While audio files encoded at the same volume level (VBR) should theoretically have consistent loudness, there are a few factors that can contribute to perceived differences in loudness:
Dynamic Range Compression: Audio mastering engineers often use dynamic range compression techniques to control the loudness variations within a recording. Compression reduces the difference between the loudest and softest parts of the audio, resulting in a more consistent perceived loudness. However, different compression settings or techniques applied during the encoding process can affect the overall loudness and dynamics of the audio, making some files sound louder than others.
Loudness Normalization: When audio files are played on various platforms or devices, they may undergo loudness normalization, where the playback system adjusts the volume to ensure consistent perceived loudness across different files. However, not all platforms or devices employ the same normalization standards or algorithms, which can lead to variations in loudness even if the files were encoded at the same volume level.
Audio Processing: Certain audio processing techniques, such as equalization (EQ) or limiting, can alter the perceived loudness of an audio file. If these processing techniques are applied differently during the encoding of the files, it can result in variations in loudness.
Human Perception: Perceived loudness is subjective and can vary from person to person. Factors like individual hearing sensitivity and listening environment can affect how loudness is perceived, leading to differences in perceived loudness between audio files encoded at the same volume level.
In summary, while encoding audio files at the same volume level (VBR) should provide consistent loudness, factors such as dynamic range compression, loudness normalization, audio processing, and human perception can contribute to perceived differences in loudness between files.