Understand the audio frequency range and audio components
Time:2022-02-23
Views:2207
Jeff smoot is vice president of application engineering and motion control at Cui devices
From cars to homes to portable devices, audio is everywhere, and its applications will only become more and more widely. When it comes to audio system design, size, cost and quality are important factors to be considered. There are many factors that affect the quality of changes, but it usually boils down to the ability to rebuild the necessary audio system for a given design. In this article, we will learn more about the basics of audio range and its subsets, the impact of shell design, and how to determine the audio range it may need according to the application.
Fundamentals of audio range
20 Hz to 20000 Hz is the common audio reference range. However, the average person can hear no more than 20 Hz to 20 kHz, and this detectable range will continue to shrink with age. The most thorough understanding of audio is through music. In music, every subsequent octave doubles the frequency. The piano‘s lowest A is about 27 Hz, while the highest C is close to 4186 Hz. In addition to these common frequencies, any object or device that produces sound will also produce harmonic frequencies. These are only higher frequencies at lower amplitudes. For example, the 27 Hz "a" tone of the piano will also produce 54 Hz harmonics, 81 Hz harmonics, and so on. Each harmonic is quieter than the previous one. In the high fidelity speaker system that needs to accurately reproduce the sound source, harmonics will become particularly important.
Subset of audio
The following table lists seven frequency subsets in the 20 Hz to 20000 Hz spectrum, which helps to define the target range used in audio system design.
Table 1: Audio subset range. (source: Cui devices)
Frequency response diagram
Frequency response diagram is a good way to intuitively understand how the buzzer, microphone or speaker reproduces various audio. Since the buzzer usually outputs only audible tones, it usually has a narrow frequency range. On the other hand, speakers usually have a wider frequency range because speakers are usually used to reproduce sound and speech.
The Y-axis of the frequency response diagram of audio output equipment such as speakers and buzzers is in dB SPL, which is basically the loudness of the equipment. The Y-axis of an audio input device, such as a microphone, indicates sensitivity in decibels because they are used to detect sound rather than produce sound. In Figure 1 below, the x-axis represents the logarithmic scale of frequency and the y-axis is listed in decibel sound pressure level, so this is a chart of audio output devices. Note that since DBS is also expressed in logarithm, both axes are expressed in logarithm.
The figure shows how many decibels of sound pressure level will be generated by inputting constant power at different frequencies. The figure is relatively flat and the change in the whole spectrum range is the smallest. Except for the sharp drop below 70 Hz, the audio device will produce a stable sound pressure level in the range of 20 Hz and 70 kHz under the same input power. Any device below 70 Hz will produce a low sound pressure level (SPL) output.
For example, the css-50508n speaker frequency response diagram of Cui devices (Figure 2) better illustrates the more typical speaker properties. The diagram includes different peak and valley values, indicating the points where resonance strengthens or reduces the output. This 41 mm × The specification of 41 mm loudspeaker lists the resonance frequency of 380 Hz ± 76 Hz, which can be regarded as the first main peak on the frequency response diagram. This drops rapidly from about 600 Hz to 700 Hz, but then achieves stable sound pressure level performance from about 800 Hz to 3000 Hz. Due to the size of the speaker, the designer can speculate that the performance of css-50508n in the low frequency range will not exceed the high frequency range, which is confirmed in the figure. By knowing how and when to refer to the frequency response diagram, the design engineer can determine whether the speaker or other output device can reproduce its target frequency.
Figure 2: css-50508n 41 mm x 41 mm speaker frequency response diagram of Cui devices. (source: Cui devices)
Audio range and enclosure considerations
The audio range affects the housing design in several ways, as described below.
speaker size
Compared with large speakers, small speakers move faster, enabling them to produce higher frequencies with fewer unnecessary harmonics. However, when trying to achieve a similar SPL output at a lower frequency, a larger speaker diaphragm is required to move enough air to match the same perceived dB SPL as the higher tone. Although the larger diaphragm is much heavier, it usually does not cause problems at low frequencies because it moves very slowly.
The use of smaller or larger speakers will ultimately depend on the application requirements, but smaller speakers usually only need a smaller shell to reduce cost and save space. Browse Cui devices blog to learn more about how to design micro speaker shell.
resonance frequency
The resonance frequency represents the natural vibration frequency of an object. When a guitar string is plucked, the string vibrates at its resonant frequency. That is, if you put the speaker next to the guitar and play the resonance frequency of the string, the guitar string will begin to vibrate and the amplitude will increase over time. However, when it comes to audio, this phenomenon will also lead to unnecessary abnormal sound with surrounding objects. Cui devices‘ blog on resonance and resonance frequency provides more information on this topic.
In order to avoid both nonlinear output and unnecessary harmonics, it is particularly important to confirm that the natural resonance frequency is not in the same spectrum range as the expected audio output in the shell design.
Material balance
The speaker and microphone design achieves a delicate balance between components that must remain stationary, flexible and rigid during movement. The loudspeaker diaphragm (or paper basin) should be very light for rapid response, while maintaining rigidity as much as possible to prevent deformation during movement. Cui devices speakers usually use light weight, hard paper and Mylar film. Mylar film is a kind of plastic, which also has the advantages of moisture-proof and moisture-proof. In addition to the diaphragm, rubber is also used to connect the diaphragm and the frame. In order to prevent breakage under extreme movement, the material must be strong and flexible so that the movement of the diaphragm is not limited.
Figure 3: basic structure of loudspeaker. (source: Cui devices)
When comparing microphone technology, you can also see the trade-off between the same components. Electret condenser microphone and MEMS microphone have good durability, compact package and low power consumption, but their frequency and sensitivity are more limited. On the other hand, ribbon microphones have better sensitivity and frequency range, but also sacrifice durability.
Material is also an important choice in shell design, which will affect the resonance and absorption of sound. The main objective of the housing is to suppress the non phase sound generated at the rear, which means that the selected material must be able to absorb sound effectively. This is particularly critical in lower frequency sound applications where it is difficult to suppress non phase sound.
epilogue
In the final analysis, the number of audio systems is limited, and no single audio output device can span the entire audio spectrum with any fidelity. In general, most applications do not require this fidelity and may not require a fully linear output. Understanding the audio frequency range will still play an important role in selecting the appropriate audio components for a design. With this understanding, engineers can better balance cost, size and performance. Cui devices provides a range of audio solutions with different frequency ranges to support a full range of applications.
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