̽��ֱ�� of Cambridge - Anurag Agarwal

Handheld device could transform heart disease screening

sc604 — Tue, 08 Apr 2025 08:10:47 +0000

̽��ֱ��researchers, from the ̽��ֱ�� of Cambridge, developed a device that makes it easy for people with or without medical training to record heart sounds accurately. Unlike a stethoscope, the device works well even if it’s not placed precisely on the chest: its larger, flexible sensing area helps capture clearer heart sounds than traditional stethoscopes.

̽��ֱ��device can also be used over clothing, making it more comfortable for patients – especially women – during routine check-ups or community heart health screening programmes.

̽��ֱ��heart sound recordings can be saved on the device, which can then be used to detect signs of heart valve disease. ̽��ֱ��researchers are also developing a machine learning algorithm which can detect signs of valve disease automatically. ̽��ֱ��results are reported in the IEEE Journal of Biomedical and Health Informatics.

Heart valve disease (valvular heart disease or VHD) has been called the ‘next cardiac epidemic,’ with a prognosis worse than many forms of cancer. Up to 50% of patients with significant VHD remain undiagnosed, and many patients only see their doctor when the disease has advanced and they are experiencing significant complications.

In the UK, the NHS and NICE have identified early detection of heart valve disease as a key goal, both to improve quality of life for patients, and to decrease costs.

An examination with a stethoscope, or auscultation, is the way that most diagnoses of heart valve disease are made. However, just 38% of patients who present to their GP with symptoms of valve disease receive an examination with a stethoscope.

“ ̽��ֱ��symptoms of VHD can be easily confused with certain respiratory conditions, which is why so many patients don’t receive a stethoscope examination,” said Professor Anurag Agarwal from Cambridge’s Department of Engineering, who led the research. “However, the accuracy of stethoscope examination for diagnosing heart valve disease is fairly poor, and it requires a GP to conduct the examination.”

In addition, a stethoscope examination requires patients to partially undress, which is both time consuming in short GP appointments, and can be uncomfortable for patients, particularly for female patients in routine screening programmes.

̽��ֱ��‘gold standard’ for diagnosing heart valve disease is an echocardiogram, but this can only be done in a hospital and NHS waiting lists are extremely long – between six to nine months at many hospitals.

“To help get waiting lists down, and to make sure we’re diagnosing heart valve disease early enough that simple interventions can improve quality of life, we wanted to develop an alternative to a stethoscope that is easy to use as a screening tool,” said Agarwal.

Agarwal and his colleagues have developed a handheld device, about the diameter of a drinks coaster, that could be a solution. Their device can be used by any health professional to accurately record heart sounds, and can be used over clothes.

While a regular or electronic stethoscope has a single sensor, the Cambridge-developed device has six, meaning it is easier for the doctor or nurse – or even someone without any medical training – to get an accurate reading, simply because the surface area is so much bigger.

̽��ֱ��device contains materials that can transmit vibration so that it can be used over clothes, which is particularly important when conducting community screening programmes to protect patient privacy. Between each of the six sensors is a gel that absorbs vibration, so the sensors don’t interfere with each other.

̽��ֱ��researchers tested the device on healthy participants with different body shapes and sizes and recorded their heart sounds. Their next steps will be to test the device in a clinical setting on a variety of patients, against results from an echocardiogram.

In parallel with the development of the device, the researchers have developed a machine learning algorithm that can use the recorded heart sounds to detect signs of valve disease automatically. Early tests of the algorithm suggest that it outperforms GPs in detecting heart valve disease.

“If successful, this device could become an affordable and scalable solution for heart health screening, especially in areas with limited medical resources,” said Agarwal.

̽��ֱ��researchers say that the device could be a useful tool to triage patients who are waiting for an echocardiogram, so that those with signs of valve disease can be seen in a hospital sooner.

A patent has been filed on the device by Cambridge Enterprise, the ̽��ֱ��’s commercialisation arm. Anurag Agarwal is a Fellow of Emmanuel College, Cambridge.

Reference:
Andrew McDonald et al. ‘A flexible multi-sensor device enabling handheld sensing of heart sounds by untrained users.’ IEEE Journal of Biomedical and Health Informatics (2025). DOI: 10.1109/JBHI.2025.3551882

Researchers have developed a handheld device that could potentially replace stethoscopes as a tool for detecting certain types of heart disease.

This device could become an affordable and scalable solution for heart health screening, especially in areas with limited medical resources

Anurag Agarwal

Acoustics Lab

Person demonstrating use of a handheld device for heart disease screening

̽��ֱ��text in this work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 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 – 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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AI algorithm accurately detects heart disease in dogs

sc604 — Tue, 29 Oct 2024 02:20:22 +0000

̽��ֱ��research team, led by the ̽��ֱ�� of Cambridge, adapted an algorithm originally designed for humans and found it could automatically detect and grade heart murmurs in dogs, based on audio recordings from digital stethoscopes. In tests, the algorithm detected heart murmurs with a sensitivity of 90%, a similar accuracy to expert cardiologists.

