̽��ֱ�� of Cambridge - acoustics

What causes the sound of a dripping tap – and how do you stop it?

sc604 — Fri, 22 Jun 2018 08:00:20 +0000

Using ultra-high-speed cameras and modern audio capture techniques, the researchers, from the ̽��ֱ�� of Cambridge, found that the ‘plink, plink’ sound produced by a water droplet hitting a liquid surface is caused not by the droplet itself, but by the oscillation of a small bubble of air trapped beneath the water’s surface. ̽��ֱ��bubble forces the water surface itself to vibrate, acting like a piston to drive the airborne sound.

In addition, the researchers found that changing the surface tension of the surface, for example by adding washing-up liquid, can stop the sound. ̽��ֱ��results are published in the journal Scientific Reports.

Despite the fact that humans have been kept awake by the sound of dripping water from a leaky tap or roof for generations, the exact source of the sound has not been known until now.

“A lot of work has been done on the physical mechanics of a dripping tap, but not very much has been done on the sound,” said Dr Anurag Agarwal of Cambridge’s Department of Engineering, who led the research. “But thanks to modern video and audio technology, we can finally find out exactly where the sound is coming from, which may help us to stop it.”

Agarwal, who leads the Acoustics Lab and is a Fellow of Emmanuel College, first decided to investigate this problem while visiting a friend who had a small leak in the roof of his house. Agarwal’s research investigates acoustics and aerodynamics of aerospace, domestic appliances and biomedical applications. “While I was being kept awake by the sound of water falling into a bucket placed underneath the leak, I started thinking about this problem,” he said. “ ̽��ֱ��next day I discussed it with my friend and another visiting academic, and we were all surprised that no one had actually answered the question of what causes the sound.”

Working with Dr Peter Jordan from the ̽��ֱ�� of Poitiers, who spent a term in Cambridge through a Fellowship from Emmanuel College, and final-year undergraduate Sam Phillips, Agarwal set up an experiment to investigate the problem. Their setup used an ultra-high-speed camera, a microphone and a hydrophone to record droplets falling into a tank of water.

Water droplets have been a source of scientific curiosity for more than a century: the earliest photographs of drop impacts were published in 1908, and scientists have been trying to figure out the source of the sound ever since.

̽��ֱ��fluid mechanics of a water droplet hitting a liquid surface are well-known: when the droplet hits the surface, it causes the formation of a cavity, which quickly recoils due to the surface tension of the liquid, resulting in a rising column of liquid. Since the cavity recoils so fast after the droplet’s impact, it causes a small air bubble to get trapped underwater.

Previous studies have posited that the ‘plink’ sound is caused by the impact itself, the resonance of the cavity, or the underwater sound field propagating through the water surface, but have not been able to confirm this experimentally.

In their experiment, the Cambridge researchers found that somewhat counter-intuitively, the initial splash, the formation of the cavity, and the jet of liquid are all effectively silent. ̽��ֱ��source of the sound is the trapped air bubble.

“Using high-speed cameras and high-sensitivity microphones, we were able to directly observe the oscillation of the air bubble for the first time, showing that the air bubble is the key driver for both the underwater sound, and the distinctive airborne ‘plink’ sound,” said Phillips, who is now a PhD student in the Department of Engineering. “However, the airborne sound is not simply the underwater sound field spreading to the surface, as had been previously thought.”

In order for the ‘plink’ to be significant, the trapped air bubble needs to be close to the bottom of the cavity caused by the drop impact. ̽��ֱ��bubble then drives oscillations of the water surface at the bottom of the cavity, acting like a piston driving sound waves into the air. This is a more efficient mechanism by which the underwater bubble drives the airborne sound field than had previously been suggested.

According to the researchers, while the study was purely curiosity-driven, the results could be used to develop more efficient ways to measure rainfall or to develop a convincing synthesised sound for water droplets in gaming or movies, which has not yet been achieved.

Reference:
Samuel Phillips, Anurag Agarwal and Peter Jordan. ‘ ̽��ֱ��Sound Produced by a Dripping Tap is Driven by Resonant Oscillations of an Entrapped Air Bubble.’ Scientific Reports (2018). DOI: 10.1038/s41598-018-27913-0

Scientists have solved the riddle behind one of the most recognisable, and annoying, household sounds: the dripping tap. And crucially, they have also identified a simple solution to stop it, which most of us already have in our kitchens.

We were all surprised that no one had actually answered the question of what causes the sound.

Anurag Agarwal

What causes the sound of a dripping tap – and how do you stop it?

