Did you correctly identify the freebie with the 64 kbps MP3?

It has an aggressive high frequency cut, beginning just below 11 kHz. Put simply, it contains far less treble information than the other files, which makes it sound noticeably dull or muffled by comparison. This cutoff frequency can vary depending on the encoder used and specific encoding settings, though 11 kHz is typical for 64 kbps.

Spectrum analyzers provide information about the frequency content of an audio source, and their output changes throughout the song. Though only a snapshot of a specific point in the track, the following provides an idea of the high frequency roll-off occurring in the 64 kbps MP3 of Michael Jackson’s hit song:

The 128 kbps MP3 is a bit trickier to correctly identify.

Although it still exhibits a noticeable high frequency roll off, the cutoff occurs much closer to the upper limits of human hearing (near 20 kHz) making the difference less immediately obvious than in the 64 kbps file. Above 128 kbps, we’re going to need to listen for other signs of a lower quality file. Try the Hearing Test at the halfway point of this test to see how well you can hear above 10 khz. One specific artifact to listen for in lower quality MP3s is ‘pre-echo’ – a faint ghost of loud transients that appears just before the actual sound. This is more common in 128 kbps and below, and varies by encoder quality. At 128 kbps, pre-echo and smearing are often more noticeable than the high-frequency cutoff itself.

Hear is a snapshot of “Teardrop”, demonstrating the high frequency roll-off of a 128 kbps MP3 compared to the lossless file:

In terms of frequency content, the 320 kbps MP3 is nearly indistinguishable from the WAV.

As shown below, both formats preserve virtually the same spectral detail, with the MP3 exhibiting only a slight roll-off near the upper threshold of human hearing. This does not mean that there’s no difference – but the variation is usually beyond the threshold of conscious perception.

As you can see, in the range of human hearing, the frequency content of a well encoded, 320 kbps MP3 is identical to the WAV file:

Tip: Try adjusting your listening volume, either up or down!

For some listeners, lowering the playback volume can actually enhance your ability to detect subtle details, like high frequency content, transient shape, and low level background artifacts. Reducing volume forces the ear to focus more precisely on small variations.

Conversely, listening at very high volumes can trigger the acoustic reflex (a physical change in the ear), which primarily attenuates low frequency content by approximately 15 dB. While this protective mechanism doesn’t specifically target high frequencies, the overall change in perception and potential listening fatigue at high volumes can affect your ability to detect subtle differences.

To get the most accurate impression, experiment with different volume levels throughout your comparison.

Try focusing on individual points in the track, while alternating between them, using Simple or Point of Interest (POI) mode!

When you’ve narrowed it down to WAV or 320 MP3, the frequency content is going to be very similar. Focusing your ears on a specific, repeated point can make it easier to pin point the extra differences in stereo width, high frequency content and transient punch.

Of all ten challenges, this might be the hardest for you to identify. Here’s why…

Fewer Transients: Slow, sustained notes lack sharp attacks, making common MP3 artifacts like smearing and pre-echo less noticeable.
Sparse Mix: Minimal instrumentation and wide dynamic range mean less spectral overlap, reducing encoder stress.
Natural Reverb: Huge ambient space masks subtle high-frequency losses and distortion.
Emotional Focus: Immersive, cinematic context draws attention away from analytical listening.

After performing a null test, subtracting bit for bit the 320 MP3 from the WAV, we’re left with a very low residual amplitude: True peak: -47.76 dBFS, RMS: -64.29 dBFS. This indicates the residual is at or below the noise floor of many playback systems. Any audible difference between the WAV and 320 MP3 is extremely subtle, nearly inaudible.

In the residual, the information in the left and right channels are nearly 100% unique (99.66%). This indicates the residual is highly stereo in nature, and nearly entirely composed of phase and spatial alterations – consistent with MP3 compression behavior. MP3 compression discards more stereo detail than it does mono.

The low quality, 64 kbps MP3 should still be easy to spot – did you find it?

Let’s zero in on the transients!

Modern Digital Audio Workstations allow us to pair what we think we hear, to what we can clearly see. From a bird’s eye view, the transients appear to be nearly identical in the WAV and 320 MP3 file. This similarity in transients at 320 kbps is typical for most modern encoders like LAME 3.1, though some older or lower quality encoders may show more noticeable transient smearing even at high bitrates. As you proceed from 320 to 128, and from 128 to 64, you can see a clear reduction in amplitude of the initial cycles of the waveform.

When we compare the WAV and 64 MP3 side by side, the difference is clear – you have significant softening of the transient.

This should be the easiest challenge to achieve a perfect score! Use POI mode to alternate between qualities while looping the initial transient punch of the drop.

Listen to the difference between the transient impact, stereo width and overall feel throughout this song. The quality of 320 kbps MP3 can vary significantly depending on the encoder used. Modern encoders like LAME 3.1 produce substantially better results than older encoders like the one built into iTunes, 1990s technology. When discussing MP3 quality, the encoder and version matter as much as the bitrate.