Published 20 Nov 2012
AXINO TECH HOME

Is the Russell Hobbs Whisper kettle really quieter?

Our original Russell Hobbs kettle was so noisy, we couldn't hear the TV or even each other when it was boiling. When the TV was on, we had to boil water only during ad breaks. So, we bought a new Russell Hobbs Whisper model. The new Whisper model is very similar in appearance to the old kettle. Whisper kettle picSame size, shape and stainless construction. They did find a cheaper way to weld on the spout, but the major difference is that there is a plastic bubble dispersion ring inside attached to the bottom of the kettle.

The new kettle did make a big difference initially. It was obvious that it made much less noise. But, gradually, over three months, the noise increased. It still wasn't as noisy as the old kettle, but it was curious that it had noticeably become louder. A Google search found that other people had observed this too, and that the only reaction from the supplier was that they recommend periodic de-scaling to reduce the noise level. There was very little observable scale inside our new kettle as would be expected after only three months, but it was time to check if de-scaling could reduce the noise.

Kettle noise testing

I made four sound recordings. First was to record the old Russell Hobbs while it boiled 1 litre of water. Then, the Whisper kettle was measured in exactly the same way. This had been in use about 3 months from new at this point. Third, I de-scaled the Whisper kettle with lemon juice in the water for three hours as suggested, then reproduced the test by boiling another litre of water. Just to be sure it was de-scaled, I actually repeated with white vinegar as someone else suggested and rechecked the noise. As it turned out, there was no difference between the two attempts at de-scaling, so I have presented only the three tests.

Each sound recording is close to three minutes in length. I analysed the loudness against time for each, using the recently approved EBU R128 standard, which uses the ITU BS1770-2 methodology. I have not reported a single figure of dB(A) as this would be largely meaningless and this test is meant as a comparison only. The result is the graph below, showing loudness values against time.

kettle loudness plotsPlots of loudness against time for the three tests

Looking at the right hand end first, the three plots are aligned at the point just prior to where the kettle steam switch automatically clicks off. The water is bubbling furiously just before that and is fairly noisy. Just before the water really starts bubbling madly, there is a quiet point. I think all kettles show this characteristic. It is do do with energy equilibrium and steam back-pressure briefly calming the waters, but the science of that process is better left to those who know about such things. The significant part of each test was between about 60 seconds and 120 seconds after starting and that is the bit which is hard to hear the TV over.

Loudness conclusions

The original kettle as shown by the green line is definitely much louder than the Whisper kettle. In fact it is about 8 decibels louder at the middle part of the cycle. This is significant. The blue line is the Whisper kettle after it was de-scaled. De-scaling made a minor improvement of noise in the early phase of the boiling cycle and also right at the end of the cycle, but measured the same maximum loudness in the middle part of the cycle as it did before being de-scaled (red line). So, the advice to descale is of some use, but the overall result is not significantly different and there is more going on that just that.

So, we are left with the question of why the Whisper kettle got louder over three months of use. It appears that de-scaling helps a little, but does not restore it to the out-of-the-box quietness. I do not have that answer because I never made an as-new measurement. My theory involves the plastic ring-thing sitting inside the kettle. I suspect the chemistry of the plastic changes with use and evidence of that is given by the fact the water is less tainted now than what it was when the kettle was new.

Loudness and spectral distribution

Loudness of a sound does not tell the full story and noise of kettles is no exception. From the loudness plot above, the noise right near the end is loudest for the Whisper kettle, although not for the old non-Whisper kettle. The sound at the end of the boiling cycle is quite boomy and in fact can resonate when on a kitchen bench due to the bench acting as a sounding-board with the cupboard space underneath. However, it is not the boomy, bass noise that stops you hearing the TV, or another person speaking. Rather it is the noise caused by large quantities of smaller bubbles forming in the water during the middle part of the cycle. This part is not necessarily the loudest, but it is the fact that the sound spectrum of that part overlaps the human speech spectrum responsible for intelligibility. See the two spectrum plots below.

mid boil spectrumThis top graph is the spectrum of the three tests at around the mid-boil part. As above, the green plot is the original kettle, the red is the Whisper before descaling and the blue is the Whisper after descaling. The spectral distribution is similar for each and the concentration of sound falls between 800Hz and 4000Hz, which is also where the spectra of human spoken consonants lie. So, this wide-band 'hissing swhishing' noise will mask human speech quite well.

Contrast this with the plot below which is taken right at the end of the boiling cycle after the quiet part. end boil spectrumThis second graph is the spectrum of the three tests at the end-boil, just before the kettle switches off. Again, the green plot is the original kettle, the red is the Whisper before descaling and the blue is the Whisper after descaling. Now, the spectrum energy is significant down in the bass region between 40Hz and 70Hz. This is due to the much larger bubbles that are rapidly forming in the water at this stage. The bass energy is rather loud, but does not interfere with human speech and so is less intrusive.

Can I minimise the sound by positioning the kettle?

In practical terms there is not much to be done. You can isolate the kettle base from the benchtop with a foam pad of some sort, which might reduce the bassy notes that occur near the end of the boiling cycle. Keeping the kettle away from corners of the kitchen might mitigate a little of the sound. If you were willing to change the decor of the kitchen, you could go to the trouble of putting acoustic tiles on the walls in a corner and then put the kettle in the corner. In this case, the tiles act as absorbers and diffusers. Some trial and error would be sensible before any outlay.

Axino-Tech Consulting and Services
November 2012

From:

Comments

From: Adele
I feel that my Breville quietboil kettle has grown a lot louder over the last year and was googling to see if others have found the same whisper-style kettles. This was interesting, thanks for doing the hard work!

Make a comment using the form on the contact page