Audio sounds best when each speaker is handling the frequency it was specifically designed for. For higher frequencies, this means the tweeter.
Human hearing is extremely sensitive to frequencies that are in the range of a tweeter, which typically kicks in around 2kHz. A quality tweeter well integrated into a system will make voices more intelligible and bring out the attack on percussion and rhythm instruments. However, if a tweeter’s frequency response is accentuating some sounds in this range it can quickly create listener fatigue.
Guiding Questions for Tweeter Design
When selecting or designing a tweeter for your system or application these are the three questions you need to ask yourself:
To what low frequency will the tweeter need to play?
How loud do I need it? You can also think of this as what sound pressure level (SPL) do I need it to achieve.
What dispersion pattern do I want for the sound?
Dispersion is more important with tweeters than other types of speakers because higher frequencies have shorter wavelengths and can be much more directional when compared to the longer soundwaves coming out of a subwoofer.
With a tweeter, you can decide how important it is for the sound to be good right on the axis in front of the speaker (aimed directly at a single listener) or whether it will be listened to primarily off-axis. In general, the smaller the surface area of a tweeter’s cone, the better its off-axis performance will be.
Tweeter Design Decisions
The vast majority of tweeters incorporated into consumer audio systems have a domed diaphragm. The dome shape helps with sound dispersion. Dome tweeters may come with or without phase plugs, as well as waveguides. Phase plugs help to equalize upper-frequency response (typically above 6 kHz) while having a side-benefit of protecting the tweeter’s diaphragm, while waveguides provide a “horn loading” effect to increase lower frequency output (typically below 6 kHz). Both phase plugs and waveguides have an effect on a tweeter’s dispersion.
Another tweeter style is a ribbon (or planar) design. These use a very thin diaphragm, usually made from metalized film or aluminum, suspended in a magnetic field. Because the mass is evenly spread across the whole surface area, they have the potential for ultra-fast transient response. They can also be oriented so that their dispersion pattern is either primarily horizontal or vertical.
Some tweeters incorporate a rear chamber behind the diaphragm to lower the resonant frequency. This is an effective design choice if you want to achieve a lower frequency crossover to the other speakers in your system.
Tweeters move fast, vibrating up to, and above, 20,000 times in a second, so their cones need to be light. The most popular cone materials for tweeters are textiles like silk or lightweight metals like titanium, aluminum and in high-fidelity applications, beryllium.
If your speaker will need to be outside, possibly get wet and withstand temperature extremes, or just be more robust in general, a PEI (Polyetherimide) dome is a good solution.
A tweeter with a mylar or PEI cone can typically be made smaller in size. Mylar tweeters will also have a higher resonant frequency (possibly above 3kHz), so calculating for crossovers and choosing the right woofer to work with them is important with these speakers. Many times these tweeters require the use of a dedicated midrange speaker to transition between the woofer and tweeter.
Crossovers and System Incorporation
When you pair a tweeter with a woofer or subwoofer in a system, the speakers will need to reproduce frequencies in their ideal bands. This is where crossovers are used to roll-off frequencies not suited for particular speakers. The rule of thumb with a tweeter is you want to crossover no lower than three times its resonant frequency when using a -12 dB/octave 2nd-order high-pass crossover. It may be possible to use a crossover at a lower frequency with a higher rate (such as -24 dB/octave 4th-order high-pass crossover).
You can handle crossovers in one of two ways: with passive components or digital signal processing (DSP). A passive crossover relies on physical components (capacitors, inductors, and resistors) to block or steer the wrong frequencies away from the tweeter so it isn’t damaged. Depending upon which components you use and how you wire them, the other frequencies will be grounded (when components are wired in parallel) or sent to the other speakers in the system (when components are wired in series).
Making a steep crossover with passive components can become complex (and expensive) very quickly. Here is where DSP shines. It gives you an amazing amount of control over how a signal is routed within a speaker. You can easily adjust specific frequencies to boost or lower output as well. An amplifier with DSP is essentially a mini audio mixing board.
As you tie a tweeter into a system, keep in mind that it will be more sensitive than the subwoofer or woofer it's working alongside (often in the range of 5-10 dB more sensitive). An amplifier will often have an HF (high frequency) channel to handle the minimal power required of a tweeter and will be able to adjust its output compensatory to the full system.
Test and Verify
Remember, humans can notice if a tweeter sounds good or bad, so it's critical that yours is designed and manufactured well. When you’re working with your speaker manufacturer, make sure they have the testing capabilities to guarantee the resonant frequency is at spec. This is an absolute must and if you have any questions about how this is achieved, please reach out to us here at MISCO and we’d be happy to explain the process.