An audiometer is a machine used for evaluating hearing loss. Audiometers are standard equipment at ENT (ear, nose, throat) clinics and in audiology centers. They usually consist of an embedded hardware unit connected to a pair of headphones and a test subject feedback button, sometimes controlled by a standard PC. Such systems can also be used with bone vibrators, to test conductive hearing mechanisms. Audiometer requirements and the test procedure are specified in IEC 60645, ISO 8253, and ANSI S3.6 standards. An alternative to hardware audiometers are software audiometers, which are available in many different configurations. Screening PC-based audiometers use a standard computer and can be run by anybody in their home to test their hearing, although their accuracy is not as high due to lack of a standard for calibration. Some of these audiometers are even available on a handheld Windows driven device. Clinical PC-based audiometers are generally more expensive than software audiometers, but are much more accurate and efficient. They are most commonly used in hospitals, audiology centers and research communities. These audiometers are also used to conduct Industrial Audiometric Testing. Because these audiometers can typically be calibrated to an accuracy of fractions of a decibel, calibration is more accurate than hardware audiometers. Some audiometers even provide a software developer's kit that provides researchers with the capability to create their own diagnostic tests.