Why jeongak

1 · What reaches the womb

A sheep-model study that quantified sound transmission into the uterus found that below 1kHz the abdominal wall, tissue and fluid together take away only 2–3dB, while above 1kHz 20–40dB is lost. What reaches a fetus is the prosody and rhythm of the low end, not the crispness of the highs.

2 · So it had to be jeongak

Three criteria follow directly: a low key, a string ensemble, a slow tempo. Jeongak satisfies all three. Pyeongjo-hoesang, our first choice for prenatal listening, is Hyeonak-yeongsanhoesang transposed a fourth down, and Sangnyeongsan is the slowest piece in the whole suite — a ten-beat cycle.

3 · Measured in our own audio

We measured what actually ships. Below 1kHz is the band that reaches the womb losing only 2–3dB, and the 30-second intro puts 98.1% of its energy there. The music under the story sits at 73.3%, and the three jeongak pieces in the outro at 88.5%. Nothing above 4kHz exceeds 0.3% in any section.

The three pieces are not equal, though. The daegeum in the outro is a wind instrument and carries almost nothing below 250Hz (0.1%) — it does not meet the “string ensemble” criterion we set out. But measured against 1kHz, the line that decides what reaches the womb, the three pieces sit at 98.8%, 75.6% and 98.7%, with under 0.5% above 4kHz — all three are in the band that arrives.

4 · Built differently for each age

Only the prenatal preset keeps the low end intact and cuts the top. Lifting a band that never reaches the womb would add nothing for the fetus and only tire the mother’s ear. As age rises the design goes the other way — from the forties, hearing above 4kHz fades first and consonants blur, so consonants and clarity are pushed higher instead.

This table is read from the actual presets, so it changes when they do. The court music column is how many dB it sits under the narration — closest for prenatal, further with age.

5 · How it was made — two different things

The 30-second intro and the music behind the story were built here, note by note, from single notes and phrases released by the National Gugak Center, following the scales, cycles and instrumentation of jeongak (tonic gung B♭2 = 116.54Hz). No existing melody was transcribed. The three jeongak pieces in the outro are different: those are performance recordings, used as recorded. They are credited below.

6 · What we do not claim

We do not say jeongak makes a child smarter or develops the brain. We found no evidence for that. The study in section 1 is a sheep model, and the paper itself frames unexpectedly good high-frequency transmission as a concern — that a fetus may be exposed to non-physiological noise — not as a benefit.

We verified what reaches the womb. We did not verify what it does once it gets there.

7 · How loud

The American Academy of Pediatrics sets no decibel ceiling for children. It gives a practical test instead: if it is too loud for an adult, it is too loud for a child — keep it at a level where you can talk without raising your voice.

The broadcast that raised the question

In 2005 an MBC anniversary documentary, “The Sound of Life,” looked at the difference between compressed audio and the original. MamaTalk’s Korean subtitle comes from it.

We do not use the figures from that programme as claims — it was not peer-reviewed research.