A tale of three vacuum cleaners
2 July 2017

This is a brief discussion of the important metrics of domestic vacuum cleaners and some test results. For some time I have had issue with salespeople equating the merit of a vacuum cleaner solely with the wattage. Although the effectiveness of a vacuum cleaner clearly has some relationship with AC power (One that had zero power drain would be a paperweight), the merits of a vacuum cleaner should be related more to the design of the path of the air through the machine, all else being equal. I usually bait the salesperson with something along the lines of 'So, it really needs to take that much power to work properly'. Since I had three machines to hand, I made some informal measurements to see if performance did relate to wattage, or not. Obviously three machines only does not make a comprehensive test, but should show whether a clear trend exists. At the same time, some other useful metrics were measured.

Machines in this study

Philips Mobilo 8514
Hoover Hurricane Plus
IS Cleaner CWD16L

By observation, the Hoover had less 'suck' than the other two and also needed the most attention to cleaning of the various filters in order to maintain what suckiness it did have.

What makes a good vacuum cleaner

There are many aspects of performance, features and ease of use that could be considered for a vacuum cleaner, but only a few really important ones.

A top-rating vacuum cleaner would have low power consumption, be very sucky(1), have low noise and filter even the smallest dust particles. It would include a cord retractor. The main head would swivel, tilt and rotate. A large range of fittings would be included. Length of AC cord should be sufficient; the machine should be light and would come apart readily for cleaning and emptying.

(1) This term is used in the original English context before it was hijacked as a colloquialism. For this article, it is a good and desirable quality.

What about cost?

OK I should have included low cost as a desirable feature. For this limited test, all three machines fit into the same cost category - low. They all cost between $150 and $400 NZ dollars (US$110-295). What you get by paying more than this is questionable. A very expensive vacuum cleaner might have lower noise; it might have a better quality HEPA filter. There might be a better brush head and the machine might be built to last longer than a 'cheapy'. Many 'mights' in there. It would probably not have lower power consumption or higher suckiness than the low cost machine. I don't know why someone needs to pay a lot of money for a vacuum cleaner; even for the most house-proud out there. Clean enough to be healthy; dirty enough to be happy -right.

Many years ago, I almost fell for a door-to-door scam. Having accepted an offer of a 'free house vacuum demonstration', I was haranged and cajoled into buying a Kirby vacuum cleaner by a salesman with a well-rehearsed routine which even involved him calling his 'boss' to get authorisation to give me a good trade-in for my old, useless, heap of garbage vacuum cleaner. Once I had recovered from the shock of hearing the $3500 price for the Kirby, a small voice reminded me I didn't value a clean carpet quite that much.

Measurements

Part 1: Power Consumption

Model Power Power factor
Philips Mobilo 1238 watts 1.0
IS Cleaner CWD16L 1164 watts 1.0
Hoover Hurricane 1378 watts 1.0
Hoover at low speed 624 watts 0.2

Most jurisdictions restrict domestic vacuum cleaners to a power of 1500 watts, so these all fall within that restriction. Cost of running a vacuum cleaner is not that important in a household budget. You might operate one for an hour a week? That would cost under 50c a week, or $26 per year. The power factor of 1.0 means that the current drawn is largely sinusoidal and in phase with the voltage, which means the power quality is good. The Hoover, however, has a speed control. For the most part, it is left at max because the suck at anything below max is poor. However, for interest, the power was checked at the minimum setting. That is shown on the fourth row of the table.

At minimum setting, the Hoover takes 624 watts but with an extremely low power factor. This waveform shows why:

Hoover waveform min power

Red trace is one cycle of the mains voltage, while the blue trace is the current drawn by the Hoover at minimum setting. Not only is the current not in phase, it is non-sinusoidal, thus creating a large number of current harmonics. The peak current there corresponds to 6.9 amps, and the rms current is only 2.6 amps. System voltage is 230Vac. Not only is this current unfriendly to the grid, the high peak and rapid rise will cause interference to AM radio, plus will shorten the life of the motor in the vacuum cleaner. Even if the machine produced useable suck at this setting, which it doesn't, I would recommend never dialling down the power with the rotary control.

Part 2: Airflow

Airflow is the most important metric. That blast of air being drawn into the vacuum cleaner at the end of the pipe is what drags in the dust and rubbish. The faster the air, the more stuff will be sucked in. Airflow is usually calculated as a volume of air that is moved, measured in cubic feet per minute (cfm). Although we are a metric country, and cubic metres per second is the valid equivalent, one finds that usage of cfm is far more common and so this is what we use. With typical domestic cleaners, the pipe internal diameter is around 30mm; give or take 1 or 2mm. Air volume will be proportional to airspeed, and this is all we need to measure. I used a hand-held anemometer for this via an adaptor which expanded the pipe to 50mm for measurement. From the airspeed readings, the volume of air is calculated as below.

Model Flow (cfm)
Philips Mobilo 76.7
IS Cleaner CWD16L 102.6
Hoover Hurricane 52.8

There is a clear distinction between models and it is no surprise that the Hoover produced the lowest readings. Now, as mentioned, this is taken at the end of the pipe and does not include the head attachments. The design of the primary head attachment will have an influence on just how well the flow of air gathers in dust and dirt. It was beyond the scope of this test to include the head for measurements and some leeway should be allowed in assessing the effectiveness. A reading over 100cfm is very good, while less than 54 is poor.

