Mounting microphones and loudspeakers in intercom systems

For successful implementation of our OEM intercom modules, we would like to provide an overview of general considerations and best practice tips for the installation of microphones and loudspeakers.
In addition we are happy to provide detailed, individual advice, analysis or parameterisation for your application.

Intercoms are generally used for one-way or bi-directional information exchange according to the sender-receiver principle. The speaker side is called the "near end" the listener side the "far end".

In order for the information being transmitted to be fully understood at the receiving end, the following requirements must be met:

• Sufficiently high recording level for speakers of different volumes
• Low levels of noise and interference
• Repeatable and stable signal transmission
• As little distortion as possible
• Sufficiently high playback level for listeners of varying sensitivity

In order to meet these general requirements, some basic design, acoustic and electrical aspects should be considered during the design and development phase. This blog post is intended to help you with this.

The acoustic transmission channel is usually the more challenging, as it is subject to changing conditions:

• Speakers with different volumes
• Speakers at different voice pitches
• Varying speaking distances
• Room reverberation
• Dynamic background noise
• Changing noise levels

In addition to these influences, which can only be controlled to a very limited extent, there are often constructive requirements that must be met when installing loudspeakers and microphones, which can lead to compromises in acoustic quality:

• Design specifications
• Vandal resistant or concealed installation
• Predefined loudspeaker and microphone positions
• Small enclosure volume
• Mounting specifications

Due to these and other constraints, it is necessary to select the appropriate components for the application and implement them to achieve maximum acoustic quality.

The electrical transmission channel covers the entire transmission path from the microphone at the near end, through the internal signal processing and transmission path, to playback at the loudspeaker at the far end. Again, there are parameters that can be influenced and those that must be accepted as given.

General conditions that cannot be influenced may include the following:

• Type of signal transmission path between transmitter and receiver
• Changes in the signal encoding of the signal during transmission (e.g. mobile communication)
• Existing interfaces
• Signal conversion during transmission
• External interference

However, the following characteristics can be influenced during developme

• Definition of bandwidth, target level, headroom, acceptable inherent noise
• Electrical design and layout
• Reduction and/or shielding of interferences variables in the device
• Structured amplification of microphone and loudspeaker
• Microphone sensitivity and self-noise
• Mean sound pressure level, frequency response and power rating of loudspeakers
• Adjustable signal processing parameters (e.g. noise reduction, echo cancellation, compression)

In general, care should be taken to ensure that the signal level at the inputs and outputs of all processing blocks in the entire transmission path has sufficient SNR and headroom.

When implementing components that are not used for voice transmission, attention should also be paid to their effects on the communication paths (e.g. EMC behaviour). This is particularly important for analogue systems in order to achieve high voice quality.

Microphone Types and Parameters

Microphones are electro-mechanical transducers that convert sound waves into electrical signals, forming the interface between the acoustic and electrical transmission channels. There are many different types of microphones in an almost endless number of variants, sizes and characteristics.

The most common types currently used in intercom devices are electret microphone capsules and MEMS microphones. The following table compares typical values for the most common parameters.

 

Of course, there are microphones available that differ significantly from the values listed. In principle, both analog and digital microphones can be found for almost any application. The key is to identify the parameters that are relevant to the planned application and to define suitable ranges of values.

Instead of individual microphones, microphone arrays are now often used, which allow a wide range of directional effects to be achieved, with strong directional attenuation of more than 20 dB, source tracking, localisation of several speakers, etc. This enables a significant improvement in speech intelligibility at the far end.

In addition to the frequency response, sensitivity, inherent noise and maximum sound pressure level are usually the most important factors. As microphone signals are usually amplified quite highly before further processing, the inherent noise and any interfering signals are also increased in the same proportion at this point and thus become audible. The sensitivity should therefore be selected so that the microphone generates a sufficiently high output signal for an averagely loud speaker at the intended speaking distance in order to keep the amplification low. If high amplification is necessary, the inherent noise should be as low as possible.

The low-distortion maximum sound pressure level comes into play at the feedback level. This results from the far-end signal that is coupled back into the microphone at the near end  and is often more than 20 dB louder than the actual near-end speaker signal. It must not cause significant distortion or even clipping of the microphone input, as echo cancellation algorithms can only remove undistorted signals from the transmitted signal. Excessive distortion will result in clearly audible echo breakthroughs and artefacts at the far end.

Due to the typically very low output voltages of a few hundred microvolts to a few millivolts and the relatively high output impedance of analog microphones, the transmitted signal is very sensitive to external interference from electromagnetic or capacitive coupling. For this reason, special attention must be paid to grounded shielding and low-interference cable paths, especially for longer cable lengths.

To send small signals over longer distances or along potential sources of interference, symmetrical data transmission using shielded 2-wire cables should be used. Although this may require additional circuitry, it is very reliable in preventing interference coupling. If this is not possible, it is important to ensure that the microphone ground does not create a ground loop.

This is where digital microphones offer a significant advantage. As long as the cable design meets the requirements for high-frequency, pulsed signals, the data transmission is extremely immune to interference. As many audio codecs and DSPs work internally with digital I2S streams and provide the appropriate interfaces, the A/D conversion takes place directly in the microphone, reducing the analogue path to a minimum. For transmission over longer distances, the digital signal may need to be converted to a suitable format such as S/PDIF or A2B.

The following points should be considered when positioning and mounting the microphone:

• To reduce the feedback level, the microphone should be mounted approximately 20 cm from the loudspeaker. However, this distance may vary depending on the application and the intended output level.
• The microphone must be isolated from the inside of the housing by a suitable seal. The seal should be made of soft solid rubber and not of porous materials if the loudspeaker is mounted in the same housing without a closed rear volume. The microphone seal should be slightly compressed during installation to ensure a good sealing to the inside of the housing.
• There must be no transmission paths in the housing between the loudspeaker and the sound channel of the microphone (e.g. due to air gaps in multilayer housings).
• The sound channel should be as short as possible and have a constant or increasing diameter towards the outside.
• The microphone should be mounted so that it does not pick up any housing vibrations.
• Directional microphones with several sound openings must not be mounted in a closed housing, as noise from inside the housing will then be picked up.
• Even at maximum playback level, the device housing must not generate any noise of its own (rattling, booming, humming, etc.). These would be transmitted to the far end as loud interference noise.

Loudspeakers are available in a similar variety to microphones. The most commonly used are dynamic loudspeakers.

Depending on the intended use and the technical conditions of the target device, loudspeakers should be used that meet the following requirements:

• Frequency response suitable for the application
• Mechanical data suitable for the device concept
• Mean sound pressure level matching the application
• Sufficient RMS power to achieve the intended target level
• Low distortion at the intended target level
• Sufficient power reserves to reproduce signal peaks without significant distortion

The available amplifier power should be in the range of the peak power of the loudspeaker or slightly higher. If the signal of the playback path is amplified in several stages, attention should be paid to suitable structuring.

The following points should be considered when mounting loudspeakers:

• Stable mounting in the cabinet to prevent vibration noise even at high levels.
• Sufficient free space in front of the speaker to allow the cone to move freely at high levels without hitting the cabinet.
• The speaker basket must not be deformed during installation.
• Front grilles should have as much open airflow area as possible.
• The rear of the loudspeaker must be isolated from the front.
• The rear volume of the loudspeaker must be sufficiently large.
• If possible, a closed rear volume should be provided to separate the rear of the speaker from the microphone.