Question 1

What is a shelf filter, and what is a peaking filter?

Accepted Answer

A shelf filter uniformly boosts or cuts everything above (highshelf) or below (lowshelf) a cutoff frequency, it is the bass and treble control of a home stereo. A peaking filter, used here for the mid band, acts around a centre frequency, and how wide that action is depends on Q: a low Q affects a broad range, a high Q makes a surgical adjustment. That is why the mid band has a Q control and the other two do not: Web Audio's shelf filters ignore Q entirely.

Question 2

Why does the preview change instantly now?

Accepted Answer

Because the filters live inside the playback graph rather than in an offline render of the whole file. Changing a BiquadFilterNode's gain is writing a number to an AudioParam: it takes effect on the next block of samples (under 3 ms), with nothing re-encoded and no playhead reset. The file is rendered in full exactly once, when you click download.

Question 3

What does auto-gain actually do?

Accepted Answer

Boosting bass by +10 dB on an already-mastered file will almost always exceed 0 dBFS, and everything past that becomes distortion. Auto-gain takes the largest boost on the curve (the highest point of the response), adds it to the original file's peak, and applies the attenuation needed to land the result at -1 dBFS. Because that is a mathematical upper bound, the output cannot clip; in exchange it can be slightly conservative when the file's peak does not sit at the boosted frequency.

Question 4

What is clipping and why is it a problem?

Accepted Answer

Clipping happens when the signal exceeds 0 dBFS (Full Scale), the maximum the digital representation can hold. Samples beyond that ceiling are chopped off, producing harsh distortion. The Predicted peak readout tells you where your settings will land before you export, and the output meter turns red whenever what is playing goes over the ceiling.

Question 5

Can I use this tool for mastering?

Accepted Answer

For broad tonal work, yes: three bands with a visible curve and gain compensation cover most fixes (dull audio, a voice without presence, too much low end). Professional mastering involves more bands, multiband compression, limiting and loudness (LUFS) analysis. For that, use a DAW, or this site's audio normalizer to set the final level.

Question 6

Does equalizing an already-compressed MP3 hurt quality?

Accepted Answer

Processing happens in PCM, after decoding, so the filters work at the best quality that file can offer. Loss only appears if you re-export as MP3, since that is a second generation of lossy compression. To preserve as much as possible after the EQ, export as WAV.

Question 7

What is the mid band for?

Accepted Answer

The mids are where the human voice and most instruments live. Cutting between 200 and 500 Hz removes the boxiness of a recording made in a small room; boosting between 2 and 4 kHz adds presence and makes speech easier to understand. That is precisely the adjustment a two-band EQ (bass and treble only) cannot make.

Question 8

Does processing change the duration or the sample rate?

Accepted Answer

No. The output render uses the same sample rate and the same number of samples as the original; only the spectral content changes. The one exception is MP3 export, whose encoder only accepts a few standard rates and resamples when needed, telling you when it does.

Bass & Treble Booster