Soundscape. The noise characteristics for receive level and peak frequency vary based on differences in traditional fishing boat (Table 1). The highest noise intensity was produced by 3 GT fishing boat, where the receive levels reached 153.0 dB re 1 µPa. With almost similar distance to the receiver position, receive level of noise for 5 and 10 GT fishing boats reached 146.8 and 149.8 dB re 1 µPa, respectively. These variations were closely related to the difference in source level, ship size, engine’s type and strength, as well as the operational speed of the vessels. The 3 GT fishing boat emitted the highest noise intensity because it was equipped with 20 HP gasoline-fueled outboard engine. With a small size and equipped engine power, it can travel at speeds up to 3.1 m s− 1. The high operational speed causes the resulting noise intensity to be high due to the combination of engine, propeller and hydrodynamics. It was different from the 5 GT fishing boat, which can only operate at a speed of 1.9 m s− 1 leading to a decreased intensity of noise. In other cases, where a 10 GT fishing boat equipped with a different type of engine (diesel-fueled inboard engine) and a power of 120 HP produced a higher noise intensity than a 5 GT type. Despite having the highest engine power, the ship's operational speed of only 0.9 m s− 1 influenced the receive level of this vessel to be lower than the 3 GT fishing boat. Therefore, the intensity of noise produced by the ship was more dominantly influenced by the operational speed and other factors such as engine’s type and strength.
Table 1
Noise characteristics for variety of traditional fishing boat.
Type of traditional fishing boat | Receive level (dB re 1 µPa) | Peak frequency (Hz) |
3 GT (8.25 m of length and 1.2 m of width) with gasoline-fueled outboard engine (20 HP and 3.1 m s− 1 of speed operation) | 153.0 | 1,100 |
5 GT (11.00 m of length and 2.6 m of width) with diesel-fueled inboard engine (45 HP and 1.9 m s− 1 of speed operation) | 146.8 | 1,886 |
10 GT (13.50 m of length and 2.8 m of width) with diesel-fueled inboard engine (120 HP and 0.9 m s− 1 of speed operation) | 149.8 | 1,632 |
In contrast to noise intensity, the highest peak frequency was produced by a 5 GT fishing boat that reached 1,886 Hz. However, it had the highest peak frequency that can be heard at 8 kHz. The 3 and 10 GT fishing boats have frequency range which can reach the audible sound threshold (20 kHz). Their peak frequency was lower which is only 1,110 and 1,632 Hz respectively. Furthermore, gasoline-fueled outboard engine type produced higher noise frequency than diesel-fueled inboard engine. The power of the engine contributed greatly to this condition. Also, the wide range of noise frequencies produced by the 3 GT fishing boat was more reasonable because the ship's operating speed was higher. Because of this, the sound produced was more varied due to the combination of engine, propeller and hydrodynamics. The variation in frequency produced by fishing vessels 10 GT was caused by the larger size and tonnage of the boat. Therefore, the frequencies produced by each source (engine, propeller and hydrodynamics) were also more varied.
Noise intensity. Figure 1 shows further differences in noise intensity for each variety of traditional fishing boats. Generally, the changes in intensity vary based on the ship’s type and movement (Fig. 1, left). Following the waveform recorded by the receiver, the intensity increases since the noise was detected. In addition, it reaches the peak when in the closest distance with the receiver, and then decreases as the ship moves away. Despite having the same pattern, the intensity and rate of change differ between one type of vessel to another. These include the highest receive level and the increased change pattern of noise intensity seen on 3 GT fishing boat (Fig. 1A, left). This is inseparable by the speed of the ship and higher than others since the intensity of the source level was also high. The rapid movement of the vessel as a consequence of such speed causes the distance with the receiver to change rapidly. The longer the distance of the ship with the receiver, the lower the intensity due to the presence of sound absorption, which may lead to increased transmission loss. Furthermore, similar reason was also directed to 5 and 10 GT fishing boats, where noise intensity also changes based on recording time due to changes in distance (Fig. 1B and 1C, left). The 5 GT fishing boat had a slower change in intensity even though the speed is higher than that of 10 GT. This is because the source level is lower as a result of decreased engine power that complements the ship.
