Following is a compilation of short articles and lessons provided by DPA Microphones. Their online Microphone University and Microphone Technology Guide contain several practical educational pieces about microphones and their uses in various settings. Bruce Myers dons his professor’s cap to teach us some valuable lessons.
HOW TO READ MICROPHONE SPECIFICATIONS
When you read microphone specifications, it is extremely important that you understand how to interpret them. In most cases the specifications can be measured or calculated in many different ways. This article section is designed to help evaluate specifications in a meaningful way.
WHAT YOU CANNOT DETERMINE FROM SPECIFICATIONS
While microphone specifications provide an indication of a microphone’s electro-acoustic performance, they will not give you the total appreciation of how it will sound. Specifications can detail objective information but cannot convey the subjective sonic experience. For example, a frequency response curve can show you how faithfully the microphone will reproduce the incoming pure sinusoidal frequencies, but not how detailed, well dissolved or transparent the result will be.
THE DECIBEL (DB) SCALE
The basis for most microphone specifications is the decibel scale. The dB scale is logarithmic and is used because of its equivalence to the way the human ear perceives changes in sound pressure. Furthermore, the changes in dB are smoother and more understandable than the very large numbers that might occur in pressure scales (Pascal, Newton or Bar). The dB scale states a given pressure in proportion to a reference pressure mostly 20 micro Pa. The reference pressure 20 micro Pa is chosen equal to 0 dB. Please note that 0 dB does not mean that there isn’t any sound; it only states the lower limiting sound pressure level of the average human ear’s ability to detect sounds.
The frequency response curve illustrates the microphone’s ability to transform acoustic energy into electric signals, and whether it will do so faithfully or will introduce coloration. Take care not to mistake frequency response for frequency range. The microphone’s frequency range will only give you a rough indication of which frequency area the microphone will be able to reproduce sound within a given tolerance. The frequency range is sometimes also referred to as “bandwidth”.
MULTIPLE FREQUENCY RESPONSE CURVES
Manufacturers of professional equipment will always provide more than one frequency response curve, as it is essential to see how the microphone will respond to sound coming from different directions and in different acoustic sound fields.
The on-axis response demonstrates the microphone’s response to sound coming directly on-axis towards its diaphragm (0°). Be aware that the on-axis response may be measured from different distances, which may influence the response on directional microphones because of the proximity effect.
DIFFUSE FIELD RESPONSE
The diffuse field response curve will illustrate how the microphone will respond in a highly reverberant sound field. This will be an acoustic environment where the sound has no specific direction but where all directions are equally probable. The reflections from walls, floor, ceiling etc. are as loud or louder than the direct sound and the sound pressure level is the same everywhere. This is especially interesting when considering omnidirectional microphones, because they are able to register the full frequency range in the lower frequencies. The diffuse field response will show a roll-off in the higher frequencies, partly due to the air’s absorption of higher frequencies.
The off-axis responses will reveal the microphone’s response to sound coming from different angles. This is particularly interesting when you want to discover how a directional (i.e. cardioid) microphone will eliminate sound coming from other angles than directly towards the diaphragm. Even though the off-axis responses are attenuated on directional microphones, it is of extreme importance that these curves also show a straight frequency response, as it will otherwise introduce an off-axis coloration.
A polar diagram is used to show how certain frequencies are reproduced when they enter the microphone from different angles. The polar diagram can provide an indication of how smooth (or uneven) the off-axis coloration will be.
A reference point on the outer circle is defined, often by a 1kHz sinusoidal tone aiming the microphone directly towards its diaphragm (0°=on top of the circle). Each shift between emphasized circles normally indicates a -5 dB step, unless otherwise indicated. In this way you will be able to determine how much weaker the signal will be around the microphone for certain frequencies, commonly 5kHz, 10kHz, 15kHz and 20 kHz.
The response curves should be smooth and symmetric to show an uncolored sound. Extreme peaks and valleys are unwanted and the response curves should not cross each other. From the polar diagram you can also see how omnidirectional microphones usually become more directional at higher frequencies.
EQUIVALENT NOISE LEVEL
The equivalent noise level (also known as the microphone’s self-noise) indicates the sound pressure level that will create the same voltage, as the self-noise from the microphone will produce. A low noise level is especially desirable when working with low sound pressure levels so the sound will not “drown” in noise from the microphone itself. The self-noise also dictates the lower limitation in the microphone’s dynamic range.
