Speaker Resonant Frequency
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Every speaker only produces sound within a given range of sound frequency as per the designer’s specifications. If check the specs of most speakers, you may notice that they are labeled with a certain frequency, 55Hz or 75Hz for example. This number is known as the speaker’s resonant frequency.

What Is A Speaker’s Resonant Frequency?

A speaker’s resonant frequency is the frequency at which the speaker will resonate, meaning the frequency at which it will continue to produce audio even without additional energy input. The resonant frequency is designated f0 (pronounced: F-naught).

In other words, the speaker’s cone will continue producing oscillations other than those induced by the motion of the voice coil in the magnet.

Another way to describe a speaker’s resonant frequency is the frequency below which a loudspeaker is increasingly unable to generate sound output for a given input signal.

Frequency Response And Performance Of A Speaker

The frequency response is a measure of how wide a range of sounds it can reproduce. Generally, sounds between 20 to 20,000 Hz can be perceived by the human ear. Most speakers can, however, respond from 45 to 20,000 Hz. The quality of sound will, however, vary depending on the range. 

The performance of a speaker, which is a measure of the quality of sound it can reproduce, depends on the range of sound frequencies available and the sensitivity of the speaker drivers used. 

Other factors that will determine the response of your speaker include impedance, power handling ability, sound staging ability, placement of the speaker, receivers and amplifiers used, brackets, stands and mounts, and wiring styles. 

What Does Resonant Frequency Mean?

You must be asking yourself, “What does resonant frequency mean?” Represented by the letter f0 and read as “F-naught,” resonant frequency is the frequency below which a loudspeaker is increasingly unable to generate sound output for a given input signal. 

What Is a Resonant Speaker?

A resonant speaker, also called a vibration speaker, is a horn without a diaphragm designed to break the limitations of the sound effects of ordinary speakers.

Traditional typical speakers transmit sound through the horizontal vibration of the cone or speaker diaphragm. On the other hand, resonant speakers transmit sound through 3600 cycle vibrations. 

The principle of resonant speakers is to convert the sound to vibrate the surface of the medium after resonating mechanically. When this happens, the whole structure of the speaker medium resonates to play a piece of melodic music. 

See some features of resonant speakers below:

  • They are smaller in size but produce loud sounds anyway. 
  • All the vocal music is original. Besides, the sound quality of each frequency is outstanding. This is because there are no standing waves in the speaker enclosure. 
  • The transmission of sound is far more reaching with the 3600 directional delivery.
  • They deliver different auditory feelings due to the other contact interfaces. 
  • The volume of the resonant speaker majorly depends on the system interface. The sound, however, propagates more strongly from solid interfaces compared to other materials.
  • The resonant horn of the speaker has a unique characteristic wavelength penetration. The resonance sound can deliver sound effects through the medium, through the medium side, and the sound is still audible on the other side of the medium. For instance, if a resonant speaker is installed on a door or wall, people will enjoy the melody from both sides of the door or wall. 

Free Air Resonance

The free-air resonant frequency of a speaker is the point at which the weight of the moving parts of a speaker balances with the force of the speaker suspension when in motion.

What is free air resonance and why is it important?

The free air resonance of a speaker is the resonance when measured for the driver in free air. This measurement is indicated by fS.speaker free air resonance frequency chart

This point directly compares with a string uncontrollably whirring in the wind. If you have seen this happen in the air, then you have seen the effect of a speaker reaching resonant frequency.

Music lovers should have adequate information about the resonant frequency and free air resonant to help you prevent your speaker enclosures from “ringing.” Woofers, for instance, should therefore have a lower free air resonance (Fs) to support the reproduction of low-frequency sounds.

This, therefore, means that speakers with higher free-air resonance frequencies will not be in a position to reproduce low-frequency sounds in a better way. 

However, the quality of sound reproduced by a loudspeaker depends on other factors such as the type of enclosure too. 

Speaker Diameter Vs. Frequency 

Speakers with larger diameters indeed produce lower frequency sounds much better than the speakers with a smaller diameter. Also, speakers with smaller diameters produce sounds with higher frequencies much louder than their larger counterparts. 

How To Measure Speaker Resonant Frequency

In this section, we will show you how to find the resonant frequency of a speaker. The resonance frequency of a speaker can be measured. However, check that you have the following items: 

  • Audio frequency oscillator
  • Power amplifier rated between 1 to 10 watts. The amplifier must, however, have a lower impedance rating of less than 0.1 ohms. 
  • Digital multimeter
  • FourFour soldiered alligator clip leads 
  • An accurate 10 ohms test resistor

Resonance causes a massive increase in speaker impedance. At resonance, therefore, the speaker impedance is pure resistance.

How To Measure Speaker Resonant Frequency

Usually, the impedance characteristic is inductive while the frequency increase towards resonance. The impedance characteristic, however, becomes capacitive as impedance falls above resonance. 

