Several factors can result in an attenuator’s performance being less than expected. The most significant of these are:

• Regenerated Sound: When the velocity of air through the attenuator creates sufficient sound that the its performance is compromised.
• Break In/Out: This occurs when sound from the noisy area before the attenuator can break into the quieter area after the attenuator.
• Flanking: When acoustic energy travels around the attenuator elements with little attenuation and then re-enters the quieter area after the attenuator.
• Vibration: Acoustic energy that is transmitted structurally rather than through the air. Vibrational energy can then be re-radiated as sound in an area that where the airborne sound has been attenuated to a lower level.
• Gaps: These allow sound to break out of noisy areas or into quiet areas with little or no attenuation. Even a small gap can significantly compromise the performance of an attenuator, particularly where higher performance is required.
• Incorrect Data: This may be a sound-pressure level stated as a sound-power level, or an incorrect plant selection. It can also be many other less obvious factors, such as rate of airflow, which will affect the aerodynamic resistance imposed level of regenerated noise produced by the attenuator.
• Damage: This may seem obvious but it is not always so. For example, if the acoustic infill is not protected by perforated metal it may erode over time, resulting in its gradual loss and a resultant loss of attenuation performance.