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Class is a tolerance envelope, not a rank. A sound level meter conforming to a class has been shown to behave within defined limits across a long list of tests covering frequency weighting, directional response, time weighting, level linearity and the effect of temperature and humidity. Class 1 and Class 2 apply the same tests with different permitted deviations, and the two envelopes are similar near one kilohertz and diverge sharply towards the ends of the frequency range. So the practical difference between the classes is not a single decibel figure. It is how much the instrument is allowed to wander where the sound you are measuring happens to live.
Class 1 and Class 2 are tolerance envelopes, not quality grades
It is tempting to read Class 1 as professional and Class 2 as basic, and that reading is misleading in both directions. A Class 2 instrument is held to a documented specification across a long battery of tests and is a genuine measuring instrument. A Class 1 instrument is not categorically better built, more durable or easier to use; it is held to narrower limits on the same tests, which usually requires a more expensive microphone and more careful acoustic design of the case.
The tests themselves are the substance. The standard specifies how closely the frequency weighting must follow the defined curve, how uniformly the instrument must respond to sound arriving from different directions, how accurately the fast and slow time weightings must behave, how linear the response must be across the level range, and how much the readings may shift with temperature, humidity and static pressure. Conformity means the instrument stayed inside the limits on all of them, not just the headline one.
A listing that states a class with a standard reference beside it is making a specific claim. The CRYSOUND CRY2831 states conformity with IEC 61672-1 Class 2, a self developed condenser microphone, a measurement range of twenty nine to one hundred and thirty eight decibels, a frequency range of twenty hertz to twelve and a half kilohertz, A, C and Z frequency weighting, fast and slow time weighting, IP54 ingress protection and a calibration certificate. That is the full shape of a class claim: the class, the document it refers to, and the operating envelope it applies within.
The tolerance widens with frequency, and that is the real difference
At the reference frequency of one kilohertz the two classes are close. Published acceptance limits there are commonly quoted as about one decibel for Class 1 and a little under one and a half for Class 2, which is a difference most users would never notice. If a sound source consisted only of a pure tone at one kilohertz, the class question would be nearly academic.
Move away from the reference and the envelopes separate. Towards the low end, below a couple of hundred hertz, the permitted deviation for Class 2 grows considerably faster than for Class 1. Towards the top of the range the same happens, and the standard allows the Class 2 limits to become very wide indeed at the upper extreme, to the point of being effectively open at the top of the range. The reason is practical: holding a small microphone to a tight response at high frequency, and holding a whole instrument to a tight response at low frequency, is where the hard engineering sits.
The consequence is that the class you need depends on the spectrum of what you are measuring rather than on how important the measurement feels. Noise dominated by low frequency content, which describes most outdoor environmental sound and a great deal of plant and machinery noise, is measured with much more confidence on a Class 1 instrument. Noise concentrated in the middle of the range, which describes speech, office environments and many hand tools, is far less sensitive to the difference. Class also interacts with the usable range: an instrument that cannot read low enough cannot characterise a quiet space whatever its class, which is why the twenty nine decibel floor quoted by the CRYSOUND listing is a separate and relevant specification.
Type and Class describe the same idea under two standards
Older instruments and older documents say Type 1 and Type 2. Those designations come from the earlier national standard for sound level meters, and the corresponding international documents of the same era used Type as well. The current international standard uses Class, applies a more thoroughly specified set of tests, and introduced an explicit framework for periodic verification. The two naming schemes describe the same two tiers of tolerance and are routinely treated as equivalent in ordinary conversation.
The equivalence is close enough for most purposes and loose enough to matter in a specification. An instrument designed against the older document was evaluated against a somewhat different list of tests, so a Type 2 label is evidence of the same intent and not proof of conformity with the current Class 2 requirements. Where the distinction has consequences, the right question is which document the instrument was evaluated against and whether that evaluation was carried out by the maker alone or by an independent body.
This is also the point at which product listings become unreliable, because Type and Class are short words that look like specifications and cost nothing to print. A label with no standard reference beside it is a statement of category, not a claim of conformity, and it should be read that way until a manual says otherwise.
