Planar magnetic drive system
Planar magnetic drive system
Embedded Files
■Planar Magnetic Drive System
At first glance, the planar magnetic drive system appears similar to the VCD (Voice Coil Diaphragm) method, as both employ a structure that drives the entire vibrating surface. However, their physical behavior is fundamentally different, resulting in significant differences in both measured characteristics and sound quality.
In this section, to clarify the origin of these differences, both systems are examined from the perspectives of structure and operating principles, and it is systematically explained how these differences manifest in time-domain responses such as Impulse, ETC, and STEP.
In planar magnetic drive systems, a wide range of structural variations exists depending on the configuration of magnets, conductor patterns, diaphragm shape, and conductor layout. The primary design parameters are as follows:
1. Magnet configuration: magnets placed only on the rear side of the diaphragm, or on both front and rear sides
2. Conductor configuration: conductors arranged on one side or on both sides of the supporting structure
3. Diaphragm shape: circular or rectangular
4. Conductor pattern: single-direction (radial or circular) or folded (serpentine) structures
In contrast to the VCD method, where the coil pattern is directly formed on the diaphragm to integrate the vibrating element and driving element, planar magnetic systems employ a structure in which thin aluminum ribbon conductors are arranged on the diaphragm (film) with a certain spacing.
As a result, the conductor coverage is limited, and the proportion of the diaphragm that contributes directly to driving force becomes relatively small compared to the total diaphragm mass. Consequently, the system tends toward a structure in which a comparatively heavy diaphragm is driven by relatively low driving force.
To address this, various combinations of magnet configuration, conductor arrangement, diaphragm shape, and conductor pattern are employed in the design. However, each of these approaches introduces new issues, and in practice, planar magnetic drive systems are realized as a set of trade-offs among these competing factors.
The main issues are as follows:
1) Insufficient driving force density (BL/M)
Because the conductors on the diaphragm must be thin and spaced at regular intervals, it is difficult to obtain sufficient driving force relative to the total mass and radiating area of the diaphragm. Even with various design measures, the tendency for a low BL/M ratio is hard to avoid, and this appears as slower initial transients and reduced energy concentration.
Furthermore, simply increasing the conductor thickness to improve BL/M tends to induce undesirable vibrations in the lateral direction. Suppressing these vibrations requires increasing the thickness of the supporting structure, which in turn leads to new issues such as increased stiffness and added diaphragm mass.
2) Lack of separation between diaphragm and suspension structure
In planar magnetic drive systems, the regions where no conductors are present also function as the movable support (i.e., suspension). As a result, the diaphragm and the supporting structure are not clearly separated.
･ The support regions between conductors require a certain level of stiffness to hold the conductors in place. Consequently, vibration propagation across the diaphragm cannot be sufficiently suppressed, leading to in-plane propagation and modal (break-up) vibration.
･ Meanwhile, rear radiation is emitted and reflected with time delay, causing phase and waveform distortion. Suppressing the leakage of this rear radiation to the front requires adding mass to the support structure; however, this increases the overall moving mass and consequently reduces BL/M.
3) Mismatch between driving force distribution and support distribution
When folded (serpentine) conductor patterns or double-sided conductor configurations are employed, the spatial distributions of conductors and support regions become non-uniform. As a result, both the driving force distribution and the mechanical support distribution become non-uniform, leading to localized variations in driving force and time delays. This contributes to increased temporal dispersion of the response and enhanced modal vibration.
As described above, in planar magnetic drive systems, the elements of driving force, support mechanism, and vibration control are not clearly separated in structure. Instead, they interact with one another and operate as a distributed system. This structural characteristic is the fundamental cause of the temporally dispersed response observed in Impulse, ETC, and STEP measurements.
For details on how these issues are addressed by the VCD method, please refer to “Introduction to the VCD Speaker.”
■Headphones for Characteristic Measurement
Based on measurement results obtained using REW, the characteristics of the planar magnetic drive system are presented. First, the unit used in this evaluation is introduced.
The unit used for this measurement is the planar magnetic headphone OPPO PM-3.
■OPPO PM-3
The OPPO PM-3 is a closed-back planar magnetic headphone developed by OPPO.