Heart murmurs are a key indicator of mitral valve disease, the most common heart condition in adult dogs. Roughly one in 30 dogs seen by a veterinarian has a heart murmur, although the prevalence is higher in small breed dogs and older dogs.

Since mitral valve disease and other heart conditions are so common in dogs, early detection is crucial as timely medication can extend their lives. ̽��ֱ��technology developed by the Cambridge team could offer an affordable and effective screening tool for primary care veterinarians, and improve quality of life for dogs. ̽��ֱ��results are reported in the Journal of Veterinary Internal Medicine.

“Heart disease in humans is a huge health issue, but in dogs it’s an even bigger problem,” said first author Dr Andrew McDonald from Cambridge’s Department of Engineering. “Most smaller dog breeds will have heart disease when they get older, but obviously dogs can’t communicate in the same way that humans can, so it’s up to primary care vets to detect heart disease early enough so it can be treated.”

Professor Anurag Agarwal, who led the research, is a specialist in acoustics and bioengineering. “As far as we’re aware, there are no existing databases of heart sounds in dogs, which is why we started out with a database of heart sounds in humans,” he said. “Mammalian hearts are fairly similar, and when things go wrong, they tend to go wrong in similar ways.”

̽��ֱ��researchers started with a database of heart sounds from about 1000 human patients and developed a machine learning algorithm to replicate whether a heart murmur had been detected by a cardiologist. They then adapted the algorithm so it could be used with heart sounds from dogs.

̽��ֱ��researchers gathered data from almost 800 dogs who were undergoing routine heart examination at four veterinary specialist centres in the UK. All dogs received a full physical examination and heart scan (echocardiogram) by a cardiologist to grade any heart murmurs and identify cardiac disease, and heart sounds were recorded using an electronic stethoscope. By an order of magnitude, this is the largest dataset of dog heart sounds ever created.

“Mitral valve disease mainly affects smaller dogs, but to test and improve our algorithm, we wanted to get data from dogs of all shapes, sizes and ages,” said co-author Professor Jose Novo Matos from Cambridge’s Department of Veterinary Medicine, a specialist in small animal cardiology. “ ̽��ֱ��more data we have to train it, the more useful our algorithm will be, both for vets and for dog owners.”

̽��ֱ��researchers fine-tuned the algorithm so it could both detect and grade heart murmurs based on the audio recordings, and differentiate between murmurs associated with mild disease and those reflecting advanced heart disease that required further treatment.

“Grading a heart murmur and determining whether the heart disease needs treatment requires a lot of experience, referral to a veterinary cardiologist, and expensive specialised heart scans,” said Novo Matos. “We want to empower general practitioners to detect heart disease and assess its severity to help owners make the best decisions for their dogs.”

Analysis of the algorithm’s performance found it agreed with the cardiologist’s assessment in over half of cases, and in 90% of cases, it was within a single grade of the cardiologist’s assessment. ̽��ֱ��researchers say this is a promising result, as it is common for there to be significant variability in how different vets grade heart murmurs.

“ ̽��ֱ��grade of heart murmur is a useful differentiator for determining next steps and treatments, and we’ve automated that process,” said McDonald. “For vets and nurses without as much stethoscope skill, and even those who are incredibly skilled with a stethoscope, we believe this algorithm could be a highly valuable tool.”

In humans with valve disease, the only treatment is surgery, but for dogs, effective medication is available. “Knowing when to medicate is so important, in order to give dogs the best quality of life possible for as long as possible,” said Agarwal. “We want to empower vets to help make those decisions.”

“So many people talk about AI as a threat to jobs, but for me, I see it as a tool that will make me a better cardiologist,” said Novo Matos. “We can’t perform heart scans on every dog in this country – we just don’t have enough time or specialists to screen every dog with a murmur. But tools like these could help vets and owners, so we can quickly identify those dogs who are most in need of treatment.”

̽��ֱ��research was supported in part by the Kennel Club Charitable Trust, the Medical Research Council, and Emmanuel College Cambridge.

Reference:
Andrew McDonald et al. ‘A machine learning algorithm to grade canine heart murmurs and stage preclinical myxomatous mitral valve disease.’ Journal of Veterinary Internal Medicine (2024). DOI: 10.1111/jvim.17224

Researchers have developed a machine learning algorithm to accurately detect heart murmurs in dogs, one of the main indicators of cardiac disease, which affects a large proportion of some smaller breeds such as King Charles Spaniels.

Jacqueline Garget

Huxley, a healthy volunteer Havanese, undergoes a physical examination at the Queen's Veterinary School Hospital, Cambridge.

