We explore the foggy world of humidity, explaining how humidity affects cellos and how it can be controlled safely.

2010 was the most severe winter in Europe for thirty years and this spring we received a host of phone calls from cellists who were worried that their cellos had lost their tonal beauty and ease of response. We also spent a lot of time re-gluing seams which had opened during the harsh winter. Most of us are aware of SAD (seasonal affective disorder) and the havoc it can play on our moods but cellos also suffer from seasonal changes. They don’t mind the dark winter days but they hate the dry air in our centrally-heated homes when the temperature outside drops near or below freezing.

Humidity and cellos

The level of moisture or humidity in the air affects the tone and condition of cellos because wood is porous and will – like a very stiff sponge – automatically adapt to the humidity in the air around it. When the level of humidity in the air rises, wood expands as it takes in some of the increased moisture; when the humidity drops, moisture is leached from wood by a process of diffusion, causing it to shrink.

What is Relative Humidity? Relative humidity (expressed as a percentage) indicates the amount of water vapour held in the air at a specific temperature, compared to the maximum amount of water vapour that the air could hold at that temperature before condensing into dew, fog, rain or snow. 40% relative humidity means that the air is holding 40% of what it is capable of holding before reaching the dew point, when the moisture in the air will start to condense.

Fortunately, cellos are designed to adapt to modest changes in humidity because the arched shape of the front and back will rise and fall as the wood expands and contracts. However, even modest changes in relative humidity can have a noticeable effect on the sound of the cello and extreme changes can cause damage. The best humidity for your cello depends on the prevailing humidity level when the cello was last glued together and/or set up. Cellos are generally most comfortable between 40% and 60% relative humidity. 30% is too low for comfort and 20% is hazardous.

In dry conditions (below 40% relative humidity) the arching of the front and back will shrink down onto both ends of the sound post, which can make the response of the cello dry, harsh and excessively resistant. If the wood contracts further, stresses from the changes of shape will tend to build up near the top and bottom of the instrument because the arching is fairly flat in these areas and therefore cannot change shape. If these stresses increase gradually, the animal glue attaching the front and back to the ribs will release at the points of tension. The tone of the cello will suffer due to the newly-opened seams, but no damage will have been done to the plates. However, if a sudden and extreme reduction in humidity occurs and the seams do not come unglued, cracks may form in the front or back. Another common symptom of low humidity is a reduction in string heights; differential rates of shrinkage in the neck root cause the neck angle to chumidity - crackshange, encouraging the bottom of the fingerboard to rise up closer to the strings. (see table below showing common symptoms of low and high humidity in cellos)

In conditions of high humidity (over 60% relative humidity) the cello may lose resistance and projection as the plates slacken and the sound post becomes too loose. The neck root will expand on the varnished side, thus making string clearances higher than usual.

Air temperature has a big impact on relative humidity. For example, if the relative humidity of the air outside in winter is 40% and the outside temperature is 8°C, as soon as the air moves into a heated house and is warmed to 20°C, the relative humidity of the air will drop to a dangerously low 18%. For this reason, the most hazardous time of year for cellos is during the winter months when central heating drives down the relative humidity of air which is already lessened by the cold conditions outside. Relative humidity in the USA plummets as low as 10-15% in heated homes and even in the UK, a centrally heated home on a freezing January day set to 20°C could drive the indoor environment down to a dangerously low relative humidity.

Relative humidity too low (perhaps less than 30%)Relative Humidity OKRelative Humidity too high (perhaps more than 60%)
Sound post is tight, so the point of bow contact moves further away from the bridge.The cello plays normallyPoint of bow contact comes closer to bridge to achieve full tone when playing
Increased resistance under the bowLess resistance under the bow
Treble response is harsh, bass response is constrainedBass response is strong and free, treble response is weak
Tone may be strident: powerful treble response and unresponsive bassTone may be over-resonant and lacking core
Seams may openSeams may open

The best way to monitor humidity levels is to use a battery powered digital hygrometer in the room or case where you keep your cello. Hygrometers are not expensive and they are small and portable. If the humidity in your room is too low you can use a room humidifier but a far cheaper, simpler solution is to keep your cello in its case whenever you are not playing it and to use a case humidifier to keep the humidity at a suitable level. It is also sensible to use a digital hygrometer in your case so that you can check the environment around your cello and be reminded when you need to moisten your case humidifier. Built-in circular (non-battery powered) case hygrometers are notoriously inaccurate but some modern digital case hygrometers are very reliable.

