When a scream is not a scream.

Biology is the fun science. Furry creatures, microbes that make beer, the birds and the bees and all that. But when push comes to shove, it is just another physics category. Making beer from fermentation primarily involves converting energy from one form to another within a biological system. It is called fermentation, a metabolic process that produces chemical energy stored in sugars and converts it into other chemical energy stored in ethanol. Ethanol moves through the bloodstream to the brain and engages in another exchange of chemical energy, causing the movement of dopamine and serotonin, neurotransmitters associated with pleasure and happiness. Happily for humans, thankfully is not physically quantifiable.

Neither is sound. We can quantify sound in numerous ways, but it brings us no closer to understanding why some people weep from pain listening to Mozart's Mass in C Minor while others weep from a genuine sense of loss and longing. The human mind is as elusive, it seems, as justice itself.

The process of hearing is a complex and intricate system that involves the transformation of sound waves in the air into electrical signals, which are then interpreted by the brain as sound. Something has to create the vibrations of air, to begin with. These waves of air enter the outer ear, which consists of the pinna (the visible part of the ear) and the ear canal. The pinna helps to collect sound waves and funnel them into the ear canal towards the eardrum. The waves reach the eardrum (tympanic membrane) and cause it to vibrate, replicating the original sound source. The vibrations from the eardrum are transmitted to the middle ear, which contains three tiny bones called the ossicles (malleus, incus, and stapes). These bones amplify the sound vibrations and transmit them to the inner ear. These bones are connected to the cochlea, an oval, snail-shaped, fluid-filled structure in the inner ear. The cochlea contains thousands of tiny hair cells sensitive to different sound cycle frequencies. If an object vibrates 20 times per second, it will create a frequency of 20Hertz (Hz), the lowest detectable frequency for a human ear. The upper limit is 20,000 Hz. Progressively smaller hairs vibrate at higher frequencies as the pressure waves move through the cochlea, causing the hair cells to move. This movement of hair cells converts the mechanical energy of sound waves into electrical signals. The electrical signals generated by the hair cells are picked up by the auditory nerve and transmitted to the brain. The auditory nerve carries these signals to various brain parts, including the auditory cortex, where they are processed and interpreted as recognizable sounds.

The best part, though, is the interpretation of sound, which is not much different than our explanation of mottling. The brain processes the electrical signals, allowing us to recognize and understand sounds such as speech, music, noise, etc. This process involves comparing the signals from both ears, which helps determine the direction and distance of the sound source. Even though they come from the same speaker, low frequencies will sound further away. Higher frequencies sound closer. Sound engineers use terms like "muddy" ( 250-500 Hz) and "harsh." 2000-4000 Kz. Boosting a signal at 12000 Hz will make it sound "airy".

But like mottling, it is all perception.

Dr Bohin said that a preterm baby "screamed" for 30 minutes. Apart from the fact that she wasn't there, "scream" is not a medical term; it's an emotive term. Is it the same thing to "scream" at the Beatles as "scream" when we are hurt? Physically, yes, it is. Humans perceive screaming when hearing frequencies around 3000 Hz; we are programmed to do it.

Humans are most sensitive to frequencies ranging from 2000 Hz to 5000 Hz. Specifically, the human ear shows peak sensitivity around 3500 - 4000 Hz, associated with the auditory canal's resonance frequency. A resonance frequency is when an object subject to a sound begins to vibrate at the same frequency as the sound source. A classic example is when the singer shatters a wine glass.

Any sound at that frequency will cause distress in a human, which causes further distress as the resonant energy multiplies itself.

Preterm infants, including those born very prematurely, can produce distinct vocalizations, such as cries. The fundamental frequency (F0) of these cries can vary significantly.

The fundamental frequency of infant cries, including those of preterm infants, is generally higher than that of adult speech. For instance, one study found that the fundamental frequency of spontaneous cries in healthy preterm infants at term-equivalent age was significantly associated with shorter gestational age. However, neither smaller body size at recording nor intrauterine growth retardation (IUGR) was related to increased F0 in preterm infants. This suggests that their smaller body size does not cause the increased F0 of spontaneous cries but might be due to more complicated neurophysiological states owing to their different intrauterine and extrauterine experiences.

The typical frequency range for infant cries is broad, often extending from 250 Hz to 500 Hz in the lower range and can exceed 600 Hz in conditions associated with distress or pathology. 250 -500 Hz. 440Hz is the A below middle C of a piano or the second lowest string on a guitar. Neither of these notes is a "screaming notes."

However, all fundamental tones have harmonics ( overtones) created one octave above and below the fundamental. 500hz will generate overtones at 1000Hz, 1500HZ, 2000 Hz etc. Harmonic frequencies typically have a lower amplitude ( volume) than the fundamental frequency. The fundamental frequency provides the sound with its strongest audible pitch reference and is the predominant frequency in any complex waveform. Harmonics contribute to the timbre or quality of the sound, making it possible to distinguish between different sounds and instruments. While harmonics are present and contribute to the overall sound, they usually have a lower amplitude than the fundamental frequency. Harmonics lose energy as they become higher in pitch.

Harmonics significantly affect a sound's overall loudness by contributing to the total sound pressure level (SPL) and perceived loudness. Their presence and relative levels can influence how loud a sound appears to the listener. The peak sensitivity is around 2,000 to 5,000 Hz. Harmonics that fall within this range can make a sound seem louder in the same way that blue light scattering from skin can make blood appear blue.

Fading harmonics do not lose effect, though. A mother who hears a baby cry at 500 Hz will hear the 7th harmonic at 3500 Hz, much louder than the fundamental frequency, and react to the resonant frequency with great alarm. A gentle moan at 500Hz may as well be a scream at 3500.

It is not physically possible for a preterm baby to cry continuously for 30 minutes because it needs to breathe in between. It is perfectly possible for a mother to feel as if her baby has been crying not just for 30 minutes but for hours. All practising doctors are well aware of this.

Like everything else in the Letby story, 30 minutes is too neat. It is long enough to sound serious, intentional and calculated. But it wasn't Letby who was calculating.

Unless the sound was recorded or measured, we can safely put it down to perception or the wicked desire of an "expert" witness to engage in a little more storytelling.

I look forward to exposing more of Sandie Bohin's irrational brain; it is truly worth the study.