Measurements of sound velocity and attenuation changes at the spin‐reorientation region are reported. The measurements were carried out with a magnetic field applied along theaaxis of the orthorhombic crystal. A cw ultrasonic interferometer, which is described here in detail, was employed for these measurements. The observed velocity shifts are satisfactorily accounted for using a phenomenological model with a quadratic magnetoelastic term. The effect of magnetic domains on the attenuation peaks is also discussed.