Figure 1 (right) shows the changes in receive levels of fishing boat noise for each frequency (1, 5, 10, and 15 kHz) based on the distance. Generally, it had the same pattern at the recording time changes as a consequence of distance. However, the intensity of noise clearly seen is different when the ship approaches the closest distance to the receiver. Receive level for noise frequency of 1 kHz was not much different from 5 kHz. Therefore, the noise frequency of 10 kHz was similar with 15 kHz in both 3, 5 and 10 GT fishing boats. The noise frequencies up to 5 kHz can be detected well by receiving levels above 135 dB re 1 µPa. Meanwhile, the receive level above 10 kHz were below 135 dB re 1 µPa. As the ship move away from the receiver, there was no significant intensity difference for each frequency. This is because the existing receive level was a combined intensity of broadband frequency.
The noise changes of each frequency as a function of distance for 3 GT fishing vessels are shown in Fig. 1A (right). At the closest distance (42.6 m), the receive level was the highest intensity (153.0, 138.9 and 135.7 dB re 1 µPa for frequency up to 5,10 and 15 kHz respectively). As the vessel moves away 52.7 m (10 s), the noise intensity decreases proportionally for each frequency. The decrease continues to occur when it moves away to 75.2 m (20 s), the receive level was the intensity of the broadband frequency with ranges to 114.6 dB re 1 µPa. Different intensity changes were shown by 5 GT fishing boat, where receive levels did not have significant changes to a distance of 57.5 m (20 s) with an intensity of 138.1 and 127.7 dB re 1 µPa for frequency up to 5 kHz and above 10 kHz respectively (Fig. 1B, right). Meanwhile, a significant decrease was seen as the ship's distance continues to drift away where the intensity of sound is a combination of broadband frequency. Changes in the same pattern occurred at 10 GT fishing boat, where intensity changes were seen when the vessel stayed away for 20 s (61.8 m), as shown by Fig. 1C (right). The difference in noise intensity changes for each of these frequencies showed that variations in ship size, engine’s type and power, and operational speed affect the source level and transmission loss of noise.
Noise spectra. Figure 2 shown the fishing boat noise spectra based on the ship's travel time which represents the distance. Spectra noise appears to vary based on the type of fishing boat, where the frequency of 3 GT had a wider range and experiences faster loss of spectra based on distance compared to the 5 and 10 GT (Fig. 2, left). However, the frequency had increased quadratically based on the distance, and the pattern of settlement varies. This variation was due to differences in the source frequency and operating speed, ship types and engines. The smaller 3 GT fishing boat, which is equipped with a 20 HP gasoline-fueled outboard engine can be operated at a higher speed, influencing the spectra noise to be produced in wider broadband frequencies and disappear faster with recording time (Fig. 2A, left). Also, 5 and 10 GT fishing boats, which are both equipped with a diesel-fueled outboard engine, have the same spectra pattern. Small differences may be observed in the frequency range and the increasing pattern based on the distance, where the 5 GT fishing boat has a smaller range and the change pattern is slightly larger than the 10 GT (Fig. 2B dan 2C, left). This occurs due to differences in engine power and operational speed of the two types of vessels.
Figure 2 (right) shown a further change in frequency for each vessel based on distance. Generally, the group (< 1, 2–3, and > 4 kHz) experienced a quadratic increase along with the recording time, which represented distance. The higher the frequency, the clearer is the reduction pattern for the three types of fishing vessels. In 3 GT, the frequency changes were seen to be lower in the < 1 kHz group, with a quadratic coefficient of 0.0008 (Fig. 2A, right). On the contrary, the pattern of increasing frequency slightly changed in the 2–3 kHz group, where the quadratic coefficient increased to 0.0009. The changes were observed in the higher group, where the coefficient was above 0.0252 when the noise frequency was > 4 kHz. Even though it had the same tendency, the quadratic coefficient of increasing frequency was lower on the 5 GT fishing boat (Fig. 2B, right). In the same frequency group, the quadratic coefficients for this type of vessel only ranged from 0.0002, 0.0003 and 0.0012, respectively. This value was still higher than the quadratic coefficient for each group produced by 10 GT fishing boats, were 0.00002, 0.00009 and 0.0020 belonged to the frequency group of < 1, 2–3, and > 4 kHz, respectively. Therefore, the variation of the source frequency as a representation of ship and engine type, as well as operation speed influenced differences in receive frequency and change patterns.