What the four listings here state about class and what they leave out
| Listing | Class or type stated | Standard referenced | Accuracy and resolution stated |
|---|---|---|---|
| CRYSOUND CRY2831 | Class 2 | IEC 61672-1 | Not stated in the listing bullets |
| Extech 407730-NIST | Not stated in the listing bullets | Not stated in the listing bullets | Two decibel accuracy, tenth of a decibel resolution |
| Extech 407730 | Not stated in the listing bullets | Not stated in the listing bullets | Two decibel accuracy, tenth of a decibel resolution |
| Extech 407732 | Type 2 in the product title only | Not stated in the listing bullets | Not stated in the listing bullets |
The pattern in that table is worth dwelling on, because it is typical rather than unusual. One listing states a class and a standard but no numeric tolerance. Two state a numeric tolerance but no class. The fourth is the most instructive case of all: the Extech 407732 carries Type 2 in its title, while its bullets describe acceleration, velocity and displacement measurement, a remote vibration sensor on a one metre cable, a frequency range of ten hertz to one kilohertz and peak or root mean square modes. Those are the specifications of a vibration meter, so nothing in the bullets supports or contradicts the sound level claim in the title, and the class detail has to come from the manual.
Between the two Extech sound meters there is a different lesson. The 407730-NIST version carries a reference to a traceable certificate in its name while stating the same two decibel accuracy and tenth of a decibel resolution as the plain 407730. A certificate and a class are complementary rather than interchangeable: one ties this particular unit to a reference at a point in time, the other describes the design envelope. Neither substitutes for the other, and a specification that asks for only one of them has left something open.
Matching the class to the claim you need to make
Work backwards from the sentence you intend to write at the end. If the sentence is a comparison between two positions or two machines, measured minutes apart on one instrument, the class matters less than consistency, because the shared offset cancels in the difference. If the sentence will be quoted to somebody who may dispute it, the class and the certificate are the two things that make it hold, and the applicable rule usually names which class it expects.
Then check the three specifications that sit alongside class and are easy to overlook. The measurement range has to cover the quietest and loudest levels you will encounter, since a floor above the ambient level makes a quiet space unmeasurable. The frequency weightings available have to include the one the rule asks for, and the background to that choice is in what dBA means on a sound meter. The time weighting and the averaging functions have to match the quantity being asked for. A display showing more digits than the tolerance supports is a separate trap, examined in what resolution means on a digital meter.
Start from the measurement rather than from the catalogue. What a sound level meter is sets out the instrument itself, how to choose a sound level meter walks through the selection in order, and the sound level meter overview covers what the market actually offers at each tier.
Frequently Asked Questions
Is Type 2 the same thing as Class 2?
They are the same idea written under different standards, and in practice the labels are used interchangeably, but they are not identical documents. Type designations come from the older national standard for sound level meters, while Class designations come from the current international standard that superseded the earlier international documents. The envelopes are similar in shape and intent. Where a specification matters, the standard reference next to the label is the part to read, because the label alone does not say which document the instrument was evaluated against.
Does a Class 2 meter satisfy occupational noise rules?
In many jurisdictions it does for routine workplace surveys, while measurements intended for enforcement or for legal dispute often call for the stricter class. The requirement is set by the regulation that applies to the situation rather than by the instrument, so the order of work is to read what the applicable rule demands and then choose the class. Buying the stricter class when the rule accepts the looser one is never wrong technically, but it does not make a measurement valid if the method was not followed.
What does a calibration certificate add to a class claim?
A class claim describes a design and a set of acceptance limits. A certificate describes one specific instrument at one specific time, recording what it read against a reference and with what uncertainty. The two answer different questions, which is why serious specifications ask for both. An instrument that conforms to a class on paper but has not been checked for a long period has an unknown relationship to any reference, and the class label does not cover that gap.
Why does class matter more for outdoor and environmental work?
Because the sound you are measuring outdoors contains much more energy at low frequencies, and the low frequency end is exactly where the class envelopes diverge most. Traffic rumble, plant machinery, wind turbine noise and distant industry all put substantial content below a few hundred hertz. A looser envelope at those frequencies translates directly into a wider uncertainty on the overall level. Indoor measurements of speech and of machinery noise concentrated in the middle of the range are less sensitive to the difference.
Can a phone application be a Class 2 meter?
Not as a complete instrument, because class applies to the microphone, the case, the filtering and the detector as one evaluated system. A phone microphone is designed for voice, its response is shaped for that purpose, the case diffracts sound in an uncontrolled way and the operating system may apply processing the application cannot see or disable. An application can produce useful comparative numbers and can be surprisingly consistent with itself. It cannot carry a class designation, and no reading from one should be written into a record that needs to stand up.