The impulse response waveforms of three tweeters—Mundorf AMT21CM2.1-C, Dayton Audio AMT2-4, and the VCD-type VCD-DT63—are overlaid and displayed, and their respective characteristics are described below.
Also, the impulse response provided with the Mundorf AMT21CM2.1-C is shown, and the two graphs are compared under consistent conditions. 

■Main Peak (Impulse 1st Peak)  
The sharpness of the initial rise is an important indicator that directly reflects the reproduction of initial transients.

The peak amplitude (energy concentration) is highest for the VCD-DT63, indicating that the energy is released in a more concentrated manner within a shorter time.
The sharpness of the rise is evaluated by the temporal slope (dA/dt) leading up to the peak, with the VCD-DT63 exhibiting the steepest rise.
In comparison, both the AMT21CM2.1-C and the AMT2-4 exhibit a slight temporal broadening. Furthermore, the AMT2-4 shows a larger post-peak rebound and stronger oscillatory components, resulting in a more pronounced tendency for the energy to be dispersed over time.

Characteristic of the AMT type]
This is considered to result from multiple vibration modes inherent to the folded structure and non-uniform in-plane velocity distribution, which prevent the energy from being concentrated at a single moment and instead cause it to be dispersed over time.

■Rise Immediately After the 1st Valley(approx. 80–150 µs)
Clear differences among the drive types are observed.
In the AMT21CM2.1-C, approximately two to three cycles of residual oscillation are observed after the valley, while in the AMT2-4 these oscillations are larger in amplitude and exhibit longer periods, with wider spacing between successive peaks. 

In contrast, the VCD-DT63 shows a smaller rebound after the valley, followed by very rapid convergence, indicating the most effective damping behavior.

[Characteristic of the AMT type]
In both the AMT21CM2.1-C and AMT2-4, a relatively large positive peak reappears after the 1st Valley. This is considered to result from multiple vibration modes inherent to the folded diaphragm structure and its interaction with air loading, causing the energy not to dissipate in a single event but to be redistributed and re-radiated over time.

■100–500 µs Region (Residual Oscillation)  
The 100–300 µs region (initial residual) is a critical range in which differences in tweeter performance appear most clearly, and high-performance tweeters exhibit significantly lower residual vibrations in this range.

In the AMT21CM2.1-C, clearly larger residuals are observed in the 100–300 µs range compared to the VCD-DT63, and additional peaks tend to remain around 500–600 µs. However, within the perceptually important initial range of 100–300 µs, it shows better characteristics than the AMT2-4.

The AMT2-4 exhibits the largest initial peaks and valleys, with the most pronounced oscillations in the 150–300 µs range, although its longer tail in the later region appears slightly shorter than that of the AMT21CM2.1-C.

In contrast, the VCD-DT63 shows that the main components have largely converged by approximately 230 µs, demonstrating the fastest decay characteristics.

[Characteristic of the AMT type]
Multiple periodic peaks and valleys are observed to persist. This is considered to result from multiple vibration modes inherent to the folded diaphragm structure, where different portions do not stop simultaneously, causing energy to be distributed and released over time. As a result, the impulse energy does not concentrate at a single point but appears spread over several cycles.

In addition, the small peaks and valleys observed around 200–300 µs may be partially attributed to sound reflected within the back chamber, which re-acts on the diaphragm and is re-radiated to the front. Similarly, the small fluctuations observed around 300–500 µs are likely due to a combination of back-chamber reflections and multiple vibration modes of the diaphragm, where the re-applied energy is radiated again to the front. 

■Beyond 500 µs (Late Residual Oscillations)   
In the region beyond 500 µs, the differences in late residual behavior among the various designs become clearly apparent.
In the AMT21CM2.1-C, a distinct re-rising peak is observed around 500–600 µs, indicating that residual energy persists into the later time domain. In contrast, although the AMT2-4 exhibits the largest initial residual, its behavior beyond 500 µs differs from that of the AMT21CM2.1-C, showing a relatively smoother and more gradual decay.
On the other hand, the VCD-DT63 demonstrates very rapid convergence after the first valley, with almost no significant residual components beyond 500 µs, indicating the most well-damped behavior among the compared units.

[Characteristic of the AMT type]
A tendency for energy to persist even beyond 500 µs is observed. In the AMT21CM2.1-C, a distinct peak appears around 500–600 µs, while in the AMT2-4, residual energy is observed around 400–500 µs. These behaviors are likely caused by multiple vibration modes originating from the folded structure, as well as interactions with the air load, resulting in delayed energy release. In conventional piston-type tweeters, the energy in this time region is generally much lower.
Furthermore, the distinct peak observed around 500–600 µs in the AMT21CM2.1-C represents a delayed component that is difficult to explain by a single vibration mode alone. It is likely that this includes components generated by sound reflected within the back chamber, which, after multiple reflections, re-excite the diaphragm and are subsequently radiated to the front.