Part 3: Static Vacuum

This is another important specification. At first, one would think that the vacuum and the airflow should be strictly related, but this is not the case. When air is flowing, only a partial vacuum exists. Vacuum created when the end of the pipe is completely blocked is more a measure of how well the machine performs when the bag is full, or the filters are clogged. The static vacuum in these tests is measured with a small electronic sensor connected to a 3mm flexi pipe which passes through a neat hole in the vacuum cleaner pipe. Vacuum readings are taken while the vacuum pipe end is sealed.

Model Static Vacuum
Philips Mobilo 23.1 kPa
IS Cleaner CWD16L 19.8 kPa
Hoover Hurricane 21.4 kPa

The Philips model creates the highest vacuum here, but the models are not that different to each other. Manufacturers' figures are hard to come by, but a consensus is that around 20kPa is typical, so we are in that ballpark here. I see manufacturers now claiming 'no loss of suction' for their machines. There is no clear definition for that, but one presumes the claim is that even when bags are full and filters are clogged up that the machine still performs as well as it did with an empty bag or canister. Frankly it isn't possible unless the motor is vastly over-rated; something that would be expensive, power-hungry and not even permitted in most places. Perhaps some design finesse around the air path through the machine might minimise loss of suction. An more appropriate phrase for the marketers would be 'minimal loss of suction'.

Part 4: Noise levels

Vacuum cleaners are uncomfortably noisy. That fan whirring around at high speed and the rushing air through the machine are hard to muffle. Some machines do better jobs than others. A test was made of the noise by placing a calibrated mic 1 metre from the machine at an angle of 45 degrees behind it, while it was running in an outdoor environment in order to avoid reflections altering the result. A full test would mean doing several readings around the machine and combining the result to get total noise power. Nonetheless, the noise levels are similar from front to side to rear, so a single reading has no significant error.

Model SPL unweighted SPL K-weighted Tone PR Annoyance Index
Philips Mobilo 70.9dB SPL 72.4dB SPL 4.6dB 75.5dB SPLeq
IS Cleaner CWD16L 80.9dB SPL 83.2dB SPL 8.7dB 89.6dB SPLeq
Hoover Hurricane 70.5dB SPL 72.6dB SPL 16.6dB 87.1dB SPLeq

While the noise levels measured would not exceed health and safety guidelines for exposure, they are uncomfortable and definitely inhibit conversation in the vicinity. There is a fair range between these three models and the results columns need some explanation.

SPL unweighted: This is the measured sound pressure level at 1 metre. It is unweighted. Now it is very common to see A-weighted noise levels for industrial measurements, however A-weighting is not appropriate. A-weighting is only useful to help analyse very quiet sound levels, so with levels up around 70dB-80dB SPL, C-weighting, or none at all is correct. For those A-weighting zealots out there, the figures reduced by between 1dB to 3.5dB when A-weighting was applied. This minimal reduction is due to the fact that most of the noise spectrum is in discrete bands with generally less energy at low frequencies.

SPL K-weighted: This column applies the weighting curve that broadcasters use to measure loudness. It has a different shape to the usual Fletcher-Munson curves. While this is not in common usage in industrial noise measurements, it was of interest here to try and better map noise readings to subjective loudness.

Tone PR: This value is very useful to account for the presence of discrete tones amongst wideband noise. Such tones are known to be subjectively more annoying than an equivalent increase of the wideband noise level. It is not in common usage, but should be. Many specifiers simply state that equipment having discrete tones or other tonal qualities should have 6dB lower noise level to account for it. The tone PR (Prominence Ratio) calculates the equivalent dB of each significant discrete tone of the acoustic spectrum, taking into account sound masking bands and the prominence of the tone above the general noise. A high value of tone PR is bad and will be more annoying than a machine with low tone PR, if all else were equal.

Looking at the noise results, the Philips Mobilo is the clear winner. It has low wideband noise and a single tone at 5046Hz that is not too prominent. The IS Cleaner noise is 10dB louder than the Philips and in addition has significant tones at 406Hz and 1218Hz, making it the most annoying of all. The Hoover Hurricane machine had similar wideband noise to the Philips but had a lot of discrete tones in the spectrum, making it rather annoying. As a matter of interest the Hoover noise was also recorded when set to its lowest speed. The overall noise reduced only by 1dB but the number and loudness of discrete tones vastly increased as the motor began shrieking. That made the equivalent noise even louder than the IS Cleaner.

Hoover acoustic spectrum

At left is the spectrogram of the Hoover at max setting. Note the discrete bands of tones and noise.

Correlation of AC power and Suckiness (Airflow)

airflow vs watts graphThe plot of the three vacuum cleaners airflow against AC power is quite revealing. It seems that there is an inverse relationship for these three machines. That is, the one with the most suck has the least power demand. Quite the opposite to what one is usually told by salespeople. This does not guarantee that the trend is always the case, however, we can say that higher power consumption does not guarantee more suckiness .

Other features

A few of the other useful features of each machine are listed:

Feature Philips Mobilo IS Cleaner CWD16L Hoover Hurricane
Cord retractor Yes No Yes
Cord length 6.1m 4.6m 5.0m
Head range of movement 3-axis 1-axis 2-axis

Having used all three of these machines, I can say that a power cord less than 6m in length is inconvenient. The head fittings on the IS Cleaner are very basic and having now disposed of the Hoover, it's fittings have been adapted to use with the IS Cleaner. Weight of the three was very similar , but the Philips model is generally easier to tow around the floor. I cannot say much about longevity, however all three of these machines have been around for years. The Hoover has left the building due to it's ineffectiveness but it never actually failed. This suggests that using a cheap vacuum cleaner is quite cost-effective since even if you replaced it every 5 years, it would take many years before the accumulated cost approached that of a machine of the expensive category. Of these three, I prefer the Philips Mobilo.

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