound levels varied significantly from the SPL level. That is, when two tones were played at the exactly the same SPL level, one sounded louder than the other. And the results were very dependent on how loud the tones were to begin with. The vertical axis is the objective SPL sound level.

Each of the curves in the graph represents a constant subjective sound level, which are in units called “phones.” The lowest curve is the minimum audible level of sound. As noted above, the ear is most sensitive around 2-5 kHz. To be audible at this minimum level, a sound at 20Hz must be 80 dB (100 million times!) more powerful than a sound at 3 kHz.

“Human hearing weakly perceives that high frequency sounds are nearby, while low frequency sounds are distant. This is because sound waves dissipate their higher frequencies as they propagate long distances.”

Near the top, the curve at 100 phones is a fairly loud level. To sound equally loud at this level the sound at 20 Hz must be about 40 dB more powerful. This change in subjective level for different loudness levels means that music played softly will seem to be lacking in bass.

Sound Masking

Shown are the masking effects of 1200 Hz tones of various intensities. Note that it is effective in masking sounds above it in frequency, but not below. The dips at 1200 and 2400 come from the effects of beats, which make the masked tone easier to detect.

Low-frequency, broad banded sounds (like water running) will mask higher frequency sounds which are softer at the listener’s ear (a conversational tone from across the room). For a single frequency masking tone, masking curves can be determined experimentally. Also, from the idea of the just noticeable difference (jnd) in sound intensity, one can approximately calculate the amount of added second sound that would exceed the jnd and thus be audible.

Broadband white noise tends to mask all frequencies, and is approximately linear in that masking. By linear you mean that if you raise the white noise by 10 dB, you have to raise everything else 10 dB to hear it.

Critical Band

When two sounds of equal loudness when sounded separately are close together in pitch, their combined loudness when sounded together will be only slightly louder than one of them alone. They may be said to be in the same critical band where they are competing for the same nerve endings on the basilar membrane of the inner ear.

Human hearing can distinguish separate tones when they are in different critical bands (4c). When tones are in the same critical band (4a and 4b) the ear perceives a modulation or beating.

The metrics “Roughness and Fluctuation Strength” describe how two or more tones create a modulation or beat frequency based on these critical bands.

• Fluctuation Strength – When the tones are about 4 Hertz apart, the ear hears a single tone with a low frequency modulation or beating.
• Roughness – When the tones are about 70 Hertz apart, the ear hears a rapid modulation or beating.
• Two Tones – With a separation of 350 Hertz, the two tones are in different critical bands, and the ear can distinguish them from each other.

Beating phenomenon occurs because the ear cannot resolve inputs whose frequency difference is smaller than the bandwidth of the critical band.

When one sound is produced and another sound is added, the increase in loudness perceived depends upon its frequency relative to the first sound. Insight into this process can be obtained from the place theory of pitch perception. If the second sound is widely separated in pitch from the first, then they do not compete for the same nerve endings on the basilar membrane of the inner ear.

Adding a second sound of equal loudness yields a total sound about twice as loud. But if the two sounds are close together in frequency, within a critical band, then the saturation effects in the organ of Corti are such that the perceived combined loudness is only slightly greater than either sound alone. This is the condition which leads to the commonly used rule of thumb for loudness addition.

One difficulty with this “rule of thumb” for loudness is that it is applicable only to adding loudness for identical sounds. If a second sound is widely enough separated in frequency to be outside the critical band of the first, then this rule does not apply at all.

Listeners can tell that two tones are different if their frequencies differ by more than about 0.3% at 3 kHz. This increases to 3% at 100 hertz. For comparison, adjacent keys on a piano differ by about 6% in frequency.

For low frequencies the critical band is about 90 Hz wide. For higher frequencies, it is between a whole tone and 1/3 octave wide.

Critical bands are used to quantify the ability of the human ear to distinguish between individual frequency tones. The human ear can hear from 20 to 20,000 Hertz, but the ability to distinguish individual tones varies as a function of frequency.

At low frequencies, the human ear can distinguish changes in frequency more easily than at high frequencies. For example, the ear can distinguish a 20 Hertz difference between 500 and 520 Hertz tones more readily than a 5000 and 5020 Hertz tones.

Center freq (Hz) : 100 – 200 – 500 – 1000 – 2000 – 5000 – 10000

Critical BW (Hz) : 90 – 90 – 110 – 150 – 280 – 700 – 1200

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