■Impulse Decay Speed (Visual Estimation)

Clear differences in decay behavior are observed among the various designs.

The AMT21CM2.1-C exhibits residual components with re-rising peaks even beyond 500–600 µs, indicating that energy persists for a relatively long duration.

The AMT2-4 shows the largest initial residual, with energy persisting up to around 500 µs overall, followed by a relatively gradual decay.

In contrast, the VCD-DT63 shows that the main components have largely settled within approximately 200–250 µs, exhibiting the fastest decay characteristics. 

■Acoustic Implications

These differences are considered to directly affect the clarity of image outlines, the transparency of the soundstage, and the resolution of high-frequency components (such as cymbals and consonants).

The AMT21CM2.1-C offers high resolution but exhibits slight temporal spreading, which tends to produce a relatively softer impression.

The AMT2-4 has larger initial and mid-stage residuals, and the temporal dispersion of energy may be perceived as blurring of the sound image.

In contrast, the VCD-DT63 demonstrates a high degree of temporal energy concentration, resulting in sharp imaging and superior spatial reproduction. 

■Overall Evaluation (Impulse Response)
In this comparison, the VCD-DT63 exhibits a clearly faster settling in its impulse response than the Mundorf AMT21CM2.1-C, along with a higher degree of temporal energy concentration. Specifically, the main energy is released within a short time window of approximately 200–250 µs, and the subsequent residual components are minimal, indicating that energy dispersion in the time domain is significantly suppressed.
In other words, unlike conventional tweeters—which typically exhibit distributed energy release over multiple cycles (residual vibrations and re-radiation)—the VCD-DT63 demonstrates a characteristic in which energy is emitted in a highly concentrated manner within an extremely short time. Consequently, it is suggested that the VCD-DT63 may achieve a new level of performance in the time domain.
[Characteristic of the AMT type]
Both the AMT21CM2.1-C and AMT2-4 exhibit a common tendency in which, rather than converging rapidly to zero after the main peak, the response decays gradually through multiple peaks and valleys. This behavior is considered to result from multiple vibration modes inherent to the folded (pleated) structure, as well as non-uniform in-plane velocity distribution, causing the vibrational energy to be released not in a single event but dispersed over time.
As a result, the impulse energy is not concentrated into a single peak, but instead appears distributed over multiple cycles.

Similarly, the ETC (Energy Time Curve) responses of three tweeters—Mundorf AMT21CM2.1-C, Dayton Audio AMT2-4, and the VCD-type VCD-DT63—are overlaid, and the characteristics of each are described below.

■Initial Decay (0–200 µs)
In terms of coherence, the AMT21CM2.1-C outperforms the AMT2-4, while the AMT2-4 tends to exhibit longer persistence of initial energy.
VCD-DT63 decays steeply immediately after the peak and has already dropped to nearly −30 dB at around 200 µs. 

■Periodic Peaks and Valleys Appearing Around 100–400 µs

[Characteristic of the AMT type]
Multiple periodic peaks and valleys are continuously observed, indicating that the energy is dispersed over time.
These periodic features are considered to result from the presence of multiple vibration modes inherent to the folded structure, where each fold does not come to rest simultaneously, causing the energy to be released in a temporally distributed manner. As a result, the impulse energy is not concentrated into a single peak but instead appears as multiple temporal components.

■Mid-Term Decay (200–500 µs)
This region is particularly important among the Impulse Response, ETC, and STEP Response. The decay characteristics in this range are reflected in the ETC −40 dB decay time and have a significant impact on the transparency, spatial openness, and clarity of the soundstage.
In the AMT21CM2.1-C and AMT2-4, residual energy is clearly observed, with multiple peaks and valleys continuing throughout this region.
In contrast, the VCD-DT63 exhibits faster decay even in this range, reaching −40 dB at approximately 380 µs and continuing to decay relatively smoothly thereafter.
[Characteristic of the AMT type]
Multiple peaks and valleys are continuously observed. This is considered to result from multiple vibration modes inherent to the folded structure and internal reflections, which cause the energy to be intermittently re-radiated. 

 
■Intermittent Energy Re-Rise Around 300–800 µs

[Characteristic of the AMT type]
In both the AMT21CM2.1-C and AMT2-4, multiple re-rising peaks are continuously observed during the decay process, indicating that energy is being intermittently re-radiated. This is considered to result from in-plane vibrations and multiple vibration modes inherent to the folded structure, as well as pressure fluctuations and reflected components within the back chamber acting again on the diaphragm, causing the energy to be radiated in a temporally dispersed manner.  

