An acoustic device, comprising: an open audio device structure that is configured to be carried on the head of a user without occluding an ear canal opening of the user; a housing carried by the open audio device structure, the housing having opposed front and rear faces and opposed first and second ends; a flat diaphragm in the housing and comprising a front face and a rear face, the diaphragm configured to radiate front acoustic radiation from its front face and into a front acoustic volume defined between the front face of the diaphragm and the front face of the housing and rear acoustic radiation from its rear face and into a rear acoustic volume defined between the rear face of the diaphragm and the rear face of the housing, wherein the front and rear acoustic radiations are out of phase; a flexible structure that supports the diaphragm such that the diaphragm can move relative to the housing; a primary magnet proximate the rear face of the diaphragm; a magnetic circuit that defines a path for magnetic flux of the primary magnet; a voice coil that is exposed to the magnetic flux and is configured to move the diaphragm up and down along a radiation axis that is normal to the front face of the diaphragm; and first and second sound-emitting outlets in the housing, wherein the first sound-emitting outlet is in or proximate the first end of the housing, defines a center, and is acoustically coupled to the front acoustic volume so as to emit from the housing front acoustic radiation, and wherein the second sound-emitting outlet is in or proximate the second end of the housing, defines another center, and is acoustically coupled to the rear acoustic volume so as to emit rear acoustic radiation; wherein a distance between the centers of the first and second sound-emitting outlets is greater than a distance along the radiation axis between the front and rear faces of the housing, wherein the open audio device structure is configured to be worn on the user's head such that the radiation axis is transverse to a side of the head, and wherein the open audio device structure comprises a temple piece (660;680;802) of eyeglass headphones (650;800), and wherein one of the first and second sound-emitting outlets is configured to be close to the user's ear and the other of the first and second sound-emitting outlets is configured to be farther from the ear. an open audio device structure that is configured to be carried on the head of a user without occluding an ear canal opening of the user; a housing carried by the open audio device structure, the housing having opposed front and rear faces and opposed first and second ends; a flat diaphragm in the housing and comprising a front face and a rear face, the diaphragm configured to radiate front acoustic radiation from its front face and into a front acoustic volume defined between the front face of the diaphragm and the front face of the housing and rear acoustic radiation from its rear face and into a rear acoustic volume defined between the rear face of the diaphragm and the rear face of the housing, wherein the front and rear acoustic radiations are out of phase; a flexible structure that supports the diaphragm such that the diaphragm can move relative to the housing; a primary magnet proximate the rear face of the diaphragm; a magnetic circuit that defines a path for magnetic flux of the primary magnet; a voice coil that is exposed to the magnetic flux and is configured to move the diaphragm up and down along a radiation axis that is normal to the front face of the diaphragm; and first and second sound-emitting outlets in the housing, wherein the first sound-emitting outlet is in or proximate the first end of the housing, defines a center, and is acoustically coupled to the front acoustic volume so as to emit from the housing front acoustic radiation, and wherein the second sound-emitting outlet is in or proximate the second end of the housing, defines another center, and is acoustically coupled to the rear acoustic volume so as to emit rear acoustic radiation; wherein a distance between the centers of the first and second sound-emitting outlets is greater than a distance along the radiation axis between the front and rear faces of the housing, wherein the open audio device structure is configured to be worn on the user's head such that the radiation axis is transverse to a side of the head, and wherein the open audio device structure comprises a temple piece (660;680;802) of eyeglass headphones (650;800), and wherein one of the first and second sound-emitting outlets is configured to be close to the user's ear and the other of the first and second sound-emitting outlets is configured to be farther from the ear.