In 2007, Nike introduced a driver called the Sumo. It was immediately identifiable because of its square head shape as well as the signature sound it made when impacting the ball. The noise in question was a loud “clank” that many players found annoying. “The Sumo had a very strong frequency content around 2,000-3,000 hertz,” says Daniel A. Russell, a professor of acoustics at Pennsylvania State University’s College of Engineering. “That’s right where the human ear is most responsive to sound.”
From a performance standpoint, a square driver actually had some tangible advantages. Moving weight to the corners of the clubhead reduced the amount of twisting that happens when the ball doesn’t hit the sweet spot on the face. Less rotation around the center of gravity meant straighter shots, even on less-than-ideal swings. But, looking back on reviews, it’s clear that the sound (and to a lesser extent, the aesthetics) was a dealbreaker for many players.
In an effort to avoid future cacophony, Nike went to Russell in 2015, hoping to leverage his then 16-year history studying acoustics in sporting equipment, including everything from baseball bats to tennis rackets. The company would ultimately get out of the golf manufacturing game altogether in 2016, but Russell has continued to delve into the world of golf club sound.
The lessons of the ill-fated square driver fad were not lost on the other big golf club manufacturers. “In the early to mid-2000s, there were a lot of square drivers out there,” said Brandon Woolley, senior research engineer for TaylorMade Golf. “They had a very loud, piercing sound. You could identify those from two holes away if you heard someone hit it on the tee.” You won’t find any square drivers on stores shelves in 2017.
Trying to articulate the perfect golf club sound is a difficult and sometimes amusing endeavor. Club designers use words like “clacky” and “thwack.” Audiologists drop terms like “tink” and “thonk.” I even had a salesman at a large retail sporting goods store refer to a popular current club as “splanky.” But, while the verbiage may be entertaining, the implications for players and companies trying to sell them gear are quite serious. Golf equipment sales was a $2.55 billion business in 2015 in the U.S. alone, yet players won’t buy clubs that don’t sound good, even if the sticks could possibly improve their game.
The big ping
Sound first became a focal point for golf club manufacturers back in 1959 with the introduction of the Ping 1A putter. Designed by Karsten Solheim, the 1A had a partially hollowed out blade, which helped enlarge the sweet spot and reduce the number of mishits. It also produced a signature ringing—or more appropriately pinging—sound when struck. The putter’s performance solidified its reputation, but its sound inspired the Ping name that has persisted across a equipment empire that still exists today.
Sound was a side effect back then, but modern club manufacturers are very deliberate about tuning their sticks. “Almost as soon as we have a prototype modeled in the computer, we will try and predict how it’s going to sound,” says Woolley of TaylorMade’s design process. “We test each prototype in various configurations until we have all the final parts in.”
Callaway goes through a similarly rigorous process with its club acoustics, starting with computer modeling before a pre-production stick makes it into the real world. “Once we have physical prototypes, we then measure the sound with a microphone and use pretty complex software to analyze the signal,” says Evan Gibbs, R&D director for woods at Callaway. To capture raw sound data, the company sometimes has players hit balls, and other times uses a large pendulum to drop a metal ball onto the club face, allowing engineers capture the resulting frequencies. The whole process takes place in an anechoic chamber designed to prevent errant vibrations from muddying the data.
In addition to the sounds, researchers capture a large number of other diagnostics about each shot to get a more-complete picture of the club’s performance. Some of the companies use proprietary cameras, but there are also advanced commercially available options like the Track Man, which Russell uses at PennState. The $19,000, device uses radar to measure a litany of variables like clubhead speed, launch angle, ball rotation, and other crucial stats.
In all cases, various flavors of commercial and custom sound software analyze the recordings, but the clubs also undergo subjective tests by both players and those involved with the development process. “We don’t necessarily play the sound for focus groups or anything like that,” says Woolley. “The lead engineer and the head of R&D who has a stake in the product will come up and listen and make sure they think it sounds good.”
Sound and fury
The number of variables that can be tracked regarding a golf shot is staggering, but when it comes to acoustics, there are a few key elements: frequency, amplitude (usually referred to as volume), and duration. The relationship between these elements is crucial for getting good sound; if even one is out of whack, ball-strikes might go from a satisfying “clink” to a groan-worthy “clonk” in a hurry.
“Frequency is probably the most-important aspect we measure,” says Woolley. “We try to stay above 3,000 hertz. If you go lower than that, we find that it starts to sound bad.”
The frequency of a club is also intertwined with its perceived volume, thanks to a phenomenon called the equal-loudness contour. Sounds with certain frequencies will ring louder, even if they maintain a consistent decibel level, simply because the ear is more-sensitive to specific parts of the audible spectrum. This is part of the reason some phone ring tones are easier to hear than others, even when both are played at the same volume.
A typical driver strike will generate a sound pressure somewhere between 80 decibels and a top end of about 110, which is roughly equivalent to a jackhammer. Once that number tops 116, it starts introducing the potential for long-term hearing damage.