Photo by Luis Tosta on Unsplash

Water on tap

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – as here, on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

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Mice sing like jet engines to find a mate

sc604 — Mon, 10 Oct 2016 15:00:00 +0000

An international group of researchers have found that mice use a mechanism similar to that of a jet engine inside their throats in order to make high frequency whistles – the first time such a mechanism has been observed in any animal.

Mice, rats and many other rodents produce ultrasonic songs that they use for attracting mates and territorial defense. These ‘singing’ mice are often used to study communication disorders in humans, such as stuttering. However, until now it was not understood how mice can make these ultrasonic sounds, which may aid in the development of more effective animal models for studying human speech disorders.

Now, new research co-authored at the ̽��ֱ�� of Cambridge and published in the journal Current Biology has found that when mice ‘sing’, they use a mechanism similar to that seen in the engines of supersonic jets.

“Mice make ultrasound in a way never found before in any animal,” said the study’s lead author Elena Mahrt, from Washington State ̽��ֱ��.

Previously, it had been thought that these ‘Clangers’-style songs were either the result of a mechanism similar to that of a tea kettle, or of the resonance caused by the vibration of the vocal cords. In fact, neither hypothesis turned out to be correct. Instead, mice point a small air jet coming from the windpipe against the inner wall of the larynx, causing a resonance and producing an ultrasonic whistle.

Using ultra-high-speed video of 100,000 frames per second the researchers showed that the vocal folds remain completely still while ultrasound was coming from the mouse’s larynx.

“This mechanism is known only to produce sound in supersonic flow applications, such as vertical takeoff and landing with jet engines, or high-speed subsonic flows, such as jets for rapid cooling of electrical components and turbines,” said Dr Anurag Agarwal, study co-author and head of the Aero-acoustics laboratories at Cambridge’s Department of Engineering. “Mice seem to be doing something very complicated and clever to make ultrasound.”

“It seems likely that many rodents use ultrasound to communicate, but very little is known about this - it is even possible that bats use this cool mechanism to echolocate,” said the study’s senior author Dr Coen Elemans from the ̽��ֱ�� of Southern Denmark. “Even though mice have been studied so intensely, they still have some cool tricks up their sleeves.”

Reference:
Elena Mahrt et al. ‘Mice produce ultrasonic vocalizations by intra-laryngeal planar impinging jets.’ Current Biology (2016) DOI: 10.1016/j.cub.2016.08.032.

Adapted from a press release by the ̽��ֱ�� of Southern Denmark.

Mice court one another with ultrasonic love songs that are inaudible to the human ear. New research shows they make these unique high frequency sounds using a mechanism that has only previously been observed in supersonic jet engines.

Mice seem to be doing something very complicated and clever to make ultrasound.

Anurag Agarwal

diamond geezer

Clangers

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How the kettle got its whistle

tdk25 — Thu, 24 Oct 2013 07:28:57 +0000

It may come as a surprise to some, but in all the years that people have been brewing tea, no-one has ever quite been able to work out why kettles whistle. In a basic sense, of course, the reasons are pretty clear, but the physical source of the noise and the specific reason for the whistling sound have both remained elusive.

Elusive, that is, until now. Writing in the October issue of the academic journal, ̽��ֱ��Physics Of Fluids, two Cambridge ̽��ֱ�� researchers claim to have solved the conundrum, and in the process developed the first accurate model for the whistling mechanism inside a classic stove kettle.

Perhaps reassuringly for those who never felt that this was a significant problem, the ramifications reach far beyond kettles themselves. Using the knowledge gained from the study, researchers could potentially isolate and stop similar, but far more irritating whistles - such as the noise made when air gets into household plumbing, or damaged car exhausts.

“ ̽��ֱ��effect we have identified can actually happen in all sorts of situations - anything where the structure containing a flow of air is similar to that of a kettle whistle,” Ross Henrywood, from the ̽��ֱ�� of Cambridge Department of Engineering, and the study’s lead author, explained.

“Pipes inside a building are one classic example and similar effects are seen inside damaged vehicle exhaust systems. Once we know where the whistle is coming from, and what’s making it happen, we can potentially get rid of it.”

Henrywood carried out the research for his fourth-year project as part of his engineering degree, under the guidance of his supervisor, Dr Anurag Agarwal, a lecturer in aeroacoustics. Drawing on previous research by Agarwal, which identified the source of noise in jet engines, the pair were able to show how sound is created inside a kettle as the “flow” of steam comes up the spout.

Having identified the source of the sound itself, they were then able to pinpoint two separate mechanisms, which not only create the sound but specifically cause a kettle to whistle, rather than making the rushing noise a flow might create in other household items, such as a hairdryer.

