How does altering the audio bitrate impact spectrograms?

I experimented with reducing the bitrate of MP3 files from 160kbps to 80kbps to understand its effect on audio quality and frequencies. After plotting spectrograms for both versions, I noticed distinct differences (with the 80Kbps version on top). Initially, I thought spectrograms might be connected to audio bitrates, but a quick search suggested otherwise. I’m curious about why these frequency differences appear in the spectrograms if they’re supposedly unrelated to bitrates. Additionally, I’d like to know the best method to analyze the variations between these two audio versions if spectrograms aren’t the right tool.

As someone not specialized in signal processing but eager to learn, I would appreciate any explanations or resources that could clarify these aspects. Thank you in advance for any help!

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Lowering the bitrate from 160kbps to 80kbps does impact the audio quality, often leading to loss of high-frequency components. This loss is visible in spectrograms, which show frequency content over time. While spectrograms don’t directly reflect bitrate, they show the effect of bitrate changes on audio frequencies.

For analyzing audio differences, besides spectrograms, you can also use tools like waveform analysis and listening tests to compare perceptual differences.

I understand the frequency loss, but how exactly does bitrate reduction cause this? And are waveform analyses better than spectrograms for this purpose?

Bitrate reduction often involves compression, which can remove certain frequencies, especially higher ones, to reduce file size. This is why you see those changes in the spectrogram. As for analysis tools, waveform analysis is different from spectrograms.
It shows amplitude variations over time but doesn’t give frequency-specific information. Spectrograms are more informative for understanding frequency loss due to compression.

So, spectrograms are suitable for this analysis. But how can I interpret the specific changes in the spectrogram due to bitrate reduction?

In the spectrogram, pay attention to the high-frequency areas. A lower bitrate like 80kbps will show fewer details or energy in higher frequency bands compared to 160kbps. This manifests as less bright or missing areas in the high-frequency region of the spectrogram. This visual difference is a direct consequence of the lossy compression removing certain frequencies to reduce file size.

Got it. Lastly, are there any specific tools or software you recommend for a more in-depth analysis of these bitrate effects?

For detailed audio analysis, software like Audacity or Adobe Audition can be quite helpful. They offer both waveform and spectrogram views, along with various analysis tools. Online resources like iZotope’s educational guides or music production forums can also provide valuable insights into understanding audio compression and its effects.