Acoustics Tutorial
Acoustics is the interdisciplinary science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound. Acoustics is also a term that is used to describe the properties or qualities of a room or building with respect to how sound is transmitted or perceived in it. This tutorial will cover the following aspects of Acoustics:
Definition of Sound
Sound is a variation in local atmospheric pressure at a rate which we can hear.
The rate (or frequency) of pressure variation is from about 20 Hz to 20,000 Hz, give or take, depending on age and presence or absence of a hearing disorder. The pressure variation is quite small, so we could never see it on a barometer or standard pressure gauge. Special instruments are required to “see” sound.
Attributes of Sound
Sound has both objective and subjective attributes. Objective attributes are the things we can measure, such as …
Qualities of Sound
The subjective attributes of sound are related to what our brain tells us about what our ears are sensing. Hearing is a percept, and what is perceived is sometimes only loosely related to objective qualities.
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Loudness (how loud or soft), in phons
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Pitch (how high or low), in mels
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Quality, which is multidimensional
Loudness and pitch, as qualities of sound.
Transmission of Sound
Sound requires a medium for propagation. No medium: no sound.
You can’t hear the alarm clock ring inside the bell jar when all the air is pumped out of it and a vacuum is created. Air is one of the media for sound transmission.
Some media are better at transmitting sound than others.
Air and air/oxygen mixtures are the two media for sound transmission that we are most likely to encounter in pulmonary medicine and respiratory therapy. They are not the best; but they are what we must work with.
Sound can be reflected ...
Sound can be focused ...
Sound can also be absorbed and transmitted through other materials ...
Waveguides
But most importantly, sound can also be directed through the use of a “waveguide,” and this becomes one of the important operative principles of acoustic airway clearance.
Some waveguides are fairly simple …
Some waveguides are quite complex …
A Familiar Waveguide
Those of us who are clinicians use a simple waveguide known as a stethoscope to transmit sounds emanating from the lungs, heart, bowel and certain blood vessels to our ears.
Another Familiar Waveguide
The tracheobronchial airways are also waveguides. We know from our clinical experience that the airways are very capable of transmitting sounds throughout their entire length.
Bi-Directional Sound
A useful analogy for the manner in which waveforms may coexist in the respiratory tract would be the way ripples on a pond, from different sources, readily pass through one another without disturbing each other.
Even ripples emanating from different directions, and with different amplitudes, can pass through each other and continue along their intended pathway (unless they are of especially large and unusual magnitude, which would constitute “distortion”).
This concept is important not only because it explains why we can have a multitude of sounds existing simultaneously, but it also explains why we can have the local atmospheric pressure perturbations known as sound coexisting with the pressure changes inside the tracheobronchial tract associated with “breathing.”
Pulmonary Auscultation
Pulmonary auscultation, and the fact that we depend upon it diagnostically, is proof that the airways can conduct an extensive range of sound frequencies with high fidelity and, to a great extent, absence of distortion.
Electronic stethoscope recordings of various breath sounds.
Resonance
In physics, resonance is the tendency of a system or a structure to oscillate at a greater amplitude at some frequencies than at others.
Tuning forks are good examples of objects that resonate and create a nearly pure frequency. When struck and caused to vibrate, a tuning fork resonates at a specific frequency, according to its design. Different notes or frequencies can be deliberately achieved by making forks of different sizes, lengths and thicknesses.
The tuning fork resonates at a “pure” frequency and makes a pleasing musical tone, unlike other objects that may resonate at a variety of frequencies and will produce “noise.”
Resonant Frequency (RF)
The Resonant Frequency (RF) is the frequency at which a structure vibrates at the greatest amplitude. Objects, structures and systems can have multiple RFs.
Sympathetic Resonance
Sympathetic resonance or sympathetic vibration is a harmonic phenomenon wherein a formerly passive vibratory body responds to external vibrations to which it has a harmonic likeness. The classic example is demonstrated with two similar tuning-forks of which one is mounted on a wooden box. If the other one is struck and then placed on the box, then muted, the un-struck mounted fork will be heard vibrating at the same tone or frequency.
Watch the video below for a demonstration of Sympathetic Resonance (be sure to turn on your speakers and adjust the volume).
As will be seen on subsequent pages, we use the principle of sympathetic resonance to describe how the Vibralung Acoustical Percussor operates. A portion of the acoustical energy applied to the airway opening, which is vibrating the column of gas in the airways, is also transferred to the airway surfaces and/or the secretions in the airways, using the physical principle of sympathetic resonance, thereby causing them to vibrate as well.