We all know that close miking boosts the low end. But why? New research from DPA Microphones explains that some instruments are more affected by proximity effect than others.
'Proximity effect' is the tendency for low end to become exaggerated as a directional microphone is moved closer to the source. Sometimes we can use it to our advantage in recording; other times it’s a nuisance that has to be worked around. Yet, although anyone who’s ever used a microphone will undoubtedly have come across it, proximity effect remains poorly understood. How many of us can confidently say that we understand why proximity effect happens, or how that theory translates to practice?
Eddy Brixen and DPA Microphones have carried out some fascinating research into proximity effect, which we’ll be sharing later on in this article. As we’ll see, it turns out that real‑world sources and microphones don’t always behave quite as you’d expect, and that proximity effect is highly dependent on the type of source you’re miking, as well as on the mic choice and placement.
To understand this research properly, though, we need a proper explanation of the phenomenon in question. How, and why, does proximity effect occur?
What Is Proximity Effect?
There are two fundamental types of microphone. One measures air pressure, like a very fast‑acting barometer; in effect, its output reflects the excursion of the diaphragm from its rest position. The other measures the rate of change of air pressure, and its output represents the velocity with which its diaphragm is moving.
A pressure mic has a diaphragm that is open to the air only on one side, and is intrinsically omnidirectional. By contrast, the diaphragm of a velocity or ‘pressure gradient’ mic is open on both sides, and the mic has a figure‑8 polar pattern. A pressure mic does not exhibit proximity effect, but a pressure gradient mic does.
The diaphragm of a pressure‑gradient mic is set in motion by pressure differences between the front and back of the diaphragm. These are created because the front and back encounter an acoustic wave at slightly different points in its cycle. The path length from the front to the rear is typically around 1cm. Peaks and troughs arriving at the front of the mic thus reach the rear of the diaphragm fractionally later, so at any given moment, a pressure difference is created across the diaphragm.
However, there’s another factor that can cause a pressure difference across the diaphragm: the inverse square law. This states that, for a source that radiates sound equally in all directions, sound level falls according to the square of the distance from the source. Put another way, the sound pressure level falls 6dB for each doubling of the distance from the source.
If we place a pressure‑gradient mic just 1cm from a such a source, the rear side of the diaphragm is twice as far from it as the front side. This means the rear of the diaphragm isn’t only encountering the sound wave slightly later than the front: it’s also encountering a version of it that’s half as loud! By contrast, if our mic is 10 metres from the source, the difference in acoustic power between front and back due to the additional 1cm path length is negligible.
But why does this affect the low‑frequency response of a microphone? Our hypothetical 1cm difference in path length from front to back is fixed, but the wavelength of sound varies with frequency. A 5kHz sine wave has a wavelength of 7cm or so, whereas the wavelength of a 50Hz sine wave is almost 7 metres. If both waves have the same peak‑to‑peak amplitude, the 5kHz sine wave will generate a far greater pressure gradient across a 1cm gap than the 50Hz sine wave. Any pressure gradient created by the inverse square law will be negligible compared to that created by the 5kHz wave, but not when compared to the very small pressure gradient generated by the 50Hz wave. In other words, the relative contribution of the inverse square law doesn’t only depend on distance, but also on frequency.
This discrepancy between the pressure gradient created by low and high frequencies gives the simplest form of pressure‑gradient microphone a naturally rising frequency response: its sensitivity increases at 6dB per octave. To flatten the response, this must be compensated for, either using an electrical filter or by damping the membrane. And this compensation further increases the contribution of the inverse square law at low frequencies.
The Magic Metre
In practice, with a pure pressure‑gradient microphone, proximity effect begins to become noticeable at about 1m from a point source, and steadily increases as the mic is moved closer.
Secondary polar patterns such as cardioid, hypercardioid, wide cardioid and so on are obtained by combining pressure and pressure‑gradient operation in different proportions. The pressure component remains immune to proximity effect, so the overall degree of ‘bass tip‑up’ depends on the relative contributions of each. In a cardioid microphone, for example, pressure and pressure‑gradient components are combined equally, so the overall proximity effect is half that of a figure‑8 microphone.
Large‑diaphragm capacitor mics often have two diaphragms either side of a central electrode, each with a cardioid pattern. This allows the overall polar pattern to be varied; for example, by combining the contributions of these two diaphragms in the same polarity, we get something resembling omnidirectional pickup. By reversing the polarity of one, we get a nominally figure‑8 pattern. The contribution of the inverse square law is cancelled or enhanced respectively, so proximity effect behaves as you’d expect.
Managing Proximity Effect
Many directional microphones are designed to be used close up to the sound source either some or all of the time. In this context, proximity effect can result in a very boomy and muddy sound. Again, therefore, designers allow this to be compensated for. In a typical studio mic, we might have an optional high‑pass filter that can be switched in for close placement or left out for distant use. Stage mics, however, are often intended only for close‑up use, and will have their low‑frequency response curtailed at source, either through the acoustical design of the mic or occasionally by the use of a transformer with a restricted low‑frequency response.