A basic kettle whistle consists of two plates, positioned close together, forming a cavity. Both plates have a hole in the middle, which allows steam to pass through.

Although the sound of a kettle is understood to be caused by vibrations made by the build-up of steam trying to escape, scientists have been trying for decades to understand what it is about this process that makes sound.

As far back as the 19th century, John William Strutt, 3rd Baron Rayleigh and author of the foundational text, ̽��ֱ��Theory Of Sound, was trying to explain it. In the end, he posited an explanation that Henrywood and Agarwal have proven to be flawed. And Lord Rayleigh was forced to concede that “much remains obscure as regards the manner in which the vibrations are excited.”

Henrywood and Agarwal started by making a series of slightly simplified kettle whistles, then tested these in a rig, in which air was forced through them at various speeds and the sound they produced was recorded.

This enabled them to plot the frequency and amplitude of the sound, and the data was then subjected to a non-dimensional analysis, effectively a set of calculations using numbers without any units, which allowed them to identify trends in the data. Finally, they used a two-microphone technique to determine frequency inside the spout.

Their results showed that, above a particular flow speed, the sound itself is produced by small vortices – regions of swirling flow – which at certain frequencies can produce noise.

As steam comes up the kettle’s spout, it meets a hole at the start of the whistle, which is much narrower than the spout itself. This contracts the flow of steam as it enters the whistle and creates a jet of steam passing through it. ̽��ֱ��steam jet is naturally unstable, like the jet of water from a garden hose that starts to break into droplets after it has travelled a certain distance. As a result, by the time it reaches the end of the whistle, the jet of steam is no longer a pure column, but slightly disturbed.

These instabilities cannot escape perfectly from the whistle and as they hit the second whistle wall, they form a small pressure pulse. This pulse causes the steam to form vortices as it exits the whistle. These vortices produce sound waves, creating the comforting noise that heralds a forthcoming cup of tea.

Henrywood and Agarwal also explain why this effect makes a whistle, rather than another noise, by showing that the mechanism is similar to that seen in an organ pipe or flute. A specific frequency dominates among the sound waves because the note is determined by the size and shape of the opening, and the length of the spout. ̽��ֱ��longer the spout, the lower the note will be.

̽��ֱ��researchers also found, however, that kettles will whistle below the flow-rate at which the vortices emerge. Just as the water begins to boil, they found an entirely different mechanism, which also makes a sound. ̽��ֱ��difference was that the tone at this stage was fixed at one frequency.

“ ̽��ֱ��fixed frequency was intriguing and not something that we had expected to see,” Henrywood said. “We eventually established that below a particular flow rate the whistle behaved like a Helmholtz resonator – the same mechanism which gives you a tone when you blow over an empty bottle.”

When air is blown over the open neck of a bottle, the Helmholtz resonator mechanism causes sound to radiate from the neck. ̽��ֱ��air just inside the neck is bouncing up and down – the air in the main body of the bottle being compressed and released each time like a spring.

For the kettle, the spring is the air inside the whistle, while the air within the whistle opening reverberates like the air in the neck of a bottle. “In a kettle, of course, the air is blown through, rather than over, the neck – the effect is similar to whistling by mouth,” Henrywood added. “In some kettles, both these mechanisms are happening. Because our study enables us to assess the mechanisms in action, we can potentially make modifications to stop the noise – if we want to.”

Henrywood and Agarwal are now working on a project to make quieter high-speed hand-dryers, by looking at how the jet of air released by these devices creates noise. Their paper on kettles - ̽��ֱ��Aeroacoustics Of A Steam Kettle – can be found in the October issue of ̽��ֱ��Physics Of Fluids.

For more information about this story, please contact Tom Kirk, Tel: +44 (0)1223 332300, thomas.kirk@admin.cam.ac.uk

Researchers have finally worked out where the noise that makes kettles whistle actually comes from – a problem which has puzzled scientists for more than 100 years.

Once we know where the whistle is coming from, and what’s making it happen, we can potentially get rid of it

Ross Henrywood

Kaitlin Foley, via Flickr

Tea kettle whistle. Homepage banner image: Dwayne Bent (Att-SA)

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String theories: the mathematics of the violin

fpjl2 — Thu, 07 Mar 2013 13:08:17 +0000

It was as a mathematics undergraduate in the early 1970s that Jim Woodhouse came across Cambridge’s amateur violin making workshop. Always a keen ‘maker’ of things, Woodhouse developed a love of crafting string instruments that quickly spilled into his academic life, as he went on to do a PhD at the ̽��ֱ�� in noise and vibration theories - focusing on the violin.

