Understanding resonance for audio component selection

Resonance is a double-edged sword in audio design. It can provide useful benefits, such as producing high sound pressure levels (SPLs) from speakers for relatively small inputs or it can introduce unwanted vibration modes into their enclosures and mounting systems. Often, it does both – at least in the prototypes.

To understand the impact of resonance, we first need to understand resonance itself says Jeff Smoot, VP of Engineering at Same Sky. Resonance occurs when a physical object or electronic circuit absorbs energy from an initial impulse or source, and then keeps vibrating even though no further force or energy acts upon it. This happens at the resonant frequency, designated as F0.

There are many forms of resonance. For example, a child’s swing acts as a resonant pendulum whose frequency is determined by the length of its rope. An LC filter can be stimulated into resonating as a tuned tank circuit. Electromechanical resonance in a piezoelectric crystal oscillator can be used as a frequency reference.

Resonance in Audio Components

When it comes to audio design, the mechanical resonance of a system is affected by its total weight and the stiffness of the materials that connect its masses. The cone is the most important mass in a speaker and the material that connects the cone to the frame provides the most important stiffness. Since speakers are made in many ways, with differing materials and approaches to supporting the cone, they have many different resonant frequencies.

Figure 1: Standard speaker construction

Standard speakers, which usually operate from 20 Hz to 20 kHz, have a cone connected to a frame via a suspension “spider.” An electromagnet attached to the rear of the cone affects the overall suspended mass. Variations in these components affect the speaker’s resonant frequency. Lighter and stiffer cone materials with more flexible suspensions produce higher resonant frequencies. That’s why high-frequency tweeters are usually relatively small and have rigid mylar cones and very flexible suspensions.

Magnetic transducer buzzers separate the driving mechanism from the sound-producing mechanism in a different way to that of a standard speaker (See Figure 2). The diaphragm is lighter and is attached to the frame in a more rigid way. This shifts the normal operating frequency range of the buzzer to a narrower range of higher frequencies. Magnetic transducer buzzers usually produce sound in the 2 kHz to 3 kHz range and don’t need as much current as a standard speaker to produce the same SPL.

Figure 2: Typical magnetic buzzer construction

Piezoelectric transducers use a varying electric field to cause a material to bend back and forth. Piezo materials are usually stiff, and the pieces used in piezoelectric transducers are quite small and thin. Because of this, they produce a high-pitched output, typically between 1 kHz and 5 kHz, over a narrow frequency range. They usually produce even greater SPLs than magnetic buzzers from the same amount of input power.

Figure 3: Typical piezoelectric buzzer construction

 How to Read Frequency Response Graphs

Most speaker manufacturers show the resonant frequency of their devices as well as the overall frequency response on a graph of SPL versus frequency in the datasheet. In practice, though, this resonance can be altered by the size of the enclosure into which the device is mounted, the materials of the enclosure, the rigidity of its mounting system, and so on.

Speakers can also have multiple resonances. For example, Same Sky’s CSS-10246-108 speaker states a resonant frequency of 200 Hz ± 40 Hz on its datasheet, but its frequency response graph (Figure 4) shows another resonant spike at roughly 3.5 kHz. There is also a resonant zone from approximately 200 Hz to 3.5 kHz.

Figure 4: Frequency response curve for the CSS-10246-108 speaker

Figure 5 shows the frequency response graph for Same Sky’s CMT-4023S-SMT-TR magnetic transducer buzzer. Its datasheet lists a resonant frequency of 4000 Hz, and this is confirmed by the peak on its frequency response chart.

Figure 5: Frequency response curve for a magnetic transducer buzzer (Image source: Same Sky)

On the other hand, audio indicator buzzers leveraging magnetic or piezoelectric principles offer a streamlined solution. These buzzers integrate internal drive circuitry, effectively eliminating the need for external frequency response characterization. These units are calibrated to operate at a fixed, nominal frequency, optimizing sound pressure level (SPL) performance within that narrow band. This design approach simplifies resonance tuning and ensures consistent acoustic output, making them ideal for applications where reliability and simplicity are paramount.

Final Design Considerations

When working with audio devices, particularly in applications involving resonance, electrical engineers face two key challenges after identifying the resonant frequency:

While theoretical design calculations and initial assessments are crucial, the iterative prototyping process remains indispensable. Empirical testing is vital for refining the enclosure design to achieve optimal performance tailored to the specific audio source. Additionally, production tolerances and component variability must be factored into the design to ensure consistent output across manufactured units.

For speakers, adequate enclosure volume is critical to prevent audio attenuation. Insufficient internal space can limit the distribution of acoustic energy, leading to SPL losses—potentially as significant as a 3-dB reduction, which corresponds to a 50% decrease in acoustic power output. Engineers must carefully evaluate the enclosure’s design, ensuring minimal interference while optimizing for efficiency.

From a systems-level perspective, it is essential to analyze the device’s full-spectrum response. This includes leveraging the broader frequency range surrounding the resonant frequency to ensure maximum efficiency and acoustic fidelity. To achieve this, the drive frequency should align with the device’s resonance while also considering the spectrum’s broader characteristics. Resonant frequency is inherently variable—it’s not a fixed point but rather a range that allows for effective response tuning on either side of the nominal value specified in datasheets. The ultimate goal is to optimize SPL and frequency response relative to the power input to the audio device, achieving peak performance within system constraints.

Conclusion

Every audio device has frequencies at which it amplifies sound, and frequencies at which it attenuates sound. By driving a buzzer or speaker with an input signal that has a frequency at or near the audio device’s resonant frequency or resonant zone, designers can create the greatest SPL with the least input power. However, most applications operate across a range of frequencies. While the resonant frequency is where the maximum SPL is achieved, a speaker or buzzer can be used across the frequency range per its specification, provided the SPL is sufficient for the intended application.