There are two typical standards:
1. The dB(A) scale will weight the SPL according to the ear’s sensitivity, especially filtering out low frequency noise. Good results (very low noise) in this scale are usually below 15 dB(A).
2. The CCIR 468-1 scale uses a different weighting, so in this scale, good results are below 25 – 30 dB.
Sensitivity expresses the microphone’s ability to convert acoustic pressure to electric voltage. The sensitivity states what voltage a microphone will produce at a certain sound pressure level. A microphone with high sensitivity will give a high voltage output and will therefore not need as much amplification (gain) as a model with lower sensitivity. In applications with low sound pressure levels, a microphone with a high sensitivity is required in order to keep the amplification noise low.
According to the IEC 268-4 norm, the sensitivity is measured in mV per Pascal at 1 kHz (measuring microphones at 250 Hz). As an alternative, the sensitivity can be submitted according to the American tradition, which states the sensitivity in dB, relatively to 1 V/Pa, which will give a negative value. A serious microphone manufacturer will also state tolerances in sensitivity, according to production differences – such tolerances would normally be in the region of 2 dB.
SPL HANDLING CAPABILITY
In many recording situations it is essential to know the maximum Sound Pressure Level (SPL) the microphone can handle. Please note that in most music recording maximum peak SPL’s easily supersede the RMS value by more than 20 dB. The RMS value indicates an average SPL and will not show the true SPL peaks.
It is important to know
1. The SPL where a certain Total Harmonic Distortion (THD) occurs.
2. The SPL where the signal from the microphone will clip, that is the waveforms will become squares. This is the term: Max. SPL and it refers to peak values in SPL.
A commonly used level of THD is 0.5% (1% is also often seen), which is the point where the distortion can be measured, but not heard. Ensure that the THD specification is measured for the complete microphone (capsule + preamplifier), as many manufacturers only specify THD measured on the preamplifier, which distorts much less than the capsule. The distortion of a circular diaphragm will double with a 6 dB increase of the input level, so you can calculate other levels of THD by using this factor.
Microphone specifications do not tell the whole story about a microphone’s quality, and are no substitute for the sonic experience. Although microphone specifications may not be fully comparable between manufacturers, when properly evaluated they do provide useful objectivity and will help in the search for the optimal microphone.
LARGE VS. SMALL DIAPHRAGMS IN OMNIDIRECTIONAL MICROPHONES
Before choosing between a large and a small diaphragm microphone it is important to know the difference in features between them. Microphone behavior cannot be compared with that of a loudspeaker.
The difference between a small and a large diaphragm microphone can in short be outlined as:
A large diaphragm microphone has less self-noise than a small diaphragm microphone. This is due to the fact that the self-noise in a microphone is mainly due to Brownian Movements; i.e. air molecules bombard the diaphragm creating an equivalent noise pressure. The smaller diaphragm behaves as a hard surface and the air molecules hitting it exchange a greater amount of their energy, producing greater sound pressure levels relative to the area and the sensitivity of the diaphragm.
The sensitivity of the large and more compliant microphone diaphragm is generally higher than the small and stiff diaphragm. The large diaphragm is easier to move, even with low sound pressure levels, and will therefore provide a larger output.
A condenser microphone’s capability of handling large SPLs is limited by two things:
1. The microphone capsule, where the distance between the diaphragm and the back plate, together with the rigidity of the diaphragm, sets limits for how much a diaphragm can move before the distortion is too high.
2. The power supply for the microphone preamplifier sets limits for the amount of signal that can be handled before clipping occurs.
The smaller and stiffer diaphragm will therefore be able to handle relatively higher SPLs than the microphone with a large diaphragm.
As the omnidirectional microphone is sensing small differences in the air pressure (sound waves), both large and small diaphragms will, in principle, be equally capable of picking up low frequencies. The lower limiting frequency (LLF) of the pressure microphone is set by a small vent, to prevent the diaphragm from moving due to changes in the ambient barometric pressure. According to the dimensions of the vent (i.e. diameter and length) it will act as an acoustic low cut filter. The upper limiting frequency (ULF) is set by several factors, all related to the dimensions of the diaphragm.
1. A large diaphragm tends to break up and will no longer act as a true piston. This phenomenon is also known from loudspeaker technology and is the reason why loudspeakers are manufactured with different sizes of diaphragms to handle different frequency areas.