■Late Region (0.7–2.2 ms)

[Characteristic of the AMT type]
In the AMT21CM2.1-C, relatively large peaks continue up to approximately 1.5–1.8 ms, while in the AMT2-4, periodic residual components tend to persist up to around 2.0 ms. These phenomena are considered to result from internal vibrations of the diaphragm and reflected sound within the back chamber, which act on the diaphragm with a temporal delay and are re-radiated to the front, thereby prolonging the decay of energy over time.  

■Overall Evaluation

Although the direct sound peak levels are largely similar among the three, clear differences are observed in the subsequent energy convergence behavior.

The AMT21CM2.1-C shows relatively good initial decay; however, residual peaks continue up to approximately 1.5–1.8 ms.
The AMT2-4 exhibits the slowest initial decay, with periodic residual components persisting up to around 2.0 ms.
In contrast, the VCD-DT63 demonstrates the highest degree of temporal energy concentration and exhibits the fastest decay and convergence.

[Characteristic of the AMT type]
During the decay process, periodic peaks and valleys are continuously observed. This is considered to result from multiple vibration modes inherent to the folded structure and internal reflections, which cause the energy to be dispersed over time. 

Furthermore, the STEP response waveforms of three tweeters—Mundorf AMT21CM2.1-C, Dayton Audio AMT2-4, and the VCD-type VCD-DT63—are overlaid and displayed, and their respective characteristics are described below.
Also, the STEP response provided with the Mundorf AMT21CM2.1-C is shown, and the two graphs are compared under consistent conditions. 

■Rise (0–50 µs)
In the initial rise, both the VCD-DT63 and the AMT21CM2.1-C exhibit relatively large peaks, indicating that energy is concentrated within a short time interval.
In contrast, the AMT2-4 shows a slightly lower peak, suggesting a relatively lower degree of energy concentration during the rise. 

■Depth of the 1st Valley (First Negative Peak)

The AMT21CM2.1-C and AMT2-4 both drop to nearly −100%, indicating that strongly oscillatory behavior is present in the transient response.

In contrast, the VCD-DT63 shows a relatively shallower valley, indicating that transient oscillations are more effectively suppressed.
[Characteristic of the AMT type]
The 1st Valley tends to be significantly deeper in AMT type drivers. This is considered to result from the folded diaphragm pushing air and subsequently generating a strong reverse velocity component due to air loading and the elasticity of the diaphragm. As a result, a pronounced oscillatory transient response appears in the system.  

■Initial Residual Oscillation (100–300 µs)

This region in the STEP Response is particularly important, similar to the −40 dB arrival time in the ETC. The rate of decay after overshoot directly affects the clarity of the sound field and the sense of silence.

The AMT2-4 exhibits relatively large periodic oscillations that persist, while the AMT21CM2.1-C, although more controlled in comparison, still shows clearly observable oscillatory components.

In contrast, the VCD-DT63 shows smaller amplitudes and shorter oscillation periods, indicating a faster decay tendency.
[Characteristic of the AMT type]
Multiple peaks and valleys are continuously observed. This behavior is considered to result from multiple vibration modes inherent to the folded structure, where each fold does not stop simultaneously but decays with temporal offsets. As a result, periodic oscillatory components appear in the STEP Response. 

■Convergence Process (300–800 µs)

The AMT21CM2.1-C and AMT2-4 continue to exhibit multiple oscillation cycles in this region. In particular, the AMT21CM2.1-C shows a relatively large re-rising component around approximately 600 µs, suggesting delayed re-radiation of energy.

The AMT2-4 also exhibits sustained periodic oscillations in the range of 400–700 µs.

In contrast, the VCD-DT63 shows a significant reduction in amplitude in this region and exhibits faster settling behavior. 

[Characteristic of the AMT type]
These re-rising components are considered to result from in-plane vibration of the diaphragm and pressure fluctuations and reflections within the back chamber, which re-act on the diaphragm and cause delayed radiation of energy over time. 

■Residuals Beyond 800 µs
Periodic oscillatory components continue to be observed in both the AMT21CM2.1-C and AMT2-4 even beyond approximately 1 ms.
In contrast, the VCD-DT63 settles to near zero in this region, with residual components being extremely small.
[Characteristic of the AMT type]
These residual components are considered to arise from the superposition of multiple vibration modes inherent to the folded structure and reflections within the internal cavity, causing energy to be dispersed over time. 

■Overall Evaluation

The AMT21CM2.1-C shows a relatively well-controlled response in the initial region; however, a re-rising component remains around approximately 600 µs.

The AMT2-4 exhibits the largest initial oscillations and tends to show slower overall settling behavior.

From the perspective of the STEP Response, the VCD-DT63 exhibits the highest degree of energy concentration at the rise, the smallest subsequent oscillatory components, and the fastest overall settling. 

[Characteristic of the AMT type]
In AMT type drivers, vibration energy does not dissipate at a single point in time but tends to be distributed across multiple temporal components. This behavior is clearly reflected in the STEP Response.