The duration of the sound is more-subjective from player to player. “You can have clubs that have too short of a duration and it makes them sound kind of plastic,” says Woolley. “If they ring too long, it can have a tuning fork type effect, which can feel strange.” The typical duration range is around 50 to 150 milliseconds.
Grip it and rib it
The lion’s share of the sound made by a driver doesn’t come from the club’s face, but rather from the residual ringing in the walls of the hollow club head. The USGA rules state that a clubhead can’t displace more than 460cc of water when submerged. Most manufacturers push this spec to its absolute limit in pursuit of greater distance. That leaves lots of surface area to vibrate and make sound.
“Most drivers—in order to meet the USGA maximum performance requirements—have a face resonance frequency around 4,500 to 5,000 hertz,” says Russell. Most club faces are titanium, and their designs are driven primarily by performance. Engineers do, however, have some tricks for tuning the sound of the rest of the head.
“One of the key technologies we use is ribs,” says Woolley. In fact, TaylorMade owns several patents regarding the use of metal ribs crisscrossing the interior surfaces of a driver head. As the walls of the clubhead get thinner to reduce weight, they also become more prone to vibration that needs damping. “We use [ribs] to stiffen certain parts of the head to raise the frequencies ranges we like better.” TaylorMade is tight-lipped about how the arrangement of the ribs specifically affects the sound, but you can easily see the ribs if you crack open one of the club heads.
In addition to the internal substructures, TaylorMade has begun using something it calls a Geocoustic sole. A section of the sole (the bottom surface of the head) has a slight indent, which allows for a bigger club face without exceeding the USGA’s volume restrictions. The combination of the pronounced convex curvature and some externalized ribs on the sole add stiffness and inhibit vibration, which bumps the frequency into a more-desirable range.
Many modern drivers now include composite materials like carbon fiber, which profoundly changes the overall sound performance of a club. “Composite adds a whole other layer of complexity,” says Woolley. Both of TaylorMade’s 2017 flagship M1 and M2 drivers have composite crowns, which cuts down on the amount of titanium that might act like a bell or a tuning fork in the club.
While the use of composite is now seen as a benefit in terms of acoustics, there are cautionary tales in that arena as well. Early composite drivers like the Callaway Big Bertha C4 were criticized by reviewers for sounding “dead” or like a “thud.”
Up the irons
While drivers tend to make the loudest and most easily identifiable sounds, the acoustics of the irons are also just as crucial and require almost as much clever engineering.
Many pros use svelte blade-style irons, which are forged, smaller, and thick, so their sound is inherently high-pitched and short. The average player, however, tends to gravitate toward a larger, cavity-back iron, which offers a bigger sweet spot, but less overall control over the ball’s flight path.
According to Woolley, the faces of the cavity-back irons have been getting thinner in recent years as club design has evolved. This redistribution of weight in the club head helps to enlarge the sweet spot and give the ball a better launch angle for increased distance. But, it also makes for a worse overall sound that has too much resonance. Manufacturers tweak the size, shape, and material of the branding badges found on the back of the club to try and keep the frequencies in check.
“Of course, badges are meant to look nice,” Woolley explains. “But there’s a surprising amount of engineering that’s gone into those to help stiffen and damp certain portions of the head.”
While the big, loud noises that come out of fairway clubs get a lot of attention, Russell has turned his attention to the much more-subdued sounds of a putter. “People will say a putter is ‘hard’ or ‘fast,’ or, on the other hand, ‘soft’ or ‘slow.’ Those are terms that are informed by the acoustics of the interaction," Russell explains. The frequency and loudness of the impact sound has a correlation with a player’s description of the club’s performance.
“Putters have the advantage in that they can be very heavy when compared to a driver, so you don’t typically have these thin-walled structures that ring and vibrate for very long,” says Woolley. “It’s easier to get good sound in a putter.” He went on to say, however, that as putter designs get more exotic, the company has an increased motivation to monitor each club’s acoustics to stay within the sound range that players prefer.
While there’s a ton of hard science that goes into analyzing golf club sounds, landing on a final tone isn’t a cut-and-dry decision.
“That’s one of the problems with engineering sound is that there is some variation in what different players like,” Woolley laughed. “I’m sure there was somebody out there who thought those square drivers sounded cool.”
Sound perception can be affected by everything from the age of the player to the altitude of the location of the course. “In Japan,” says Callaway's Gibbs, “Golfers typically prefer a much louder sound and longer duration on their drivers than golfers in other markets.”
There’s more to it than just using sound as a marketing tool, as well. An oft-cited 2005 study showed that there was a strong correlation between the sound of a golf club and its overall “feel,” which is a slightly nebulous metric, but an omnipresent concept in the golf world that encompasses a variety of elements about a particular club.
“Three things matter in terms of player preference and player perception of quality when it comes to a golf club or any other object used to impact a ball: The way it feels, the way it sounds, and the initial perceived trajectory of the ball,” Russell explains. In his experience, it creates a tangible psychological effect. “Sometimes you’ll talk to players and they’ll say, ‘Wow, this has a lot of pop,’ when it didn’t actually go any farther.”