2. The weight of the diaphragm will attenuate the displacement of the diaphragm for higher frequencies.
3. The diffractions around the edges of the microphone capsule will limit the microphone’s capability of handling very high frequencies.
The conclusion is that a large diaphragm microphone will have a more limited frequency range than a small diaphragm.
When a microphone is placed in a sound field, its mere presence will influence the sound itself. This is due to the acoustic phenomena that can occur around the microphone due to the size of the microphone capsule, how it is positioned, the shape and the size of the microphone body containing the preamplifier, and the connector and design of the protecting grid.
All flat fronted omnidirectional microphones become increasingly directional for higher frequencies. High frequency sound waves coming directly from the front of the microphone will be reflected at the surface of the diaphragm, creating a sound pressure build-up between incoming and outgoing sound. This phenomenon occurs when the wavelength of the sound becomes comparable with, or is smaller than the diameter of the diaphragm.
A small diaphragm microphone can usually offer a higher dynamic range than a large diaphragm microphone. To explain this it is useful to understand how the dynamic range is calculated. The most sensible method of calculation is establishing the difference in dB between the noise floor and SPL where the microphone produces a certain amount of total harmonic distortion (THD). We have seen earlier how the noise floor of the microphone rises if the diaphragm is small, but the SPL handling increases even more compared with the large diaphragm. This is illustrated in the table below, which shows how the dynamic range is calculated for omnidirectional studio microphones.
Small diaphragm microphones can therefore have an equal or better dynamic range. The dynamic range is just shifted to cover different SPLs.
Both diaphragm sizes have their respective advantages and disadvantages. This is illustrated in the table below, which compares the specifications of small, medium and large diaphragm microphones.
HOW TO TEST THE PERFORMANCE OF A MICROPHONE
Normally the manufacturer encloses a product description with the microphone. It is a good idea to read the description carefully and prepare a focused test of the manufacturer’s listed features and of the product specifications. Make sure you are using the product for an appropriate application. If no graphs or curves are enclosed with the literature, do not hesitate to contact the manufacturer for this information.
CHOOSING A REFERENCE MICROPHONE FOR THE TEST
The reference microphone is often chosen for more personal reasons – “My favorite microphone”, than for scientific/application comparability. Make sure the manufacturer has informed you about the purpose, application and characteristics for the test microphone and then choose the most appropriate microphone according to the application. A more scientific approach is to also use a true reference microphone i.e. a measurement microphone. The probe-like design of these microphones enables them to be positioned extremely close to the test microphone without any influence on the sound field. Furthermore these microphones have totally linear frequency and phase responses, which will help you to “calibrate” your ears in between listening to other microphones.
POSITIONING THE TEST AND REFERENCE MICROPHONES
It is important to bear in mind that the acoustic memory of the human being is only a few seconds, which leads to the so-called simultaneous A-B test- or A-B-C test if more microphones are to be considered. The microphones need to be present simultaneously, picking up exactly the same sound source. You need to align the test and reference microphones bringing the diaphragms as close to the same point as possible. Note that the distance to the sound source needs to be at least four (4) times greater than the maximum distance between the centers of the microphone diaphragms. Some microphone housings and bodies are quite bulky. Use one microphone stand for each microphone in the set-up to find a position, which ensures a minimum influence on the acoustic field around the diaphragms from the more bulky microphone bodies. Do not hesitate to use a pop-filter if you intend to test the microphone with vocals, but use one pop-filter only.
TESTING MICROPHONES WITH VOCALS
The most common tests of studio microphones are done with vocals, but do not hesitate to use more complex sound sources like guitar, piano, and wind or percussion instruments to spice up your evaluation. Most microphones at least have a decent on-axis response and you will only be able to evaluate the true quality of a microphone if you also test its off-axis qualities. Musical instruments are extremely qualified sound sources for testing both the on- and off-axis qualities of microphones simultaneously, but you can also get a good idea of the microphone’s performance when using speech or singing using the following procedure. Make sure the headphone feed is from one microphone only, as it could otherwise influence the vocalist’s performance.
30 cm ON-AXIS (REFERENCE POSITION)
Start here. This is more or less the normal distance to a studio microphone when used for vocals. Adjust the sensitivity on test and reference microphones to exactly the same level using voice or tone generation as the sound source, double-checking the levels with the peak meters in the console. Make sure that all equalizers are bypassed or in neutral position. Select the microphone you want to listen to by using the MUTE button in the console – not by using the faders. If you send a feed to the vocalist’s headphones the MUTE function will also mute the AUX SENDS on most consoles, in these cases find alternative ways to switch the MUTE function on, again not using the faders (i.e. using the L+R routing function).
