Audio guides

Short answers to questions about cutting, joining and converting audio, backed by tests we ran with the tools on this site.

• How long should a fade out be, and why does it seem to drop off at the end?A linear fade out is 6 dB down at half time and 20 dB down at 90%; on a 10 s fade the last 0.99 s goes from -20 dB to silence. Fading twice gets -20 dB at 6.83 s.
• What do you lose when you convert stereo to mono?Centre sound keeps its level; one-sided sound drops 6.02 dB, wide sound 3 dB; opposite-polarity sound cancels to silence, and a 1 ms offset cuts notches at 501, 1502, 2506 Hz. Measured.
• 44.1 or 48 kHz, 16 or 24 bit: which should you pick, and what does converting cost?Keep the rate the file has unless the destination asks for the other. Converting is flat to 18 kHz, -3 dB at 19.2 kHz, with no seams; 16-bit adds -95 dBFS of rounding noise, 24-bit -149 dBFS. Measured.
• How do you add audio to Google Slides, and how do you cut the clip to fit?Slides accepts .mp3 and .wav from Drive, per Google's help page. Cut the clip in the audio cutter and save MP3 at 192 kbps: 1.44 MB per minute, 30 s plays as 30.04 s. Fade 1 s in and 2 s out if it loops. Measured.
• How do you make a Discord soundboard sound that fits the 5.2-second limit?Cut the clip in the audio cutter to 5.1 s or less and save MP3 at 320 kbps: a 5.2 s MP3 is at most 210 KB, so only the length limit bites. A 5.200 s cut decodes to 5.251 s. Measured.
• How do you convert an M4B audiobook to MP3, and how long a book fits in one go?Drop the .m4b on the M4A to MP3 converter: mono books up to 101 minutes and stereo up to 60 converted; longer ones must be split first. Chapters are lost, tags and cover kept. Measured.
• How do you convert an iPhone voice memo to MP3, and which bitrate should you pick?Export the memo as .m4a, open it in the M4A to MP3 converter, pick 192 or 256 kbps. The MP3 comes out mono and bigger than the memo; Lossless (ALAC) memos don't open. Measured.
• How do you turn a BPM into milliseconds for a delay, and how exact does the BPM have to be?Beat ms = 60000 divided by BPM; at 120 that is 500 ms, an eighth 250, a dotted eighth 375, a triplet 166.67. Table 60 to 200 BPM, plus 15 test tracks through the BPM finder to see what a wrong reading does to the delay.
• How do you convert an AAC file to MP3, and how is .aac different from .m4a?Open the .aac in the audio cutter and save MP3 at 256 kbps (320 for AAC of 192 kbps or more). Every .aac we made opened; it carried 23.2 ms of lead-in the .m4a hides. Measured.
• How do you convert a WhatsApp voice message (.opus) to MP3?Open the .opus in the audio cutter and save MP3 at 128 kbps. Every Opus voice note we made opened and stayed mono; the MP3 was 4 to 9 times bigger. Measured.
• How do you convert a WebM screen or voice recording to MP3?Drop the WebM on the video-to-MP3 page and pick 48 kHz. Every WebM we made opened, including recordings that carry no length. Measured.
• How do you compress a WAV file, and how much smaller does each option make it?A 24-bit 48 kHz WAV became 61% as 16-bit 44.1 kHz WAV, 50% as mono or 22.05 kHz, 23% as 320 kbps MP3 and 9% at 128 kbps. Zip only got 89%. Measured.
• How do you compress an MP3, and how much smaller can it get?128 kbps made a 320 kbps MP3 40% of its size; mono at 22.05 kHz got it to 15% (sound up to 10 kHz); 32 kbps to 10%. Re-saving at the same bitrate only lost quality. Measured.
• How do you convert OGG to MP3, and which bitrate keeps the sound?Both kinds of OGG opened. An MP3 cannot be better than the OGG it came from: 256 kbps stayed within 0.8 dB of every OGG we tried, 128 kbps lost 3 to 14 dB. Measured.
