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Resonance in audio design is when an object takes in a sound wave from an initial source and continues to vibrate without any added force. The frequency at which this happens for a speaker is called its resonant frequency, represented on most spec sheets as f0 or Fs.
An overly simple way to think about resonant frequency is that it is the frequency at which a speaker naturally moves easiest. A speaker typically becomes increasingly unable to produce frequencies the further below they are from its resonant frequency. For instance, many midrange/voice speakers have too high of a resonant frequency to allow them to reproduce bass.
For reference, here are some rough guidelines for the resonant frequency ranges to expect for various types of speakers:
- Subwoofer: 20–47 Hz
- Woofer: 20–85 Hz
- Full range: 45–700 Hz
- Midrange: 50–530 Hz
- Guitar speaker: 77–155 Hz
- Voice speaker: 200–800 Hz
Material Selection and Resonant Frequency
Everything that goes into a speaker will impact its resonant frequency in some way. Therefore, selecting a speaker with the correct resonant frequency is essential, so your speaker can perform well in its intended application. This should be considered early in the design process.
A speaker manufacturer may ask you, “What kinds of sounds does your speaker need to handle?” With this question, one thing they’re trying to get at is what resonant frequency you will need in your speaker. The goal for resonant frequency will influence many design factors.
The material used for the cone, the weight of the suspension, and the speaker driver will all have an impact. In general, lighter and stiffer materials, as well as, suspensions produce speakers with higher resonant frequencies. This is why tweeters are smaller, are lighter, have more rigid cones, and have less flexible suspensions. The opposite is true for woofers: they’ve got more mass (heavier cones) and flexible suspensions (spiders and surrounds), generally, and are designed to travel further.
Speaker Enclosures and Resonant Frequency
Resonant frequency is influenced by the speaker enclosure, so the enclosure must also be considered. A speaker manufacturer should ask you where and how a speaker will be housed when they’re planning your project. They should also be able to make recommendations if they will be creating the enclosure along with the driver. A good way to mess up audio design is to put a speaker into an enclosure that is too small, because it may “ring” when playing low frequencies due to a peak in output at resonant frequency.
The size of the enclosure isn’t the only factor. The material it is made out of, and any dampening material that is added inside, will impact the resonant frequency too. Speaker engineers can also use ports or a passive radiator to help lower the resonant frequency of the speaker in an enclosure to help the speaker to play louder and lower.
Resonant Frequencies and Audio Systems
In a multi-speaker system the resonant frequencies of each speaker must be considered in order to create the best overall sound. This is where the concept of filters (or crossovers) comes into play. A crossover is the point where frequencies begin to roll from one speaker in a system to another (i.e., as the pitch of a signal goes up, a sound may move from a subwoofer to a full-range speaker).
A good way to get the best performance out of a sound system is by taking advantage of the digital signal processing (DSP) system that is built into many of today’s amps. With DSP, you can easily set crossovers and adjust their slopes to produce exactly the performance parameters you want. That way, you know your speakers are handling the frequencies they were designed for.
