Case study on a TPCD tweeter

 This article explores how one of the key factors influencing sound quality — frequency response — can be optimized by engineering the diaphragm construction. Specifically, by changing the thickness and stiffness profiles of Composite Sound’s Thin-Ply Carbon Diaphragms (TPCD) in a 1” tweeter.

Before we get into the details on how to optimize performance, we need to understand what factors are critical in achieving great sound quality in a tweeter. To do so I asked three experts in the field of loudspeaker engineering and design: Jack Oclee-Brown, VP of Technology at KEF; Laurence Dickie, legendary loudspeaker designer and the man behind Bowers & Wilkins’ Nautilus, now founder and head of engineering at Vivid Audio; and Erik Wiederholtz, co-founder and CTO of Perlisten Audio.

I asked Jack, Laurence, and Erik what the most critical factors for sound quality in a tweeter (dome) are.

Jack Oclee-Brown: In my view, the key to a great sounding tweeter is the combination of controlled directivity and a passband that’s as free from resonance as possible. This is quite a challenge because the acoustic and structural wavelengths involved are small, and this means that small geometric details have a big impact. Achieving controlled directivity typically means using waveguides and possibly phase plugs too. If one has the ambition to make a tweeter with extremely wide bandwidth, then getting good acoustical behavior in a waveguide becomes challenging and in practice there aren’t too many possible geometries that can support a homogenous wave over a wide bandwidth. Optimizing the mechanical and acoustical design in harmony is the key.

Rigidity is critical for avoiding structural resonance so that the dome moves uniformly at all frequencies, and this comes down to two factors. First, the geometry of the dome has a huge influence. Second, the material of the dome and specifically the modulus of elasticity compared to the material density. The biggest challenge is that, especially when working with waveguides, the geometry of the dome is also dictated by acoustical requirements, and this means it cannot be optimized completely freely. Another consideration is the mass of the dome, which directly relates to the efficiency that can be achieved. Material selection is very important, and besides the underlying material properties, other factors are very important such as any restrictions on the geometry due to manufacture or processing techniques.

Laurence Dickie: When I first started with Vivid, we created the 50mm and 26mm dome drivers simply making every effort to keep the first break-up as high and out-of-band as possible. This was achieved with the catenary dome and high modulus carbon fibre edge stiffening ring. However, I did consider that a compromise might exist between highest break-up and better damping and, for the D50, we chose to use a lossy adhesive for the carbon to aluminum bond rather than a stiff one. Our goal is to minimize any audible break-up as much as possible. The other factor, which is clearly of great importance, is to minimize the moving mass.

Erik Wiederholtz: In general, we want high sensitivity, low mass, low distortion, broad bandwidth, and high dynamic range tweeter designs. When we discuss sensitivity, what we really mean is efficiency, we need a low mass to aid in that efficiency.

Related to frequency response and distortion: achieving high stiffness while keeping low mass, and well damped, are usually opposites in design targets and can be a hard thing to achieve in real life. We want to balance all of those [stiffness, mass, damping] really well, and ideally to be able to tune these factors so that we can tweak the frequency response for the needs of each application. Having the modes controlled, consistent, and well damped is critical.

Then we want different types of behavior or frequency response depending on how we design the loudspeaker, for instance if we use a waveguide or a flat baffle. We use waveguides to help with sensitivity gain and directivity as the two largest benefits. The waveguide is a benefit in low-frequency gain performance but can change the response of the tweeter in the upper frequency bands as the waveguides add air mass to the diaphragm. Here again, we want to be able to tune the behavior of the diaphragm to tweak the frequency response.


Summarizing Jack’s, Laurence’s, and Erik’s inputs, the following factors are important:

• Break-up as high in frequency and out-of-passband as possible.
• Well damped, or how I like to describe it, well controlled break-up.
• Low mass.
• The ability to tailor the behavior and performance for specific loudspeaker design and requirements.

What Happens in a Tweeter Dome During Break-Up?
Break-up is where the diaphragm starts resonating and no longer moves like a piston at certain frequencies. Some areas of the diaphragm move in the direction of the voice coil but amplified, causing increased sound pressure level and peaks in the frequency response.

Similarly, some diaphragm areas may move in the opposite direction of the voice coil, thereby causing reduced sound pressure level and dips in the frequency response. Break-up is typically associated with distortion and has negative effects on sound quality. The consequences are often described as harshness, ringing, and a lack of clarity.

In a stiff tweeter dome, for instance made of aluminum, beryllium, or Composite Sound’s Thin-Ply Carbon, the first break-up mode appears in the outer area of the dome. This mode typically shows up as a peak in the frequency response, appearing around 20kHz to 25kHz for a 1” tweeter dome made from aluminum or titanium.

Figure 1 and Figure 2 show the simulated behavior during break-up of a 1” titanium tweeter dome in 2D and 3D, respectively.

Engineering to Control Resonances
When the thickness or stiffness is changed in any point in a diaphragm, the way resonances occur in the diaphragm will change and naturally the frequency response will also change. Composite Sound’s Thin-Ply Carbon Diaphragm (TPCD) technology enables freedom of engineering so that different thickness and stiffness profiles in three dimensions can be varied. Specific modes/resonances can furthermore be addressed by changing the diaphragm properties in specific points or areas of the diaphragm.

This enables the same tweeter driver to be tested to isolate the effect of different Composite Sound TPCD constructions on specific metrics, for instance frequency response on- and off-axis, sensitivity and distortion characteristics. If tested in a loudspeaker, listening tests comparing different constructions can be conducted.

