Fabric vs Metal Dome Tweeters: Which is Better?

There has always been an ongoing debate about the sonic characteristics of tweeter dome materials. Do metal dome tweeters really sound "harsh," as some claim? Do silk domes sound, well, "soft" or  "silky"?

Different materials do have different physical properties as it relates to diaphragm performance and these can and do influence the audible behavior of the driver. With full knowledge that many well-respected designers and engineers have opposing views and opinions on the subject, there are a few basic generalizations that can be made about different tweeter dome materials.  We discuss these points in our recent YouTube Video below.

Soft vs Hard Dome Tweeters Comparison: Which one is better?

Metal dome tweeters are generally constructed of alloys of either beryllium, aluminum or titanium. (All of which are chosen for their high stiffness to mass ratio.) They tend to have larger amplitude high frequency break-up modes than soft domes. Softer domes tend to have more break up modes which occur at lower frequencies than metal domes. When a metal dome breaks up, it does so much more dramatically than do soft dome tweeters which have the advantage of having more compliance and damping distributed throughout the dome. The more severe break up modes involve a much higher percentage of the tweeters surface area than a soft dome which tends to do a better job of localizing the disturbance, and isolating the other areas of the surface from the break up. The more dramatic metal dome break ups often will show up as a large dip and/or a narrow spike in the upper end frequency-response. There may be one or several peaks and dips which may be minimized by octave or fractional octave SPL averaging common to PC based measurement programs. Silk-dome tweeters tend to break up more often, but more gently. Some audiophiles feel that makes them more pleasing to the ear even when the break up mode is lower in the audio band than modes commonly seen in metal dome diaphragms.

The metal dome breakup manifests itself in the frequency response curve as a sharp peak, or a sharp dip in its usable frequency range.  You can see a prime example of this in a recently reviewed Infinity speaker by Sound and Vision Magazine. Notice the frequency peaking of +10dB out to 20kHz.  Some folks may perceive this as detailed or airy while others with more sensitive hearing may find the speaker to sound harsh, especially at loud listening levels where the tweeter can get overdriven.

Infinity R162 Speaker Measurement Courtesy of Sound and Vision Magazine

In contrast look at how smooth the frequency response is of the speakers below.  One of them uses a properly engineered metal dome tweeter (the RBH MC-6C) while the other uses a soft dome tweeter (the Status Acoustics Decimo).  Both speakers are excellent examples of products that utilize high quality tweeters with well controlled dampening properties.

RBH Sound Frequency Response Measurement

Red trace: Decimo (soft dome) ; Green: MC-6C (metal dome)

The "harsh" character often attributed to metal domes is not necessarily always because of elevated frequency response levels in the highest audible octave (10-20 kHz). Since metal by its nature is not well-damped, (meaning the energy is not absorbed in the material) if a metal tweeter sounds harsh, it’s likely due to ringing or resonances in the dome that manifest themselves as audible artifacts, often times at frequencies lower than the curves would suggest.

Silk, treated cloth, polyamid, Kortec, Sonatex, are inherently better damped than single layered metal diaphragms. (The best metal dome designs get damping from their surrounds and absorption from acoustical treatments placed under the domes). Cloth and soft materials do not ring the way metal does, and the frequency response of a cloth tweeter is generally smoother. Soft domes with a lower and less resonant response roll-off at lower frequencies at the top of their usuable bandwidth.

So What's the Bottom Line?

Regardless of the dome material chosen, a lot comes down to the implementation of the technology and, overall, how the speaker has been designed. Another thing to consider is that ringing at lower frequencies in a tweeters bandwidth may result from the tweeter's motor being under damped. If a higher order high pass filter is being used on the tweeter it may also induce ringing unless it is critically damped. (Which is not what is typically used in filters. What is typically used are Butterworth filters which all ring at any order above the first). Ringing of this nature, indeed any low frequency ringing has nothing to do with the dome material but rather its mass, mechanical resistance, and its motor force.

There are excellent-sounding speakers using all kinds of tweeters. There are also hybrid dome tweeters that use a metal dome overlay over a soft material underneath in an attempt to get the best of both worlds.

In the end, the overall sonic character of the speaker has as much—if not more—to do with the designer’s voicing decisions and other design choices as it does with any single arbitrary tweeter dome material. What one hears in the highest frequency ranges covered by the tweeter is a complex combination of materials, geometry of the parts and the tweeter frontplate, the baffle, the grille, and the crossover network, even if we eliminate the clear contribution of the amplifier and source material. This can vary significantly from one angle to another. Often moving 30 degrees off axis will radically affect the response you hear in the top octave. There is NO guarantee that a resonance will produce a peak more than a dip. This is a 50/50 split. There is NO guarantee of the Q or amplitude of that resonance. These things will often confound confuse and surprise even designers that have done it many times before. The amount of small issues that can go wrong and create real audible measurable artifacts is large enough to warrant a chapter on its own.