Re: What are the hearing ranges of other species of life ?

There will always be confusion about the exact frequency range of hearing for any species. Here are three reasons that account for the confusion:

First, frequency is encoded on a logarithmic scale. For virtually all mammals, the frequency (Hz) of a sound is encoded spatially along a membrane in the inner ear called the basilar membrane. The lengths of this membrane differ across species but are around 35 mm in humans and of similar length in many small mammals. Because the encoding is always logarithmic, a fixed distance along this membrane will encode vastly different frequency ranges depending on the frequency region. A one mm section in the low frequency may encode only a 100 Hz, while an equivalent one mm length in the high frequency region may encode 10s of thousands of Hz, a phenomenon due simply to the logarithmic frequency scale. Normal variation in the length of the basilar membrane easily can cause variations of 10,000 Hz in the high frequency region. However, this normal variability represents only a very small proportion of the total length of the basilar membrane and only a very small proportion of total hearing ability.

Second, the hearing sensitivity function is almost vertical at the limits of hearing. On a graph of sound level as a function of frequency (on a log scale), the curve showing hearing sensitivity is fairly flat for the middle frequencies but approaches nearly a vertical function at the frequency limits for both the low and the high frequency regions. Because this portion of the function is very steep (and because the frequency scale is logarithmic, as well) any infinitesimal variability in the length of the basilar membrane will have a large effect in determining the frequency range. These large frequency effects (30,000 to 40,000 Hz in the example that you mentioned) are really just normal variability.

Finally, we humans have difficulty interpreting logarithmic scales because of our familiarity with linear scales. Let me give an example. Vision acuity is defined by comparing the distance in feet between what normal individuals see and the distance for an individual to see the same thing, all scaled to 20 feet, hence 20/20 vision. In reality, a person with 20/22 vision and a person with 20/18 vision both have normal vision but because the distance scale is linear, we have no difficulty in dealing with a such variability. If we now compare this analogy to identifying a frequency range of hearing on a logarithmic scale, differences between 30,000 Hz and 40,000 Hz become simply normal variability. However, this variability is much harder for humans to deal with because the numbers are so large due to the logarithmic scale.

To solve your dilemma, you have two choices. Either define the upper limit of hearing with a single number (you can pick either 30,000 Hz or 40,000 Hz and still be correct) which is what most people do, or define the upper limit as a range (from 30,000 Hz to 40,000 Hz). Many people do not opt for the second choice because it requires too much explanation for interpretation.