【Introduction】Designers of resonant audio systems face two key challenges. The first challenge is to use the resonant frequency and resonant region of the loudspeaker or buzzer to generate the maximum output sound pressure level (SPL). The second challenge is to avoid the hum and rattle that resonance introduces in the enclosures and mounting systems of audio devices. While resonance is a familiar concept, this article will review what resonance means for audio design, including the challenges mentioned above, factors affecting 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 pulse and subsequently vibrates at the same frequency. However, without more applied force, the amplitude will get smaller and smaller. The frequency at which resonance occurs is called the resonance frequency of the system, denoted as F0.
Resonance can occur in a variety of situations. A good common example is the guitar, which produces sound entirely by vibrating. When a player plucks the strings of an acoustic guitar, the strings vibrate and transmit sound energy to the hollow wooden body of the instrument, causing it to resonate and amplify the sound produced. Likewise, if an LC filter is excited with a signal of the appropriate frequency, the filter will resonate as a tuned oscillatory circuit. In basic radio, this effect can be used to capture broadcast signals by simply adjusting the capacitance or inductance value 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.
Overview of Audio Output components
Factors affecting mechanical resonance include weight, as well as the stiffness that connects different masses together. For standard speakers, this mass is the diaphragm (or cone), and stiffness is determined by the flexibility of the suspension connecting the diaphragm to the frame. Due to the variety of ways speakers are made, each speaker type can have a different resonant frequency.
Other factors that cause a speaker to have different resonant frequencies include diaphragm material, suspension thickness, and electromagnet size. The electromagnet is attached to the rear of the cone and affects the weight. In general, lighter, stiffer materials and flexible suspension components result in higher resonant frequencies. For example, tweeters are small and lightweight, with rigid Mylar cones and highly flexible suspension components. By modifying these factors, standard speakers can have a frequency range of 20 Hz to 20,000 H.
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 sounding mechanism in a different way than speakers. Magnetic sensors have a higher normal frequency range due to a lighter diaphragm that is more firmly attached to the frame, but with a reduced range. They typically produce sound at 2 to 3 kHz, with the added benefit of requiring less current than speakers to produce the same sound pressure level.
Figure 2: Standard magnetic buzzer structure (Image credit: CUI Devices)
Finally, there’s the piezo sensor buzzer. They are more efficient at producing higher sound pressure levels at the same current than their magnetic equivalents. The buzzer uses the piezoelectric effect to bend the piezoelectric ceramic element in different directions by changing the electric field, thereby producing a sound wave output. This piezoelectric material is generally rigid, and the components used in these types of buzzers are small and thin. Piezo sensor buzzers, like magnetic products, are capable of producing high-pitched noise in a narrow frequency range of 1 to 5 kHz.
Figure 3: Standard Piezo Buzzer Structure (Image Source: CUI Devices)
Resonance Design Considerations
Designing a loudspeaker or buzzer that can take advantage of resonance is a complex task, considering the desired resonant frequency or range of resonant frequencies, the characteristics of the loudspeaker or buzzer that will be used, and the packaging of the loudspeaker or buzzer the shape and size of the speakers of the device. These factors significantly influence each other.
For example, when a small speaker is installed in a very large enclosure, where the speaker can move freely, 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 interacts with the speaker cone and affects 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 efficient designs.
Given this complexity, the best way to do any kind of audio design is often to build a few prototypes, measure the characteristics of those prototypes, and then adjust them to produce the best output for the audio source of your choice. This prototype-based approach can also help designers understand and compensate for the reality that component characteristics vary within manufacturing tolerances, and speaker geometry and stiffness will also be affected by production differences. The achievable performance of loudspeakers that are handcrafted from the best components selected from a batch is often difficult to reproduce with mass production techniques and standard components.
Cabinets (especially for speakers) must also be designed with enough interior space to allow the generated audio energy to function without attenuation. A slight 3 dB reduction in sound pressure level caused by speaker covering or material will halve the output sound power. CUI Devices’ “How to Design a Micro Speaker Enclosure” blog discusses this in more detail.
In general, it is important to look at the full spectral response of an audio component and take advantage of its performance at frequencies either side of the resonant frequency peak. Since the resonant frequency is not an exact number, nor is it necessarily a very narrow frequency band (especially for loudspeakers), on either side of the peak specified in the spec, there may be useful things that the designer can take advantage of Frequency response. This concept aims 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 CUI Devices’ CSS-10246-108 loudspeaker shows a resonant frequency of 200 Hz ± 40 Hz, but its frequency response graph shows another resonant spike at approximately 3.5 kHz. In addition, there is a resonance region around 200 Hz to 3.5 kHz. Designers can use this information to select the right loudspeaker for their application.
Figure 4: CSS-10246-108 speaker frequency response curve (Image source: CUI Devices)
As another example, CUI Devices’ CMT-4023S-SMT-TR magnetic sensor buzzer’s specification lists a resonant frequency of 4000 Hz. This can be confirmed by the buzzer frequency response graph below. In addition, to simplify resonance problems, the buzzer can also be used as an audio indicator with a built-in drive circuit. Since their operation is set to a fixed nominal frequency, these internally driven devices do not require a frequency response plot as they are designed to achieve maximum sound pressure levels within 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 resonant frequency of the device to ensure that it produces the maximum sound pressure level without causing unwanted vibrations. This means using the data provided by the supplier (specifically the resonant frequency) as a starting point for the design and then optimizing the design in the resonant region that exists around this value. Once the preliminary design is complete, a prototype should be used to check that the audio device interacts with its speakers and mounted components for the performance of the design. CUI Devices offers a range of audio solutions across the spectrum to help engineers find the right components for the job.
Source: Digi-Key, by Jeff Smoot
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