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 The strength of sound(sound volume) varies according to the amplitudes
of the sound wave. Sound is heard high when the amplitude of the sound wave is high. Pronunciation devices such as piano show differences in the feeling of sound according to whether they are played with power or not. The differences is attributed to the strength of sound whose physical
standard unit is a sound pressure level, that is, decibel(dB).
■ Decibel(dB)
The sound pressure level of a low sound as just detectable is about 10
million times as low as that of a high sound which makes us feel a pain.
Such big difference is very difficult to be treated as a absolute unit. Unlike the scale of a ruler, the feeling of sound is
related with the changed proportion of sound pressure level. In other words, the use of a measure saying what times of the initial level of sound pressure is better consistent with our feeling of sound. The standard unit of the measure is
decibel(dB). Therefore, dB is not an absolute unit but a proportion of difference between a specific value and a
measured sound volume.
Level of Sound Pressure(LP) = 20 log (p/p1) (dB)
Like in the equation, the twice of a sound is 6dB in size, the triple is 9dB, the four times is 12 and the six times, 20.
The increase of sound size is similar with that of a stereo volume. In general, the sound size which may call our attention is at least 80dB or over. Our Signal Phone makes an output from 80 up to 130 dB.
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 High or low sound is a feeling depending on the vibration number of the
sound wave per second. When the vibration number is large, the sound is heard high while when the number is small, the sound is heard low.
The standard unit of vibration number is Hz. Man's ears can hear 20 to
2000Hz, the range of audible sound. The height of sound is related with
the vibration number of the sound wave, but not proportionate to the
number. When vibration number becomes double, it is described that the height f sound becomes one octave higher than its initial value, not that the height of sound become double.
The height of sound(frequency) is based on a principle like in paino keys whose sound becomes higher as they proceed
to the right.
Our Signal Phone has its own frequency band on which warning sound, alarm, melody and vocal signal proceed in order. The warning signal is highest in frequency(high sound) and the vocal signal, lowest(low sound).
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Timbre is defined according to differences among momentary changes in the wave form (over tone, harmonics),
based on the proportion of high-frequency components of the sound, and sound volume. The reason why the sound of the piano is different from that of the violin despite
equal in strength is because the two instruments are
different with each other in timbre. This also explains that there are differences in voices among individuals and that birds' singing and tigers' roaring can be discriminated.
Passing through the prism, the sun light is divided into rainbow colors. This suggests that the sun light consists of several different colors(or, several different numbers of vibration). This is similar in case of sound. Sign waves can be regarded as sound materials. The timbre of a sound depends on the size of the sign wave for the sound.
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The felt size of sound is not equal to the physical strength of sound(dB). Even sounds whose physical strength is equal
are differently heard by man's ears in case that they are different in height(vibration number, frequency). A sound
whose frequency is high is more stressful to the ark shell and louder to the ears than that whose frequency is low.
Thus the felt size of sound increases as the strength(dB) and height(vibration number, frequency) of sound become
high. In other words, the size of sound goes up when the vibration number and amplitude of sound increase.
For our some products making sounds in equal size, or 115dB, the size of warning sound, whose frequency is high, is felt largest while that of vocal signal, whose frequency is low, smallest.
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