This blog will discuss audio frequencies and their various subsets as well as how they impact the design of audio enclosures to ensure a proper audio range for your design.
Despite a diverse range of applications, audio systems of all types often struggle to balance cost, size, and quality. Ensuring the system's ability to output the entire range of audio frequencies needed is crucial in this process. In this blog from Same Sky, discover the various audio frequencies, their subsets, and how they affect the design of audio enclosures. Learn more about when different audio ranges are needed or not in an end application to ensure proper speaker, microphone, or buzzer selection and maximize performance for your audio system.
When designing an audio system, whether it's for residential use, automotive applications, or embedded/portable devices, striking a balance between cost, size, and audio quality is crucial. Audio quality hinges on several factors, notably the system's capability to reproduce a wide spectrum of frequencies. This article explores these frequency ranges and their subsets, highlighting their influence on the design of audio enclosures. It also clarifies the necessity of different audio ranges in various applications.
The typical audio frequency range spans from 20 Hz to 20,000 Hz, though many individuals do not hear the entire spectrum, and this range often diminishes with age. In music, each octave corresponds to a doubling of frequency. For instance, the lowest note on a piano, an A, resonates at approximately 27 Hz, while the highest note, a C, reaches about 4186 Hz.
In addition to these primary frequencies, virtually all sound-producing sources generate harmonic frequencies, which are multiples of higher frequencies but at reduced amplitudes. For example, the 27 Hz "A" note on a piano generates quieter harmonics like 54 Hz and 81 Hz. These harmonics are crucial for high-fidelity speaker systems aiming to reproduce the original sound source.
In the audio frequency spectrum ranging from 20 Hz to 20 kHz, there exist seven distinct subsets of frequencies. These subsets are pivotal in guiding the design of systems tailored for recording or playback purposes.
| Frequency Subset | Frequency Range | Description |
|---|---|---|
| Sub-bass | 16 to 60 Hz | This is the low musical range - an upright bass, tuba, bass guitar, at the lower end, will fall into this category |
| Bass | 60 to 250 Hz | This is the normal speaking vocal range |
| Lower Midrange | 250 to 500 Hz | In the lower midrange are typical brass instruments, and mid woodwinds, like alto saxophone and the middle range of a clarinet |
| Midrange | 500 Hz to 2 kHz | The name may be midrange, but it is on the higher end of the fundamental frequencies created by most musical instruments. Here, one can find instruments like the violin and piccolo |
| Higher Midrange | 2 to 4 kHz | As mentioned, harmonics are at multiples of the fundamental frequency, so if expecting the fundamentals for a trumpet to be in the lower midrange, one can expect the harmonic to be at 2 times, 3 times, and 4 times that fundamental, which would put them in this range |
| Presence | 4 to 6 kHz | Harmonics for the violin and piccolo are found here |
| Brilliance | 6 to 20 kHz | Above 6 kHz is where sounds become more like whines and whistles because they are so high pitched. In this range, sibilant sounds (the unwanted whistle when sometimes pronouncing an ‘s’) and harmonics for certain percussive sounds like cymbals are found |
An effective method to observe how speakers, buzzers, or microphones reproduce various frequencies is through a frequency response graph. Generally, buzzers exhibit a narrower frequency range as they primarily emit audible tones, whereas speakers offer a broader range to accurately replicate sounds and voices.
For speakers, buzzers, and similar output devices, the y-axis on a frequency response chart measures dB SPL (decibels of Sound Pressure Level), indicating loudness. In contrast, for microphones which detect sound rather than emit it, the y-axis measures sensitivity in dB. It's important to note that the x-axis represents frequency on a logarithmic scale, and since the y-axis denotes dB SPL, this chart pertains to a speaker or similar output device. Remember, dB values are logarithmic, hence both axes operate on a logarithmic scale.
This graph illustrates the dB SPL produced with a consistent power input across various frequencies. In this instance, the output remains relatively consistent with a notable decline below 70 Hz and a gentler decrease above 20 kHz. This indicates that the audio device, under uniform power input, maintains a similar sound pressure level between 70 Hz and 20 kHz but exhibits lower sound pressure levels outside of this range.
