When we open a song, we always subconsciously slide the phone’s volume button – turn it up to hear the details, turn it down to avoid disturbing others. But have you ever wondered: what kind of “volume code” is hidden in the audio file itself? Why does the same song sound so different in loudness when played through headphones and speakers? To solve these puzzles, we have to start with the core concept of “decibels”. It is both the “internal scale” of digital audio and the “intensity scale” of physical sound, but it is often confused as the same thing. Today, we will disassemble the dual identity of audio decibels, tell you how to understand the “natural volume” of the file, and control the “actual loudness” heard by the ears.
I. “Volume Genes” in the Digital World: dBFS in Audio Files
When we download an MP3 or export a recording, the file itself is already engraved with a set of “volume genes” – this is the decibel unit dBFS (Decibels Full Scale) dedicated to digital audio. It is like a ruler, measuring the “fullness” of the sound signal in the file, but it has no direct connection with how “loud” we hear with our ears.
The unique feature of dBFS is its “reverse scale”: taking the maximum undistorted signal that digital audio can carry as 0dB (full scale), all actual signal dBFS values are negative. For example, if the peak value of a song is -6dBFS, it means that its signal intensity is 6dB away from the “full scale”; if the peak value is close to 0dBFS, it indicates that the signal is very full, and a little amplification may cause harsh distortion.
This value is the “innate attribute” of the audio file and is not affected by the playback device. Just like the pixels of a photo are fixed, whether opened with a mobile phone or a computer, the pixels will not change. It is also easy to check: import the file with the free audio software Audacity, right-click the waveform scale and select “dBFS”, and the value at the top of the waveform is the peak value; through “Effect – Statistics”, you can also see the average intensity (RMS dBFS), which is closer to the human ear’s perception of “loudness”.
II. “True Loudness” Heard by the Ears: dBSPL in the Physical World
The moment the play button is pressed, the digital signal vibrates the air through the speaker, and what finally reaches the ear is the physical “sound pressure level”, with the unit dBSPL (Decibels Sound Pressure Level). This is what we really feel as “loud or not”. It is like a water thermometer measuring water temperature, directly reflecting the pressure of sound on the eardrum.
The value of dBSPL is determined by three factors:
- dBFS potential of the file: Under the same device, a file with -3dBFS can output higher sound pressure than a file with -20dBFS (like a high-pressure water gun can shoot farther than a low-pressure one);
- Volume setting of the playback device: The volume knob is essentially a “signal amplifier”. For the same file, turning up the volume by 3dB will increase the sound pressure level by approximately 3dB;
- Distance and environment: There may be 80dB at 1 meter from the speaker, and only 70dB when retreating to 3 meters (sound attenuates significantly with distance). A noisy environment will also “mask” part of the sound pressure, making people unconsciously turn up the volume.
In daily scenarios, 60-85dBSPL is a comfortable range: a soft conversation is about 60dB, and the climax of a movie theater is about 85dB. Long-term exposure to more than 85dB may damage hearing – for example, a 100dB concert has a safe listening time of only 15 minutes, and for every 3dB increase, the safe time is halved.
III. From File to Perception: How to Precisely Control “Dual Decibels”?
To match the “potential” of the audio file with the “comfort” of the actual perception, two steps are required:
Step 1: Measure the dBFS of the file to know the “family background”
After opening the file with Audacity, observe the peak dBFS:
- If the peak is lower than -10dBFS (such as -15dB), it indicates that the file signal is relatively “weak”. You may need to turn up the device volume when playing, which is suitable for quiet environments;
- If the peak is around -6dBFS, it is a standard recording (the setting of most pop music) with a good dynamic balance;
- If the peak is close to 0dBFS, play it carefully. It may damage hearing if the volume is too high (common in uncompressed live recordings).
Step 2: Measure the actual dBSPL to calibrate perception
Use a mobile APP (such as “Sound Meter”) or a professional sound level meter, place the microphone at the ear position, and observe the value when playing audio:
- Listening to songs at home: Keep it at 60-75dBSPL, which can not only hear the details but also not disturb the family;
- Commuting outdoors: It can be temporarily increased to 80-85dBSPL, but avoid exceeding 1 hour;
- Playing before bed: It is recommended to be lower than 60dBSPL, equivalent to a whisper, which does not interfere with sleep.
It is worth noting that mobile APPs need to be calibrated first: open the APP in a quiet room (about 30dB). If the display deviation exceeds 5dB, you can manually adjust it in the settings (some APPs support calibration by comparing with professional equipment).
IV. Pitfall Guide: These “Decibel Misunderstandings” Should Be Alerted
- “Full volume bar = maximum decibel”?
The volume bar of the player is a relative scale (such as 0-100%) and does not represent the actual sound pressure. For the same song, it may be 80dB full with headphones, and 100dB full with speakers. It should be based on the sound level meter.
- “High file decibel = good sound quality”?
Excessively pursuing high dBFS (such as forcibly pressing the file to 0dBFS) will compress the dynamic range, making the music lose its sense of hierarchy (just like adjusting all colors to the most vivid, but it is difficult to see the details).
- “Mobile phone decibel measurement is inaccurate, so it is unnecessary to use”?
Although professional sound level meters have an accuracy of ±1dB, the error of mobile APPs of ±3dB is sufficient for daily reference – as long as you know that “over 90dB needs to be turned down”, you can avoid most hearing risks.
The “dual decibels” of audio are like the “rated power” and “actual power consumption” of electrical appliances: the dBFS of the file is the rated parameter, determining the upper limit; the actual dBSPL is the power consumption in use, which requires us to actively control. Mastering their relationship can not only make each song release the best perception but also protect the long-term health of the ears – after all, a good sound experience is always based on “being able to hear clearly” and “being able to hear safely”.