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What the mother’s coo can tell us about safety in sound

By Susanna Coss
Reviewed by Rebecca Knowles, OTD, OTR/L, RYT

What makes the Safe and Sound Protocol (SSP) music special? How is it different from other music? These are two questions we’re often asked at Unyte.

The simplified answer is that the music of the SSP Core Pathway has been algorithmically filtered to highlight characteristics of sound that are used in safe human communication. This sends cues of safety to the nervous system that help the listener feel more regulated.

But what are these safe characteristics of sound?

One clue is found in how a mother soothes their infant. By changing the pitch, rhythm and volume of their voice, commonly referred to as the mother’s coo, they are able to convey security, which contributes to a nurturing relationship with the baby. 

Jacek Kolacz, Ph.D., a research assistant professor at the Ohio State University College of Medicine, and colleagues investigated this phenomenon by exploring the impact of maternal vocal prosody on the regulation of infants’ nervous systems after experiencing a social stressor (Kolacz et al., 2021).

The “Still-Face Paradigm,” developed by Ed Tronick, Ph.D. (1978)
Kolacz et al., 2021

The study involved 96 mothers and their infants between 4 and 8 months old. The “Still-Face Paradigm,” developed by Ed Tronick, Ph.D. (1978), was used as the social stressor. It began with the mothers interacting normally with their infant in a face-to-face play phase, followed by a two-minute still-face phase, where the mother withdrew interaction and sat still with a neutral expression. After two minutes, normal interaction was resumed in the reunion phase. Throughout the experiment, the researchers monitored the infants’ heart rates and behaviors. During the reunion phase, they assessed the prosody of the mothers’ voices by examining two qualities: frequency and spectro-temporal variability.

Frequency, measured in hertz (Hz), correlates to pitch. Based on the way the human ear is shaped, sounds within the 500-to-5,000 Hz range are easier to understand and detect, especially during speech. These frequencies fall in the middle range of what humans are able to hear and are known as the “band of perceptual advantage.” This frequency band encompasses the pitches used when humans communicate calmly with each other, as opposed to distress calls or screams, which are typically at a higher pitch of more than 5,000 Hz.

Examples of different sounds and general frequency range.

Safety is also communicated vocally through rhythm and volume, with spectro-temporal variability playing a crucial role. In this context, “spectro” pertains to spectral modulation, which denotes the change in frequency (pitch) components over time. Consider how pitch changes as a musical scale is played, or how pitch rises at the end of a sentence to indicate a question being asked. Temporal modulation is the change in sound wave amplitude (volume) over time and is used to create rhythm and emphasis. Our brains pick up on variations in volume and pitch to add complexity and nuance to the meaning of the words being spoken. 

In this study, the mothers’ voices with more middle-range frequencies (500-to-5,000 Hz) and changes in pitch and volume (spectro-temporal variability) were associated with reductions in infant heart rate and behavioral stress, and linked to increases in cardiac vagal tone following exposure to stress. These results suggest that prosodic features in sound can improve nervous system regulation, pointing to the relationship between sound and the nervous system. 

“As mammals, there’s something about prosodic, melodic vocalizations that signal safety to our bodies. What SSP does is it takes those properties and puts them into music in a way that serves as a progressive exercise to get those systems [involved in regulation] going.”

— Seth Porges, in Clinical Conversations: Practical Applications of Polyvagal Theory

How does this relate to the SSP?

The features of the mothers’ voices measured in this study were based on the same principles as the music filtering in the SSP. In summary, the SSP amplifies the way sound is modulated around middle-frequency bands associated with safety to help the listener feel regulated, safe and connected to themselves, their environment and others.

Continue your learning

Dr. Stephen Porges and Seth Porges delve further into this topic in an on-demand webinar, titled Clinical Conversations: Practical Applications of Polyvagal Theory. Watch a snippet of the webinar below, and access the full recording here.

Dr. Stephen Porges and Seth Porges in a recent webinar, titled Clinical Conversations: Practical Applications of Polyvagal Theory.


Kolacz, J., daSilva, E. B., Lewis, G. F., Bertenthal, B. I., & Porges, S. W. (2021). Associations between acoustic features of maternal speech and infants’ emotion regulation following a social stressor. Infancy : the official journal of the International Society on Infant Studies, 27(1), 135–158. https://doi.org/10.1111/infa.12440

Tronick, E. Z., Als, H., Adamson, L., Wise, S., & Brazelton, T. B. (1978). The infant’s response to entrapment between contradictory messages in face-to-face interaction. Journal of the American Academy of Child Psychiatry, 17(1), 1–13. https://doi.org/10.1016/S0002-7138(09)62273-1

About the author Susanna Coss

Susanna Coss, Unyte’s Research and Training Associate, contributes to advancing the empirical foundation of Unyte tools by cultivating research collaborations, exploring relevant subjects, and converting insights into practical solutions and training materials. With a foundation in neuroscience and a dedicated enthusiasm for assisting individuals in optimizing their nervous systems amid the complexities of modern life, Susanna has a keen interest in topics such as heart rate variability, biophilic design, and psychedelic-assisted therapies. 

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