As a planar magnetic design, it is notable for being a lightweight and high-efficiency model specifically intended for portable use. It features relatively high sensitivity and low impedance, making it easy to drive even with devices such as smartphones.
Structurally, it employs a diaphragm with aluminum conductor patterns formed on its surface, which are driven across the entire radiating area by Lorentz force within a magnetic field. In general, planar magnetic systems are described as using a double-sided magnetic circuit, with magnets placed on both sides of the diaphragm to achieve uniform driving force.
However, actual disassembly observations reveal that the magnets are located only on the rear side (cylindrical magnets with outer diameters of φ37–inner φ26 and φ11–inner φ2, each with a thickness of 2 mm, arranged concentrically). Magnetic materials are placed on both sides of the diaphragm, including the magnet plate, to distribute the magnetic field.
In addition, the aluminum conductor patterns are arranged on both sides of the diaphragm in a single-direction layout. However, because the winding directions are opposite on each side, the regions without conductors (support regions) are not uniformly distributed, resulting in a structurally non-uniform configuration.
This structure achieves a certain level of efficiency and portability; however, non-uniformity in magnetic flux distribution and conductor arrangement tends to remain. As a result, the design prioritizes listening comfort over precise transient reproduction.
In the market, it has been recognized as a “portable planar magnetic headphone.”
■Impulse Response
Impulse represents the time-domain characteristic showing how a loudspeaker responds to an instantaneous input signal.
The ideal response exhibits a sharp single peak, followed by a rapid decay of subsequent vibrations.
If significant subsequent oscillations remain, this indicates the presence of unwanted reflections or resonances within the system. These appear as components delayed relative to the direct sound (“delayed sound”), and as their amount increases, they can lead to blurred image outlines and reduced clarity of the soundstage.
【Measurement Conditions 】
●Acoustic Measurement Software: REW (Room EQ Wizard)
●Analysis Items: Impulse Response
●Measurement Distance: 2 cm (near-field measurement) ※Room reflections and spatial effects are excluded to evaluate the intrinsic characteristics of the unit
●Bandwidth: 3 kHz – 96 kHz (Butterworth HPF, 2nd order ×2; no LPF applied)
●Sampling Frequency: 192 kHz
●Normalization: Peak Normalization
The characteristics of the impulse response waveform of the planar magnetic headphone OPPO PM-3 are described below. For comparison, the characteristics of the Mundorf AMT21CM2.1-C and the VCD-type VCD-DT63 are overlaid.
■Initial Region (0–50 µs) = High to Ultra-High Frequencies
The sharpness of the initial rise is a critical indicator that directly reflects the accuracy of initial transient reproduction.
In the PM-3, the initial rise slope is somewhat gradual, and the peak width is broader. This is attributed to insufficient initial acceleration due to low driving force density (BL/M), as well as energy dispersion caused by inadequate in-plane synchronization.
In terms of sound quality, this appears as softer attack edges, reduced extension in the ultra-high frequency range, and diminished reproduction of fine micro-transients.
■First Valley (Inversion) (50–100 µs) = Upper-Mid to High Frequencies
In the PM-3, the arrival at the first valley is delayed, resulting in an overall temporal shift of the first cycle. This behavior is attributed to time delay in vibration, i.e., distributed-system behavior, indicating that motion across the diaphragm is not synchronized and instead propagates as a wave.
In terms of sound quality, this appears as weaker post-attack coherence and a tendency for the sound image to become blurred.
■Second Peak (100–150 µs) (Most Critical)
In the PM-3, the most prominent feature appears in this region, where energy re-concentrates as a second peak. This indicates that the response is not a single event but is temporally divided.
In terms of sound quality, this manifests as a bloated sound image and degraded separation.
It should be noted that the 100–200 µs region is a critical time window that strongly affects perceived sound quality, and in high-end tweeters, the energy in this region is suppressed to extremely low levels.
■150–300 µs
Large oscillations persist in this region, and the amplitude is greater than in other drive types. This indicates that the locations where driving force is applied and where damping occurs do not coincide, resulting in residual localized vibrations.
In terms of sound quality, this appears as a lack of core definition, increased muddiness, and reduced resolution.
■300～600µs
The decay is slow, and vibrations persist over an extended period. This is attributed to the lack of separation between the support mechanism and the diaphragm, as well as insufficient damping.
