🎶Basics of sound🎶
The sound
Sound is a mechanical vibration that propagates as an acoustic wave through a transmission medium such as a gas, liquid or solid. The vibrations generated create pressure waves that travel outward and in all directions from the source of the sound. These can be detected by pressure sensors (e.g. hydrophones) or auditory apparatus (e.g. our ears).
The speed of sound is different according to the medium where the vibrations are propagated and several factors affect the sound speed, such as temperature, salinity, depth.
Sound travels underwater at a different speed than it does in air due to differences in density, temperature, and pressure. In general, sound travels faster and further in water than in air.
When a sound source, such as a marine mammal vocalization or a ship's engine, generates sound waves in the water, those waves travel outward in all directions. Sound waves experience attenuation, as they travel through the water. Factors such as water temperature, salinity, depth, and the presence of obstacles can affect how much sound attenuates over distance.
Characteristics of the sound
Sound is a longitudinal mechanical wave characterized by its frequency, amplitude, and wavelength.
Frequency and amplitude
- Frequency refers to the number of oscillations per second and is perceived as the pitch of the sound (higher frequency sounds are perceived as higher pitched) and is measured in Hertz (Hz).
- Amplitude refers to the magnitude or intensity of the oscillations and is perceived as the loudness of the sound (greater amplitude corresponds to a louder sound) and is often expressed in decibels (dB). Decibel scale is a logarithmic scale that measures the relative pressure of a sound compared to a reference pressure level. In water the reference level is 1 µPa (Pascal (Pa) is the unit of pressure defined as one newton per square meter (N/m2)). As a rough rule of thumb, each 10 dB increment increases the intensity by ten times, and sounds twice as loud.
- Wavelength is the distance between two consecutive points that are in phase on a wave, typically measured from crest to crest or trough to trough. It represents the length of one complete cycle of the wave. Wavelength is usually denoted by the Greek letter lambda (λ) and is inversely related to the frequency of the wave, meaning that higher frequency waves have shorter wavelengths and lower frequency waves have longer wavelengths. In different mediums, the wavelength can vary depending on the speed at which the wave travels through that medium.
Source level
The source level is the intensity of a sound source and is given in decibels (dB). For sound intensity, the reference level is typically the threshold of human hearing, which is defined as 0 dB. Each increase of 10 dB represents a tenfold increase in intensity (e.g., a sound at 10 dB is 10 times more intense than the threshold of human hearing).
Biological sounds, such as those produced by marine mammals, fish, and invertebrates, can vary widely in intensity, but often fall within the range of 80 dB to 120 dB. Shipping and vessel traffic can produce underwater noise levels ranging from approximately 120 dB to 180 dB, depending on factors such as the size and speed of the vessel. Underwater construction activities, such as pile driving and dredging, can generate sound levels exceeding 200 dB at close range.
According to the duration and prevalence in time, can be categorized in continuous sound or impulsive sounds. Impulsive sounds are very short in time, such as Pile driving noise, and continuous sounds are prevalent in time, such as Shipping noise.
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Tonal and atonal sounds
Pitch refers to the perceived frequency of a sound, which determines how high or low a sound seems to a listener. It is a subjective auditory sensation that allows us to distinguish between different tones. Higher frequency sounds are perceived as higher pitches (e.g., a whistle), while lower frequency sounds are perceived as lower pitches (e.g., a drum beat). Although pitch is closely related to the frequency of a sound wave, it also depends on other factors, such as the sound's intensity and the listener's perception.
According to the sense of pitch the sounds can be described as tonal or atonal sounds:
- Tonal sounds: sound that is a pure tone so that it is concentrated in a narrow frequency band. In other words, it has a distinct musical note or tone quality. Due to the vibration, tonal sounds can result in harmonics, a series of overtones, which are integer multiples of the fundamental frequency. For a signal whose fundamental frequency is f, the second harmonic has a frequency of 2f. The third harmonic has a frequency of 3f, and so on. For example, a signal whose fundamental frequency is 100Hz, will have a second harmonic at 2*100= 200 Hz. Figure 1 shows an example of dolphins’ whistles with harmonics.
- Atonal sounds: These sounds lack a clear sense of pitch or harmonic structure. These sounds are often perceived as noise or dissonance rather than musical tones. An atonal sound lacks a tonal centre or a preponderant tone, so the spectrogram looks as follows
Recording underwater sound
Hydrophones or autonomous recording devices are used to record underwater sounds, and they detect pressure changes. They can record in different formats (Wav and Mp3). WAV (Waveform Audio File Format) is a common uncompressed audio format known for its high quality and compatibility with a wide range of audio software and devices. WAV is a popular choice for recording audio because it preserves the original sound quality without compression.
Sound analysis
Different analysis can be performed to analyze sound files. One approach is to analyze sounds throught the spectrograms.
A spectrogram is a visual representation of the spectrum of frequencies of a signal as it varies with time. It is a two-dimensional plot where the x-axis represents time, the y-axis represents frequency, and the intensity or color of each point in the plot represents the magnitude or power of the frequency content at that time and frequency. Figure 3 is an example of a spectrogram.
Sound sources
The underwater environment is rich in different sound sources Figure 4:
Natural phenomena: Waves, wind, and geological processes
Anthropogenic phenomena: Human activities such as shipping, fishing, and underwater construction.
Ambient noise or background noise are the terms used to refer to the combination of different sources at a location. That will vary according to the characteristics of the sound sources and their propagation in the water.
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1.Natural phenomena:
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Marine mammals produce a variaty of sounds to navigate and socialize with other individuals. Dolphins produde commnly whistles, for example:
2.Anthropogenic phenomena
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This diagram helps you better understand where the sounds we hear fall on the frequency scale. Some sounds have low frequencies, while others reach very high frequencies. Understanding this range can give us insight into how different sounds are perceived and how they interact with our environment.
