Experiment with Lullaby

Sounds are an integral part of the human being right from the communication to entertainment and sometimes even as therapy. I hope everyone in their childhood life has experienced lullaby. That may be the first-ever music or song listen by every human being even before understanding verbal. When I had a discussion with our team about this, we came up with some queries such as

i. Does it make any effect when we move the cradle while the mother sings?

ii. How the song or music makes a child getting relaxed or happy without understanding verbal?

iii. How the child interprets that lullabies?

To get the answers and understand the answers for the above, we need to know something about the generation of different moods in human being. Each mood will be created depending on the brain wave signals and the generation of hormones in the brain. Before the child understands the verbal, it starts to feel the music using the brain. When the sound reaches the brain at a particular frequency, that creates the brainwave according to that. Brainwaves states are categorized into several categories such as Delta, Theta, Alpha, Beta and Gamma. Each state generates or enhances different feelings and moods. Among that, Delta and Theta are the low-frequency states (0.1-4 Hz and 4-8 Hz respectively) which helps for the deep sleep and relaxation etc.,

Then we started finding the answers for the above. One of them in the discussion introduced the term Binaural Beats (Two different frequencies of sound simultaneously introduced in the left and right ear make the brain to interpret as a single frequency sound which is a mathematical difference of those sent into the ears). If the binaural beats happen in the brain, it can help to create the different brainwave states which in turn helps the child to sleep and relax etc., With keeping some understanding of sound and above things in mind, we decided to dig inside. After brainstorming, we got the first clue that it is a Doppler Effect – a principle in sound explains that change in frequency is observed for the receiver when the observer and source move relative to the wave medium. Here the cradle holding the child moves forward and backwards from the mother who is singing the song, so there is a chance of frequency shift when the sound reaches the receiver(child). Then we started the experiment to understand the phenomenon.

First, we selected one lullaby song (Aararo Aariraro) from tamil movie, here is the link Aararo Aariraro Song for that. Then we chose the lyric part of the movie and we did the frequency analysis to find the dominant frequencies in that song. From that, we found out that dominant frequencies are 102 Hz, 348 Hz and 653 Hz. Out of that, we chose 653 Hz for the analysis.

Theoretical Calculations:

According to the Doppler effect, relationship between the observed frequency and source frequency for the classical physics (speed of the source and receiver relative to the medium are lower than the speed of the sound waves in medium),

Here is the link for the detailed theoretical calculations

From that we found,

When the receiver approaches the source, the observed frequency is 

f = 655 Hz

When the receiver decides the source, the observed frequency is

f = 651 Hz

Real-time Experiment:

First, the baby cradle kept static and passed the 653 Hz tone, the receiver(baby) should hear the 653 Hz, 

Then the baby cradle moves towards the source, the receiver(baby) should hear 655 Hz.

As expected, the receiver has shifted in frequency more than source frequency. With that decibel variation of 64.8 dB(A) has been observed when it moving towards the source(mother).

When the baby cradle moves outwards the source, the receiver(baby) should hear 651 Hz.

As expected, the receiver has shifted in frequency less than source frequency. With that decibel variation of 48.9 dB(A) has been observed when it moving outwards from the source(mother)

From the above experiment, we have found that the doppler shift is happening when the mother sings the song while moving the cradle move towards and outwards and that is also within 2 to 4 Hz with decibel variation of 10 - 20 dB(A). Finally, it is theoretically and experimentally proved with the above evidence.

If we can consider two ears of the baby as two receivers, both the receiver moving together with the same speed. Here even the doppler shift happens, but the observed frequency for both receivers are same.

Anyway, we believe that the doppler effect is somewhere helping to create the binaural beats. It may be found out by undergoing further research on this area continuing this. If the answer has been found out in future, the rest will reveal itself automatically. 

Here is the video link of the lullaby experiment.

Please contact(marirenu141@gmail.com) if you need any clarification or if you want to collaborate for further research works. 

Here you can listen to one more lullaby song by Mrs.Priyanka Varadan - Shruthilaya Kalakendira, Coimbatore.

Thanks, Mr.K.R.Balathandapani (Mentor) for supporting me in doing this.