Hygrometer test. We tested three digital hygrometers for this article: Planet Waves, Oasis and Stretto. Planet Waves consistently registered a lower humidity reading than either Oasis or Stretto and was also less sensitive to humidity changes. Since Stretto agreed most consistently with our control hygrometer, we chose Stretto as our favoured hygrometer, but Oasis was also very reliable and a little less bulky than Stretto. Further information about these models is listed below:

Humidifier test. We then tested three case humidifiers (Planet Waves, Stretto and Oasis) using Stretto hygrometers in three identical fibreglass cello cases, placing a different humidifier in each case. We kept the cases in a humidity controlled room, left each case open for about one hour each day, and took daily readings from the three cases for seven days. We used large-sized Stretto and Planet Waves humidifiers in our cases; the Oasis humidifier is only available in a small size.

In this first test the Stretto humidifier was consistently the best performer. The Planet Waves came a credible second but the Oasis did not make much impact on case humidity at all.

In a second test we used Stretto as our control humidifier and compared its performance with a ‘Dampit’ style green snake humidifier and a homemade device made from a 35mm plastic film canister with holes drilled in the ends and a piece of dampened sponge inside. In this test the ‘Dampit’ style humidifier raised humidity even more than the Stretto during the first 24 hours but its performance soon dropped away. The home made canister device was inadequate for a cello case (its performance was similar to the Oasis).

This second test showed that the traditional ‘Dampit’ style humidifier has a powerful initial effect but it must be re-moistened daily to maintain its performance and it must also be used very carefully to avoid damage to the cello and varnish. The Stretto maintained its performance throughout the test period and we would suggest re-soaking its crystal bag every week. The Stretto is less likely to damage an instrument than a ‘Dampit’ style humidifier, as moisture is absorbed by a bag of hydroscopic crystals and cannot leak onto the instrument. (See further details below).

Humidifier manufacturers recommend re-charging humidifiers with water every 1-2 weeks but the safest approach is to check the case hygrometer every morning when you open your case, and re-charge the humidifier if necessary. Another factor to consider is the air-tightness of your case; if the case is not well sealed or if you tend to leave it open for extended periods, you will need to recharge your humidifier more regularly. It is most effective to use distilled or de-ionised water in humidifiers so that the absorbent sponge or crystal does not get clogged up by mineral deposits as the water evaporates. For more detail about distilled or de-ionised water, see below.

Case study. Colin Carr and his Gofriller cello travel between the UK and the USA many times a year in a busy solo touring schedule. Colin explains how he keeps his cello happy:

‘Over the years it has become clear that the cello does not do well in extreme dryness or humidity, but I have gradually learned to control the environment around the cello so that I never have to worry about changing bridges for summer and winter. I use a room hygrometer as I find they are more accurate, and try to maintain my cello at 40-50% relative humidity. Whenever possible I use a room humidifier, but if the dryness is extreme I will use a Dampit as well. I always put a Dampit in when flying. In hotels, running the shower for ten minutes is an effective humidifier (keep the plug in so the humidity lasts longer). When I can’t use a room humidifier I drape a large damp towel over the whole cello case with the cello inside. It’s possible to raise the humidity in an instrument locker from 25% to 60% using this damp towel method and as long as the towel isn’t dripping wet, there’s no danger of over doing it. In hot concert halls I don’t worry too much about the cello as I create a lot of humidity when I play!’

© Robin Aitchison and Sarah Mnatzaganian 2010.

