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Acoustic components in medical applications

  Knowledge

Transmission of information from a machine to the end user is of great importance in a large number of applications. It allows rapid feedback based on visual or acoustic stimuli and provides the necessary input for further processing by the user. Especially in the medical industry, where very short reaction times can make the difference between life and death, a high degree of attention must be paid to signal transmission.

Acoustic signals are particularly suitable for this purpose, since, unlike optical signals, they do not require permanent monitoring and, if the speaker is dimensioned accordingly, can also function across several rooms. This includes not only the transmission of audio signals by the application, but also the recording of information by microphones.

Focus on safety requirements

Special safety and testing regulations apply to technical applications in the medical industry and are specified by the DIN standard DIN EN 60601-1-8 or the ISO standard IEC 60601-1-8. The regulation is an essential part and required. In terms to acoustic warnings, the regulations highlight two aspects in particular:

  • Differentiation between individual devices based on their different tones
  • A harmonic component in the tone that aids in the location of the device to the operator, audibly

Manufacturers of medical devices should therefore consult the DIN standard as early as possible in the design cycle, as it may prescribe a specific set of tones and a specific sound pressure level (SPL) that must be reproduced in the finished application.

Choosing the right speaker

There are several components that are capable of processing audio signals. Speakers, buzzers and microphones are the predominant elements of acoustics-based human-machine communication. The components to be used in the medical application are selected primarily according to the specifications of DIN EN 60601-1-8. Buzzers are only suitable for reproducing a single, fixed tone frequency, whereas speakers are capable of playing many different tones at once across a specific frequency band. Due to their wider range of use, speakers are used in most applications.

When choosing a speaker, it is advisable to focus on the lowest tone you need to reproduce and the respective SPL requirement. In most cases, the frequency will be lower than 500Hz. The values are indicated in the specification sheet of the respective speaker. However, the way in which these were measured must be taken into account. For example, the SPL depends to a large extent on the input power provided by the speaker and the distance at which the microphone was spaced to capture the SPL. However, there is no set standard specifying at which distance the SPL should be measured. Many speakers are listed at 10cm or 50cm away from the measuring device.

In addition to these basic values, the conditions of use must also be taken into account: For instance, the measurable sound pressure changes depending on the distance away from the speaker. A speaker with a listed SPL of 86 decibels (dB) at 50cm will only output 80dB when the distance away from the speaker is doubled.

Doubling the input power from 1W to 2W, for instance, generates an SPL gain of 3dB, which represents a sound pressure level of roughly 89dB at a distance of 50cm.

When dimensioning the speaker, it is therefore important not only to pay attention to the rated values in the data sheet, but also to consider the everyday conditions of use of the finished device. PUI Audio recommends a few basic points that may help here:

  1. Invest in a low cost SPL meter or download a suitable SPL-App.
  2. Perform multiple measurements of the environment to minimize inaccuracies.
  3. Build a use-case to determine the actual operating conditions, thereby making a realistic assessment of the speaker dimensions.
  4. Target a value of about 15 to 20dB above the specified SPL in order to achieve the prescribed characteristics even at a certain distance.

Interpreting a speaker's power rating correctly

In addition to the performance data, the operating conditions represent another important characteristic value. All PUI Audio speakers are specified in the data sheet with the rated input power and the maximum input power.

The rated input power is the level at which the speaker was designed to play at continuously. In contrast, the maximum input power describes the ability of a speaker to sustain instantaneous peaks.

If a speaker is intended to reproduce a continuous pure tone (a sine wave), it is important not to exceed the rated input power, as otherwise there is a risk of the component being destroyed.

Use the peak voltage applied to determine how much power a speaker can dissipate. When using a multimeter that displays RMS voltage, multiply the measured value by 1.414 and then square the result. Then divide the resulting value by the speaker's impedance.

Example:

According to the formula, a 2Vrms measured pure tone signal applied to an 8Ohm speaker dissipates 1W of power.

Voice and music signals do not consist of pure sine waves, since a constant tone is not reproduced. The calculation can, nevertheless, be carried out using the aforesaid formula. It is, however, a good idea to check the signal using an oscilloscope to ensure the peaks of the signal do not exceed the maximum input power of the speaker.

Selecting the right amplifier

An amplifier is essential when using a speaker. Amplifier selection is pretty straight-forward as long as you pay attention to certain specifications

The key criterion is how much power an amplifier can create with a given input voltage condition and impedance load. At the same time, the level of distortion must remain below 1% to prevent any component damage. This is referred to as 'Total Harmonic Distortion' (THD).

Besides causing an unpleasant sound, distortion can also destroy the speaker. Exceeding 1% distortion with an amplifier drives it into a 'clipping' condition. In this case, the amplifier sends DC voltage to a speaker instead of AC voltage, forcing it to its outer mechanical bounds. This cause the leads to break that connect the input terminal to the voice coil.

The clipping effect can also occur to a certain extent with an over-sized speaker whose power reserves are insufficient. It is possible to destroy a speaker rated for 5W with a 4W clipped signal. As a rule of thumb, PUI Audio recommends selecting an amplifier that has double the power capacity of what you need to prevent components being destroyed by clipping.

PUI Audio is the leading manufacturer of speakers, among others inthe medical industry. The company's experience and expertise come from decades of business relationships with medical technology firms, delivering effective client consulting and participating in product developments. Rutronik's qualified team of application engineers supports customers when selecting the right components for their projects and provides comprehensive support throughout the entire design cycle.

Find components at www.rutronik24.com.

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