Unwanted sound, or noise, such as that produced by airplanes, traffic, or industrial machinery, is considered a form. of pollution. Noise pollution is at its worst in densely populated areas. It can cause hearing loss, stress, high blood pressure, sleep loss, distraction (注意力分散), and lost productivity! Sounds are produced, by objects that vibrate (振动), at a rate that the ear can detect. This rate is called frequency and is measured in hertz (赫兹), or vibrations per second. Most humans can hear sounds between 20 and 20,000 hertz, while dogs can hear high pitched sounds (高频声音) up to 50,000 hertz. While high frequency sounds tend to be more hazardous (危险的) and more annoying to heating than low frequency sounds, most noise pollution damage is related to the intensity of the sound. Measured in decibels (分贝), noise intensity can range from zero, the quietest sound the human ear can detect, to over 60 decibels. Conversation takes place at around 40 decibels, a subway train is about 80 decibels, and a rock concert is from 80 to 100 decibels. The intensity of a nearby jet taking off is about 110 decibels. 120 decibels axe the limit at which human beings will be aware of pain, and suffer from potential heating loss. Long lasting, high intensity sounds are the most damaging to heating and produce the most stress in humans. Solutions to noise pollution include adding insulation and sound proofing (隔音) to doors, walls, and ceilings using ear protection, particularly in industrial working areas planting vegetation to absorb and screen out noise pollution and zoning urban areas to maintain a separation between residential areas and zones of excessive noise. Noise pollution can cause ______.