Re: My audible spectrum is from 56Hz to 20kHz, why isn't it from 20Hz to 20kHz?

The observation you made is very much what I would expect due to well-known 
difficulties in measuring hearing ability. In other words, it is likely 
that your hearing actually works as the textbooks say but that your 
measurements were in error.  There are several reasons why your 
measurements may have been in error.

1) An accurate measure of hearing at any frequency may require a special 
testing environment, usually a special room that looks like a meat locker.  
These special rooms reduce the surrounding noise to allow a true measure of 
hearing.  Measuring hearing in a regular classroom means that the 
surrounding noise was not controlled.  Your actual hearing ability may 
allow you to detect 20 Hz signals, though you may not have been able to 
accomplish this because of the noise in the classroom.  Typical 
environments such as a classroom have substantial amounts of low frequency 
noise that can affect the results at the lowest frequencies.  This low-
frequency noise does not even sound very loud because our ears are not very 
sensitive in the low-frequency region.

2)   An accurate measure of hearing for the extremely low frequencies 
requires a special earphone or a special loudspeaker.  Measuring at the 
high frequencies also requires a special earphone or a special loudspeaker.  
It is practically impossible to produce a single earphone or a single 
loudspeaker that works accurately at both ends of the frequency spectrum.  
This is one reason why an inexpensive a clock radio with  only a single 
loudspeaker does not sound very good; it is not reproducing the low  or the 
high frequencies very well.  Each channel (the right or the left) of a very 
expensive stereo system often has multiple loudspeakers to cover the entire 
range.  To do your experiment accurately, it would be necessary to use a 
minimum of three loudspeakers, one for the low frequencies, one for the mid 
frequencies and one for the high frequencies.  If you used earphones rather 
than a loudspeaker, a new problem can occur.  The earphone must provide an 
airtight connection to the ear.  Any air leak here will drastically reduce 
the amount of low frequency energy the earphone can supply.  So, if you 
used an earphone that was capable of producing the lowest frequencies 
(doubtful) and it was not pressed real hard against your ear, the low 
frequency energy simple leaked out compared to the mid or high frequency 
energy.

3) The normal ear is not equally sensitive at all frequencies.  If you 
graph hearing sensitivity as a function of the log of the frequency, the 
graph is "U" shaped meaning that the hearing sensitivity will differ 
depending on the frequency region.  In the low frequency region the 
function is very steep (the left arm of the "U"). In other words in the low 
frequency region, the ear rapidly becomes more sensitive with frequency.  
In the middle frequency region, the function is fairly flat (the bottom 
part of the "U"), that is, the ear is approximately equally sensitive 
across the middle frequency region.  As the frequencies are swept to the 
higher frequency region the ear becomes less sensitive at a rapid rate (the 
right arm of the "U").  So, small changes in frequency at either the low or 
the high frequency region will cause larger effects compared to small 
changes in the mid frequencies.