These days a Professor in the Department of Engineering, as well as a self-confessed “bad cellist”, Woodhouse maintains a keen interest in both instrument-making and the science of acoustics, and will be giving a pre-concert talk for the Science Festival at the West Road Concert Hall on Friday 8 March.

̽��ֱ��talk will be followed by a virtuoso violin performance from Magdalena Fitzpatrick, accompanied by the Cambridge Graduate Orchestra.

Woodhouse compares violins to guitars. Both are essentially wooden boxes with strings, although played in different way, but a note on a guitar, even if wrong, still sounds musical. “You can do bad things aurally with violins that simply aren’t possible with guitars, a fact painfully obvious to parents of children beginning to learn,” says Woodhouse. “But what is it about ‘bowing a string’ that makes a musical sound that much harder to produce?”

A violin player controls three things: the speed of the bow, the downward force applied, and position of the bow on the string. According to Woodhouse, once you choose speed and position, it is the force that primarily dictates whether the result approaches a musical note or spits out a wince-inducing “graunch” noise.

This force-based window of ‘musicality’ grows ever-slimmer as the bow moves up to the bridge, an analysis first plotted in graphical form by former Bell Telephone scientist and pioneer of violin acoustic research John Schelleng in the 1960s.

“Relatively inexperienced players need to try and learn to be comfortable hovering in the middle of this space, staying away from the minimum and maximum, but virtuosos will probe the boundaries for the best tone,” says Woodhouse.

“These factors feed into the idea of ‘playability’ of specific instruments,” says Woodhouse. Some violins are a great deal more valuable than others, but why is this?

“One aspect is ‘beauty of sound’, which is difficult to address in scientific terms as it comes down to peoples’ perceptions. But ease of playing is also a major factor. Whether an instrument is more accommodating in terms of the Schelleng model is one facet of that, and one that lends itself more easily to scientific investigation.”

Issues of playability have been the subject of increasingly sophisticated mathematical modelling, enabling computer simulations to show how strings on particular violins will respond to certain bow gestures. These techniques are increasingly used to explore design questions, but also in accurate electronic instruments - the virtual violin.

But for Woodhouse, nothing can replace a finely crafted instrument, and the evening class he took forty-odd years ago has gone from strength to strength - the Cambridge Violin Makers still run one of the country’s top violin crafting workshops and summer schools.

“I still love to work on musical acoustics from time to time. In fact, I’ve got a current violin PhD student at the moment, and it’s always popular with undergraduates as well as a great outreach subject.”

̽��ֱ��talk and concert take place at West Road Concert Hall on the evening of Friday 8 March. For more information, please visit the Science Festival website.

̽��ֱ��screeching produced by children starting out on the violin has curdled the blood of many a parent over the centuries. A pre-concert talk at the Science Festival asks what science can tell us about why the violin is so hard to play.

Virtuosos will probe the boundaries for the best tone

Jim Woodhouse

Jason Hollinger from Flickr

Violin -- closeup

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Virtual violins

tdk25 — Mon, 01 Sep 2008 00:00:00 +0000

Violins crafted 300 years ago by the master violin-maker Antonio Stradivari sell for millions of pounds on the rare occasion they reach auction; what is it about their quality of sound that makes them prized above all others? Indeed, is their sound actually discernibly different? Any experienced violinist knows that some violins respond to their bow better than others: what determines which violins are difficult and which are easy to play? Questions such as these have fascinated musicians and scientists since the 19th century.

To get to the heart of the riddle there is an added complication: sound is in the ‘ear of the beholder’. In fact, although much is now known about the acoustics of the violin, and how this is influenced by the way it is made, virtually nothing is known about how human capacities for perceiving, discriminating and judging violin sounds match up to their acoustical features. This is a very significant gap, as perceptual judgements obviously define what makes a violin different from, say, a cello, just as it makes one violin different from another, for listeners, performers and violin-makers alike.

A three-year project funded by the Leverhulme Trust that is reaching completion at the ̽��ֱ�� of Cambridge has been intent on filling this gap. ̽��ֱ��approach has involved collaboration between four departments – Professor Jim Woodhouse from the Department of Engineering, Dr Claudia Fritz and Dr Ian Cross from the Faculty of Music, Professor Brian Moore from the Department of Experimental Psychology and Dr Alan Blackwell from the Computer Laboratory.

