Analyze This: Do exotic woods make better guitars?

Some guitarists have fancy instruments made from rare, expensive woods. Those guitars look pretty. And the musicians insist that such instruments are worth the expense. They claim the guitars produce a richer sound than those made from less costly woods. A team of researchers, including a guitar maker, recently set out to test that notion. What they found might surprise musicians.

Even a simple acoustic (non-electric) guitar has a lot of parts. Major parts include strings, of course. An acoustic guitar usually has six strings. One end of each string is attached to the instrument’s so-called head. And that sits at the end of a long neck. The other end of each string is attached to a small piece called the bridge. The bridge is attached to the guitar’s body. Several parts, in turn, make up that body. The part that people can most easily see when the guitar is being played is called the soundboard. The sides and back of the guitar’s body are, as a group, referred to as the “back wood.”

Christopher Plack is a psychoacoustician (SY-ko-ah-koo-STISH-un). That means he studies how humans perceive sounds. He works at Lancaster University in England. There, he and his teammates designed an experiment to see if guitarists could truly tell the difference between instruments made from different woods.

A guitar maker is also known as a luthier. For this experiment, a luthier made six nearly identical guitars. Their soundboards, necks and heads were made from the same types of wood. And all of the instruments were shaped and glued together at the same time. So, Plack argues, the guitars were as close to identical as possible.

The big difference between them was the material used as the back wood. One was Brazilian rosewood. Luthiers love it, says Plack, because it is hard, dense and stiff. That allows it to stand up to the stresses and strains caused by music’s vibrations. But Brazilian rosewood comes from a rare species. These trees face a high risk of extinction in the wild over the next few decades. Instruments made from it can fetch thousands of dollars. Other woods used for the study’s guitars varied in hardness, strength and cost. They consisted of Indian rosewood, mahogany, walnut, maple and sapele (Sah-PEL-ay).

The researchers then recruited dozens of musicians to test the guitars. Each musician played the instruments in a dimly lit room while wearing a welder’s mask. That meant the guitarists couldn’t see much of anything. They couldn’t tell what wood the instrument had been made from. They could really only see where to put their fingers on the strings, says Plack. In all, the team collected and used data from 52 musicians (only one of whom was female).

Each musician played each guitar for two minutes. They were allowed to play any song they liked. Then, they were asked to rate each guitar on a scale of 1 to 5. Their ratings could include one decimal place (such as “3.1,” for example) if they wished. One of the most interesting questions the team asked was: “How do you like the overall sound of the guitar?”

Based on the replies, the researchers then used statistics to mathematically rate each guitar. Those ratings didn’t differ by much. To more easily show the differences between the six guitars, the team compared their ratings two at a time. Those data appear in the chart below:

Data in the chart depict the differences in the ratings between two guitars that differed only in their back wood. That difference was calculated by taking the average rating of the guitar made with the wood listed in the column on the right and then subtracting the average rating of the guitar made with the wood listed in the column on the left.

The value depicted by the blue dot is the most likely difference in ratings. But the blue bar depicts a range across which the true rating difference would most likely be found. (The orange triangle and bar refer to how the players ranked the woods on playability.)

Data Dive:

1. Take a look at the chart. Some of the differences are very small. Even the largest one is less than 0.2 point. Now consider that the maximum possible difference between two guitars is 4. (That would happen if one guitar in the pair got a maximum rating of 5.0 and the other one only got a 1.0.)
   
   For the data shown, what combination of wood types yields the largest difference in average ratings in terms of percent? Does that seem like a big difference to you?
2. The average difference in ratings between guitars is depicted by a single value. That point is calculated using statistics. Even though the average value is the most likely difference in ratings, it’s possible that the actual average could be higher or lower than that. (That’s what average implies — that there is a range of numbers.) In the chart, the range of numbers the actual average might fall within is depicted by the blue line.
   
   Now, what do you notice about the ranges of ratings differences for each match-up between woods? Does it always include a value of 0.0? (Hint: That means that even though the average difference between the two guitars in each pair probably isn’t zero, it’s possible that the true difference in each matchup could be 0.0.) That would mean that, statistically, there would be no meaningful difference between the guitars in any such matchup.
   
   How does that idea match up with the percentage difference you calculated in question 1 above?
3. Can you determine which wood musicians rated best from the chart? Or which they rated worst?
   
   Here’s one possible way to do that: Think of each comparison in the chart as a game of sports. (So, in each comparison, there will be a “winner” and a “loser.”) Now think of the entire set of comparisons as a sports tournament. The sports team that wins the tournament will have the best overall “win-loss” record.
   
   Use the chart above to figure out each wood’s win-loss record. (Remember: If the ratings difference is a positive number, that means that the wood in the right column had a higher rating than the wood listed in the left column. But if the ratings difference is a negative number, then the wood listed in the left column had a higher rating.)
   
   Fill in the following chart with checkmarks as you compile each wood’s “win-loss” record:

Wood	“Wins”	“Losses”
Brazilian Rosewood
Indian Rosewood
Mahogany
Maple
Sapele
Walnut

NOTE: Because each wood is “competing” against five other woods, the total number of wins and losses for each wood should add up to five.

Now, use the “win-loss” record to figure out the relative ranking of woods. Fill out the chart below:

Ranking	Wood
1
2
3
4
5
6

Use the original chart to do a final check of your overall rankings: In the head-to-head matchup between your top-ranked wood and your second-ranked wood, does the top-ranked wood “win”? Then, continue working your way down the rankings. Does your second-ranked wood have a higher average rating than your third-ranked wood? Is your third-ranked wood rated better than your fourth-ranked wood? If you run across any results in head-to-head matchups that don’t match your rankings, then those woods should swap places in your rankings.

 

4. Can you think of a simpler, quicker way to determine which wood is top-rated?

5. Was the team’s chart difficult to understand? Why?

6. How could you improve this graph or make it easier to read?

7. Variables are things that differ in an experiment and that might contribute to different findings. The main difference in this experiment was the tree species used for the back wood. Can you identify at least one other potential variable that might have been capable of influencing the ratings of guitars?

Note: This article was updated on May 15, 2019 to correct references in the text to the columns in the chart.

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Analyze This! explores science through data, graphs, visualizations and more. Have a comment or a suggestion for a future post? Send an email to sns@sciencenews.org.