Fundamentals of Sound and Hearing
Sonic-Shield believes an educated customer is our best customer, therefore we want to talk with you about the fundamentals of sound and hearing in order to help you understand your particular noise problem.
The fundamentals of sound and hearing outlined below may seem a little technical to some, but believe us when we tell you, these really ARE the fundamentals of sound and hearing. We have some really passionate team members on our staff who have made the study of sound and hearing their life’s work and THEIR fundamentals of sound and hearing are for MENSA members.
OK- so lets get started. Fundamentals of sound and hearing starts with sound. Sound is mechanical energy that repetitively causes collisions of molecules in a medium, resulting in periodic changes in the form of waves, which can propagate through gaseous, liquid and solid materials. Acoustics is a technical field that involves the study of how sound waves behave and how they are perceived. OUCH! First paragraph……….fundamentals of sound and hearing? O.K. Let’s keep going.
The fundamental measure of sound loudness level is the decibel (dB), which defines the change in atmospheric pressure relative to a standard pressure level. The lowest perceivable sound level is 1 dB and the upper limit of sound that a human can tolerate is 120 dB. We consider sound to be “quiet” when it is less than 40 dB, “moderate” when it is between 40 and 70 dB, and “loud” when it is 70-100 dB, and “dangerous” if it is over 100 dB. Repeated or continuous exposure to “loud” and “dangerous” noise levels can lead to temporary and permanent hearing loss.
The frequency of sound is the distance between successive peaks (or valleys) in the waves divided by the speed of sound in that medium, expressed in cycles per second, or Hertz (Hz). We interpret the frequency of sound in terms of tones or “notes”, and we hear the tones in terms of “octaves”, where an octave is twice the numerical frequency of the fundamental tone. For example, if the fundamental tone occurs at 100 Hz, an octave above this tone occurs at 200 Hz. On a musical scale, there are 12 semitones (half-notes) within an octave. Acoustical engineers will sub-divide each octave into three regions, corresponding to the lower, middle and upper one-third octave bands. The lower frequency limit of normal hearing is 20 Hz, and the higher limit is 20,000 Hz.
Our ears are not sensitive to sound uniformly over the audible hearing range. That is, at a constant sound pressure level, sound at the very low and high frequency ranges is not heard as well as sound in the mid-frequencies. This is probably due to the fact that humans evolved to better hear sounds within the speech frequency range, which normally occurs between 100 Hz and 3000 Hz. This effect is more pronounced when the sound levels are low versus when the sound levels are high. For example, when listening to low volume levels of music, we will typically need to boost the low frequencies and high frequencies (by engaging the so-called “loudness button” on our stereo receivers). The sensitivity as a function of frequency is often called the “frequency response”. The “loudness button” is not needed at higher volume levels, however, as the bass frequencies in particular will sound “booming”.
Acoustic testing equipment, such as sound level meters, measure sound with a “flat” frequency response, that is, sound across the audible range is “heard” at equal volume. In order to simulate the frequency response of the ear, acoustic engineers have developed “weighting scales”, which are useful to determine those sound levels that may be offensive for hearing.
Many community noise ordinances and occupational noise limits are provided in overall A-weighted sound pressure levels. A-weighting is valid for noise at “quiet” to “moderate” loudness levels where our ears lack sensitivity in the low frequency range. At higher loudness levels, however, our ears are much more sensitive to low frequency noise, and it is probably more valid to use C-weighted SPLs. The proper use of A-weighting versus C-weighting must therefore consider the frequency characteristics of the noise together with its overall sound pressure level.
Now- would you like a more detailed, higher level explanation of the fundamentals of sound and hearing? Charts, tables, graphs, formulas, algebra and a very good, detailed explanation is found by opening the document below.