Noise experience in Indore to Bangalore VOLVO Multiaxle Bus

Two weeks ago, I went to Rajasthan for official work. When I return back from Rajasthan, I didn't get the train ticket. So, I decided to travel by bus. The plan is that I have to take a bus from Kota to Indore, Indore to Bangalore and Bangalore to Chennai.

Kota to Indore - 320 Kms

Indore to Bangalore - 1500 kms

Bangalore to Chennai - 350 kms

From Kota to Indore, I took a normal AC private bus and from Indore to Bangalore, there is only one bus available which is a VOLVO Multiaxle Sleeper bus. I never traveled in VOLVO buses before that. So, I was in a hurry to travel by that bus. It was around 25 hours journey.

The bus started at 2.00 PM from Indore. I noticed that noise coming inside the bus. Before that I thought, VOLVO buses won't give the noise inside the bus. I took my tool that is the measurement microphone which I always kept inside my bag Since I am an acoustics engineer. I took the reading of the spectrum of noise coming inside the bus. I noticed that it was around 100 dB. Ohh! It is not good, I thought. If we continuously hear this 100 dB for 25 hours, it will damage our ears.

I decided to look at the spectrum of noise which means the frequency range of noise. Most of the dominant noise is below the 340 Hz frequency which gave me an idea about the noise. 

Yes. It is not a huge noise as what we hear as noise in 100 dB. The noise was not so heavy as what we feel as unbearable noise. Here is the spectrum of noise measured inside the bus in my seat.

This is called the Fletcher Munson Principle. The initial reading of around 100 dB is the actual noise level produced by the bus, but most of the noise produced by the bus below the 340 Hz range. So, we (Human) perceive this noise in less volume. It is because of the ear structure, the human ear is resonant to 1kHz to 4 kHz. Though the noise is 90 dB in the low-frequency range, we perceive that as a 70 dB. And at the same time, in the 1 kHz to 4 kHz range, it will be 90 dB. This is where the weighting curve comes into play. A-Weighting represents the human response to sound and C - weighting represents the microphone response to sound. Noise will be measured in the A-weighting which is represented by dBA. 

Yes, I measured the value without any weighting. After I did the weighting for my readings, I got the exact decibel(dB) level we humans perceive for the particular frequency of the noise. 

Actual noise level                 - 103 dB

Human Response to noise    - 77.3 dBA  

In this journey, I learned from a real-time scenario where how the Fletcher-Munson principle in Acoustics(Science of Sound) works. And, VOLVO bus not producing huge noise as what humans can't bear. Though it was less noisy, I noticed since I love sounds. 

Moral of the Journey:

Keep your experience with the measured results while inferring the measurements. 

If you read this article fully, you might have learned the important concept in acoustics.