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Feature : Sound Tech 1 - Intro/Frequencies
Over the last several years I have been involved in car audio in some way,
from a Civic where I was running a JBL 12" off my head unit to the present,
being my multimedia filled Wagon.
Every step and upgrade was a challege, and with that challege came knowledge.
Over the next few 'features' I am going to share that knowledge, and I
credit a great deal of it to Ian Brooks and Alan Farmer.
Sound is vibrating air, which our ears can hear. The number of vibrations
per second is referred to as 'cycles per second' (CPS) or more commonly known
as Hertz (Hz), in honour of the physicist Heinrich Hertz. Sound frequency
measurement is generally accepted as 20Hz-20KHz (20,000Hz), human hearing
being approx 30Hz-16KHz, and most sensitive at low level 2K-3KHz.
Musical notation is based on 12 notes between each doubling of the
frequency (octave): Between 110Hz and 220Hz there are 12 notes, between 220Hz
and 440Hz are 12 notes and so on. The division between the notes is based
on the 12th root of 2. Each single adjacent note is called a Semi-tone. Each
second alternate note is called a Tone. 8 (Octave) of the 12 notes can be
used to make a scale, as 2 tetra-chords. Octave is also used to describe
double or half the frequency (ratio 1:2).
Audio engineering uses the octave (frequency ratio 1:2) for frequency measurement.
Other areas of engineering use the decade (ratio 1:10), 3.3 octaves is 1 decade.
Decibel (dB), deci meaning one tenth and bel in honour of Alexander Graham Bell.
dB is a ratio for power (1:10) therefore:-
0db is 1 with 0 zero (1)
10db is 1 with 1 zero (10)
20db is 1 with 2 zeros (100)
60db is 1 with 6 zeros (1,000,000)
120db is 1 with 12 zeros (1,000,000,000,000)
Our ears expand when it is quiet to hear detail and contract when it is loud. 1dB
is the smallest discernable power change, 3db (2x power change) we only hear as a
bit louder and 10db (10x power change) we only hear as double as loud.
Audibly, in general terms, the difference between 100 - 200 watt audio system, is
minimal (3db) while the difference between 100 - 1,000 watt audio system is twice
as loud (10db).
0dB SPL (Sound Pressure Level) is threshold of hearing, and if you want to get
technical, its the smallest modulation of atmospheric pressure 0.0002dynes/cm²
(20micro Pascals) at 2KHz.
20 - 40dB SPL quiet ambient background, will allow 60db dynamic range.
40 - 60dB SPL is average noise level of the world we live and work in.
60 - 80dB SPL is loud sound needing to speak loudly to be heard.
80 - 100dB SPL is loud
100 - 120dB SPL is VERY LOUD
By paying attention to this graph, it can be seen that at he threshold of hearing,
our ears are approx 60db (one million times) more sensitive at 2KHz than at 40Hz.
This is the reason for the loud switch on most audio systems. The loud switch
boosts the bass to compensate for our hearing at low level. But at high power, all
frequencies tend to be heard at approx the same level.
Hearing damage is specified from 84 dB SPL+ for 4hrs of continuous industrial
machine noise. Time is halved for each 3db increase: 87dB = 2hrs, 90dB = 1hr, etc.
A false belief of many is that excessive loud music is not harmful. Reverberation
in vehicles and buildings can hold sound at a continuous level,adding 20-30db more
sound energy, similar to machine noise. Mainstream music is often over-compressed
within a 10db dynamic range, excessive use of compression to exaggerate loudness
causes music to sound flat and lifeless.
Sound is air vibrating, traveling as a wave motion at approx 344 meters per second
(depending on temperature, humidity, distance above sea level). Wavelength is the
distance of one complete wave cycle. Wavelength = 344 / frequency, so as an example,
the wavelegth of 42Hz is 344/42Hz = 8.2 meters.
Sound Facts:
1 Acoustic Watt at 1 meter, radiating spherically from a point source = 109dB SPL.
Inverse square law: As direct sound doubles in distance, energy decreases to 1/4
(-6db).
The critical distance is the point where direct and reverberant sound energy is equal.
An echo is sound reflected back from 10 meters or more, heard as distinct repeat.
Acoustical Masking: any sound of 6db+ masks others of similar frequency.
The spectral energy of music is flat to approx 1K Hz, then approx -6db per octave
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