Although much of the literature on the subject is concerned with just one mechanism, there are actually several different mechanisms that produce the photoacoustic effect. The primary universal mechanism is photothermal, based on the heating effect of the light and the consequent expansion of the light-absorbing material.  In detail, the photothermal mechanism  consists of the following stages: 
# conversion of the absorbed pulsed or modulated radiation into heat energy. 
# temporal changes of the temperatures at the loci where radiation is absorbed – rising  as radiation is absorbed and  falling when radiation stops and the system cools. 
# expansion and contraction following these temperature changes, which are "translated" to pressure changes. The pressure changes, which occur in the region where light was absorbed, propagate within the sample body and can be sensed by a sensor coupled directly to it. Commonly, for the case of a condensed phase sample (liquid, solid), pressure changes are rather measured in the surrounding gaseous  phase (commonly air), formed there by the diffusion of the thermal pulsations.
The main physical picture, in this case, envisions the original temperature pulsations as origins of propagating temperature waves ("thermal waves"), which travel in the condensed phase, ultimately reaching the surrounding gaseous phase. The resulting  temperature pulsations in the gaseous phase are the prime cause of the pressure changes there. The amplitude of the traveling thermal wave  decreases strongly (exponentially) along its propagation direction, but if its propagation distance in the condensed phase is not too long, its amplitude near the gaseous phase is sufficient to create detectable  pressure changes. This property of the thermal wave confers unique features to the detection of light absorption by the photoacoustic method. The temperature and pressure changes involved are minute, compared to everyday scale – typical order of magnitude  for the temperature changes, using ordinary light intensities, is about micro- to millidegrees and for the resulting pressure changes is about nano- to microbars.
The photothermal mechanism manifests itself, besides the photoacoustic effect, also by other physical changes, notably emission of infra-red radiation and changes in the refraction index. Correspondingly, it may be detected by various other means, described by terms such as "photothermal radiometry", "thermal lens" and "thermal beam deflection" (popularly also known as "mirage" effect, see Photothermal spectroscopy). These methods parallel the photoacoustic detection. However, each method has its special range of application.