In Val’s most recent plots of noise levels received from passing ships at our Lime Kiln study site, we’re seeing interesting peaks at high frequency (>20 kHz) that vary between ship classes. Most prominently, there is a peak near or just above ~40 kHz for many ship types that is sometimes quite narrow-band, other times really broad (+/- ~10 kHz).
What spectral patterns should we expect for modern commercial ships, particularly at frequencies between 10 kHz and 100 kHz?
Thus begins another dive into the ship noise literature… revealing:
- Merchant ship propeller diameter (in meters) is about equal to their length in meters divided by 25 (Gray, Greeley, 1980).
- Mean blade-rate frequency varies a bit with ship length, but basically for big ships a typical frequency is about 8 Hz.
- The blade-rate frequency shows up because cavitation noise is maximized when a propeller blade tip passes through the low velocity and pressure region of the wake field (the top of their rotation on single screw ships). There is also a lot of energy at many harmonics of the blade-rate frequency. Similar patterns of peaks are created by engines (firing rate harmonics) and generators (generator harmonics).
- The following plots from (Arveson, Vendittis, 2000) show that underwater noise data collected <~600m from a 173 m long coal carrier contain these numerous harmonics.
- All those peaks and additional broad-band noise from the collapse of the cavities add up to create spectra (e.g. 1/3-octave levels, below) which have led to the common characterization of shipping noise as having most of the energy at low frequencies (1-500 Hz). As Areveson and Vendittis put it:“In addition to the blade rate harmonic series, cavitation generates a wideband spectrum due to the chaotic collapse of cavities. This spectrum has a broad, high-level ‘‘hump’’ centered at about 55 Hz, followed by a continuum that decreases by 6 dB per octave on a constant-bandwidth plot, or 3 dB per octave as seen on a 1/3-octave plot.”
- What is commonly overlooked is the rise in high-frequency noise that occurs with increases in ship engine RPM (and correspondingly ship speed). Note that the increase in noise between the highest two RPM levels at 30,000 Hz is 2-4x the increase at 30 or 300 Hz.
- The authors note: “Above the cavitation inception speed the shape and peak frequency of the wideband cavitation spectrum do not change appreciably with ship speed. Only the overall level changes; it increases smoothly with speed according to 104 log (rpm), or about 31 dB per double speed.”
- While these spectra are interesting, they are not showing much structure above 20 kHz…
- At least for the cruise ships going 10 knots and measured at 500 yards by (Kipple, 2002) there are no dramatic peaks in the 10-40 kHz range, nor any indication that there is a peak near 40 kHz.