Question: I have friends that are very picky when it comes to MP3 bitrate, and will always look for the 320 kbps version of a file. However, I have never noticed any differences, they sound the same to me. I remember reading somewhere, can't remember where, that the human ear is simply incapable of sensing the difference, even if present.

Can anyone shed some light on the 192 vs. 320 kbps issue?

Answer: 
**There is a distinct difference, however whether it's perceivable, and how much, depends on many factors.**

Unfortunately I can't access the full articles, however here's the abstract of a [very relevant article](http://www.aes.org/e-lib/browse.cfm?elib=15074), emphasis mine.

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> Mp3 compression is commonly used to reduce the size of digital music files but introduces a number of potentially audible artifacts, especially at low bitrates. We investigated whether listeners prefer CD quality to mp3 files at various bitrates (96 kb/s to 320 kb/s), and whether this preference is affected by musical genre. Thirteen trained listeners completed an A/B comparison task judging CD quality and compressed files. **Listeners significantly preferred CD quality to mp3 files up to 192 kb/s for all musical genres. In addition, we observed a significant effect of expertise (sound engineers vs. musicians) and musical genres (electric v.s acoustic music).**
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So, this study finds that the answer is that above 192Kbit/s, further gains depend on the genre and the training of the listener.

Furthermore [another study](http://www.aes.org/e-lib/browse.cfm?elib=12896) finds that perceived quality depends on whether you are using headphones or not:

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> The impact of using loudspeaker versus headphone playback on the subjective quality of compressed audio is investigated. It is shown that reverberation and to a lesser extent cross-talk, which both are introduced naturally in loudspeaker playback, can effectively hide coding artifacts.
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[This other paper](http://www.eurasip.org/proceedings/eusipco/eusipco2006/papers/1568979126.pdf) describes the differences between different bitrates and different ways of testing. In all cases it shows a very minor difference between 192Kbit/s and 256Kbit/s and basically no difference between 256Kbit/s and 320Kbit/s.

![enter image description here](https://i.stack.imgur.com/agUKc.png)

As you can see, above 192Kbit it becomes quite hard to tell the difference.
It really depends on a number of factors. Some of them are:

* The age of the listener, the older you get, the less high frequency sounds you can [hear](http://www.roger-russell.com/hearing/hearing.htm). Besides age, there are [more individual factors](http://www.ncbi.nlm.nih.gov/pubmed/3558959).
* The quality of the music equipment, on a nice home stereo you are much more likely to note a difference than on an Ipod on a busy street.
* The quality depends on which encoder program was used. The encoder basically decides which sounds to encode in the audio stream and which sounds to omit. Encoder programs have a [psycho-acoustic model](http://en.wikipedia.org/wiki/Psychoacoustics) that tells them which sounds are important to the listener and which ones not so much. The quality of the output depends a lot on the model (and therefore the software that was used to encode the MP3).
* The kind of music you listen to makes a difference.

Conclusion: it is **not only** about the bit rates.

These and more points are addressed in this introductory article from Karlheinz Brandenburg (Co-developer of MP3): [MP3 and AAC explained](http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.26.5956&rep=rep1&type=pdf) (PDF, 143KB).