Details of some of the hygrometers and humidifiers reviewed in this article and how to use them:

Stretto digital hygrometer
• Dimensions: 76mm x 58mm x 15mm
• Dual LCD display showing both temperature and relative humidity levels
• Temperature displayed in both Fahrenheit and Centigrade
• Records maximum and minimum temperature and humidity
• Internal clock
• Supplied with a detachable, velcro fastening to fix to case interior
• Battery included.
• On/off switch

Stretto Cello Humidifier
• Perforated plastic container 95mm x 65mm x 15mm
• Attaches with Velcro to the inside of the case.
• Supplied with two microfibre humidification bags filled with hydroscopic crystals
• Fresh bags can be ordered separately

To use the Stretto humidifier, immerse one bag in distilled water for 5 minutes, until it is swollen up in size. Dry carefully and place inside the perforated container. Stretto advise fitting the humidifier and hygrometer to opposite sides of the head of the cello case. Re-charge the bag with distilled water as soon as the humidity in your case starts to drop below a comfortable level. Bags can be re-charged with water many times, but need changing when they no longer absorb water.

We stock Stretto hygrometers and humidifiers. Other online suppliers are also available including Strings Music Horizons. 

humidity - stretto

Planet Waves Humidity & Temperature Sensor
• Black plastic oval 100mm x 70mm x 30mm
• Measures relative humidity levels 20% to 99%
• Temperature displayed in both Fahrenheit and Centigrade
• Programmable set point monitors the minimum humidity desired
• A warning icon indicates when it is necessary to use a humidifier.
• Indicates the highest and lowest temperature and humidity levels
• Battery provided

Planet Waves Large Instrument Humidifier
• Black plastic perforated oval 120mm x 70mm x 30mm
• Contains open-celled phenolic foam.

The humidifier is charged using 1-2 syringes of distilled water, inserted into a nozzle point. Despite the presence of hydroscopic foam inside the humidifier, we managed to shake water out of our Planet Waves humidifier after charging it with water, so we would advise giving the humidifier a good hard shake into an absorbant towel, to ensure that no water can escape.

Both the Planet Waves sensor and humidifier can be fitted with velcro into the head of the cello case. They can be ordered from:, and other online suppliers.

Oasis® Digital Hygrometer/Thermometer
• Dimensions: 101mm x 32mm x 10mm
• Measures relative humidity and temperature
• Records high/low temperatures
• Measures temperature in Centigrade or Farenheit
• On/off switch
• 5 year guarantee
• Measures 20% to 90% Relative Humidity

We were not able to find a UK supplier for the Oasis hygrometer , but it can be ordered from the USA from:

Dampit and Green Snake products can be bought from most music shops.• Green perforated rubbery tube filled with absorbent yellow sponge• 350mm long, 10mm diameter
• The top end is plugged with a wide-headed bung which sits in the f hole.

After soaking in distilled water, make sure that you wring the sponge-filled tube out well to remove excess moisture and shake it sharply to ensure that no further drops of water escape. Then dry the outside surfaces very carefully with a towel before inserting into the f hole. Use this style of humidifier with great care as over-moistened humidifiers of this design can cause damage to the varnish around the f hole where the head of the humidifier sits. If water drips from the end of the product onto the inside onto the bottom of the cello, it could cause damage to the varnish, distortion of the rib wood and ungluing of seams at the base of the cello.

Distilled or De-ionised Water

Humidifiers work much more effectively and last for much longer if you hydrate them with pure water which does not contain minerals. Distilled water used to be widely available in chemists, but it less popular now than de-ionised water which is much cheaper to produce than distilled water. Both distilled and de-ionised water are ideal for use in a humidifier. De-ionised water is sold in car shops, garages and supermarkets, but some de-ionised water products which are sold for topping up car batteries do contain additives, so do check that you are buying plain and simple demineralised water! Accreditations to look out for include ISO 9001:2000 Quality Assurance accreditation. Apparently B&Q, Halfords, Homebase, Asda and Tesco all sell simple deionised or distilled water. Some are own-brand, and some by Tetrosyl. The Distilled Water Company delivers free within the M25 area of London; their address is:

© Sarah Mnatzaganian 2010

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