3.1 Fundamentals of Acoustics and Psychoacoustics

Sound is the produced wave by vibrating entities . It travels via a medium from one point to another one. It is a mechanical wave as it produced due to the traveling motion of the sound vibration via a non-vacuous (conductive) medium where the mechanical sound wave travels. It results from a longitudinal motion of the medium’s particles. The wave’s physics explains the process of the sound generation, traveling, and reception, where the sound waves carry the vibration (disturbance) from one position to another that originated from the wave source. The initiating source may be a stereo speaker or vocal chords. The particle interaction leads to the sound traveling, which lets the vibrating waves to transport from one position to another [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17]. Conversely, the mechanical sound waves require a receiver to widespread. The general characteristics are required for the acoustic sound waves to be transmitted:

  • The acoustic waves are mechanical waves that need a medium to carry their energy from any position to another.

  • The acoustic waves, which are mechanical waves, are incapable to travel over a vacuum.

  • The acoustic waves are longitudinal waves, which consist of repeating rarefactions/compressions patterns.

There are several methods to measure the sound waves, namely, the (i) frequency; (ii) wavelength, where the distance that the disturbance travels via the medium represents a complete wave cycle; (iii) amplitude which is related to the sound volume, loudness, and intensity; (iv) phase; and (v) speed of sound that depends on the medium state/type, which is affected by the elasticity and the inertia.

Consequently, the sound represents the wave motion with different pressure due to a vibrating source that sets particles in motion only one sound tone. The discrete particles travel about their relaxing point at the same tone frequency. Vibrating particles during their movement push adjacent other ones and put them in motion. This creates a chain effect producing areas of low and high pressure. The interchange between the high and low pressure areas transfers away from the sound source generating sound waves. On a membrane, the mechanical effect is used to sense the sound waves, such as the membrane of a microphone or a diaphragm of a stethoscope. For a real-world example, say there is a trumpet playing in the room [18,19,20].

The actual sound may be an acoustic wave from a single sound pulse, mechanical vibration, noise, or a continuous frequency sound wave. Audio sound sensors (transducers) include input sensors to transform the sound waves to electrical signal and output actuators to transform back into the electrical signals to sound. Acoustic (sound) sensors can detect and transmit vibrations and sound waves from infrasound (very low frequencies) up to ultrasound (very high frequencies). Acoustic wave sensors detect acoustic or mechanical waves produced by the human body. During the propagation of the acoustic wave through the body, the propagation path characteristics change, which affect the amplitude/velocity of the acoustic wave. Measuring the phase/frequency characteristics of sensed signals reflects the occurred changes in the velocity, which is correlated to the consistent physical measured quantity. Several expressive biosignals are carried by the body sounds to state the patient’s health. The mechanical waves within the body generate the body sounds due to the mechanical vibrations of blood/tissues, airway walls oscillation, and the heart valve vibrations. These body sounds composed of several spectral varying frequency/intensity components due to the existence of the noises. The heart sounds auscultation is useful for detecting the cardiac pathologies, while the snoring/lung sounds auscultation are used for detecting the respiratory disorders [21,22,23,24,25,26]. Figure 3.1 illustrates the electrical circuit model of the body sound formation and sensing phases of the acoustic biosignals.

Fig. 3.1
figure 1

Electrical circuit model of the acoustic biosignal formation and sensing processes

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The formation of the body sounds includes their origination and transmission through the tissue. The body sounds acoustical path initiates at the sound source, which vibrates the volumes of the blood, and oscillates the biological structures. In the biological medium , the body sound propagates with velocity v, which equals the sound propagation. In addition, in the time domain, the body sound oscillates with the sound frequency f. It also oscillates with wavelength λ along its propagation path.

3.2 Acoustic Biosignal Sources

There are several body sounds that lead to acoustic biosignals.

3.2.1 Heart Sounds

The cardiac system’s contractile activity influences the heart sounds (HS), which produce direct information on the closure of the heart’s valves. The HSs provide information on myocardial, hemodynamic, and valvular activities weakening. The normal heart sounds include the sounds related to the atrioventricular valves closing to prevent the blood backward flow. Induced mechanical vibrations are obvious as the first HS due to blood flow deceleration, ventricular myocardium jerky contraction, and any abrupt tension changes. This first HS is the longest and loudest compared to all other HSs. It includes relatively low-frequency spectral components with duration of about 140 ms. The semilunar valves closing produce the second HS, which prevents the blood backward flow. In the second HS, during the inspiration, the left-sided sounds lead by about 40 ms, while with expiration both the right-sided/left-sided sounds are overlaid or still marginally split by <30 ms [27,28,29,30,31,32].