Optimizing Frequency Response by Varying Diaphragm Construction
There are several factors that will influence the sound quality of a tweeter, for instance diaphragm material properties, geometry, size, surround, voice coil former, dispersion lenses, waveguides, cross-over, and more. This investigation focuses on the diaphragm itself.

The conventional way of improving a stiff tweeter dome has been to find the material with the highest Young’s modulus to density ratio with the aim of moving the break-up higher in frequency. Beryllium and diamond are two prime examples of this strategy. This will however not change the nature of or the effect of the break-up—it will simply be moved higher in frequency.

There is a different approach where the break-up is not only moved higher in frequency, but the way it occurs is also controlled. As can be seen in Figure 1 and Figure 2, the main break-up mode in a stiff tweeter dome originates from the outer area of the diaphragm. Therefore, it is interesting to investigate the effect on this mode and the resulting frequency response by changing the properties in this particular area of the diaphragm.

The theoretical background on how the modal behavior, and thereby resulting frequency response, can be changed and optimized in Composite Sound’s TPCD has been described in several articles in audioXpress and Voice Coil magazines.

This article investigates the effect on frequency response on-axis when changing diaphragm thickness and stiffness profile in Composite Sound’s TPCD tweeter dome to address the specific area where the break-up mode originates from.

The measurements were performed on Scan-Speak’s D3004/666000 driver (Photo 2) in Scan-Speak’s anechoic chamber using Audio Precision APx526 and APx1701 and GRAS 46BE ¼” constant current power (CCP) microphones. Acoustical output measured at 2,83Vrms and the impedance at 1 Vrms. Measurement distance 1 meter. Measurements performed by Simon Möller Nielsen, R&D engineer at Scan-Speak, during October 2025.

The Effect of Thickness
The frequency response graph in Figure 3 shows the effect of dome thickness on the frequency response. The graph shows the frequency response for 45, 67, and 90 micrometer Composite Sound TPCDs. The 45 micrometer diaphragm has fibers in two directions, the 67 micrometer diaphragm has fibers in three directions, and the 90 micrometer diaphragm has fibers in four directions. When moving from two to three and four fiber directions in a fiber reinforced composite, the equivalent Young’s modulus* is increased, regardless of thickness.

Furthermore, the level of anisotropy (how properties such as Young’s modulus varies in different directions relative to the fibers) is changed; two fiber directions result in big anisotropy, three fiber directions lower anisotropy and four fiber directions the lowest level of anisotropy. That means that the effect on the frequency response is a combination of thickness, level of anisotropy, and equivalent Young’s modulus.

We can see that increasing thickness with their resulting change in anisotropy and equivalent Young’s modulus moves the break-up higher in frequency and there is also a tendency of increased amplitude of the resonance peak. The break-up frequency is moved from 22kHz in the 45 micrometer dome to 32kHz in the 90 micrometer dome. This is however at the expense of sensitivity.

Moving Break-Up Frequency by Optimizing Thickness and Stiffness Profile
Can we move the break-up higher in frequency, control it more effectively, and do so without sacrificing sensitivity? Figure 4 shows two domes having the same 45 micrometer dome thickness. The difference is that the black curve is from a dome where the outer diaphragm area has double thickness. This modification moves the break-up from 22 kHz to 38 kHz, with only a small reduction in sensitivity.

Reducing Peak Amplitude through Focused Stiffness
Composite Sound’s TPCD technology allows for the precise targeting of stiffness enhancements at designated locations and orientations within the diaphragm. This targeted approach enables the mitigation of specific vibrational modes with minimal impact on the overall mass of the diaphragm. Figure 5 demonstrates that by strategically increasing stiffness at a particular point and along a defined direction, the amplitude of the resonance peak can be reduced by 4dB, as evidenced by the difference between the black and yellow curves.

Exploring the Potential of Engineered Composite Sound TPCD Diaphragms
Composite Sound wants to make it easy to explore the potential of our TPCD technology in your product, regardless if it is a tweeter, headphone, compression driver or something else. For that reason, we have a three-step process:

Pre-study analysis: We will simulate the performance of a Composite Sound TPCD diaphragm in comparison to your reference so you can see the potential performance. We can also assist you with information and data should you want to run simulations yourself.
Prototype testing: For you to test, measure and listen to your audio product we will provide you with engineered Composite Sound TPCD prototype diaphragms.
Production: If or when you decide to use Composite Sound TPCD in your audio product, we will produce the TPCD according to our agreed specification with short leadtime.

Contact me by email to start the exploration!
www.composite-sound.com

Resources
M. Turesson, “TPCD Technology in Headphones: Engineering to Control Diaphragm Resonances,” audioXpress, January 2024.

*Equivalent Young’s modulus is an approximation of the overall stiffness for an orthotropic material if treated as isotropic. As a simplification: it can be considered as the average Young’s modulus in an orthotropic material. An isotropic material has the same properties in all directions (examples include aluminum and titanium). An orthotropic material has different properties in different directions (for instance fiber reinforced materials).


About Composite Sound
Composite Sound’s mission is to transform the audio industry — redefining how transducers and loudspeakers are designed, and ultimately, how sound is experienced. We build long-term relationships with audio companies that are on a journey to achieve what they have not achieved before. We do that by engineering Thin-Ply Carbon to control resonances. That enables our customers to achieve better sound quality, play louder, and realize solutions not possible with conventional technology. Composite Sound develops and produces TPCD speaker and headphone diaphragms for applications where controlled resonances, high stiffness and minimum mass is critical. That includes for instance tweeters, midranges, woofers, full ranges, compression drivers, headphones and microspeakers. That same TPCD technology can also be used for other components with similar requirements such as voice coil formers, surrounds, turntable components, and more.

This article was originally published in audioXpress, January 2026