In the reference position you will probably have some kind of preference of what an uncolored voice should sound like. Here a directional microphone (i.e. a cardioid, hyper-cardioid or a figure-of-eight microphone) will normally not exhibit any or very little proximity effect. The weighting of the lower frequencies can therefore be expected to be neutral if you are testing a directional microphone. An omnidirectional microphone will not be influenced by the proximity effect, regardless of the distance, but you will use this distance as reference anyhow. The reference position will help you to discover any unwanted off-axis coloration when you move around the microphone later on. Return to the reference position as often as you like during your test to calibrate your ears.
30 cm 45° OFF-AXIS TO THE SIDE (OFF-AXIS COLORATION TEST)
It is extremely difficult to design studio microphones with no off-axis coloration characteristics, especially directional microphones. However, the off-axis qualities of a microphone are of the utmost importance if the aim is a clear and transparent recording. Off-axis sounds are allowed to be attenuated (if directional microphones). An increased attenuation of the higher frequencies can also be expected in cases of larger diaphragms, but an off-axis comb-effect is definitely unwanted.
30 cm 45° OFF-AXIS UP (OFF-AXIS COLORATION TEST #2)
If the test microphone has a bulky design and is not rotationally symmetrical, this test will reveal any unsymmetrical coloration that might occur. “Up” means talking/singing into the microphone in an angle from the top of the protection grid provoking a sonic reflection from the base of the cartridge where the capsule is connected to the preamplifier housing – the bottom of the cartridge chamber. A microphone, which exhibits unsymmetrical off-axis coloration, has an extremely limited applicability and is not suitable for the more demanding recordings like ambience or suchlike.
3 cm ON-AXIS (PROXIMITY EFFECT AND POP NOISE TEST)
If the test microphone is a directional microphone this close-up test will give you a picture of the microphone’s sensitivity to pop noises even when using a pop-screen. In this position you can also expect an extreme enhancement of the lower frequencies due to the proximity effect of a directional microphone. In cases of some male voices or rock ‘n roll bass drums this effect might be something you are looking for, but normally the proximity effect is an unwanted side effect – or at least something you try to use as discretely as possible. Omnidirectional microphones do not suffer from the proximity effect and you should not be able to hear any coloration of the lower frequencies when moving close to an omni. Furthermore, omnidirectional microphones are less sensitive to pop noises than their directional counterparts. Shouting into the microphone at close distance will reveal any possible limitation of the dynamic range of the microphone. Make sure that it is not your console or microphone amplifier that is the limiting factor in this test.
3-4 m ON-AXIS (AMBIENCE TEST)
If the recording room allows it, it is now time for the ambience test, where you move as far away from the microphone as possible – preferably at least 3-4 m. Directional microphones will again reveal the unwanted proximity effect and will now sound thin with a severe bass roll-off. Omnidirectional microphones will be able to do the job better and keep an uncolored response. The amount of sonic reflections from the walls in the recording room will now create a complex sound field at the diaphragm and the true directional quality of the microphone will reveal itself. Here it is important to cross-reference with the probe-like reference microphones.
30 cm 180° OFF-AXIS (FRONT-TO-BACK ATTENUATION AND COLORATION TEST)
The reason for designing a directional microphone is, of course, to attenuate sounds from unwanted directions. To get a good front-to-back-attenuation on a cardioid microphone is quite difficult and to obtain a perfect polar pattern on an omnidirectional microphone is also quite an achievement. Talking/singing directly into the microphone from behind will help you to discover any possible unwanted back loops of the directional polar pattern or, if an omni, any unwanted coloration of the sound besides for the expected attenuation of higher frequencies.
Double-check the sensitivity adjustment on test and reference microphones to make sure that levels are identical. Tap and/or rub the microphones (including the reference microphones) on the preamplifier housing and/or on the microphone stand to get an idea of the microphone’s sensitivity to handling noise. Generally you will find that a directional microphone is more sensitive to handling than an omni.
Original content courtesy of DPA Microphones can be found at http://www.dpamicrophones.com/mic-university/how-to-read-microphone-specifications