Maximum current therefore flows through the coil at resonance as the impedance level is at minimum. To determine the value of resonant frequency, follow the steps below:

  1. Mount the speaker or horn so that it is perfectly suspended and does not link to anything. You can use rubber bands to facilitate this.
  2. Wire the items and set up the generator for 1V p-p output.
  3. Adjust the frequency to allow the maximum voltage across the connection. The frequency that appears at the maximum voltage is the resonant frequency.

What Is Self Resonance?

Self-resonance frequency is the frequency at which the parasitic capacitance of the coil resonates with the ideal inductance of the coil, causing an extremely high impedance on it.

Parasitic impedance is an unavoidable and unwanted capacitance existing between the components of the circuit as a result of their proximity to each other. 

At self-resonance, and due to the high resistance between the circuit, the device looks like an open circuit. The value or extent of parasitic capacitance, however, varies depending on the type of coil. Multi-layered coils, wire-wound coils, and conductive film coils will have different levels of stray or parasitic capacitance.  

Lowest Resonant Frequency

Generally, the lowest resonant frequency of a speaker or a vibrating device is also known as its fundamental frequency.  

Most vibrating objects, such as the vibrating coil or diaphragm of a speaker, have more than one resonant frequency. Besides, most musical instruments are known to vibrate at harmonics of the lowest resonant frequency. A harmonic is a factor of the fundamental frequency. 

Lowest Resonant Frequency Speaker

It is very difficult to say which non-subwoofer speaker has the lowest resonant frequency. However, you can get very large full-range speakers that go very low in frequency. As an example, Klipschorns have a resonance around 25 Hz.

Speakers with much lower resonant frequencies are generally low in output and have a narrow range of frequency. Also, the lowest resonant frequency speakers experience extreme sound distortions making them sound very poor.

On the other hand, and as a rule of thumb, a lower resonant frequency shows that a speaker will have better chances of reproducing low-frequency signals than one with a higher frequency.

Why Is Speaker Resonant Frequency Important?

Ensuring that your speaker is resonating at the correct frequency is important for the following reasons:

  1. Prevents Ringing: Resonating at the correct frequency will prevent your speaker cabinet from ringing. When audio is played through a speaker at the resonant frequency, the speaker cabinet will radiate that sound. If not, the combination of sounds from the moving parts and the stiffness of the suspension will affect the overall frequency and cause ringing.
  2. Frequency Response: A speaker’s resonant frequency determines how well it will perform with low vs mid vs high frequencies. As an example, a speaker that operates at 40Hz will have deeper, bass tones than a speaker that operates at 60Hz for example. 

Most speaker cabinets take advantage of this concept, but one especially is the T-line subwoofer box. You can read our article to learn more about T-line subwoofer boxes and resonant frequency

Reading Resonance in Frequency Response Curves

A good way to determine your speaker’s resonant frequency response at different dB levels is to use a resonance frequency curve. Most manufacturers provide a graph of SPL versus frequency on the datasheet of the speaker. 

Of course, this resonance frequency can change due to varying factors such as mounting and enclosure size. However, it is generally a good estimate. 

Below, we will show you how to read some example resonance frequency charts:

Example 1:

Frequency Response Curve 100dB 20 kHz

The datasheet of the speaker above indicates a resonant frequency of 4000 Hz. This resonant frequency is indicated by the peak shown on its frequency response graph.

Example 2:

speaker resonant chart upt to 108 dB 20k Hz

The datasheet of the speaker above indicates a resonant frequency of 200 Hz with a 40 Hz margin of error. However, in reality, the frequency response chart also shows another resonant spike at about 4 kHz. In fact, the speaker is resonant within the range of approximately 200 Hz to 4 kHz.

Subwoofer Resonant Frequency

Subwoofer drivers are designed to reproduce low frequency sounds. However, as the frequency gets too low, the driver may just roll off at a certain very low frequency level. This point is referred to as the subwoofer resonant frequency.

The normal frequency range for a consumer-grade subwoofer is about 20–200 Hz. For professional live sound, the frequency can go below 100 Hz and below 80 Hz in THX-certified systems.

This may however not be a reality today as it is possible to add enough boost below the resonant frequency to compensate for the sound effect.

 

Speaker Cabinet Resonant Frequency

The resonant frequency of an object is the frequency that the object will resonate when struck. Generally, a lighter object has a higher resonant frequency than a heavy one.

Speaker cabinet resonant frequency is therefore the frequency at which the cabinet of a speaker will resonate.

Speakers should therefore be properly designed with the right cabinets to prevent the radiation of sound produced by the speaker.

If a sound is played through a speaker and that sound is at the resonant frequency of the speaker, the speaker cabinet will also radiate that sound. Generally, the tuning frequency of a speaker is usually fairly close to resonance (+/- 30%).