From cars to homes to portable devices, audio is all around and is only growing in its applications. When it comes to audio system design, size, cost, and quality are important factors to consider. Quality is influenced by many variables but typically comes down to a system’s ability to recreate the necessary audio frequencies for a given design. In this article, learn more about the basics of audio frequency range and its subsets, the impact of enclosure design, and how to determine what audio ranges might be needed depending on the application.
Audio frequency range basics
20 Hz to 20,000 Hz is the commonly referenced audio frequency range. However, the average human can hear less than this 20 Hz to 20 kHz range and, as individuals age, this detectable range only continues to shrink. Audio frequency is most well-understood through music where each subsequent octave doubles the frequency. A piano’s lowest note of A is around 27 Hz, while its highest note of C is close to 4186 Hz. Outside of these common frequencies, any object or device that produces sound also produces harmonic frequencies. These are simply higher frequencies at a lower amplitude. As an example, a piano’s 27 Hz “A” note also generates a 54 Hz harmonic, 81 Hz harmonic, and so on with each harmonic being quieter than the last. Harmonics become particularly important in high-fidelity speaker systems where accurate recreation of the audio source is needed.
Subsets of audio frequency
The table below lists the seven frequency subsets within the 20 Hz to 20,000 Hz spectrum that assist in defining the target ranges used in audio system design.
Table 1: Audio frequency range subsets. (Image source: CUI Devices)
Frequency response graphs
Frequency response graphs are a good way to visualize how a buzzer, microphone, or speaker will reproduce various audio frequencies. Because buzzers are typically only outputting an audible tone, they usually feature a narrow frequency range. On the other hand, speakers generally carry wider frequency ranges because they are commonly tasked with recreating sound and voice.
The y-axis on a frequency response graph for audio output devices, such as speakers and buzzers, is represented in decibels of sound pressure level (dB SPL), which is basically a device’s loudness. The y-axis for audio input devices, such as microphones, instead represents sensitivity in dB since they are detecting rather than producing sound. In Figure 1 below, the x-axis represents frequency on a logarithmic scale with the y-axis listed in dB SPL, which makes this a graph for an audio output device. Note, because dBs are also logarithmic, both axes are logarithmic.
Figure 1: Basic frequency response graph. (Image source: CUI Devices)
Representing how many dB of SPL will be produced with a constant power input at different frequencies, this graph is relatively flat with minimal changes across the frequency spectrum. Other than a steep drop-off below 70 Hz, this audio device provided with the same input power would produce a consistent SPL between 70 Hz and 20 kHz. Anything below 70 Hz would produce less SPL output.
The frequency response graph for CUI Devices’ CSS-50508N speaker (Figure 2) is a better example of a more typical speaker profile. This graph includes varied peaks and valleys which denote points where resonance either strengthens or reduces the output. This 41 mm x 41 mm speaker’s datasheet lists a resonant frequency of 380 Hz ± 76 Hz, which can be seen as the first main peak on the graph. This quickly drops off at around 600 to 700 Hz but then provides stable SPL performance from roughly 800 Hz to 3,000 Hz. Due to the speaker’s size, a designer could presume the CSS-50508N would not perform well at lower frequencies compared to higher frequencies, which is confirmed by the graph. By understanding how and when to reference a frequency response chart, a design engineer can confirm whether a speaker or other output device can reproduce their target frequencies.
Figure 2: Frequency response graph for CUI Devices’ CSS-50508N 41 mm x 41 mm speaker. (Image source: CUI Devices)
Audio range and enclosure considerations
Audio range can impact enclosure design in several ways as outlined in the sections below.
Smaller-sized speakers move faster compared to larger speakers, allowing them to produce higher frequencies with less unwanted harmonics. However, when trying to achieve similar SPL output at lower frequencies, larger speaker diaphragms are required to move enough air in order to match the same perceived dB SPL as higher pitches. While larger diaphragms are much heavier, this usually does not create an issue at lower frequencies where they are moving much slower.
Deciding between a smaller or larger speaker will ultimately depend on the requirements of the application, but smaller speakers typically lead to a smaller enclosure, which can reduce cost and improve space savings. Learn more in CUI Devices’ blog on How to Design a Micro Speaker Enclosure.
Resonant frequency represents the frequency at which an object naturally wants to vibrate. Guitar strings vibrate at their resonant frequency when plucked, which means that if a speaker were placed next to a guitar string playing its resonant frequency, the guitar string would begin to vibrate and increase in amplitude with time. However, when it comes to audio, this same phenomenon can lead to unwanted buzzing and rattles with surrounding objects. CUI Devices’ blog on resonance and resonant frequency provides additional information on this topic.
To avoid having a speaker with both a non-linear output and unwanted harmonics it becomes important in enclosure design to confirm that the enclosure does not have a natural resonant frequency in the same spectrum as the intended audio output.
Speaker and microphone design strikes a delicate balance between components that must remain still, flexible, and rigid during movement. A speaker’s diaphragm (or cone) should be light to allow for quick response while remaining as rigid as possible to avoid deformation as it moves. CUI Devices’ speakers commonly use paper and mylar, which are both light and rigid. As a type of plastic, mylar also has the added benefit of being resistant to moisture and humidity. In addition to the diaphragm, rubber is used to connect the diaphragm to the frame. To prevent breaking due to extreme movement, this material must be strong as well as pliable so as to not restrict movement of the diaphragm.
Figure 3: Basic construction of a speaker. (Image source: CUI Devices)
These same trade-offs can also be seen when comparing microphone technologies. Electret condenser microphones and MEMS microphones afford users durability, compact packages, and low power, but with more limited frequency and sensitivity. On the other hand, ribbon microphones offer improved sensitivity and frequency range with the trade-off of poor durability.
Material is also an important choice in enclosure design, impacting both the resonance and absorption of sound. An enclosure’s primary goal is to dampen the out-of-phase rearward generated sound, meaning the material chosen must be effective in sound absorption. This is particularly crucial in lower frequency sound applications where it is harder to dampen.
At the end of the day, there are a limited number of audio systems and no individual audio output device that can span the entire audio spectrum with any level of fidelity. In general, most applications will not require this level of fidelity, and a perfectly linear output is likely not needed. However, understanding the audio frequency range will still play an important role in selecting an appropriate audio component for a design. By having this understanding, engineers can better weigh the trade-offs between cost, size, and performance. CUI Devices provides a range of audio solutions with varying frequency ranges to support a full suite of applications.