Source: framework/components/sound/constants.js

// distance model
Object.assign(pc, {
    /**
     * @static
     * @readOnly
     * @name pc.DISTANCE_LINEAR
     * @type String
     * @description Linear distance model
     */
    DISTANCE_LINEAR: 'linear',

    /**
     * @static
     * @readonly
     * @type String
     * @name pc.DISTANCE_INVERSE
     * @description Inverse distance model
     */
    DISTANCE_INVERSE: 'inverse',

    /**
     * @static
     * @readonly
     * @type String
     * @name pc.DISTANCE_EXPONENTIAL
     * @description Exponential distance model
     */
    DISTANCE_EXPONENTIAL: 'exponential'
});