In terms of sound quality, this appears as overly long decay, smearing of sound, and a lack of tightness.
■After 600 µs
Minute vibrations persist over an extended period, exhibiting the characteristic tailing of a distributed system (multi-mode residuals).
As a result, the sound image becomes diffused, leading to reduced clarity.
■Overall Evaluation (Impulse Characteristics)
The PM-3 exhibits a slightly slower initial response, a pronounced second peak, and slow decay. As a result, its response shows energy distributed over time.
The waveform is not a single peak but a multi-peak structure, which is the result of multiple time-delayed components and multi-mode vibrations being superimposed. In other words, rather than releasing input energy in a single event, it exhibits an impulse response in which the energy is released in a temporally divided manner.
The main factors contributing to these characteristics include insufficient driving force density (BL/M), the lack of separation between the diaphragm and the support mechanism, and the non-uniformity of both driving force distribution and support distribution.
In terms of sound quality, while it offers advantages such as smoothness, low harshness, and a sense of spatial spread, it also exhibits drawbacks including slower transient attack, blurred sound imaging, and reduced reproduction of fine micro-transients.
These tendencies were also confirmed through actual listening evaluations, where the perceived sound characteristics corresponded closely with the measured results.
■ETC（Energy Time Curve）
ETC is a metric derived from the impulse response that shows how the energy of the input signal decays over time.
The faster the unwanted energy converges, the less blurring occurs in the sound image, and the clearer the localization and spatial reproduction become.
Differences in sound quality and soundstage reproduction are determined by the amount of components that arrive later than the direct sound (“delayed sound”). In the ETC, the energy components that appear after the direct sound are observed as sounds arriving with delay (“delayed sound”).
【Measurement Conditions 】
●Acoustic Measurement Software: REW (Room EQ Wizard)
●Analysis Items: Impulse Response / ETC (Energy Time Curve)
●Measurement Distance: 2 cm (near-field measurement) ※Room reflections and spatial effects are excluded to evaluate the intrinsic characteristics of the unit
●Bandwidth: 3 kHz – 96 kHz (Butterworth HPF, 2nd order ×2; no LPF applied)
●Sampling Frequency: 192 kHz
●Normalization: Peak Normalization
Similarly, for the ETC (Energy Time Curve) of the OPPO PM-3, the characteristics of the Mundorf AMT21CM2.1-C and the VCD-type VCD-DT63 are overlaid, and their features are described below.
■Initial Decay (0–100 µs)
In the PM-3, the decay immediately after the initial rise is relatively gradual, and a tendency toward temporal dispersion is observed from the earliest stage. As a result, the concentration of initial energy is reduced.
■Approximately 100–300 µs (Most Critical Region)
In the PM-3, energy remains in the −15 to −25 dB range, clearly persisting at a higher level compared to other drive types. This corresponds to the magnitude of the second peak observed in the impulse response, indicating that energy is concentrated and retained in this time region.
■Approximately 300–800 µs (Midrange-Dominant Region)
The PM-3 exhibits a temporally dispersed response with multiple peaks rather than a monotonic decay, indicating that energy is released in several stages over time.
This behavior shows that the diaphragm does not move uniformly as a whole; instead, the driven vibration propagates across the diaphragm as a wave.
The primary causes include non-uniform driving force distribution due to uneven conductor placement, as well as residual and re-radiated local vibrations arising from a structure in which the support region also functions as part of the vibrating element.
As a result, this system operates not as a single vibrating body, but as a “collection of multiple vibrating elements with time delays.”
In terms of sound quality, this appears as diffusion of the sound image, reduced separation, decreased resolution, and midrange muddiness.
These behaviors are not simply due to insufficient decay, but are considered to originate from the characteristic time-segmented energy release of a distributed system.
■Overall Evaluation
The PM-3 exhibits a typical distributed-system response in which energy continues to be released with delay over the entire time range. In particular, within the 50–400 µs region, it is characterized by slower decay and a higher level of residual energy compared to other drive types.
This behavior is not merely due to insufficient decay, but is attributed to the time-segmented release of energy. It is reflected in the impulse response as a delayed arrival of the first valley and an increased second peak. In other words, the response does not converge into a single peak but is divided into multiple temporal components.
