acoustics.test.ts

1	import { describe, expect, it } from 'vitest'
2	import {
3	SOUND_SPEED_AIR, chirp, groupSpeed, phaseSpeed, singingFrequency, thicknessFromFrequency,
4	travelTime, wavenumber,
5	} from '../src/ice/acoustics.js'
6
7	describe('ice acoustics', () => {
8	it('agrees with the ice measured in the paper the model is from', () => {
9	// Romeyn et al. 2021: the film's lake, about 725 Hz from about 4.3 cm; and a lake
10	// whose ice quakes had a 190 Hz component, about 16 cm.
11	expect(thicknessFromFrequency(725)).toBeCloseTo(0.043, 2)
12	expect(thicknessFromFrequency(190)).toBeCloseTo(0.16, 2)
13	})
14
15	it('sings higher the thinner it is, and very nearly in inverse proportion', () => {
16	expect(singingFrequency(0.03)).toBeGreaterThan(singingFrequency(0.05))
17	for (const h of [0.02, 0.04, 0.08, 0.2]) {
18	expect(singingFrequency(h) * h).toBeCloseTo(30.3, 0)
19	expect(thicknessFromFrequency(singingFrequency(h))).toBeCloseTo(h, 6)
20	}
21	})
22
23	it('sings at the frequency whose bending wave keeps step with sound in air', () => {
24	const h = 0.041
25	const note = singingFrequency(h)
26	expect(phaseSpeed(wavenumber(note, h), h)).toBeCloseTo(SOUND_SPEED_AIR, 3)
27	// Above the note the wave outruns sound in air and can radiate; below it, it cannot.
28	expect(phaseSpeed(wavenumber(note * 2, h), h)).toBeGreaterThan(SOUND_SPEED_AIR)
29	expect(phaseSpeed(wavenumber(note / 2, h), h)).toBeLessThan(SOUND_SPEED_AIR)
30	})
31
32	it('carries high frequencies faster than low ones', () => {
33	const h = 0.041
34	let before = 0
35	for (const f of [200, 740, 1500, 3000, 6000]) {
36	const speed = groupSpeed(wavenumber(f, h), h)
37	expect(speed).toBeGreaterThan(before)
38	before = speed
39	}
40	expect(travelTime(6000, h, 100)).toBeLessThan(travelTime(740, h, 100))
41	})
42
43	it('makes a chirp that falls onto the note, and falls for longer from further off', () => {
44	const h = 0.041
45	const near = chirp(h, 50)
46	const far = chirp(h, 400)
47	for (const { frequencies } of [near, far]) {
48	expect(frequencies[0]).toBeCloseTo(6000, -1)
49	for (let i = 1; i < frequencies.length; i++) expect(frequencies[i]).toBeLessThan(frequencies[i - 1]!)
50	expect(frequencies[frequencies.length - 1]! / singingFrequency(h)).toBeCloseTo(1.02, 2)
51	}
52	expect(far.duration / near.duration).toBeCloseTo(8, 5)
53	// A crack 400 m off falls for a third of a second: long enough to hear as a "pew".
54	expect(far.duration).toBeGreaterThan(0.25)
55	expect(far.duration).toBeLessThan(0.5)
56	})
57	})