I.3.1. THE SOUND
We live in a world of sounds, undoubtedly essential in terms of communication and/or transfer of knowledge. Nature furnishes us with an abundant variety of sound sources, but it’s the man-made ones that often raise problems for the environmental health and will be discussed here. Besides the sources of sonic energy, the existence of sound implies one or more media through which this energy is transmitted. Thus, the sound may be defined as “a form of radiation which involves pressure waves in matter”.
The pressure fluctuations (as seen in Figure I.3.1.) are of a vibrational nature, causing the neighboring air pressure to change with no apparent movement of air taking place. These pressure variations create in humans the aural sensation called commonly “the sound”.
Figure I.3.1. Representation of sound
Short History of Acoustics
The word “acoustics” is derived from the Greek word meaning hearing. Although interest in acoustics was shown through music and architecture as early as ca. 4000 b.c. by ancient Egyptians, Hindus, Chinese and Japanese and later by renowned antique Greek figures such as Phytagoras, Aristotle, or the amphyteatre architect Marcus Vitruvius Pollio, it was not until the seventeenth century that the nature of sound was critically examined with carefully planned experiments. Thus, in 1636, a Franciscan friar named Marin Mersenne (who is often called today “the father of acoustics”) performed the first experiments which aimed at the determination of the speed of sound in air. These measurements were repeated in different circumstances during the next three centuries with greater refinements about the World War I due to the usage of microphones. Nevertheless, the experiments regarding the velocity of sound in solids and liquids were not performed until early in the nineteenth century. In 1808, J. B. Biot, a French physicist measured the speed of sound in iron, while Daniel Calladon, a Swiss physicist determined its speed in water. The temperature dependence of the velocity of sound was established during one of the Arctic expeditions.
Although the science of acoustics was continuously developed over the centuries, the system that has changed our civilization forever was telephony, the principle that allowed human speech to be transmitted at great distances. It was in the summer of 1874 that Graham Bell, a professor of vocal physiology at Boston University conceived the first speaking phone. Today, only in Bucharest, the number of fixed telephones raises to 1 797 556, with a number of 3 457 000 in the whole Romania for Mobile phones sold only by Connex.
Another revolutionary discovery which enveloped the world with speech and music was radio broadcasting. The young Italian physicist Guglielmo Marconi put together in 1896 a number of electronic devices and demonstrated his system to the Italian Navy in the Mediterranean. After this event, the development of broadcasting was so rapid that in 1927, the Federal Radio Commission of the United States was granted legal authority to control wavelengths and regulate power. In 1939 the TV invention followed, and today the broadcasting all over the world is stronger than ever.
Whatever the period of history, men seem to have an insatiable desire for change that music can easily offer even in a constant environment. Talking about acoustics invariably relates to talking about music, therefore a summary of its history may be required here.
The development of music is divided by historians into three different periods:
(1) homophobic or unison music (up to the eleventh century);
(2) polyphonic music with several parts, but without any independent musical significance of the several voices (up to the seventeenth century);
(3) modern polyphonic music in which many melodic lines were superposed.
Modern music has benefited greatly from Thomas Edison’s two inventions, the phonograph (1877) and the motion picture machine (1891). Also, in 1889, the first magnetic recorder was invented by Valdemar Poulsen in Denmark.
New sounds of music in the twentieth century were only made possible by the developments of electrical engineering and have continued to grow in size and diversity.
This is the number of vibrations or pressure fluctuations that a sound source undergoes per second. The sound emitted from the source will propagate with the same frequency. The unit is the Hertz (Hz).
Example: 440 cycles/sec = 440 Hz for a violin string.
This is the distance traveled by the sound during the period of one complete vibration (see Figure I.3.1). The unit is the meter (m).
Velocity of Sound (c)
The frequency and wavelength of a sound wave are related by the simple formula:
c = f (1)
Equation (1) shows that the velocity of sound (c) in a given medium is a constant. It is sufficiently accurate for the purpose of building acoustics to consider this value equal to 330 m/s. However, sound propagates in different media at different speeds with small temperature dependence that is the principle responsible for the bending of the sound in the atmosphere.
Examples: the velocity of sound in brick is equal to 3650 m/s or in a steel bar, 5060 m/s; the velocity of sound in water is at 20ºC, 1482 m/s.
