“What Makes Pleasant Sound?”

By William Abou-Rjaili ’22

What is Sound?

Sound is just a pressure wave which is propagated through space. Sound is known as a longitudinal wave, which means that the displacement is in the same direction as the wave. Since sound is also a mechanical wave, it is also called a compressional wave. 

How Do we “Hear” Sound?

Sound waves travel through the air into your ear. They then vibrate your eardrums, which translate the mechanical waves in the air to solid mechanical waves. The eardrum is directly connected to a small bone called an ossicle, which is in the cochlea, or middle ear. Hair patches connected to the nervous system then translate the mechanical waves into electric signals, which are then sent to the brain through the nervous system. 

These hairs, instead of vibrating with all of the sound waves coming in and sending that data to the brain, instead each have different resonant frequencies, which make them vibrate very vigorously at certain frequencies. 

Resonant Frequencies

The resonant frequency of an object is the frequency where objects vibrate most at. The reason for this is that waves add up. For example, if you have a wave which interacts with its negative counterpart, it would cancel out, as seen below. When an  object vibrates at its resonant frequency, all of the waves entering the object are adding up instead of cancelling out. There, the object vibrates a lot.

Harmonics

Harmonics occur when multiple waves are combined which have simple ratios. For example, the octave has a ratio of 2:1 (the lower note has half the frequency of the higher note). A perfect fifth has a ratio of 3:2. A major third has a ratio of 4:5. 

The reason why we consider harmonics as consonant (or pleasurable) is debated, and is still an open question. This was proven to not be a societal reason, since babies and even monkeys have been shown to hear the difference. Instead, scientists originally thought that this was some kind of mechanical reason, such as the way the tones would combine, or the way they were converted to electrical impulses. Instead, researchers from the State University of Nizhniy Novgorod say they may have found a more consistent reason. They say that the brain works like a set of neurons, with two listening for waves and one taking them both together and processing it. Each of the first two neurons send electrical impulses to the third neuron (named the interneuron), which sends the signals to the rest of the brain. The interneuron works as an OR gate, firing if either of the first two neurons fire. If the notes are consonant, they arrive at the first neurons at the same time very often (proportionally to the ratio of the sounds), meaning they both send signals to the interneuron at the same time. If this is the case, the interneuron sends out only one signal instead of two, then waits to “recharge,” and then fires again. This causes a regular pulse, which is pleasing. If the tones are dissonant, the electrical signals from the first neurons arrive at different times, making the interneuron’s pulse irregular.

Overtones and Timbre

Timbre is what makes us hear the difference between different notes from different instruments. When an instrument plays a note, the resulting wave has a different shape than, for example, a sine wave. Each instrument is different, which creates different sounds and tone qualities for different instruments. 

The overtone series of a note also reflects the instrument being played. Overtones are harmonic notes above the fundamental frequency (the one you hear most) which play with the fundamental frequency.