Strings and body

̽��ֱ��tone, pitch and loudness of a violin are the product of many components: drawing a bow across tightly stretched violin strings forces them into complex harmonic vibration; a significant fraction of this acoustical energy is transmitted, via a structure called the bridge, into the violin body. Here, the sound is amplified by the vibration of the wooden box and the air inside it.

̽��ֱ��team’s approach relies on the fact that the acoustical behaviours of the strings and the violin body can be treated separately, and that it is the latter that distinguishes different violins. In fact, on its own, a string makes hardly any sound and the acoustical behaviour is much the same from one instrument to another. ̽��ֱ��main acoustical feature that ‘colours’ the sound in ways that are unique for each violin is the way in which the violin body responds to the different frequencies input from the bridge and radiated from the body. This characteristic transformation is known as the violin’s ‘frequency response characteristic’.

Virtual violins

̽��ֱ��first stage of the project was to create a ‘virtual violin’. To carry out any comparative study of musical instruments it is important to rule out variations caused by the player. Instead of achieving this by using a robotic violinist that repeats the same piece on a variety of real violins, in this project the tests themselves are virtual.

Sensors on a violin bridge record the string waveforms arising as a player performs normally. ̽��ֱ��recordings are stored as standard force functions, which can then be applied to different violins to hear how they sound without having to worry about any complications caused by variations in playing. So, by ‘playing’ these recordings through computer models of different violins’ frequency response characteristics using digital filters, a prediction of the sound of the violin can be created. This makes it possible to ‘play’ exactly the same performance on different ‘virtual violins’. ̽��ֱ��frequency response characteristics can be derived from empirical measurements made on a range of real violins.

̽��ֱ��psychoacoustics of the violin

Once the violin response is represented in digital filter form, it becomes very easy to make controlled variations of a kind that would be almost impossible to achieve by physical changes to a violin. This gave the researchers an opportunity to focus on what features of violins’ response characteristics determine how listeners discriminate between different violins. In particular, the psychoacoustical experiments looked at just-noticeable differences of alterations made to the acoustical response characteristics of two violins: an excellent violin made by David Rubio and a mass-produced student violin that was informally rated as low quality. Psychoacoustical test methods can be used to find the threshold for detection of any particular change, and also to obtain statistically significant data on quality judgements made by the listeners.

Using groups of listeners that spanned expert string players, expert non-string-playing musicians and non-musicians, it was found for both instruments that the alteration of individual low-frequency resonances needs to be fairly large in order to be perceptible. Even for the listeners who were expert players, a resonance needed to be shifted (in terms of frequency) by about a semitone to be perceptibly different. However, if several resonances are shifted simultaneously, a smaller shift becomes audible.

Testing timbre

̽��ֱ��sound of an instrument is not just about pitch and resonance but is also about a somewhat elusive quality known as timbre. It is, in effect, the richness of the sound. Similar to the manner in which a wine taster conveys the flavour and aroma of a fine wine, there are many different descriptors for the timbre of an instrument: from ‘warm’, ‘sonorous’, ‘clean’ and ‘free’, to ‘unbalanced’, ‘heavy’, ‘dull’ and ‘dead’. In fact, a data-mining exercise from ̽��ֱ��Strad, a classical music magazine covering string instruments, came up with a list of 61 words that are commonly used by players, critics, makers and listeners to describe the quality of the sound.

This list of descriptors was used as the basis for a series of experiments in which players located the words in two-dimensional spaces, the results being analysed by multidimensional scaling methods (MDS) to produce maps of families of terms. Some relevant descriptors can therefore be selected on the basis of their distribution in the MDS spaces. This is now allowing the team to test timbre in a more methodical way than has been possible before, asking questions such as: does an increase of amplitude in the frequency range between 650 Hz and 1300 Hz really make the violin sound more ‘nasal’?

Probing the mysteries of music

̽��ֱ��aim is to provide researchers, violin-makers and repairers with an evidence-based means of assessing what it is necessary to adjust on a violin to achieve improved sound. This rigorous analysis of descriptors and their relations will not only be useful to specialists in discussions with performers, but will also have pedagogical value and might lead to new ways for composers and arrangers to annotate musical scores. Perhaps one day, when describing how one violin sounds different to another, we will be able to say exactly why.

For more information, please contact the author Dr Claudia Fritz (cf291@cam.ac.uk) at the Faculty of Music.

Why does one violin sound different to another? Investigating this question has brought together researchers from music, engineering, experimental psychology and computer science.

̽��ֱ��aim is to provide researchers, violin-makers and repairers with an evidence-based means of assessing what it is necessary to adjust on a violin to achieve improved sound.

firepile on flickr

Violin

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