A Little Bit of Sound is a Good Idea

Rutronik’s Jochen Neller discusses the properties of piezo and electromechanical micro speakers.

In today’s human-machine interface designs, the focus is clearly centered on optical indicators and information display. However, sound quality is also very important to guarantee well-balanced visual and acoustic signals. In some systems, the sound is more important than the optical aspect, as this particular quality is perceived even without visual stimuli. A diverse and extensive range of micro speakers on the market today can cater for all aspects of these requirements.

Micro speakers are ideal for single-frequency signal sounds and the normal sound range (speech/music signals). In literature, they are often referred to as buzzers, signal emitters, beepers, etc. There are two types: electromechanical and piezoceramic versions.

A piezo speaker places an alternating voltage across a ceramic layer. The ceramic base material is usually bonded onto small brass or nickel plates. Models where the feedback electrode is connected to an external circuit have three connections. The oscillator of these can be operated at an exact resonant frequency and thus at the highest volume. The advantages of piezo buzzers are their robustness and low power consumption as a result of the high impedance of the piezo disk. They are therefore generally used in mobile handheld devices that output an alarm signal, e.g. blood pressure monitors. A piezoceramic diaphragm is available, for instance, from Murata: 7BB-20-6 delivers a resonant frequency of 6.3kHz and is used, for example, to output a signal sound for an electric cooker clock.

Electromechanical speakers (also referred to as dynamic speakers) are based on a varying current flowing through an oscillator coil. The diaphragm is made of paper or fabric to achieve the best sound possible; plastic is also used in less favorable ambient conditions. Due to their low coil resistance they have a relatively high power consumption, but usually provide better sound quality. For instance, the AS04008C0-2-R from PUI Audio is applied in high-quality video intercoms. Even small diameter speakers offer a relatively pleasant, low-frequency or broadband sound, e.g. in MP3 players.

Who's That Talking?

Since micro speakers are usually integrated in a small housing, it is usually not possible to distinguish which type has been fitted. Quite often even the data sheet is not explicit on this. However, this is the first and most fundamental decision a developer needs to make when selecting a speaker for a specific application: if low power consumption is crucial, a piezo speaker is recommended. If, on the other hand, the sound quality is of paramount importance, an electromechanical model should be used. An indication of the exact type is provided by the coil resistance - as this is always stated for electromechanical speakers in the specification.

Advantages:

Some micro speakers come with an integrated oscillator and are referred to as indicators. They only have to be operated with a DC voltage power supply. The data sheet for these speakers specifies the characteristic curve of the voltage/acoustic pressure, and the frequency is predetermined by the factory set oscillator. Piezo buzzers with an integrated oscillator usually also have a piezo disk with feedback electrode. The oscillator (feedback circuit) is therefore operated at an exact resonant frequency and thus at the highest possible volume. The PKB24SPCH3601-B0 from Murata reaches a maximum volume of 90dB/10cm thanks to an integrated piezo disk operating at a resonant frequency of 3.6kHz and a 12V supply. It is applied, e.g., in level gauges.

However, speakers without an integrated oscillator, referred to as transducers, offer the widest range of choice. Their lower price is definitely a factor that can influence customers. The world's arguably smallest SMD micro speaker is KSSGI3J12-A from Kingstate. The electromagnetic buzzer measures just 4mm in length, while achieving a maximum volume of 70dB/10cm at 3Vss. Its compact size makes it ideal for portable and medical handheld devices.

Developers must generate and set the frequency for these models themselves, if not already available. The characteristic curve of the frequency/acoustic pressure is stated in the specification. It is not easy to compare the volume of the various types, and it is necessary to always pay attention to the varying measuring conditions.

A rule of thumb for the acoustic pressure is: A difference of 10dB is defined as double or half the volume. Doubling the distance thus results in a level difference of 6dB. In other words, if the volume is 80dB at a distance of 1m, it will be 74dB at 2m, and still 68dB at 4m. The sound pressure level is, however, a purely technical factor, and exact conclusions on the actual perceived sensation can thus only be drawn to a limited extent.

Sound Pressure Level Calculator

However, the speaker is not solely responsible for the quality of sound transmission. It is additionally influenced by the material and dimensions of the housing (Helmholtz resonator) into which it is built. Furthermore, no two ears are the same, meaning our own sense of hearing is also a decisive factor for sound quality.

Rutronik offers a wide range of various micro speaker types. They are available with stranded wires, fully wired and, increasingly, with SMD technology. With the latter version, the sound release hole is either on the top or the side. To protect against possible damage during assembly, it is often equipped with a peel-off film. Rutronik specialists are happy to help customers make the right choice.

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