Audio Surveillance Guide

Author: Ethan Ace, Published on Apr 30, 2014

Audio for surveillance is a complex topic with a lot of confusion and poorly implemented systems out there. We regularly get questions from users, such as:

  • What kind of mic should I use?
  • Where should it be mounted?
  • How do I configure my system for best possible audio quality?

And finding the answers to these questions and others can be nearly impossible for those without an audio background.

So, in this guide, we aim to clear up the following topics:

  • Microphone types: Internal vs. external, mic vs. line level, mounting location, and directional vs. omnidirectional.
  • Sensitivity and range: Testing 3 cameras with internal microphones vs. a ceiling mount external units we bought from Louroe to see which perform best at distances up to 20'.
  • Audio configuration: Setting up the camera, client, and VMS for best performance.
  • Basic use: How to use your camera and VMS with audio inputs and outputs.
  • VMS considerations: Differences in operation, configuration, and export of audio from VMS systems. 
  • Door intercom: How to configure a camera and VMS for use as a door intercom, one of the most common audio applications. 

In this guide, we bought and tested two popular Louroe Electronics products, a ceiling mic with audio interface and a door intercom station:

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Comments (11)

Love it. Thanks!

Hi Ethan!

You state "a mic mounted on the ceiling of a cell block or interview should be omnidirectional, as sound may travel in multiple directions".

Since sound may come from any direction, would not an omnidirectional mic pickup reflected sound from the ceiling and the upper portions of the walls, thus leading to arrival of both direct from the source sound as well as possibly out-of-phase reflected sound from the ceiling etc. leading to a final output which may not be as clear as what may have been the case if a cardioid/hyper-cardioid microphone had been used, or are we talking of using PZM omni-directional microphones?

Yes, omnidirectional mics may pick up reflective sound, but typically the delay has to be fairly significant to make it a real problem. When I was doing sound systems, for example, we'd consider it when we did churches, but not in even the largest board rooms. 20' doesn't make all that much of a difference, as the delay is a few milliseconds at most. 100'? Different story.

20' can make a difference. 1' is approximately 1ms so 20' would be about 20ms. Small room acoustics can be much harder to overcome than large room acoustics. If two loudspeakers are 20' apart on the same plane, setting the outboard speaker 20ms makes a huge difference in perceived directionality of the sound source.

I've used the Louroe Verifact A and door stations many times with great success. Good product...

John:

We monitor our cameras live from central location, and we have also begun using simplex audio as an interactive tool to effect outcomes at apartments (to control after hours breaches with loitering, swim pools, fitness cntrs, etc.)

Do you have any resources to recommend for equipment choices and best practices set-up for this type of application?

Many thanks,

Bill Coberly

When it comes to loudspeakers for talkdown you have a couple of options.

First, you can use a powered speaker which connects directly to the camera. Sony has one that takes 24V power and is very simple to connect. You can also use powered horns from Valcom or Bogen but they don't make it as easy. You need to split out wires to connect things, and I don't recommend it.

You can also use a small amplifier with non-powered speakers. Wattages vary, but honestly, unless the environment is very loud, you can get away with 5-10W speakers, usually. The advantage of the amp is that you can use more than one speaker if you need to, for more power or wider coverage. I think if you stick to the trusted paging brands for this (Atlas, Bogen, Valcom) you can't really go wrong.

Does that help?

Thanks for an excellent article - there is literally ZERO information (well, easy to find information) out there on this topic - Suppliers (ADI, Tri-Ed) also know very little about it.

It would be worth mentioning in the clip demonstrating 2-way audio on an Axis Q1604 via the web interface that this only works on a PC, only works on Internet Explorer and requires the Axis Media plugin to be installed. Mac users would have to use Exacq for 2-way audio, and I believe this is a feature of Exacq PRO only, not Start. Not trying to nit-pick but these details may be a big deal for some.

Thanks, Scott. Good points worth noting.

Great article guys! We have used both the PZM-11LL and Verifact (various models) with the IF amps with great sucess mounted in both ceiling or wall. We have some installs that the audio signal is encoded at the camera then streams to the NVR, in others where we have a hybrid recorder we let the video capture card handle the audio encoding and we couple that with the video stream in configuration to deliver good video with well syncronized video.

So far our experience with built in microphones lines up with the results of this test Ok for applications that do not potentially go to court or involve someone talking at a very low level in an interview. An issue we just discovered in a recent low budget deployment using a small Axis indoor PTZ product which has built in audio if the user moves the camera while recording / listening the mic picks up the mechanical drive sounds over the conversation.

I have done a few projects where audio recording was desired. One of them was a police academy where the desire was to record training helped in their classrooms and then archive the video to DVD. I used a DVR with analog cameras and multiple ceiling microphones. One of the rooms used 4 microphones, another used 2 microphones, and the last one used 6 microphones. Keep in mind that the desire was for a classroom setting and not for a room full of people talking at once. Each room was mixed together using a mixer and then fed to the single view microphone for the room. The microphone was just an analog camera with an overview FOV. The desire was to get not only the instructors point of view but also students asking questions. I looked at Louroe but I ended up using ETS

The ceiling height was about 10' and the mics were surface mounted. One thing in placement is to watch out for HVAC diffusers.

I liked the fact that they had built in circuitry to factor out noise, and had the option of built in compression to help normalize the dynamic nature of speech. I used their mixers to sum the audio feeds together. In the room with 6 microphones I used the DNR-1 To help pick up speech related frequencies. High fidelity was not the object here. The objective was to get speech recordings and it worked quite well. Now there website is not the best and needs help, but the products did perform well. They also include RCA interface cables that are on the low cost side and don't provide adequate shielding for RF environments. This location gets interference from a local AM tower and better shielding got rid of the radio interference in the line since RCA is an unbalanced interface, it is better to keep the cables as short as possible.

For more advanced audio requirements, one can use the Crown boundary mics for surveillance with a really good DSP from companies such as Bi-amp which contain AEC (acoustic echo cancellation) algorithms generally for use with telephone conferencing. I demoed one of these in an interview room to be able to pick up whispering as well as add sound masking to the room to help prevent outside noise from being intelligible. The AEC was able to filter out the sound masking very well.

Please note that I am not affiliated with any of the companies listed above and I am simply posting on personal experience and use of the actual product.

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