• How do you convert FLAC to MP3, and which bitrate should you pick?Every FLAC we tried opened, 16- or 24-bit, 44.1 to 192 kHz. 320 kbps is half the size of a 16-bit FLAC, 128 kbps a fifth; hi-res sources made no better MP3. Measured.
• How accurate is a live BPM detector listening through the microphone?The right tempo stayed on screen in 90-96% of readings with noise as loud as the music, echo or a voice at music level; noise 5 dB louder dropped it to 70%. Measured.
• How much headroom does a WAV need before you convert it to MP3?A loud master peaking at 0 dBFS came out up to 1.67 dB over full scale as MP3. 95% volume fixed it at 192-320 kbps; 128 kbps needed 80%. Measured.
• How long does it take to convert a whole M4A library to MP3, and are the originals safe?100 three-minute M4A songs converted to MP3 in one batch; tags and covers kept on every file and all 100 originals byte-for-byte unchanged. Measured.
• Why does the same song show different BPMs, and which reading is right?On 35 test songs at known tempos the main number was right 18 times; 14 times the right tempo was the half or double reading, 3 times a 2/3 or 4/3 one. Measured.
• Mixing in key: which keys go together, and can you trust a key reading?Same number or one step on the wheel keeps 6 or 7 of 7 notes in common. On 96 test songs the key finder was right on 78; all 18 misses were the relative key, which shares every note. Measured.
• How many MP3 files can you merge into one, and in what order?100 MP3s merged into one, in order with no samples lost or added; 16 min 40 s of audio worked, 30 min crashed the tab on our test machine. File count did not matter. Measured.
• How do you create an iPhone ringtone from a song?Cut 29.96 s as MP3 or 30 s as WAV, save to Files, share it as a ringtone from GarageBand. Measured file lengths at the 30 s limit.
• How do you get the audio out of a video file?Open the video in the audio cutter and save the part you want as WAV. 12 of 17 test videos opened; the limit is the file size, 2,145,386,496 bytes.
• How long a file can an online MP3 cutter open?A 60-minute stereo MP3 opened; 90 minutes ran out of memory. Chromium refused anything over about 101 minutes at 44.1 kHz, whatever the memory.
• How do you split one MP3 into several tracks without losing anything?One region per track, each end typed as the next start. WAV tracks add back to the source sample for sample; MP3 tracks each gain 31 ms.
• How big is a merged audio file, and should you save it as MP3 or WAV?Size depends only on length and export setting: 0.96 MB per minute at 128 kbps, 2.4 MB at 320 kbps, 10.6 MB as WAV. Measured, with what does and does not change it.
• How long should a crossfade be when you join audio?Any length from 0.5 to 4 s dips 3 dB in the middle; length only sets how long the blend lasts. Measured, plus the two settings that make the dip much deeper.
• Why are there gaps between joined MP3 files, and how do you remove them?The gap is inside each MP3, not added by the joiner: 31 ms per seam for untagged 128 kbps files, 0 ms for LAME-tagged ones or WAV. Measured, with the fix.
• Why does a cut click, and how long should the fade be?The click is the step where the audio jumps to or from zero. Measured on 2,000 random cuts: a 2 ms fade removes it. Set both fades to 0.1 s.
• What happens when you combine audio files with different sample rates?8 to 96 kHz, mono and stereo clips joined into one 44.1 kHz file kept their speed, pitch and level. Where the leftovers are, and how 5.1 is folded.
• How do you make an MP3 ringtone for Android?Cut, fade out 1 to 2 s, save MP3, put it in Ringtones. Exact file sizes per length and bitrate, and what a missing fade does to the end.
• Can you trim an MP3 to the exact millisecond?Typed cut times landed on the exact sample in every WAV export we tested. What moves an MP3 trim by 25.1 ms, and how to avoid it.
• Does cutting an MP3 lose quality?Yes, when the cutter re-encodes. Measured loss per bitrate over 10 re-cuts, and why WAV or 320 kbps export avoids most of it.
• Does an online MP3 cutter upload your file?How to check in two minutes whether an online cutter sends your file anywhere, and what our own network capture showed: 0 bytes sent.