Figure 3.2 illustrates the different heart’s sound waveforms. The normal first HS is louder than the second HS in the mitral valve [33,34,35,36,37,38]. The normal minimally split first HS is a normal variation of the first HS (Fig. 3.2b). Abnormal right bundle branch block can be detected if the first heart sound splitting takes>50 milliseconds. In the tricuspid area, the splitting is obviously heard. In addition, due to several heart abnormalities, a decreased intensity first HS can be produced as shown in Fig. 3.2d. Figure 3.2e shows an aortic ejection click abnormality caused by stiffness and thickened of the aortic valve cusps.

Fig. 3.2
figure 2

Heart sounds different waveform (a) normal first heart sound, (b) normal minimally split first heart sound, (c) abnormal markedly split first heart sound, (d) decreased intensity first heart sound, and (e) abnormal first heart sound plus aortic ejection click

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The second HS has about 110 ms duration with more snapping quality, higher frequency components, and lower intensity compared to the first HS. Other various HSs occur based on the existence of abnormality or the age effect, such as:

  1. (i)

    The HS due to the rapid filling of the ventricle, which is comparatively short and comprises very low-frequency components of 25–50 Hz range.

  2. (ii)

    The HS due to the active ventricular filling and the atrial contraction. This sound contains 20–30 Hz very low-frequency spectral components.

  3. (iii)

    The ejection sounds due to the semilunar valve opening, where a HS is generated from the abrupt valves opening or the occurrence of sudden tensing leading to the clicky sounds of high frequency.

  4. (iv)

    The opening sounds due to the atrioventricular valves opening.

  5. (v)

    The murmurs are abnormal sounds due to the convinced turbulent blood flow in the backward regurgitation progress, which are high-frequency noisy sounds .

Figure 3.2 illustrates the different waveforms of the heart sounds .

3.2.2 Breath Sound

The lungs and the large airways produce in/out breath sounds (BSs) that can be received by stethoscope, which can be normal or abnormal. Lung sounds provide information about the ventilation/dynamics of the upper airways. Abnormal BSs specify a lung problem, including infection, inflammation, asthma, obstruction, and fluid in the lungs. Identifying such medical conditions requires listening to the BSs. The lung sounds’ time amplitude plots can be represented by expanding or unexpanding ways, where the expanded time scales (at 800 mm/sec) illustrate distinct patterns not appearing in the unexpanded speed plots (at 100 mm/sec) as demonstrated in Fig. 3.3 [39,40,41,42,43]. The right waveforms in Fig. 3.3 represent the expanded form, while the left waveforms represent the unexpanded form, where the amplitude and time are represented on the Y- and X-axis, respectively.

Fig. 3.3
figure 3

Inspiratory sounds: (a) normal vesicular inspiratory sound, (b) normal tracheal inspiration, (c) abnormal inspiratory crackles, (d) abnormal sonorous rhonchus, and (e) abnormal inspiratory wheeze

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Figure 3.3 shows more details of the acoustic phenomena in the time expanded analysis waveforms compared to the unexpanded ones as they are stretched out to provide an overall view of the acoustic characteristics of the inspiratory sounds in the real time. However, the unexpanded display is similar to the phonocardiographic presentation. Figure 3.3 illustrates different normal and abnormal inspiration sound waveforms. Generally, the lung sounds cannot be recognized clearly in the time domain, while the heart sounds can be easily observed due to their comparatively high intensity. The HSs are about 30 dB stronger compared to the inspiratory sounds when auscultated on the chest. The tracheobronchial normal sounds happen during both the inspiration and expiration processes, although the vesicular sounds dominate during the inspiration process only. Figure 3.3 reveals that the tracheobronchial sounds have wider frequencies which range up to 1 kHz compared to the vesicular sounds that have components up to 500 Hz. Typically, the normal breathing sound is similar to the air sound, which can be categorized according to the sound source location as follows [44,45,46,47]:

  1. (i)

    Tracheobronchial sounds are originated in the tracheal and bronchial tracts and heard close to the large airways. They are dominated on the neck. The source of this sound is given by the air turbulence that flow in the trachea and bronchi due to high airflow velocity causing air vibrations. The sounds’ propagation distance near the skin is comparatively short. Thus, the produced sounds are relatively loud similar to the air blown over a tube and hold up to 1 kHz frequency components, where the amplitude component at the baseline has frequency 1.2–1.8 kHz.

  2. (ii)

    Vesicular sounds are heard distant positions from the large airways, where the sources of this sounds are spread through the lungs and create in air turbulences. They are dominated at the chest. During the inspiration, the vesicular sounds are originated mainly when the air moves through gradually smaller airways, where the inspiratory airflow hits the airway branches creating the air turbulences. Directional fluctuations of the local airflow occur due to the air turbulences that convinced by branching and bronchospasm of the airways. Nevertheless, the air movements through gradually larger airways occur during the expiration leading to less turbulence and hence less sound during the expiration. The propagation of the vesicular sounds through the lung toward the skin experiences a comparatively large damping, which emits soft sound. The vesicular sounds contain frequency components in the range of 100–400 Hz and mainly at about 100 Hz. The vesicular sounds have narrower spectral range and lower intensity compared to the tracheobronchial sounds.

