The Bernoulli Effect and Vocal Fold Vibration
The vocal folds primary function is to protect the airways. During respiration the vocal folds are separated (abducted), allowing air to move freely in and out of the body. The fact that the vocal folds can intrude on the airstream (adduct) and form a constriction of the airway is a critical part of phonation. The airstream is forced to make a detour around the vocal folds, and it is under these conditions that the Bernoulli Effect occurs.
The Bernoulli Effect
Daniel Bernoulli was an eighteenth-century Swiss scientist who discovered that as the velocity of a fluid increases its pressure decreases. This is demonstrated when a constant flow of fluid or gas is passed through a tube and a section of the tube is constricted. At the point of constriction the flow will speed up and there will be a drop pressure against the walls of the tube. This principle has become widely known as the Bernoulli Effect.
One way to explain this is that at the point of constriction more energy is used up as the air or fluid molecules accelerate, leaving less energy to exert pressure, and thus pressure decreases. This phenomenon explains why aeroplanes fly and why when standing in the shower the curtain always manages to stick to your leg. The two results of the Bernoulli Effect can be explained with two examples.
*Flow increase: when you place your thumb over the end of a running hosepipe, the flow of water speeds up and travels further across the garden. At the point of constriction velocity increases.
- Air pressure drop: when air flows across the top of an aeroplanes wing, in travels faster and the lower pressure creates lift. Remember that with the shape of an airplanes wing, the bottom is flat, while the top is curved. Air travels across the top and bottom in the same time, so air travels slower on the bottom (creating more pressure) and faster on top (creating less pressure). This keeps the plane in the air! At the point of constriction air pressure drops.
Awareness Exercise: Bernoulli Experiment
To witness the Bernoulli Effect for yourself try the following.
- Take two strips of paper. Hold them in front of you and let them rest about 5 cm apart.
- Blow between the two strips of paper and observe what happens.
You may have expected the pieces of paper to be blown apart. Instead you will have seen them come closer together. As the airstream passes down the flexible duct, formed by the paper, it begins to accelerate and the pressure drops. The lower pressure then causes the paper duct to collapse in, as there is higher pressure on the outside of the duct.
What does this have to do with the voice?
The Bernoulli Effect provides an explanation of how the vocal folds actually vibrate. It is a common misconception that the vocal folds vibrate through repetitive muscular contractions. Clearly this is not possible if you consider that when a soprano sings a top C her vocal folds are vibrating 1047 times per second.
The vocal folds are constructed from soft tissues that are in layers. Each layer has different properties. Ingo Titze has modelled these layers as a series masses linked together by springs. Each layer is capable of some independent movement and has a degree of elasticity. The outermost layers have the greatest degree of elasticity. When the vocal folds are adducted during phonation, the airstream is momentarily stopped by the vocal folds. At this point subglottic pressure begins to build up below the vocal folds. When the pressure is high enough, the soft tissues of the vocal folds are forced to separate and the airstream is allowed to flow through the vocal folds. Like the paper duct above, the airstream through the vocal folds then accelerates causing a drop in pressure. This drop in pressure then sucks the vocal folds back together. Subglottic pressure then builds up again and the process continues. This cycle of vocal folds motion create the air compressions and rarefactions that cause sound.
What creates pitch?
The rate the vocal folds come together and separate over a period of time is perceived as the pitch of the tone. The exact number of repetitions can be measured and is referred to as frequency. Frequency is usually expressed in hertz (Hz) or cycles per second. The higher the frequency, the higher the perceived pitch, and likewise, the lower the frequency, the lower the perceived pitch.
Many factors affect the rate at which the vocal folds come together and separate. It is a complex system. The elastic forces within the vocal folds themselves should be considered first. These elastic forces are altered depending on the laryngeal set-up the physiological state of the muscle, ligament and epithelium that form the vocal folds. Remember that not only is the muscle of the vocal fold that is important here but also all the other muscles that can change the positioning of the vocal folds. All these muscles are often referred to collectively as the intrinsic muscles of the larynx and affect dynamically the physical properties of the vocal folds. The myoelastic aerodynamic theory of vocal fold vibration states that greater mass, lesser tension or lesser elasticity of the vocal folds will tend to reduce the frequency of vibration, whereas less mass, greater tension or greater elasticity will increase the frequency of vibration. Airflow will also make a difference, as the rate at which subglottic pressure builds up underneath the vocal folds will change. We must also consider the supraglottic pressures above larynx to give a complete picture.
What creates loudness?
If more of the vocal fold muscle is engaged during adduction, when airflow commences, it takes more force a higher subglottic pressure to separate them. This higher force causes more air to be displaced and the resultant sound wave has greater amplitude. Think back to the hosepipe analogy. When the thumb is placed over the end of the hosepipe, the water comes out at a higher velocity and travels further across the garden. It is more difficult for the water to leave the hosepipe, due to the tighter constriction, resulting in the water jet travelling faster and with more force. With sound, the more air that is displaced in each cycle the larger the amplitude of the wave. When the sound wave reaches the ear drum, the larger amplitude is perceived as loudness. Intensity is measured objectively using decibels (dB).
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