This is caused by in-plane propagation due to the diaphragm not accelerating and stopping uniformly, non-uniformity in both driving force distribution and support structure, and the redistribution of energy through the excitation of multiple vibration modes.
As a result, in terms of sound quality, this appears as weaker post-attack coherence, blurred sound image contours, reduced separation, and midrange muddiness (a phenomenon in which temporally dispersed components are perceived as a midrange sound image).
■STEP応答■STEP Response
STEP is a characteristic that shows how the output changes over time when the input signal rises instantaneously and is then maintained.
The ideal step response rises rapidly and settles to a stable state without oscillation.
However, if unwanted reflections or delays exist within the system, oscillations and fluctuations appear in the output. These are observed as the effects of components that occur later than the direct sound (“delayed sound”).
【Measurement Conditions 】
●Acoustic Measurement Software: REW (Room EQ Wizard)
●Analysis Items: Impulse Response / Step Response
●Measurement Distance: 2 cm (near-field measurement) ※Room reflections and spatial effects are excluded to evaluate the intrinsic characteristics of the unit
●Bandwidth: 3 kHz – 96 kHz (Butterworth HPF, 2nd order ×2; no LPF applied)
●Sampling Frequency: 192 kHz
●Normalization: Peak Normalization
Furthermore, for the STEP response waveform of the OPPO PM-3, the characteristics of the Mundorf AMT21CM2.1-C and the VCD-type VCD-DT63 are overlaid, and their features are described below.
■Rise (around t ≈ 0)
The PM-3 shows a slower rise compared to the other types, indicating insufficient initial acceleration in the transient response. This is attributed to insufficient drive force density (BL/M) and inadequate in-plane synchronization.
■Two-Stage Structure (Most Critical)
The PM-3 exhibits a deep undershoot followed by a re-rise rather than settling rapidly. This re-rise corresponds to delayed energy—associated with the second peak in the impulse response—that has been integrated over time, indicating that the energy is released in a temporally distributed manner. As a result, the response does not converge as a single event but instead forms a structure in which multiple temporally separated responses are superimposed.
While the VCD-DT63 converges with an almost single response, the AMT21CM2.1-C shows intermediate behavior. In contrast, the PM-3 exhibits a clear re-rise around 150–300 µs, followed by continued fine oscillations, forming a “two-stage plus additional oscillations (two-stage + α)” structure.
■Convergence Speed
The PM-3 retains oscillations beyond 500 µs, resulting in the longest convergence time. Furthermore, even after 1 ms, small residual vibrations persist, indicating a long energy tail.
■波形の本質
PM-3 のSTEP応答は、単純な非臨界制動や単一共振では説明できるものではなく、時間遅延を伴う複数の応答成分が合成された分布系特有の挙動です。すなわち、単一の振動体ではなく、「時間遅延を持つ複数の振動体の集合」として動作しています。
■Correlation with Sound Quality
The PM-3 offers a soft, smooth, and spacious sound as its strength. However, its weaknesses appear as reduced edge sharpness in the attack, less distinct image outlines, and decreased separation of fine details.
■Consistency with Impulse / ETC
This STEP response is consistent with the Impulse and ETC characteristics. Specifically, a delayed rise and an increased second peak are observed in the Impulse response, re-rise of energy appears in the ETC, and a two-stage structure with delayed convergence is observed in the STEP response.
These results indicate that, under the present measurement conditions, the PM-3 exhibits temporally dispersed response characteristics. Furthermore, this behavior is consistent with structural characteristics of planar magnetic drivers, namely non-uniform drive force distribution due to constraints in conductor layout, a structure in which the support mechanism and diaphragm are not clearly separated, and the resulting in-plane propagation and multi-mode vibrations. Such tendencies are considered relatively common in this type of driver.
■Overall Evaluation
The PM-3 exhibits a relatively slow rise, does not settle in a single event, shows a multi-stage response, and tends to retain residual vibrations over an extended period. In other words, rather than behaving as a single-response system to a step input, it can be interpreted as producing a superposition of multiple temporally separated responses.
As a result, it can be understood that time dispersion is relatively large across all stages, including the initial rise, main response, and settling behavior.
Google Sites
Report abuse