def _get_noise_id(tr):
    """
    Get trace id for noise type data. e.g., 'starttime-endtime'

    :param tr: an obspy trace
    :return str event_id: noise id
    """
    starttime = tr.stats.starttime
    endtime = tr.stats.endtime
    event_id = "-".join([starttime.strftime("%Y%m%d%H%M%S"), endtime.strftime("%Y%m%d%H%M%S")])
    return event_id