Propagation of Sound Waves
Sound is thus a longitudinal wave, i.e. the vibrations are in the direction of motion (see Figure I.3.1.). As a wave, sound shows all wave properties: absorption, reflection, diffraction and interference. However, as a longitudinal wave, sound cannot be polarized.
Pressure (p), Intensity (I); Sound Pressure Level, Sound Intensity Level
The magnitudes of sound pressure affecting a healthy human ear vary from 2x10-5Pa at the threshold, up to 200 Pa in the region of instantaneous damage. In comparison with the normal atmospheric pressure of 10-5 Pa, these magnitudes have a wide range, therefore, for the mathematical usage, a logarithmic scale proved accurate describing also the actual non-linear ear response to pressure variations (Weber-Fechner law).
Sound intensity (I) is proportional to the pressure (p) as in the formula below:
I p2 (2)
The unit is the W/m2.
The two important parameters associated with sound are the sound pressure level and the intensity level. They are defined by the following:
(1) Sound pressure level = 20 log10(p/p0) (3)
where p0 is the pressure at the average threshold of hearing at 1000 Hz frequency (e.g. 2x10-5 Pa) and p is the applied sound pressure,
(2) Sound intensity level = 10 log10(I/Io) (4)
where I0 is the minimum threshold intensity (e.g. 10-12W/m2) and I is the applied sound intensity.
Sound intensity is a more complex measure for sound as the rate at which acoustic energy passes through the unit area perpendicular to the direction of propagation of the wave. Both types of level are expressed in decibels (dB) and have, according to definitions (3) and (4), the same value in the particular case of a plane wave. However, the sound intensity level is a directed quantity giving information also on the direction of the sound flow. Table I.3.1 shows the characteristics of few representative sounds.
Table I.3.1. Representative sound levels
Intensity level (dB)
Threshold of hearing
Threshold of feeling
100 = 1
Threshold of pain
It is convenient to express the total sound output from a source as power. This is equal to the intensity multiplied by area. The unit is the Watt (W).
Examples: A loud voice reaches about 1 mW, a large orchestra 10 W, whereas a jet plane may generate values of up to 100 kW.
I.3.1.2. Aural Environment
The auditory apparatus of man with the ear as its vital component is exceedingly complex. Its permanent function is that of converting the sound pressure waves of the surroundings into signals that are sent to the brain; the ears are never closed. Figure I.3.2 presents the main components of the ear, showing the outer, middle and inner anatomical parts.
Figure I. 3.2. Sketch of the anatomical parts of the ear
The sound reaches first the outer (visible part) of the ear, the pinna. Designed with a concave shape, the pinna scatters longer wavelengths, while reflecting the shorter ones into the auditory tube (meatus). The meatus ends into the tympanic membrane, which, due to its size (2.5 cm in length and 5-7 mm in diameter), resonates to a frequency of about 3 kHz.
The middle ear is an air-filled cavity that picks up, amplifies (20 times) and transmits the vibration motion of the eardrum to the inner ear. A chain of tiny bones (hammer, anvil and stirrup) suspended and constrained by ligaments and small muscles in here has the role of a shock absorber for the excessive overpressures associated with certain sounds (e. g. levels above 90 dB for more than 10 ms). The air in this enclosure is maintained at atmospheric pressure through the Eustachian tube that connects the ear to the mouth.
The inner ear is a system of fluid-filled canals protected acoustically inside the temporal bone of the skull. Under the influence of sound, the stirrup strikes a small opening covered by a membrane at the beginning of the inner ear causing its vibration and transmitting this way the pressure to the fluid within. Three semicircular canals are disposed as one for each dimension of space furnishing us with the sense of equilibrium. However, the most important part of the inner ear is the cochlea, which contains the final receptor for the hearing. Long of about 40 mm, cochlea supports as a result of fluid movements the bending of the little hair cells (Corti organ) within. Thus, nerve impulses (2400 endings) are initiated and transferred via the auditory nerve to the temporal lobe of the brain giving us the sense of “sound”.
This extends in frequency for humans between 20 Hz and 20 kHz and depends upon the age and physical condition of the individual. Frequencies above and below this range are known as ultrasonic and infrasonic, respectively.
