It is difficult to find information on variable bit rate (VBR) mp3 files. When one considers the reconstruction of a waveform from samples, it is logical to expect a better quality file if the sampling rate is increased when the waveform is changing rapidly. This means that the sampling rate should be proportional to the frequency. It is my understanding that VBR sampling achieves this. Thus if one assumes that a VBR mp3 file is of better quality than a fixed rate mp3 file of the same size, how do the file sizes compare when the percent VBR is changed? To investigate this, I ripped the CD "Piano, Strings & Bossa Nova" by Lalo Schifrin in several MP3 formats. This CD is a reissue of a '60s LP. The following table compares the file sizes for seven formats.
Sampling Rate File Size Difference 128k Fixed 26.0M 50% VBR 27.7M 1.7M 60% VBR 30.2M 2.5M 70% VBR 35.3M 5.1M 80% VBR 39.2M 3.9M 90% VBR 44.0M 4.8M 100% VBR 47.7M 3.7M
From the above results, it can be seen that the 50% VBR file is approximately the same size as the 128k fixed rate file. According to some sources, a 128k fixed rate mp3 is cd quality. This is speculation, but it is logical to expect a VBR file of the same size to be of higher quality than the fixed rate file. The VBR file quality can be improved further by increasing the rate. Compared to the 128k fixed rate, the largest jump in file size occurs between 60% VBR and 70% VBR. I conclude from this that the 70% VBR is a good compromise between 100% VBR and file size.