Sound and light searching

A sound/light line search is a quick and efficient method using whistle, voice and light to attract the attention of a responsive lost person at night and then listen or see a response from them.

This search is used early in a search along tracks and other lineal features during darkness or daylight (without torches) to attract a lost person.

Sound /light searchers provide good safety for searchers as they can follow defined trails and/or ridge features during night and avoid rougher terrrain and vegetation.

Resources Required

  • Loud whistle
  • Bright Torch with spare batteries


  • Two person search team with a nominated leader
  • The team spreads along the track approx 50m apart with the leader at the rear
  • Both team members shine their hand held torches slowly in a figure of eight pattern from side to side as they move forward
  • Two short whistle blasts from the Leader for team to stop
  • Both members count down – 5, 4, 3, 2, 1 while turning off their lights
  • Both members block their ears and blast on their whistles for 4-5 seconds
  • When appropriate, both members can also then call the lost person’s name
  • Both searchers listen and look for sounds or light from the lost person
  • Leader estimates a 10-15 second controlled listening and looking period
  • One short blast from the Leader and both members resume shining their torches and move further along their route until the leader is 10-15m past where the forward person was previously and repeat the process
  • A second team follows behind the first team delayed by about 15-20 minutes.
  • If the first pass has not found the person it may have alerted the lost person by way of sound or light and the second team may then hear or see light from the lost person as they pass.

Whistle Signals

  • One short blast: From the leader and the team starts walking
  • Two short blasts: Team stops walking and prepares for whistle blast
  • Three short blasts: Help, I need assistance
  • Four short blasts: Come to me (recall)
  • One or two near continuous long blasts: Alert the lost person and then listen for a response


  • Covers a large area very quickly with a small number of searchers
  • Allows searching 24/7
  • Acts as attractor for the lost person to head towards either the light or the whistle blasts
  • The lost person can be mobile or immobile and still be heard or seen, if they have source of light; even a mobile phone can be seen from a considerable distance, provided it is pointed in the right direction
  • Can be used very early in a search when very little information is available and the likelihood of the lost person being responsive is high
  • Low technology and easy to implement with simple instructions and a small number of experienced searchers
  • The sound light line search is safer to use at night on lineal features, compared with negotiating terrain and vegetation off tracks.


  • Requires a responsive lost person
  • The wind and weather can limit the sound penetration
  • Searchers need to be experienced in searching at night, there is an increased risk of injury to searchers
  • Requires multiple teams to repeat the search over time intervals
  • Successive teams need to be separated in time so the whistle blasts/light from each team do not cause
  • confusion between them
  • Extra batteries are required for the high power torch


  • Team spacing and distance between blast cycles depends on terrain, weather conditions and background noise. It could be 50-200m. To check, do a test before you start the search to see how far you short blasts travel.
  • The larger distance between searchers the greatest coverage and the quicker the search. The shorter distance between searchers increased detectability of a possible lost person response.
  • Ensure you have overlap of the sound circles. Stop spots do not need to be precise.
  • Probability of detection in increased by good listening and watching, doing so in all directions.
  • Having no light or sound during the listening watching period focuses the senses of the searchers.
  • When calling the lost person’s name always make it positive and friendly.
  • Radios should be switched off or on a channel with no other traffic to ensure you are not disturbed during the listening period . Keep talking between searchers to a minimum.
  • Given that 25% of hiker lost persons are found within 50m of a track, 50% are found within 100m and 95% are found within 500m, it provides good initial coverage of linear features using this Track Offset probability of success very early in the search while the lost person is still likely to be responsive.
  • This technique could potentially be adapted for motorbikes, ski mobiles, horses, etc.
Original author: Linda Beilharz. Notes – Bush Search And Rescue Training, November 2011.   Information source: SARINZ
24 August 2012

Last Updated on February 17, 2021