Free Volume Booster

Worked example

A quiet voice memo whose "Source peak" reads -12.0 dBFS. The gain starts at +6.0 dB, so "Before you export" shows "Gain +6.0 dB" and "Output peak -6.0 dBFS", and the summary reads "Requested +6.0 dB, applied +6.0 dB. No peaks needed limiting." A gain of 6 dB multiplies every sample by about 2 (1.995). Drag the slider to +20 dB, which multiplies by 10, and the loudest sample would reach +8 dBFS. With "Limiter" selected the output peak is held at or just under -0.1 dBFS, "Peaks pulled down" shows 8.1 dB or more, and the summary reports the largest reduction and the percentage of the audio affected. With "Hard clip" the same +20 dB cuts the peaks off, "Clipping" shows the percentage of samples at full scale, and the page says "This gain will clip. Turn on the limiter, or lower the gain." For a 10-minute stereo recording at 44.1 kHz the page estimates "About 100.9 MB" as WAV and "About 13.7 MB" as MP3 at 192 kbps. The file you download is named like memo.boosted.wav.

Which settings to use, and the limits

• Start at +6 dB, or press the Auto button. The gain runs from -20 to +30 dB in 0.5 dB steps (the "-" and "+" buttons move 1 dB), with presets at +6, +10 and +15 dB; it starts at +6 dB. As a guide, +6 dB roughly doubles the amplitude, +20 dB multiplies it by 10 and +30 dB by 31.6. "Auto: as loud as safely possible" picks the gain that brings the loud end of your file (the 99.5th percentile sample) up to -0.1 dBFS. It rounds to 0.5 dB, never goes below 0 dB or above +30 dB, and switches the mode to "Limiter".
• Choose how peaks are handled. The row "When a peak goes past full scale" has three choices. "Limiter" (the default) looks 5 milliseconds ahead and pulls down only the loudest peaks, with a 120 millisecond release and a ceiling of -0.1 dBFS, so quiet parts get the full boost and nothing clips. "Soft clip" leaves everything under 0.7 of full scale (about -3.1 dBFS) alone and rounds off what is above it, so it never reaches full scale but adds some distortion. "Hard clip" applies the exact gain and cuts off anything past full scale, which crackles on loud passages. Use "Hard clip" only if you want that sound or "Clipping" reads "None".
• Listen before you export. "Preview" plays the whole file from the start, and the "Original" and "Boosted" buttons switch between the two at the same position. Move the slider while it plays and the boosted version follows after a fraction of a second. "Before you export" shows the "Source peak", the "Output peak" and, in "Limiter" mode, "Peaks pulled down", so you can see the effect as well as hear it. If "Output peak" turns red, samples are being cut off.
• Choose WAV, MP3 or M4A by what happens next. "Output format" offers WAV (the default), MP3 and M4A. WAV is instant, 16-bit and keeps the sample rate of WAV, AIFF, FLAC, MP3 and Ogg files; M4A and AAC inputs are decoded at 44,100 Hz. MP3 and M4A are smaller but lossy, show a "Quality" choice of 96, 128, 192 (the default) or 256 kbps, and need a one-time encoder download of about 30 MB. The size line uses 44 bytes plus seconds × sample rate × channels × 2 for WAV, and seconds × kbps × 1000 ÷ 8 for MP3 and M4A. The "Cancel" button stops an encode and the page then says "Canceled."
• Video files keep their picture. MP4, WebM and MOV videos are accepted as well as audio. The sound is first pulled out with the media engine (about 31 MB on first use), boosted, and then, with "Keep video (.mp4)", put back into a copy of the video: the picture is copied untouched and the sound is replaced as AAC at the "Quality" you choose. The result is named like clip.boosted.mp4. Choose "Audio only" to get just the boosted sound as WAV, MP3 or M4A.
• Know the size limits, and that decoded audio is bigger than the file. The page refuses a file above about 2 GB on a normal computer, about 1 GB on a computer reporting 4 GB of memory or less, and about 400 MB on a phone, and shows a caution above 800 MB, 400 MB and 150 MB. The audio is also decoded to 32-bit samples that are kept in memory, plus a boosted copy, so memory use is far higher than the file size: a one-hour stereo recording at 44.1 kHz is about 1.2 GB decoded, and the boosted copy needs about the same again. For long recordings, cut the part you need first. Nothing is uploaded.

Common problems and how to fix them

It says "This file is 2.3 GB, which is too large to process in your browser on this device (limit ~2 GB). Try a smaller file or trim a shorter clip first."

The numbers are your file's size and your device's ceiling: about 2 GB, 1 GB or 400 MB. Cut a shorter piece first with an audio or video trimmer, or open the page on a computer instead of a phone. A file just under the ceiling can still show the amber caution and may be slow.

It says "Couldn't read this file."

The browser could not decode the audio, or, for a video, the media engine could not pull a sound track out of it. A video with no audio track ends the same way. Play the original to check it has sound, and if it does, save it as MP3 or WAV and load that. When the file is above the caution size, the message is followed by a line about large files running out of memory; a shorter piece will help.

It says "This gain will clip. Turn on the limiter, or lower the gain." or "0.49% of the samples in this file are clipped and will sound crackly. Try the limiter or soft clip."

You are in "Hard clip" mode and the gain pushes some samples past full scale. Switch to "Limiter" or "Soft clip", or lower the gain until "Clipping" reads "None". The output peak figure turns red whenever samples are being cut off.

It says "That is a lot of limiting and may sound squashed. Lower the gain a little for a more natural result."

This appears in "Limiter" mode when the limiter pulls peaks down by more than 6 dB over more than 5% of the audio. The file will not clip, but it can sound flattened. Lower the gain until "Peaks pulled down" falls, or use "Soft clip" for a gentler effect. For speech where loudness matters most you may choose to keep it.

It says "Couldn't put the boosted sound back into the video. Try Audio only."

This comes from "Keep video (.mp4)". The tool copies the picture as it is into a new MP4 and adds the boosted sound as AAC, which can fail when the video's picture format cannot be stored in an MP4 that way, or when the browser runs short of memory. Switch the output to "Audio only", then use a tool that adds audio to a video to put the boosted sound back on the picture.

How it compares

This tool applies one fixed gain, set by you in decibels. It does not measure loudness, so it will not match two files to the same level the way a loudness-normalizing tool does; use that when you need consistent volume across files, and this when you want a known amount of boost. Boosting raises background noise by the same amount as the voice, and the tool has no noise reduction, compression or equalizer. The limiter lets you lift quiet material without crackle, but a heavily limited file sounds squashed, which is why the page warns you. A desktop audio editor offers more control, such as compression, equalizing and batches of files. This tool works on one file at a time, and in exchange needs nothing installed.