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Guidelines for Designing in Small Speaker ComponentsThe Problem: I need a small speaker in my device, but I don’t know where to start.

Speaker or Buzzer: Buzzers or piezoelectric speakers (piezos) can produce loud sounds, but with a very limited frequency range. For playing voice, music, or a variety of tones, a speaker should be selected. Note that there are other small “speaker” components used for near-ear applications (e.g., phones, headsets) which include receivers and balanced armatures. These types of components are supported by Seltech, but not being considered here.

Dimensions and Shape: The size for all three dimensions is often the first determining criteria. The smallest speakers currently on the market are approximately 13mmØ or 8x12mm. It’s ideal to maximize the allotted space in your device to ensure that the best possible sound can be achieved. Based on the industrial design a round or rectangular speaker can be selected. Rectangular speakers tend to be used in designs that are limited in space, especially PCB space.

Connectivity: The main 2 connectivity options are wires soldered to the speaker or spring contacts to be pressed directly against a PCB. Care should be taken if soldering to spring contacts, since too much heat can damage the speaker. Other methods include flex PCB and pin contacts.

Sensitivity: This parameter gives an idea of how “efficient” the speaker is at transmitting sound. It is given in terms of decibels (dB) at a particular input power and distance from the speaker. (For example: 80 dB at 1W/1m) Even though many speakers have a power rating other than 1W, the sensitivity may be shown at 1W to provide a nominalized comparison with other speakers. The rule for changes in input power or distance from the speaker can be seen below:

PowerdB change

Distance

dB change

Doubled +3

Doubled -6

Halved -3 Halved +6

Frequency Range: The resonance or resonant frequency (Fo) is the frequency at which a speaker naturally vibrates. This value determines the low frequency performance of a speaker. Referencing the chart below, for sounds lower than the resonance frequency, the decibels of the speaker quickly decrease. Above the resonance frequency, a speaker should ideally have a flat response for most of its performance range. Many speakers are rated up to 20kHz, but most adults cannot hear sounds above ~12kHz. Although it’s difficult for small speakers to have good low frequency performance, the solutions offered by Seltech are very competitive.

Electrical: Speakers have nominal and maximum power ratings. A speaker can be driven at its nominal rating for extended periods of time, while the max rating is for short “bursts”. Speakers range in impedance up to a few hundred Ω. Some common impedance values are 4, 8, and 50Ω. The audio amplifier should match the speaker’s impedance.

Challenges: The difficulties with properly designing in a speaker are often more mechanical than electrical in nature.

Loudness and Frequency Range: Loudness and volume are also called SPL (sound pressure level). In general, speakers that are larger and have higher power ratings will produce more SPL. A speaker’s performance may not be loud enough due to several factors, including (1) inadequate amplifier (2) acoustic short circuit (3) poor mechanical enclosure design. An acoustic short circuit occurs when the out-of-phase sound waves coming from opposite sides of the speaker “cancel” each other out. This decreases the SPL and eliminates much of the lower frequencies. An acoustic short circuit can be prevented by using an enclosure and providing an acoustic seal with a gasket against the front rim of the speaker.

Distortion / Lack of Clarity: Even though a speaker is loud or has a wide frequency range, it may still not produce optimal sound. Poor audio quality can be due to distortion of the speaker itself – often called THD (total harmonic distortion). Levels of <5% THD are preferred. Additional causes of distortion are (1) improper or insecure mounting (2) being overdriven, which can lead to damage (3) unwanted vibration of the acoustic mesh (if applicable) (4) sound port or front volume is too large. More than the parameters on a datasheet, clarity may be the feature that is best observed by testing speaker samples and actually hearing each speaker’s performance.

Back Volume Design: Many small speakers are designed to require a closed back volume to have ideal acoustics. This is required in order for the speakers to have a limited amount of air to push against and to prevent them from being overdriven. Also, the size of the back volume can adjust the resonance frequency of the system. The rectangular speakers from Knowles require about 1cm3 of empty air in their back volume. The relationship between back volume, SPL, and frequency response can be seen in the graph below. An increase in back volume lowers the possible frequency response (more bass) and reduces the SPL (quieter performance). The reverse is also true. A decrease in back volume lessens the amount of available bass and increases the SPL of the system (louder performance). A front volume can also exist in the space between the speaker and the “outside world”; however, this space should usually be kept to a minimum to avoid unwanted peaks in the frequency response.

Waterproofing: A speaker can be waterproof based on the material of its membrane (e.g., silicone, mylar, plastic) and the glue attaching the membrane/cone to the speaker basket. What is defined as “waterproof” should be clarified by the manufacturer. Some speakers come with an IP (ingress protection) rating. Since a speaker operates with a moving membrane, it’s best for the membrane to be free of any water, dirt, or other particles for it to operate ideally. Therefore, it’s also recommended to have some type of acoustic vent/mesh between the speaker and the “outside world”. For speakers that are not waterproof, a waterproof acoustic mesh can typically achieve the desired IP rating.

Mounting: Very small speakers are usually designed to be held in place by a “compression fit” in the device. Alternately, some speakers come with screw holes or adhesive gaskets around the front rim. Gluing speakers in place can be problematic if the adhesive isn’t closely controlled to keep it off of the speaker membrane.

Add-on Customization: Seltech offers mesh, adhesive gasket, wire + connector, and enclosure add-ons to provide the highest quality and most cost-effective solution for your design. These solutions are provided in custom and off-the-shelf versions, for high or low volume projects. The images below are some examples of available assemblies.