Consonant and Dissonant Sounds

T has gotten really into music in the last month or so. Lately, when he hears music he likes, he’ll start dancing. We went out to eat a few weeks ago, and the restaurant had music playing in the background, and T was dancing up a storm in his high chair – it was adorable! And, I especially love when T dances when I play the piano!

T’s evident love of music made me start thinking about babies and qualities of music that they may innately appreciate. A little reading led me to a few studies (for example, this one by Trainor and Heinmiller) that have shown that even infants that are just a few months old prefer pairs of musical notes played together (a musical interval) that are consonant (pleasant sounding) rather than dissonant (harsh or unpleasant). What’s interesting to me about these studies is that babies seem to recognize and prefer musical intervals widely recognized by adults (both musicians and non-musicians) as being consonant, even without much music-listening experience. Here’s a YouTube video that gives examples of consonant (for example, an octave, a perfect fourth, a perfect fifth, etc.) and dissonant intervals (a minor second, a major second, etc.) – the difference between consonant and dissonant intervals is really striking, even if you don’t know the names of the intervals!

The study I linked to above showed that infants prefer listening to consonant intervals rather than dissonant intervals. And, this study (by Sugimoto et al.) showed than even an infant chimpanzee preferred listening to consonant intervals rather than dissonant intervals. So this seems to suggest that there’s something hard-wired in our brains that makes us prefer consonant musical intervals, even if we haven’t heard much music or had any musical training.

So, I decided to test T to see if he has a preference for consonant musical intervals over dissonant intervals! I played different examples of consonant and dissonant sounds for T, both on an iPad and on the piano to see if he had different reactions (this was not the protocol used in any research study, but it was the best I could do at home :)). When I played consonant and dissonant intervals on the piano, I got no noticeably different reaction from T, although, this may have been because he was preoccupied with trying to lick the fan. I then played consonant and dissonant sounds three different times on the iPad. Two of the times, he started smiling when he heard the consonant sounds and reaching for the iPad, and when he heard the dissonant sounds, he turned away from the iPad and even seemed a little visibly distressed (although this may have been because my experiment was running into snack time; I’m beginning to understand why research studies with babies often have a high attrition rate). The third time got no difference in reaction. So, it seem possible that T has a preference for consonant sounds over dissonant ones, but I can’t really be sure based on this.

One thing that piqued my curiosity is that these studies were done in normal hearing babies (and a normal hearing chimpanzee), so I wondered whether people with hearing loss hear consonant and dissonant intervals different than normally-hearing people. I found this study (Tufts, et al.) which showed that people with hearing loss do hear consonant and dissonant intervals differently, and their explanation of why was so interesting to me!

Before I talk about what Tufts et al. found, here’s a quick explanation of why different intervals are heard as consonant or dissonant. The big difference between different intervals is how far apart the two notes are – for example, a minor third is 3 semitones apart (an example is C and E-flat) and an octave is 12 semitones apart. I think that the accepted theory for why an interval sounds consonant is that the component notes of the interval are far enough apart to be easily resolved by the cochlea – one technical way to say this is that the two notes fall into different auditory filters. Here’s a picture I made to go along with an analogy:

intervals.jpg

Imagine you’re rolling balls down a hill into different buckets (I have no idea why you’d be doing this, but just go along with it!). How far apart two balls are at the top of the hill represents a musical interval, and each bucket at the bottom of the hill represents an auditory filter. If two balls fall into the same bucket, the interval composed of those two “balls” (notes) will sound dissonant, whereas if they fall into different buckets, they’ll sound consonant. People with hearing loss are known to have broader auditory filters – that is, the buckets at the bottom of the hill are a lot bigger, so more balls would fall into them. So, based on this theory, you’d predict that people with hearing loss would find closely spaced intervals dissonant that people with normal hearing would find more consonant.

And, to some extent, this is what Tufts et al. found – here are two cool plots (FIGS. 3 and 4):

tufts.jpg

From Tufts, et al. – Top – FIG. 3 of Tufts, et al – Consonance/Dissonance scores for Normal Hearing adults. Bottom – FIG. 4 of Tufts, et al. – Consonance/Dissonance scores for Hearing Impaired adults.

FIGS. 3 and 4 of Tufts show people’s consonance/dissonance ratings for different musical intervals (indicated by the ratio of the frequencies) – as you can see from FIG. 3 – people with normal hearing say that notes with a ratio of 1 (this is the “unison interval,” or the same note!) is very consonant, and then there’s a steep drop where very closely spaced notes (a frequency ratio between 1.0 and 1.1) sounds VERY dissonant (from the analogy above, where balls are landing in the same bucket), and then as the notes get farther apart, the intervals become more and more consonant (analogy – balls more likely to land in different buckets) as the interval approaches an octave (a frequency ratio of 2.0). While the pattern is similar for people with hearing loss (the bottom figure), the curves look slightly different – if you look at the solid line on the bottom figure, the curve is flatter and has a negative peak later, which shows that people with hearing loss generally rated all intervals as sounding less consonant (because the curve is flatter overall), and that the most dissonant intervals to people with hearing loss were ones that were already “recovering” in consonance for people with normal hearing.

This may explain why people with hearing loss often say that music doesn’t “sound right” to them, even if they wear hearing aids or cochlear implants – the musical intervals that people with normal hearing find pleasant and that are therefore used heavily in music may sound harsh and unpleasant to people with hearing loss.

(By the way, the Tufts, et al. study was done with adults with mild-moderate hearing loss; I have no idea what you would find for babies with hearing loss, which I think is a really interesting question!)

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