Resonant frequency in acoustics and audio technology
A guide for installers, expert planners and hobbyists, using speakers and speaker components as examples
Imagine standing on a bridge and feeling it swaying in time with your steps—that is resonance. In the audio world, the phenomenon of resonant frequency is a crucial building block for speaker designers and anyone working with audio technology. Let's dive deeper into the world of vibrations and sounds to understand how resonance works in acoustics and audio technology.
What is resonant frequency?
Resonant frequency is a basic concept in acoustics and audio technology. It has a decisive role when designing and optimising speaker systems. Understanding this phenomenon is useful for DIY speaker hobbyists, installers and expert planners. After all, it helps create—and also understand—high-quality audio experiences.
Real World Resonance Stories
As the Wine Glass Shatters
There's a famous party trick where singing makes a wine glass vibrate or even break. This illustrates how powerful resonance can be. Things are no different in the audio world: resonant frequency can make a speaker system "vibrate" and also metaphorically "break".
The Tacoma Bridge
The incident at the Tacoma-Narrows Bridge in the US state of Washington is a famous negative example of what resonance can do at its worst. The bridge collapsed in 1940 after being rocked by strong winds. That put the bridge in the range of its resonant frequency. The bridge then began to swing more and more uncontrollably until the structure broke. Experts refer to this as a "resonance catastrophe". This example impressively demonstrates why understanding and controlling resonance in design—whether for bridges or speakers—is absolutely crucial.
Technical Definition of Resonant Frequency
Resonant frequency is the frequency at which a system reaches the greatest amplitude of vibration. In acoustics, this refers to the natural frequency at which certain parts of a speaker, such as the diaphragm or the cabinet, vibrate most strongly. Resonant frequency is marked on many speakers and components, e.g., 560 Hz for a diaphragm.
This frequency is determined by the vibrating system:
- the mass (the part that moves)
- the overhead suspension
- the attenuation properties
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Here is another simple example, to make sure we understand:
(Playback) frequencies (e.g., voice, recording) | Resonant frequency (e.g., of a component) |
The frequency at which a speaker emits its playback (speech, music). This can cover a broad range of frequencies, depending on the music and speaker. A normal male voice is around 125 Hz. | The one frequency at which a speaker component vibrates at maximum. Example: A hypothetical resonant frequency is 125 Hz for a hypothetical speaker component. If the component vibrates at exactly this frequency during playback, the sound is particularly loud. |
Practical understanding of the resonant frequency: resonance effects lead to uneven playback
In audio, component resonances influence individual frequencies or frequency ranges of a signal.
- Amplification: Resonance (of a component) and signal (music, speech, sound) add up.
- Attenuate: Resonance and signal offset each other. However, this is not desirable for either speaker or system.
That's because signals simultaneously containing the resonant frequency of individual speaker components would then be changed. A hypothetical example:
- If the speaker diaphragm had to transmit a sound at a frequency of 560 Hz …
- … and if the component's resonant frequency were 560 Hz, sounds in this frequency range would be distorted.
In other words, this means that:
The goal is to control the resonant frequency so that it does not affect speaker sound and performance and causes no unwanted distortion or damage. Speakers are also designed with a resonant frequency outside the main playback range of the speaker to minimise negative effects on the sound.
In practice, a well-designed speaker system should have a carefully calculated and controlled resonant frequency to ensure the best possible sound quality and longevity.
Importance of resonant frequency for speaker design and acoustics
Selecting speaker components
Speaker builders always strive for the optimal solution for their particular application. Whether for
- Music with dry bass
- Crystal clear treble
- voice output with good intelligibility during announcements
Resonance should be avoided here, as it is disruptive and potentially annoying. The selection, combination and design of the components is therefore essential.
Speaker Cabinets
- Cabinet volume: The volume of the enclosure influences air attenuation. A larger cabinet allows lower resonant frequency, as the air in the enclosure has more room to vibrate. A smaller cabinet, however, limits this space and tends to increase resonant frequency.
- Cabinet type: Different cabinet types such as closed cabinets, bass reflex or "transmission line" housings have different effects on resonant frequency. With vented enclosure concepts (e.g. bass reflex, a closed cabinet with a defined opening, tube or channel), inventors utilise certain resonance characteristics to generate additional sound pressure in the bass range. This is technically referred to as a "Helmholtz resonator".
- Material and design: The stiffness and density of the cabinet material, as well as the design influence resonant frequency. Heavier and stiffer material can reduce cabinet resonance, which affects the overall resonance of the speaker system.
- Internal attenuation: Materials within the cabinet used for attenuation (such as acoustic foam or fibre insulation) can influence resonant frequency by reducing the internal reflections.
- Cabinet shape and construction: The shape of the cabinet and how it is built (e.g. the arrangement of the struts) can also have an influence on resonant frequency. Careful design can help to minimise unwanted resonance.
Crossover network selection
Crossover networks must be designed to accommodate driver and speaker construction resonant frequencies. They should distribute frequencies to prevent any possible negative effects from resonance from impairing playback. This ensures an optimum sound balance.
Even rooms have a resonant frequency
When a room is excited in its resonant frequencies (see: room modes), certain frequencies are excessively amplified or cancelled out. This results in an uneven sound. Expert planners should bear this in mind when designing concert halls or recording studios, for example.
Practical resonant frequency application in DIY speaker construction
Measuring resonant frequency
You can measure resonant frequency in various ways, such as impedance measurement or laser vibrometry. For DIY enthusiasts, software and microphones are often the most accessible tools to gain insight into the resonance characteristics of their DIY project.
The right cabinet
The material and size of a speaker's enclosure matter a great deal. A well-designed cabinet can reduce resonant frequencies to such a degree that the speaker renders certain frequencies better (or more evenly). Experimentation is key here: different materials and shapes can produce dramatic differences in sound. The use of attenuation materials in the housing can help to minimise unwanted resonance and improve sound quality.
Conclusion
Resonant frequency is more than just a technical term; it is a fundamental principle that affects everything from the structure of a building to the sound of a speaker. For DIY speaker hobbyists or expert planners, understanding resonance opens the door to a world of possibilities, whether through optimising speaker designs or improving room acoustics. It is a journey that requires both technical expertise and creative experimentation.
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