Figure I.3.3 shows the average threshold curve for the young adults with normal hearing. It can be seen that the human sensitivity varies considerably over the audible range, especially near the threshold of hearing where variation is of about 70 dB. Maximum sensitivity is reached in the range of 3 kHz as dictated by the anatomy of the outer ear. At this level, the acoustic threshold intensity is of the order of magnitude of 10-12W/m2 and the physical displacement of the eardrum has been found to be as minute as 10-9 cm. Figure I.3.3 shows also that some discomfort is apparent at 120 dB, and sensations of tickle and pain are experienced in the case of every frequency that exceeds 140 dB.
Figure I.3.3. Diagram of the approximate threshold of hearing for young people (age 18-25)
Unlike frequency, which is an objective measure, pitch is a subjective term depending mainly on frequency, but also on intensity, the wave form and the duration of the signal. The sensation of pitch is usually alluded to in terms of “high” and “low”. The unit is the mel.
Example: A tone having a frequency of 1000 Hz is arbitrarily defined as having a pitch of 1000 mels. Figure I.3.4 shows the relationship between pitch in mels and frequency.
Figure I.3.4. Relationship between pitch in mels and frequency
This is also a subjective effect that is a function of the ear and brain as well as the intensity and the frequency of the vibration. A sound that is regarded loud to one person may not appear as loud to another whose hearing is poor. Moreover, due to the ear’s variations in sensitivity to different frequencies, two notes that are equal in intensity and different in pitch may not sound equally loud, even to a single observer. The unit is the Figure I.3.5 shows a range of equal-loudness contours for a normal ear.
The curves illustrate the fact that loudness becomes less frequency dependent as the level of loudness increases. Also, at 1000 Hz, loudness and intensity levels are numerically equal.
Figure I.3.5. Equal loudness contours
Noise is often referred to as any unwanted sound, the concept varying greatly for each individual according to his interests, activities or mood. Despite its versatility, the categories of noise generally accepted are:
(1) ambient or background noise;
(2) steady-state noise;
(3) fluctuating or intermitting noise and
(4) impulsive noise.
Ambient noise describes the noise in the environment from both near and far away sources. This does not include strong sounds.
Steady-state noise often refers to sound generated by machinery with levels reasonably constant during the period of measurement.
Examples: Railway noise, industrial noise.
In case of fluctuating noise the sound may vary in level but it is “on” (above the ambient level) for times longer than 200 msec (the integration time of the ear).
Examples: Traffic noise, open-air concerts, discotheque noise.
An impulsive noise is the noise of a very short duration for its peak pressure level.
Examples: Dropping of a hard tool on the floor, beeping of a car.
Hearing defects are the effects of noise on humans that are the easiest to be clinically evaluated. Diseases such as presbycusis, tinnitus or conductive, nerve or cortical deafness are diseases of the ear which may or may not involve the factor noise. However, it is considered that the hearing ability is a function of three variables: noise characteristics, exposure time and age.
The intensity level of a sound to cause immediate permanent deafness is that of the order of 150 dB (see Figure I.3.3.). Moreover, the noise-induced hearing loss depends on frequency as shown in Table I.3.2, with narrow band noise being more damaging than broadband noise. The harmful effect to hearing of long duration noise is more difficult to assess and the limits variable. Here we will consider that a safe occupational approach would be an exposure of 5 working hours per day below the levels presented in Table I.3.2. Age alone may cause hearing loss, although other interferences are with certainty present.
Table I.3.2. Maximum levels of noise
Other Effects of Noise
It is known for several decades now that noise can induce responses in the human body that are not related to the auditory system. These are both physiological and psychological effects. The intensity level region corresponding to psychological effects begins with 30-60 dB, while that related to the physiological effects is 60-90 dB.
In most instances a physiological change is evident only during or for a short period after the noise manifestation. However, long time exposures may conduct to severe modifications of the physiological equilibrium of the body. Greatly affected are the circulatory, endocrine, digestive and respiratory systems. The most common circulatory effects of noise are the raise in blood pressure, raise or decrease in pulse according to frequency as well as a reduction in peripheral blood flow. Among the endocrine glands, the thyroid has the highest susceptibility to noise. Also, the diminishing of the gastric secretions and of the bowel movements at different levels of exposure are important effects. Changes in the breathing pattern have been noted for intermittent noise with an increase in the quantity of the carbon dioxide eliminated.
Psychological effects of noise are harder to be quantified. However, irritability, annoyance or negative emotions, disorders reflected in lack of attention or vigilance, speech interferences or finally sleep disorders are no less to be considered.