An increase in the range of molecular oscillations (Intensity) will be perceived as an increase in loudness. If the energy producing a sound is increased, the frequency may stay the same, but the range of oscillations of the molecule will increase.
Hence, the molecules striking the ear drum will have more intensity and will be perceived as being a louder sound.
A curious feature of hearing is the range of degrees of loudness to which the ear can respond.
The Threshold of hearing is the intensity level of a sound which is perceived by the listener fifty percent of the time.
At one end of the range is threshold, which may be defined as the intensity level of a sound which is perceived by the listener fifty percent of the time.
At the other end of the range is the intensity level where severe pain is experienced and damage to the ear is likely to occur.
If I arbitrarily set the threshold of hearing to a value of 1, the loudest sound the ear can handle without generating serious pain is: 10, 100, 1000, 10,000 or more intense?
(Take a guess. How much more do you think? The answer is below).
We are considerably interested in that range because it may differ between individuals, particularly in regards to the threshold value.
For some individuals, threshold may be raised considerably to the point where it is above the level of many environmental sounds.
The range of hearing for loudness is so large that we must use a logarithmic scale to the base 10 to measure it.
If we start with 1, the loudest sound we can hear is:
100,000,000,000,000,000
Our problem now, if we want to test that range, is to develop an intensity dial that goes from 0 (no sound) to that number.
Hall's attempt to make an Intensity Dial is presented below.
My intensity dial was a failure. I couldn't do it because it takes too many numbers to get to the top. That's. because I was using an Arithmetic scale.
0, 1, 2, 3, 4, etc.
A Geometric scale would involve less numbers. For example:
1, 10, 100, 1000, 10,000 etc.
But still the numbers are too large to fit on a dial. So we can use their log values. For example:
100, 101, 102, 103, 104 etc. (That is supposed to read 10 to the zero, 10 to the first, 10 to the second, 10 to the third, etc.)
Since the above is all to the base 10, we can leave out 10 and just use the exponents like this:
0, 1, 2, 3, 4, etc.
Zero decibels (0dB) is the average threshold of hearing for the healthy adult ear.
The units of this logarithmic scale are called Bells (after Alexander Graham Bell.
0Bells, 1Bells, 2Bells, 3Bells, 4 Bells, etc.
Of course, nobody uses this scale now because the units are too large, just like one dollar is too large for many things we want to purchase. Hence, just like one dollar is broken up into 10 dimes, one Bell is equal to 10 decibels (dB). This is the unit
0dB, 10dB, 20dB, 30dB, 40dB, etc.
A person with a hearing level at 4 Bells (40dB), would require a tone 10,000 times more intense than a person with a normal threshold (0 dB).
At 3 Bells (30dB), the tone would have to be 1,000 times more intense.
At 2 Bells (20dB), the tone would have to be 100 times more intense.
At 10 Bells (10dB), the tone would have to be 10 times more intense.
Although it changes a little for each frequency, the air pressure at 0dB is about .0002dynes/cm2
At 0 Bells (dB), a tone would equal the intensity that is accepted to be the standard normal threshold for the healthy adult ear.
Zero, in an arithmetic scale, would mean a total absence of sound.
But 0dB is a logarithmic scale and hence does NOT mean a total absence of sound. It represents the pressure of the sound at threshold, which for most frequencies is close to .0002 dynes/cm2 .
The dynes/cm2 unit is a more appropriate measure here than pounds/in2, because of the delicate nature of the eardrum and its sensitivity to very minute pressures.
If 0dB is a positive intensity level, then there must be smaller (lesser) intensities.
Hence, a person with a threshold of -10dB can hear a tone at 1/10 of the intensity level of a person with 0 dB threshold.
A hearing threshold of 30dB or more for most frequencies in both ears would cause a significant speech and language problem
I have known some young children to test as low as (minus) -30dB!
As a very gross rule of thumb (to serve our purposes here in describing speech and language development) I am going to say that any hearing threshold 20dB or below is within the normal range.
Also, (as a gross rule of thumb) any threshold 30dB or greater will pose a significant problem for language/speech development and use.
That is, providing the loss is for most frequencies in both ears. You can develop speech and language in a normal fashion with just one good ear.
So what are the chances that a person would have a 30db loss in most frequencies of both ears? Unfortunately very good!
The highest incidence of hearing loss in the United States is due to middle ear infections, which are very common in young children.