  3. (iii)

    Bronchovesicular sounds have intermediate characteristics between the vesicular and tracheobronchial sounds.

However, the abnormal BSs may include a (i) high-pitched BS (Crackles), (ii) low-pitched BS (rhonchi), (iii) vibratory sound due to the contraction of the upper airway (stridor), and (iv) high-pitched whistling sound due to the bronchospasm of the bronchial tubes (wheezing).

3.2.3 Gurgling/Intestinal Sound

Auscultation abdominal sounds are produced by blood flow, friction rubs, and peristalsis. The intestine produces abdominal sounds known as the rumbling, gurgling, high-pitched, and growling sounds. These sounds are related to the movement of liquids, food, juices, digestive process, and air through the intestines. The peristalsis causes rumbling sound after eating. Hunger sends signals through the brain to the intestines and stomach resulting in the muscle contraction causing sounds. Abdominal sounds can be categorized as normal, hyperactive, or hypoactive [48,49,50,51]. Hyperactive bowel sounds are louder sounds than the normal that indicate the increased intestinal activity. Conversely, the hypoactive bowel sounds occur with the slowed down intestinal activity. Hypoactive, hyperactive, or absent bowel sounds may indicate one of the following diseases: digestive tract infection, trauma, hernia, less blood flow to the intestines, tumor, abnormal potassium/calcium level in the blood, perforated ulcers, and intestinal movement temporary reducing.

The stethoscope is used to hear any abnormal bowel sounds, which is called auscultation [52,53,54]. However, bowel obstructions yield high-pitched, very loud sounds. These sounds can often be heard without using a stethoscope. Bowel sounds are variable; thus the stethoscope diaphragm is used mainly to hear the bowel sounds to note their character and frequency, where the gurgling sounds occur at a frequency of 5–34 per min. Abnormal hyperactive bowel sounds have high-pitched and increased bowel sounds.

3.2.4 Korotkoff Sounds

The Korotkoff sounds are the sounds heard during the blood pressure measurements using the stethoscope. These sounds are different than the heart sounds “dub” and “lub” as they are due to vibrations inside the ventricles during the valves’ snapping shut, which are heard with the stethoscope. The first Korotkoff sound is heard if the pressure in the cuff decreases to the level of the patient’s systolic blood pressure level produced by the heart due to the occurred turbulence. Thumping sounds have been heard since the pressure in the cuff is permitted to fall more. Ultimately, the sounds’ quality changes as the pressure in the cuff falls more till they disappear, where decreasing the pressure lower than the diastolic blood pressure will cancel the control of the cuff on the blood flow. This returns the blood flow to be again smooth without turbulence leading to no more audible sound [55,56,57].

3.2.5 Other Body Sounds

3.2.5.1 Vascular Sounds

Audible vascular (bruits) sounds are heard due to the turbulent of the blood flow in the large arteries, namely, the iliac, renal arteries, femoral arteries, and aorta. These sounds can be heard at different vascular locations for at least 5 sec each. Swishing sounds may be produced during the auscultation bruits indicating abdominal aortic aneurism, iliac/femoral artery stenosis, and renal artery stenosis [58].

3.2.5.2 Friction Rub Sounds

The stethoscope is used to hear the friction rubs over the liver and spleen. Friction rub indicates peritoneal surface’s inflammation of the organ due to tumor, infarct, or infection. Generally, from the preceding reporting of the human body sounds, we can conclude that the airways and lungs require different instruments/sensors to detect them than those used to hear the heart sounds. The stethoscope is used to detect such sounds, which is placed over the chest while breathing in/out slowly and deeply. Through the bell in the stethoscope, the listener will hear different sounds at the different positions of the chest. Afterward, in the same way, the diaphragm is used. In normal lung sounds, there will be no crackles or wheezes. Crackles are heard when the lung rubs against the chest wall, creating friction and rubbing sound. Wheeze is a whistling, high-pitched sound that is heard with the constricted airways with the existence of fluid in the lungs. In addition, the stethoscope is used to hear by the small intestines and stomach gurgling/intestinal sounds when placed over the abdomen upper left part below the ribs. However, the borborygmi noise due to the movement of the fecal material, gas, or food is also heard [59,60,61].

The overall human hearing ranges from 20 to 20,000 Hz. The human HSs have a frequency within the 20–200 Hz range, and the human lung sounds have a frequency range of 25–1500 Hz. Acoustic wave sensors are essential to sense such frequencies, especially with the existence of the different sources of noise [61,62,63,64,65]. These sensors are sensitive with varying levels to the alarms/changes from many physical parameters. The advancement in the electronics and sensor/transducer fabrication leads to rapid progress in the acoustic medical devices.