acoustics.test.ts

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`174b4a1a` 2026-10-09 Pavel Safronov	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	})