Analysis of the design scheme of resonance audio system

There are two key challenges facing designers of resonant audio systems. The first challenge is to exploit the resonant frequency and resonant region of a speaker or buzzer to produce the maximum output sound pressure level (SPL). The second challenge is to avoid the buzzing and rattling that resonance introduces in the audio device's cabinet and mounting system. Although resonance is a familiar concept, this article will review its significance to audio design, including the challenges mentioned above, factors that affect resonance, how to understand frequency response curves, and more.

Resonance and Resonant Frequency Basics

To understand the effects of resonance, it is first necessary to understand the basic characteristics of resonance. Resonance occurs when a physical object or electronic circuit absorbs energy from an initial impulse and subsequently vibrates at the same frequency. However, without more force acting on it, the amplitude of the vibrations gets smaller and smaller. The frequency at which resonance occurs is called the resonant frequency of the system, denoted as F0.

Resonance can occur in a variety of situations. A good and common example is the guitar, which produces sound purely through vibration. When a player plucks the strings of an acoustic guitar, the strings vibrate and transfer sound energy to the hollow wooden body of the instrument, causing it to resonate and amplify the sound produced. Similarly, if an LC filter is excited with a signal of the appropriate frequency, the filter will resonate as a tuned oscillating circuit. In basic radios, this effect can be used to capture broadcast signals by adjusting the capacitor or inductor values ​​of the oscillating circuit so that the resonant frequency of the oscillating circuit matches the broadcast frequency. Electromechanical resonance in piezoelectric crystal oscillators can be used as a frequency reference.

Audio Output Components Overview

Mechanical resonance is influenced by weight and the stiffness of the connections between the different masses. For a standard speaker, this mass is the diaphragm (or cone), while the stiffness is determined by the flexibility of the suspension that connects the diaphragm to the frame. Because speakers are manufactured in a variety of ways, each speaker type can have a different resonant frequency.

Other factors that contribute to different resonant frequencies in loudspeakers include the diaphragm material, suspension thickness, and electromagnet size. The electromagnet is attached to the back of the cone and contributes to the weight. Generally, lighter, more rigid materials and flexible suspension components result in higher resonant frequencies. For example, a tweeter is small, lightweight, has a rigid Mylar cone, and a highly flexible suspension component. By modifying these factors, a standard loudspeaker can have a frequency range of 20 Hz to 20,000 Hz.

Figure 1: Standard speaker structure (Image source: CUI Devices)

Another type of audio output component is the magnetic sensor buzzer. They separate the drive mechanism from the sound-generating mechanism differently than speakers. Magnetic sensors have a higher normal frequency range, but the range is reduced, due to the lighter diaphragm being more firmly attached to the frame. They can usually produce sounds between 2 and 3 kHz, with the added benefit of requiring less current than a speaker to produce the same sound pressure level.

Figure 2: Standard magnetic buzzer structure (Image source: CUI Devices)

Finally, there are piezoelectric sensor buzzers. They are more efficient at producing higher sound pressure levels at the same current than their magnetic counterparts. The buzzers use the piezoelectric effect to bend a piezoelectric ceramic element in different directions by changing the electric field, thus producing a sound wave output. This piezoelectric material is usually rigid, and the components used in these types of buzzers are small and thin. Piezoelectric sensor buzzers, like their magnetic counterparts, are capable of producing a high-pitched noise in a narrow frequency range of 1 to 5 kHz.

Figure 3: Standard piezoelectric buzzer structure (Image source: CUI Devices)

Resonance Design Considerations

Designing a speaker or buzzer that exploits resonance is a complex task that requires consideration of the desired resonant frequency or range of resonant frequencies, the characteristics of the speaker or buzzer to be used, and the shape and size of the enclosure that the speaker or buzzer will be housed in. These factors have significant impacts on each other.

For example, if a small speaker is mounted in a very large enclosure, the speaker is free to move and the resonant frequency of the system (speaker plus enclosure) may be the same as the natural resonant frequency of the speaker operating in free air. However, if the speaker is placed in a small, tightly sealed enclosure, the air inside acts as a mechanical spring that can interact with the speaker cone and affect the resonant frequency of the system. In addition to this, there are other interactions, such as nonlinear electrical drive characteristics, which must also be considered for an efficient design.

Given this complexity, the best way to approach any kind of audio design is often to build a few prototypes, measure the characteristics of those prototypes, and then make adjustments to produce the best output given the chosen audio source. This prototype-based approach also helps designers understand and compensate for the reality that component characteristics vary within manufacturing tolerances, and that speaker geometry and stiffness will also be affected by production variations. The performance achieved by a handcrafted speaker based on the best components selected from a batch is often difficult to replicate using mass production techniques and standard components.

The enclosure (especially for speakers) must also be designed with enough internal space to allow the generated audio energy to function without attenuation. A slight 3 dB reduction in sound pressure level caused by the enclosure cover or material will cut the output sound power in half. This is discussed in more detail in CUI Devices’ “How to Design a Micro Speaker Enclosure” blog.

In general, it is important to look at the full spectrum response of an audio component and take advantage of its performance at frequencies on either side of the peak resonant frequency. Since the resonant frequency is not a precise number, nor is it necessarily a very narrow frequency band (especially for speakers), there may be useful frequency responses on either side of the peak specified in the data sheet that the designer can take advantage of. The concept is to optimize the output sound pressure level and frequency for a given input power. To achieve this, the device should be driven at its resonant frequency and frequencies within its resonant region.

For example, the data sheet for the CUI Devices CSS-10246-108 speaker shows that it resonates at 200 Hz ± 40 Hz, but its frequency response graph shows another resonant peak at about 3.5 kHz. In addition, there is a resonant region from about 200 Hz to 3.5 kHz. Designers can use this information to select a matching speaker for their application.

Figure 4: CSS-10246-108 speaker frequency response curve (Image source: CUI Devices)

As another example, the data sheet for the CUI Devices CMT-4023S-SMT-TR magnetic sensor buzzer lists a resonant frequency of 4000 Hz. This can be confirmed by the buzzer frequency response graph below. Also, to simplify the resonance issue, the buzzer can also be used as an audio indicator of the internal drive circuit. Since their operation is set to a fixed rated frequency, these internally driven devices do not require a frequency response graph because they are designed to achieve the maximum sound pressure level in their specified frequency window.

Figure 5: Frequency response curve of the CMT-4023S-SMT-TR magnetic sensor buzzer (Image source: CUI Devices)

Summarize

When designing an audio device for an application, engineers must consider the device’s resonant frequency to ensure it produces maximum sound pressure levels without causing unwanted vibrations. This means using data provided by the supplier (specifically the resonant frequency) as a starting point for the design, then optimizing the design within the resonance zone that exists around this value. Once the initial design is complete, prototypes should be used to check that the audio device interacts with its enclosure and mounting components as designed. CUI Devices offers a range of audio solutions across the spectrum to help engineers find the right component for the job.