We are assuming you are talking audiometry here (as opposed to areas such as vestibular assessment that are not typically associated with sound booths.
Guidance about the maximum permissible ambient sound pressure levels for AC and BC audiometry are provided in international standards (ISO 8253-1:2010).
Certain models of headphones (e.g. circum-aural models) offer a degree of sound attenuation even without a booth. However, a key consideration when no sound booth is available is the use of audiocups. These are enclosures of the transducer that act to effectively attenuate environmental noise in cases where a south booth is not available.
Most audiometers with the appropriate transducer e.g. DD45/TDH39/TDH49 can be calibrated with audiocups, but they are most commonly associated with screening audiometers that are more likely to be used in settings without a booth e.g. doctors surgery, elderly care homes or domestic visits, or industrial screening of workers in a workplace.
In pure tone audiometry, it would not generally be considered acceptable to proceed routinely in most cases without a sound attenuating booth but that is not always the reality, particularly in the above scenarios. Audiocups can be a great solution. They are cheaper than a booth, and they are easily transportable. However, they do not address all of the issues one might encounter during audiometry so are not necessarily a complete alternative.
Some reasons why audiocups only offer a partial solution to lack of a sound booth are as follows:
Amplivox Audiocups Retrieved from http://www.amplivox.ltd.uk/category/products/audiometry/audiocups/ )
ISO 8253-1: Acoustics. Audiometric Test Methods. Part 1: Basic Pure Tone Air and
Bone Conduction Threshold Audiometry. (Identical to ISO 8253-1)