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PHYSICS OF HUMAN VOICE

 

PHYSICS OF HUMAN VOICE

by Akshat Tandon

This presentation includes:

•   What is sound

•   Physics behind human voice production

•   Why every human has different voice?

•   Whispering

•   Physics behind echo, loud voice and acoustics

What is Sound?

Sound is a phenomenon in which pressure disturbances propagate through an elastic material medium. In the context of physics, it is characterized as a mechanical wave of pressure or related quantities (e.g. displacement).

In simple words a source of sound waves bumping into air molecules which bump into neighbors which make a path and vibrations.

Fun Activity:

Take a steel bowl filled with water and when you tap the bowl with a spoon the water vibrates. These vibration waves bump into each other causing a chain of reaction which we hear as sound.

Physics Behind Human Voice

Like any other instrument, the human voice needs three main components: energy input, an oscillating system and a resonating tool. The air bulb in the organ corresponds to the lungs. The energy comes from the muscles around the lungs; these are the diaphragm below the lungs and the muscles between the ribs. The oscillating system is realized by the vocal cords embedded in the larynx. The resonating system is the vocal tract, which means the opening between the vocal cords and the lips.

When the muscles around the lungs contract, air pressure is built up, resulting in an air-stream upwards, along the trachea. The over-pressure in the trachea causes the vocal cords to move apart so that air can flow through this opening, called glottis. This process happens very quickly, and the periodicity of the oscillation determines the pitch of the generated sound. In order to produce the concert pitch of A4 (440 Hz), the vocal cords have to open and close 440 times within a second!

Vocal Cord Vibration and Formants

A special feature of this vibration consists in the vocal folds being closed for about half of the oscillation period. Because of the long time duration when the cords are closed, the vibration of the vocal cords differs distinctly from a sinusoidal oscillation. Therefore the vibration contains strong higher harmonics, and the sound produced is rich in overtones.

The resonances of the oral cavity are called formants. Applying the average length of a male vocal tract of 17.5 cm, the resonances have frequencies of 500 Hz, 1500 Hz, 2500 Hz, and so on.

Why Every Person Has a Different Voice

The voice of each person is different due to several factors including the size and shape of the vocal cords, the structure of the throat, mouth, and nasal passages, and individual differences in resonance and articulation.

•       The voice is produced by the vibration of vocal cords located in the larynx.

•       The size and thickness of the vocal cords vary from person to person, influencing pitch and tone.

•       The shape and size of the vocal tract, including the throat, mouth, and nasal passages, affect resonance.

•       Individual habits, such as speech patterns and accents, also contribute to the uniqueness of a person's voice.

Whispering

Contrary to the speaking or singing mode, the vocal cords are not completely closed while whispering. Usually, the rear part of the glottis remains open in a triangular form, forming a so-called whispering angle. The air flow through this opening does not cause the vocal cords to regular oscillations but creates turbulences. The basic sound is a noise. Therefore, some characteristics of a voice, like pitch or timbre, are lost, while others like formants remain. Because of this reason a whispered voice can still transmit textual information.

Physics Behind Echo, Loud Voice and Acoustics

Echo

Echocardiography uses very high frequency sound waves (ultrasound), which are inaudible to human ears. Sound waves are forms of pressure wave; they can travel through any medium. In echocardiography these waves are generated and detected by means of a piezoelectric crystal contained within a transducer.

In the body, echoes are generated when ultrasound waves reach tissue, where the acoustic properties (densities) change, and are then reflected back. This usually happens at organ boundaries, tissue interfaces, and cellular boundaries. Dense structures, such as the pericardium and calcified valves, appear bright (white), whereas blood filled cavities are almost echo free (black).

Ultrasound waves are sinusoidal fluctuations in pressure. For medical applications, the frequency of ultrasound tends to lie within the range of 2-20 million cycles per second (MHz), and echocardiography sits at the lower end of this spectrum (2.5-10 MHz).

Loud Voice

A loud voice is produced by high-amplitude sound waves, where greater air pressure and energy from the lungs vibrate the vocal cords more intensely.

Unit of Loudness — Decibels (dB):

The most common unit for measuring sound intensity. It is a logarithmic scale that compares the intensity of a sound to a reference level, typically the threshold of hearing (0 dB). The formula for sound intensity in decibels is: L = 10 Log10 (I / I₀)

Factors Affecting Loudness:

•       Sound pressure level (SPL): The higher the SPL, the louder the sound.

•       Frequency: The higher the frequency, the shriller the sound.

•       Duration: The longer the duration, the more likely it is to be perceived as loud.

•       Distance from source: The farther away, the quieter the sound.

•       Background noise: Can mask sounds, making them more difficult to hear.

The National Institute for Occupational Safety and Health (NIOSH) recommends that the average sound level in the workplace should not exceed 85 dB over an 8-hour period.

References

•   The Physics Hyperbook

•   Sathee

•   NBC

•   Wikipedia

•   Sparkl

•   Scribd

•   The Voice Foundation

— Akshat Tandon



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