How to Buy a Quiet Air Purifier

Air purifier noise output varies from very quiet - quiet enough to blend in with regular ambient background noise (40 dB) – to very loud - loud enough to almost be as loud as a vacuum cleaner (70 dB - eight times louder than 40 dB).

Each individual model’s noise output varies according to fan speed settings. On lower fan speed noise output is lower. On higher fan speed noise output is greater.

Some models are quieter (or louder) than others. The Honeywell HPA100, for example, can get as loud as 68.7 dB on its highest fan speed. The Honeywell AirGenius 5 doesn’t get any louder than 56.1 dB on its highest fan speed setting.

The HPA100 can only get as quiet as 51.5 dB on its lowest fan speed. The AirGenius 5 can get much quieter – on its lowest fan speed it can get as quiet as 45.2 dB.

The easy deduction to make is that the AirGenius 5 is the quieter air purifier. Or is it?

Noise Efficiency - CFM/dB

The analysis above considers only total noise output (more specifically, minimum and maximum noise output at min/max settings). What if we wanted to take this analysis a little bit further?

In evaluating energy efficiency we certainly don’t restrict any analysis to total power draw. Let’s say, hypothetically, that a small desktop air purifier draws under 10 watts even on its highest fan speed. Larger units can draw over 100 watts of power. Is the smaller unit more energy efficient because it draws less maximum power? Maybe. Maybe not.

To properly analyze each unit’s energy efficiency we have to compare its power draw to its output – its CFM. If the small unit outputs at 5 CFM and draws 10 watts of power its CFM/watt ratio is 0.5. If the large unit outputs at 500 CFM and draws 100 watts of power its CFM/watt ratio is 5.

5 is 10 times 0.5 so we can say that the larger unit is 10 times more energy efficient even though its maximum power draw is much more.

Let’s do the same for noise output. Yes, the HPA100 produces 68.7 dB of noise on its highest setting but at how much CFM? The answer is approx. 100 CFM. The CFM/noise ratio for this unit on this setting is approx. 1.5.

What about the AirGenius 5? On its highest setting it produces only 56.1 dB of noise and also with a higher CFM of approx. 175. Its CFM/noise ratio on this setting is approx. 3.

We can now say that when each unit is on its highest setting the AirGenius 5 is about twice as “noise efficient” as the HPA100. So not only is the AirGenius 5 more quiet on its highest setting and on its lowest setting but does so at a greater output – a higher CFM. This is demonstrated by its much better CFM/noise ratio. In other words, it is a much quieter air purifier not only because it doesn’t get as loud (lower maximum dB) but also because it doesn’t make nearly as much noise compared to its output (high CFM/dB ratio). Keep this in mind as you study the noise output of all of the air purifiers we’ve tested thus far.

Based on these results our recommendation as the best most quiet air purifiers currently on the market are the following:

1. Coway AirMega 400
This unit has far and away the best CFM to dB ratios of any air purifier currently on the market. We highly value total output and this unit provides tremendous output while still staying very quiet. The one caveat here is price. This unit is very expensive. This is what prevents it from being a top-rated air purifier that we recommend in our overall rankings. However, if you’re looking for the best most quiet air purifier at any price, this model is the winner.

2. Winix 5500-2 or Coway Mighty
Unlike the AirMega 400, either one of these units is highly affordable and a great value considering their filtration quality and overall output compared to their price. Either one of these units is a great option if you’re looking for a quiet air purifier that’s also a great all around performer.

A deeper analysis of air purifier noise output

So far, we’ve shown two different ways to evaluate an air purifier’s noise output:

• total noise output at different fan speeds including minimum/maximum fan speed
• CFM/dB ratio

Why do some models have a higher or lower minimum/maximum noise output? Why do some have a better or worse CFM/dB ratio? To answer these questions, we have to look at the types of noise air purifiers produce.

During testing we observed two different types of noise produced by the air purifiers we tested:

• air/wind noise – for most users this is unlikely be an unpleasant sound
• fan motor noise – for most users this is likely to be an unpleasant sound

Air/wind noise

The greatest factor affecting air/wind noise is outlet size.

As you might expect:

• Smaller air purifiers tend to have a smaller outlet size
• Larger air purifiers tend to have a larger outlet size

Large air purifiers are quieter

This is one of the primary reasons why we recommend larger units over smaller units in our general buyer’s guide.

At the same output (CFM), larger units have a larger outlet through which the air can travel. This results in lower air/wind noise in larger units at the same output. The difference in noise output is substantial.

Let’s compare two units to show you what we mean:

Our testing showed that in a small room (under 150 sq. ft.) about 100 CFM of output is required to properly lower particle concentration in the room.

It should be clear from the tabled data shown above that a larger air purifier like the Coway is a much better option for this application if low noise output is a priority.

Note: Units like the Coway can also work in larger rooms on higher settings. On these higher settings the Coway will produce just as much noise as smaller units like the HPA100 on their highest settings. The difference here is that while it’s actually possible to use larger units in these larger rooms, smaller units will not work at all. This superior versatility is one of the many additional reasons why we recommend larger units in our general buyer’s guide.

Fan motor noise

Some units we tested exhibited what we call “fan motor noise”. This can best be described as a low droning humming noise emanating from the fan’s motor.

Most models we tested did NOT make this noise during testing. All top rated models did not. Those models that did will have this fact listed in their review.

The Tower Advantage

Certain tower units like the Honeywell Airgenius 5 had two advantages over large and small air purifiers in our noise output testing.

1. Tower units usually have not one but two outlets. This distributes air/wind noise over a larger area – a larger area away from our sound meter.

2. Most air purifiers on the market have an outlet pointing upwards. Tower units have outlets pointing forward (on the front of the unit). This gave tower units an advantage in our noise output testing since we placed the sound meter mostly above and only slightly in front of the control panel of each unit we tested.

Tower units have one other advantage over all other air purifiers when it comes to noise output: Tower units output forward which allows you to direct the noise wherever you want. Of course, the noise will also be in the direction of clean air. Again, all other units output upward – with these units there’s no way to direct the noise.

Summary

First, an air purifier can really make only two different types of noise:

• air/wind noise
• fan motor noise

Second, larger units with larger outlets are going to be much quieter than smaller units with smaller outlets at the same output. So, if noise is a concern for you definitely look into buying a larger unit.

Third, definitely look out for units with fan motor noise.

What is the quietest air purifier on the market?

Chances are that you will be just fine with any larger unit – most can be set to fan speeds where they will produce very little noise – and at these low noise levels, differences between specific models is mostly insignificant. We would specifically recommend either this Coway or Winix only because they do everything else really well also – they’re a great value, have good energy efficiency, proper filtration, etc.

If you’re looking for a specific model that we would recommend as the most quiet air purifier on the market, it’s the Coway AirMega 400. It’s not a great value but it is extremely quiet with the best CFM/dB ratios of any air purifier we’ve tested so far.