This isn't something I've read a ton on; just a little.
In the olden days, instrument tuning wasn't done by a standardized pitch. During the 1700s, an instrument might be tuned to 380 Hz or 480 depending on where and what it was. As orchestras grew into their own entities (rather than being accompanying ensembles for composers), there became a need for standardized tuning. The standard tuning fork came to be, and around 1860 the standard pitch was decided to be 435 Hz, with most of the world following suit. The Italians disagreed and went with 440 Hz, Americans taking after the Italians in 1917.
This changed in 1919, when a provision of the Treaty of Versailles set the standard pitch for all signatory nations to 440 Hz. Future events would solidify the trend toward a 440 Hz standard:
https://capionlarsen.com/history-pitch/ said:In 1939, an international conference recommended that the A above middle C be tuned to 440 Hz. This standard was taken up by the International Organization for Standardization in 1955 (and was reaffirmed by them in 1975) as ISO 16. The difference between this and the diapason normal is due to confusion over which temperature the French standard should be measured at. The initial standard was A = 439 Hz but this was superseded by A = 440 Hz after complaints that 439 Hz was difficult to reproduce in a laboratory owing to 439 being a prime number.
This is where conspiracy comes into play:
https://globalnews.ca/news/4194106/440-hz-conspiracy-music/ said:There is allegedly something sinister and evil about 440 Hz. It is said that the Rockefeller Foundation had an interest in making sure the United States adopted the 440 Hz standard in 1935 as part of a “war on consciousness” leading to “musical cult control.”
Without going too far down this rat hole, this theory says that tuning all music to 440 Hz turns it into a military weapon.
I quote from one of the many online articles on the subject: “The monopolization of the music industry features this imposed frequency that is ‘herding’ populations into greater aggression, psychosocial agitation, and emotional distress predisposing people to physical illnesses and financial impositions profiting the agents, agencies, and companies engaged in the monopoly.”
That covers the surface of where questioning begins. Interestingly, there appears to be research that lends support to conspiracy. Some of it's kind of shocking...
https://pubmed.ncbi.nlm.nih.gov/31031095/ said:432 Hz tuned music was associated with a slight decrease of mean (systolic and diastolic) blood pressure values (although not significant), a marked decrease in the mean of heart rate (-4.79 bpm, p = 0.05) and a slight decrease of the mean respiratory rate values (1 r.a., p = 0.06), compared to 440 Hz. The subjects were more focused about listening to music and more generally satisfied after the sessions in which they listened to 432 Hz tuned music.
https://pubmed.ncbi.nlm.nih.gov/32401941/ said:Significantly lower anxiety level values were observed at 432 Hz (8.7±2.67) and 440 Hz (8.4±2.84) compared to the control group (17.2±4.60; p<0.05). The salivary cortisol level at 432 Hz (0.49±0.37 μg/dL) was significantly lower than 440 Hz (1.35±0.69 μg/dL) and the control group (1.59±0.7 μg/dL; p<0.05). Conclusion The use of music significantly decreased clinical anxiety levels, and the frequency of 432 Hz was effective in decreasing salivary cortisol levels before tooth extraction.
https://pubmed.ncbi.nlm.nih.gov/33263352/ said:The participants were eight males (mean age =58.12, SD ±13.62), and four females (mean age =56.25, SD ±14.17). Five were quadriplegics and seven were paraplegics. Listening times and wash-out periods were variable. The stress decreased, but not significantly, with listening to music at both frequencies. After listening to music at 432 Hz there was a significant improvement in sleep scores (+3.6, p=0.02), while there was no improvement in sleep scores listening to music at 440 Hz (-1.50, p=0.34).
https://pubmed.ncbi.nlm.nih.gov/33368583/ said:It is known that Ghrelin, a peptide hormone, regulates food intake in the hypothalamus; in a previous paper, we reported that musical stimuli at 432 Hz modified the Ghrelin expression in the rat, increasing beneficial effects on metabolism. In this study, we used this frequency and we focused our attention on body weight, Ghrelin expression, and neuron morphology in hypothalamic cultures. To investigate the role of music, we utilized newborn pups from pregnant rats that were exposed to music stimuli at 432 Hz during the perinatal period and for the postnatal period, some for 3 days (P3) and others for 6 days (P6). Some pups were not exposed to music stimuli (controls). Our results showed that music increased the body weight of pups; in addition, enhanced Ghrelin expression in hypothalamic neurons and their axonal elongation were highlighted by immunocytochemical techniques. Moreover, we found that the positive music effect started in pups at P3 and increased at P6 compared with controls. These results suggest that the musical frequency at 432 Hz could stimulate the orexigenic Ghrelin effects influencing the increase in body weight and affecting the number of hypothalamic neurons expressing Ghrelin.
https://pubmed.ncbi.nlm.nih.gov/8532483/ said:The presented tone stimulus was 440 Hz, 60 dB pressure of pure tone, and the presented tempo varied from 10 to 300 cycles per minute. The relationship between heart rate and preferred tempo was examined to calculate the density function of ratios of preferred tempi to heart rate. This density function indicated that preferred tempi were distributed mostly one, one and a half, and two times as fast as heart rate. This finding indicated preferred tempi had a simple harmonic relationship to heart rate.