Frequency response graphs can also depict more pronounced peaks and valleys, indicating areas where resonance amplifies or suppresses the output. For instance, using Same Sky’ CSS-50508N speaker as an example, the figure below illustrates a typical speaker profile. According to the datasheet, the resonant frequency is 380 Hz ±76 Hz, correlating with the initial peak, followed by a significant drop between 600 and 700 Hz. However, the response remains flat between 800 Hz and 3 kHz. Given its compact size of 41 mm x 41 mm, it's expected that this speaker may reproduce higher frequencies better than lower frequencies, as shown in the graph. Design engineers can leverage this information to ensure the speaker meets the intended frequency reproduction requirements.
Understanding the fundamentals of audio frequency is crucial when it comes to enclosure selection and design. The audio frequency range significantly influences various aspects of enclosure design. Here's how:
Speaker and Enclosure Size
Smaller speakers can move faster, enabling them to produce higher frequencies with greater accuracy while minimizing unwanted harmonics. As detailed in Same Sky’ blog on designing micro speaker enclosures, smaller speakers also require smaller enclosures, which saves space and reduces material costs.
However, to achieve the same dB SPL at very low frequencies, a larger diaphragm is necessary to move sufficient air. This is due to the inherent difficulty in moving enough air to match the perceived dB SPL of higher frequencies. The good news is that the increased weight of a larger diaphragm is less problematic at lower frequencies, where movement is slower.
Resonance
Most objects have a resonant frequency—the natural frequency at which they vibrate. For instance, a guitar string vibrates at its resonant frequency when plucked. If you play this frequency with a speaker near the string, it will begin to vibrate and amplify over time. This phenomenon also occurs with other objects, causing unwanted rattles and buzzing in surrounding items. Same Sky’ blog on resonance and resonant frequency delves deeper into this topic.
When designing an enclosure, it's crucial to ensure that it doesn't have a natural resonant frequency within the same range as the expected audio output. Otherwise, the speaker will produce a non-linear output and unwanted harmonics. However, in some applications, controlling or widening the resonance range of a box is desirable.
Materials
Designing speakers and microphones requires a precise balance of components that must remain still, flex, and stay rigid while moving. For speakers, the cone or diaphragm needs to be extremely light to ensure quick response, yet rigid enough to avoid deformation. The most common materials used in Same Sky’ speakers are paper and plastic. Both materials are exceptionally light and stiff, but plastic also resists moisture and humidity. Additionally, the rubber connecting the diaphragm to the frame must be strong enough to endure extreme movement without breaking while remaining pliable to avoid interfering with the cone’s motion.
This trade-off in sensitivity, frequency range, robustness, and SPL range applies to microphone materials as well. Microphones range from simple electret or MEMS microphones, which offer sufficient but limited frequency and sensitivity, to ribbon microphones, known for their exceptional sensitivity and frequency range. However, ribbon microphones are extremely fragile and unsuitable for many percussive instruments; they must be handled with care and carried with a cover to prevent diaphragm damage.
These trade-offs, along with material costs, vary across different audio ranges. Lower-range speakers don't prioritize cone weight as much but require suspensions capable of larger movements.
The material used for an enclosure also impacts resonance and sound absorption. When designing an enclosure, which primarily aims to dampen the out-of-phase rearward sound, engineers need materials that effectively absorb sound. This is particularly important for lower-frequency sounds, which are more challenging to dampen.
It's important to recognize that very few systems, and no single speaker or enclosure, can deliver the full audio range with high fidelity. Extreme frequencies, in particular, necessitate specialized speakers and enclosures. For truly accurate sound reproduction, a balanced array of speakers across all ranges is essential, each tuned to produce the most linear output.
Second, most applications do not require this level of fidelity, and a linear output may not be the desired outcome. For instance, a phone only needs to cover the basic human vocal range, and even when extending the frequency range to accommodate harmonics, it still falls short of the 20 Hz to 20 kHz range. Similarly, notification or security applications only require a buzz, warble, or screech within a narrow frequency range but at varying SPLs. For these designs, buzzers or sirens, which prioritize cost, size, power, and loudness over frequency range, are a suitable choice.
Ultimately, understanding the full constraints of a project is essential. Making decisions on trade-offs is a critical aspect of being an engineer and designer.
Conclusion
The audio frequency range is a significant factor in the design and component selection of speakers, buzzers, enclosures, and microphones. A fundamental understanding of this range, its implications for recording and reproduction, and its relation to the physical limitations of audio equipment is crucial for the design process. Same Sky offers a wide variety of audio components as well as audio design services, providing solutions for numerous applications with diverse frequency requirements.
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