SCHOEPS microphones in extreme weather conditions

Condenser microphones are the first choice in terms of sound quality. All Schoeps microphones work according to this principle. These are highly sensitive sensors that detect the smallest charge shifts. During development and production our microphones are tested across a wide range of environmental conditions. They are specified to work in temperatures from -10° to +60° C (14° to 140° F) and at relative humidity up to 90%. These specifications are conservative, however; we have positive feedback from customers who have used their Schoeps microphones in the Arctic or the desert without any problems. Still there are limits, which this article will describe, along with giving suggestions for using our microphones under difficult conditions.

The purely electronic parts of condenser microphones can operate reliably across a very wide temperature range. We test our microphone amplifiers from -30° to +80° C (-22° to 176° F), for example. The capsule, however, is more vulnerable. In extreme temperatures the membrane will stiffen or relax somewhat, causing the sensitivity and frequency response of the microphone to change slightly. The membranes for all our capsules are artificially “pre-aged” under tension at high temperature prior to assembly, which helps them remain stable across a wide range of conditions. But prolonged operation at extreme temperatures can have more lasting effects which resemble accelerated aging.

Air always contains some amount of water vapor, and can absorb more of it at high temperatures than at low ones. The amount of water vapor in air, expressed as a percentage of the maximum amount that could be held by air of that temperature, is the relative humidity. If the temperature changes, the relative humidity will change as well. At or above 100% relative humidity, water droplets can form (condensation).

Our microphones function very well at almost any steady level of humidity. SCHOEPS microphones are specified to work correctly up to a relative humidity of 90% as measured at 25° C (77° F). All our microphones are required to pass testing under these conditions.

But condensation within a capsule—whether it is DC-polarized like ours or RF-based—can disrupt the capsule’s functioning, since it can bridge parts which must be isolated electrically from one another. Condensation is the change of the state of water from the gas phase into the liquid phase; see Wikipedia.org/Condensation. Condensation mainly results from changes in temperature and humidity–especially if they are abrupt–rather than from any “steady-state” conditions. Condensation creates water droplets. These water droplets will attach to material which is colder than the air. Glasses wearers know what is meant here: In winter, glasses that are still cold will fog up immediately when entering a warm room. Bringing a microphone from indoors to outdoors or vice versa, can cause an abrupt change in temperature (and thus humidity) both for that microphone and the air around it. The water droplets will attach to the cold metal parts in the microphone and will make it fail.

A further consideration: Microphones used in dusty environments, and/or that are used to pick up speaking or singing voices at close range, can accumulate deposits inside them. This contamination, combined with water droplets (e.g. created by condensation of moisture), can create a leakage path for the charge placed on the diaphragm. The result can be noise--rumbling, crackling or whistling sounds, which can be quite severe—or even some temporary loss of signal. There is a mutual cause-and-effect relationship here, since high humidity both promotes such deposits and causes deterioration of performance when they are present.

Moisture problems occur primarily when microphones are used outdoors under changing conditions. We have paid considerable attention to this in recent years, particularly with regard to our shotgun microphones since they are used in such an extremely wide range of settings. We have therefore further developed the capsule of the CMIT series so that it has a high humidity and condensation resistance, starting with generation D (2021, roughly since SN #9200, see CMIT 5 Product history and MiniCMIT Product history). Of course, this development has no influence on the frequency response and the sound of the capsule. We introduced these improvements without any public announcement, and watched as the number of reported humidity-related problems decreased to almost nil. Now (2023) we are finally ready to say something publicly about these improvements.

Condensation measurement: In addition to the temperature test described above, we now carry out a precisely defined condensation test on the analog models of the CMIT series (MiniCMIT, CMIT 5) during production, in which the microphone undergoes a rapid change from cold and dry to hot and humid. Clearly visible condensation droplets appear on the microphone housing, as shown in the photos below.

Most condenser microphones, regardless of manufacturer, have problems with this test, expressed as loud crackling noise or in extreme cases, a complete loss of signal. A CMIT capsule from generation D continues to run unaffected. All CMIT capsules that fail this test are discarded. Microphones that are carefully stored and not exposed to dirt and dust retain this property for a long time. Since such exposure cannot always be prevented in practical, outdoor use, there may be some deterioration over time. We generally recommend regular cleaning here; see section 4.

General:

• Keep your microphones in their cases when not in use; don’t leave them out on stands, especially in a dusty environment. The membrane and backplate of a DC-polarized condenser microphone capsule will continue to attract small particles for some time after it has been disconnected from powering. Case inserts made of foam material that can give off small particles should definitely not be used.
• Use a high-quality windscreen or pop screen 100% of the time when close-miking a person who is speaking or singing, or in any dusty recording environment. If a windscreen or pop screen shows signs of deterioration, replace it promptly with a new one.
• If your microphones or capsules are used in ways that could lead to contamination, return them to us periodically for checking and cleaning. We purposely apply the lowest allowable fee category for this service (see below). Customers should never attempt such cleaning themselves; it requires the capsule to be disassembled completely and then rebuilt. Particularly for directional microphones certain small parts often are replaceds, and adjustment and retesting are required.
• For outdoor use we recommend using the foam windscreen that we provide as an accessory, or (even better) a closed, basket-style windscreen. We advise against the use of inferior foam windscreens, because they give off flakes which can get into the microphone.

Temperature extremes and very high humidity:

• While operation at extreme temperatures should normally be reliable, please consider that this causes a capsule to “age faster”.
• To avoid condensation problems, avoid sudden changes from cold to warm environments.
• Storing/pre-warming or acclimating the microphone: the microphone should be stored warmer than the ambient temperature. Alternatively, the microphone should be removed from its case half an hour before use on site to acclimate it.
• If a microphone has been used in a damp environment, it may be useful when putting it back into its case to include a packet of desiccant with silica gel. This attracts moisture and dehumidifies the air in the case and the microphone.
• The microphone should always be kept clean and dry, because condensation on dust grains, microparticles (e.g. from bad foam), salt or dirt deposits cause moisture films and leakage currents to form much faster. A new microphone, or one that has been serviced recently and/or always carefully stored is less prone to condensation than one that has been used time and time again in extreme conditions such as humidity, dust, salt and dirt.

To make microphones more resistant to high humidity, we recommend:

• for CMIT 5 / MiniCMIT microphones produced before 2021 (SN < ~9200): We offer an upgrade to the latest capsule generation D at a fixed price; see here and here.
• for other capsules and microphones (MK, CCM, CMH, SuperCMIT): Cleaning of the capsule membrane, along with any necessary replacement of small parts, will always fall within the "small service fee" category as long as no more significant repairs are needed; see here. However, there is currently no way to bring these microphones to the high condensation resistance of a new-generation CMIT capsule.