are: The equipment used was not properly calibrated. The criteria used for judging equal loudness at different frequencies had differed. Subjects were not properly rested for days in advance, or were exposed to loud noise in traveling to the tests which tensed the tensor tympani and stapedius muscles controlling low-frequency mechanical coupling. Side versus frontal presentation Real-life sounds from a reasonably distant source arrive as planar wavefronts. If the source of sound is directly in front of the listener, then both ears receive equal intensity, but at frequencies above about 1 kHz the sound that enters the ear canal is partially reduced by the head shadow, and also highly dependent on reflection off the pinna (outer ear). Off-centre sounds result in increased head masking at one ear, and subtle changes in the effect of the pinna, especially at the other ear. This combined effect of head-masking and pinna reflection is quantified in a set of curves in three-dimensional space referred to as head-related transfer functions (HRTFs). Frontal presentation is now regarded as preferable when deriving equal-loudness contours, and the latest ISO standard is specifically based on frontal and central presentation. Because no HRTF is involved in normal headphone listening, equal-loudness curves derived using headphones are valid only for the special case of what is called side-presentation, which is not how we normally hear. The Robinson–Dadson determination used loudspeakers, and for a long time the difference from the Fletcher–Munson curves was explained partly on the basis that the latter used headphones. However, the ISO report actually lists the latter as using compensated headphones, though it doesn't make clear how Robinson–Dadson achieved compensation. Headphones versus loudspeaker testing Good headphones, well sealed to the ear, provide a flat low-frequency pressure response to the ear canal, with low distortion even at high intensities. At low frequencies, the ear is purely pressure-sensitive, and the cavity formed between headphones and ear is too small to introduce modifying resonances. Headphone testing is, therefore, a good way to derive equal-loudness contours below about 500 Hz, though reservations have been expressed about the validity of headphone measurements when determining the actual threshold of hearing, based on the observation that closing off the ear canal produces increased sensitivity to the sound of blood flow within the ear, which the brain appears to mask in normal listening conditions. At high frequencies, headphone measurement becomes unreliable, and the various resonances of pinnae