Something that’s not always appreciated is that the proximity effect can actually work to our advantage in a noisy environment such as a busy stage, because it helps to discriminate wanted from unwanted audio. A directional mic equalised to give a flat frequency response at, say, 5cm, will sharply attenuate low‑frequency spill from more distant sources. Furthermore, the proximity effect is only manifested in the pressure‑gradient component of the mic’s pickup and hence within a figure‑8 pattern, even if the mic itself is cardioid. So, in effect, when a singer addresses the mic from the front, his or her voice is boosted in the low frequencies, but sources to the sides of the mic are not.
This property can also be exploited by the performer talking ‘across’ rather than directly into the mic to reduce proximity effect where it’s too prominent. Likewise, when miking sources such as guitar cabs, the mic can be angled to reduce proximity effect.
Points, Planes And Lines
The explanation of proximity effect outlined above makes some theoretical assumptions. Most importantly, it assumes we are miking up a source that radiates sound equally in all directions: a so‑called point source. Staying in the realm of theory, however, we can also hypothesise two other types of sound source. A line source radiates sound 360 degrees in the horizontal plane, but has no dispersion in the vertical plane. A plane source, meanwhile, shows no dispersion at all. It simply projects a coherent wavefront directly forwards.
The inverse square law doesn’t apply to line or plane sources. Because a plane source exhibits no dispersion, there is no loss of power with distance: in theory, its output forms a perfect beam that is equally loud at all distances from the source. A line source is a ‘halfway house’ between plane and point, with a theoretical attenuation of 3dB for each doubling of distance, rather than the 6dB seen with a point source. Hence, in theory, a directional microphone will encounter no proximity effect at all with a plane source, and with a line source, the proximity effect will be halved compared with a point source.
These ideals can be useful approximations for the behaviour of real‑world sound sources. For example, many PA systems are designed to share the property of a line source whereby sound disperses only in the horizontal plane and not in the vertical plane. This minimises unwanted reflections from sound radiating upwards and downwards, and helps to ensure that the sound level does not drop off too much between the front and back of the hall.
However, ideals are just that, and no real sound source behaves exactly like a perfect point, plane or line. Musical instruments and loudspeakers are complex radiators that exhibit elements of all three characteristics, often varying with distance and frequency. Adding to the fun is the fact that we never encounter them in free space: our microphones always capture a mixture of directly radiated and reflected sound.
Proximity In Practice
Eddy Brixen and DPA set out to measure the actual contribution of proximity effect when real‑world sound sources are recorded with real microphones. Five common sound sources were captured using six different DPA microphones, ranging from the omnidirectional 4007 to the supercardioid 4099. Of the mics on test, five are designed to produce a flat frequency response at 30‑60 cm from the source, whereas the 4018V is a specialised vocal mic intended for close‑up use. Measurements were made, where possible, at seven different distances, starting at 1cm and doubling every time until 64cm was reached; however, the physical design of some of the mics precluded use at 1 or 2 cm.
The sound sources tested were a coaxial two‑way loudspeaker, a bass drum, a trumpet, an acoustic guitar and a grand piano. The results were intriguing and in some cases counter‑intuitive. Least affected by proximity effect was the trumpet, which produces relatively little information below 200Hz in any case. However, the loudspeaker, the grand piano and the acoustic guitar all displayed a similar tendency. As you’d expect, low‑frequency pickup is exaggerated as the distance from mic to source is reduced — but only up to a point. With the loudspeaker and grand piano, for example, there’s little change in the low end once the mics are 16cm or closer.
What this research shows is that within the normal range of close‑miking, there isn’t always a straightforward relationship between mic distance and ‘bass tip‑up’.
A Sense Of Perspective
We can understand what’s happening here by means of a visual analogy. The further away we get from something, the more it behaves like a point source. The planet Jupiter is massive, but because it’s so far away, it appears as a mere dot in the night sky. Conversely, even small items can start to fill up our field of vision when we get close to them. In exactly the same way, sound sources that only subtend a tiny angle at distance become less and less point‑like from a closer perspective. As we get closer to the source, it becomes more plane‑like, and eventually this tendency takes over, meaning that the proximity effect plateaus.
This, incidentally, is why large bass drivers aren’t necessarily a good thing in nearfield monitors. The tweeter behaves as a point source at all listening distances, but the output of the woofer becomes increasingly plane‑like as the listener moves towards it. The net result is that the level balance between woofer and tweeter changes, and with it the overall frequency response of the monitor. (Eddy’s measurements show that the bass level actually falls at 4cm, perhaps suggesting that the shorter mic‑speaker distance is outweighed by a reduced contribution from the speaker’s bass ports.) It also means that proximity effect is relatively negligible when miking large speakers such as are found in typical guitar amps. The complex acoustical phenomena that are generated around the dome in the centre of the cone have a much greater influence on the tone than any variation in bass level with distance.
Compared with the other sources, the bass drum behaves much more like a true point source, with proximity effect continuing to increase as the mic is moved closer.
What this research shows is that within the normal range of close‑miking, there isn’t always a straightforward relationship between mic distance and ‘bass tip‑up’. What’s more, this relationship can change depending on the angle of the mic relative to the source.
The acoustic guitar is a good example. Miked behind the bridge, as in this research, the soundboard acts like a plane source; but the soundhole may behave more like a point source, which is one reason why miking directly in front of it can yield a very boomy sound. It’s just one more example of that universal principle of sound recording: if it sounds right, it is right. And, as Joe Meek didn’t go on to say, if it sounds wrong, the proximity effect is only one possible cause!
More info: www.dpamicrophones.com