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Identification of ‘violent’ processes that cause wheezing could lead to better diagnosis and treatment for lung disease

sc604 — Wed, 24 Feb 2021 00:01:31 +0000

̽��ֱ��researchers, from the ̽��ֱ�� of Cambridge, used modelling and high-speed video techniques to show what causes wheezing and how to predict it. Their results could be used as the basis of a cheaper and faster diagnostic for lung disease that requires just a stethoscope and a microphone.

Improved understanding of the physical mechanism responsible for generating wheezing sounds could provide a better causal link between symptoms and disease, and help improve diagnosis and treatment. ̽��ֱ��results are reported in the journal Royal Society Open Science.

At some point, most of us have experienced wheezing, a high-pitched whistling sound made while breathing. For most people, the phenomenon is temporary and usually the result a cold or mild allergic reaction. However, regular or chronic wheezing is often a symptom of more serious conditions, such as asthma, emphysema, chronic obstructive pulmonary disease (COPD) or certain cancers.

“Because wheezing makes it harder to breathe, it puts an enormous amount of pressure on the lungs,” said first author Dr Alastair Gregory from Cambridge’s Department of Engineering. “ ̽��ֱ��sounds associated with wheezing have been used to make diagnoses for centuries, but the physical mechanisms responsible for the onset of wheezing are poorly understood, and there is no model for predicting when wheezing will occur.”

Co-author Dr Anurag Agarwal, Head of the Acoustics lab in the Department of Engineering, said he first got the idea to study wheezing after a family vacation several years ago. “I started wheezing the first night we were there, which had never happened to me before,” he said. “And as an engineer who studies acoustics, my first thought was how cool it was that my body was making these noises. After a few days however, I was having real trouble breathing, which made the novelty wear off pretty quickly.”

Agarwal’s wheezing was likely caused by a dust mite allergy, which was easily treated with over-the-counter antihistamines. However, after speaking with a neighbour who is also a specialist in respiratory medicine, he learned that even though it is a common occurrence, the physical mechanisms that cause wheezing are somewhat mysterious.

“Since wheezing is associated with so many conditions, it is difficult to be sure of what is wrong with a patient just based on the wheeze, so we’re working on understanding how wheezing sounds are produced so that diagnoses can be more specific,” said Agarwal.

̽��ֱ��airways of the lung are a branching network of flexible tubes, called bronchioles, that gradually get shorter and narrower as they get deeper into the lung.

In order to mimic this setup in the lab, the researchers modified a piece of equipment called a Starling resistor, in which airflow is driven through thin elastic tubes of various lengths and thicknesses.

Co-author and computer vision specialist Professor Joan Lasenby developed a multi-camera stereoscopy technique to film the air being forced through the tubes at different degrees of tension, in order to observe the physical mechanisms that cause wheezing.

“It surprised us just how violent the mechanism of wheezing is,” said Gregory, who is also a Junior Research Fellow at Magdalene College. “We found that there are two conditions for wheezing to occur: the first is that the pressure on the tubes is such that one or more of the bronchioles nearly collapses, and the second is that air is forced though the collapsed airway with enough force to drive oscillations.”

Once these conditions are met, the oscillations grow and are sustained by a flutter mechanism in which waves travelling from front to back have the same frequency as the opening and closing of the tube. “A similar phenomenon has been seen in aircraft wings when they fail, or in bridges when they collapse,” said Agarwal. “When up and down vibrations are at the same frequency as clockwise and anticlockwise twisting vibrations, we get flutter that causes the structure to collapse. ̽��ֱ��same process is at work inside the respiratory system.”

Using these observations, the researchers developed a ‘tube law’ in order to predict when this potentially damaging oscillation might occur, depending on the tube’s material properties, geometry and the amount of tension.

“We then use this law to build a model that can predict the onset of wheezing and could even be the basis of a cheaper and faster diagnostic for lung disease,” said Gregory. “Instead of expensive and time-consuming methods such as x-rays or MRI, we wouldn’t need anything more than a microphone and a stethoscope.”

A diagnostic based on this method would work by using a microphone – early tests were done using the in-built microphone on a normal smartphone – to record the frequency of the wheezing sound and use this to identify which bronchiole is near collapse, and whether the airways are unusually stiff or flexible in order to target treatment. ̽��ֱ��researchers hope that by finding changes in material properties from wheezing, and locations that wheezes come from, the additional information will make it easier to distinguish between different conditions, although further work in this area is still needed.

Reference:.
A. L. Gregory, A. Agarwal and J. Lasenby. ‘An Experimental Investigation to Model Wheezing in Lungs.’ Royal Society Open Science (2021). DOI: 10.1098/rsos.201951

A team of engineers has identified the ‘violent’ physical processes at work inside the lungs which cause wheezing, a condition that affects up to a quarter of the world’s population.

Since wheezing is associated with so many conditions, it is difficult to be sure of what is wrong with a patient just based on the wheeze

Anurag Agarwal

Hey Paul Studios

Dimensional Lungs

̽��ֱ��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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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

Yes

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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