A brief study on thunder claps J.A.P. Bodhika Department of Physics, University of Ruhuna, Matara, Sri Lanka article info Article history: Received 21 January 2018 Received in revised form 20 September 2018 Accepted 25 September 2018 Keywords: Thunder sounds Thunderclaps Quantify Relative amplitudes abstract Thunder is the acoustic emission associated with lightning discharges. In describing the features of thun- der, a few subjective terms such as clap, roll and rumble have been used in the literature with no proper definitions. In this study the features of pressure pulses and their relative amplitudes were analyzed to understand claps of thunder. The subjective term clap was quantified along with relative pulse ampli- tudes and confirmed by listening to the recorded thunder signals carefully. The pulses with amplitudes greater than 40% of the peak amplitude were identified as claps. The most significant contribution to the sound in a thunder flash is due to claps and has been investigated separately in this article. The number of claps in a thunder flash, their frequency variation, time to start first clap, clap duration, and there pulse characteristics have also been studied. The frequency of pressure oscillations within these claps has been found to be less than 300 Hz. Besides, this study has revealed that 62% of the flashes consist of 1 to 2 claps. While it has been observed that the activity of the thunder signal is higher in the initial half than in the latter half. Ó 2018 Published by Elsevier Ltd. 1. Introduction Thunder is the acoustic emission associated with a lightning discharge. The pressure created inside a rapidly heating lightning channel generates a shock wave. After propagating a few meters, the shock wave turns into a thunderflash. Investigating the differ- ent sounds associated with thunder is essential to recognise them from ambient noise. It is worthy to note that in describing thunder, subjective terms such as clap, peal, roll, and rumble have been used in literature without proper definitions. Peals or claps are the sudden loud sounds that occur against a background of prolonged roll or rumble, which are separated by time intervals of 1 sec or more. A clap is sometimes considered to be less intensive and of shorter duration than a peal. Duration of claps is about 0.2–2 sec [1], and are (apparently) produced when individual thunder signals from portions of sufficiently long sec- tions of a lightning channel arrive more or less simultaneously at the observer’s location [2–4]. Thus, a clap is produced from a sec- tion of the channel having a relatively high overall length that is oriented or partially oriented to the observer’s line of sight. The time interval between claps has been reported to be typically 1– 3 sec [1], and there are about two to four claps per flash [1,5,6]. Details of relative clap amplitudes are found in [1,5], and [6]. In Florida, Uman and Evans (1977) reported approximately 2 or 3 claps per ground flash [7]. They further revealed that the first clap in ground flashes is generally the largest, the second clap the sec- ond largest, and the third clap the third largest. However, this result is not consistent with the data of Latham (1964) for a com- bination of cloud and ground flashes [1]. According to [8] and [9], the formation of thunderclaps has three contributory effects. A macro tortuous segment of a lightning channel will direct the acoustic radiation from its constituent meso tortuous pulse-emitting segments into limited annular zones. An observer located in this zone (near the perpendicular plane bisect- ing the macro tortuous segment) will perceive the group of pulses as loud as a clap of thunder, whereas another observer outside the zone will perceive this same source as a lower-amplitude rumbling thunder. This has been confirmed by experiments [2] and in com- puter simulations [10]. A second effect, which occurs only very close to the plane, is the juxtaposition of several pulses in phase, which increases the pulse amplitude to a greater extent than would be a random arrival of the same pulse. The third effect con- tributing to thunderclap formation is merely the bunching effect in time of the pulses. In a given period of time, more pulses will be received from a nearly perpendicular macro tortuous segment of a channel than from an equally long segment perceived at a higher angle, owing to the overall difference in the travel times of the composite pulses. Again, [3] and [4] discussed in detail the generation of thunder- claps that are to be associated with the sound emitted by relatively long sections of the main channel and by channel branches that are approximately perpendicular to the line of sight of the observer. https://doi.org/10.1016/j.apacoust.2018.09.018 0003-682X/Ó 2018 Published by Elsevier Ltd. E-mail address: jbodhika@phy.ruh.ac.lk Applied Acoustics 145 (2019) 98–103 Contents lists available at ScienceDirect Applied Acoustics journal homepage: www.elsevier.com/locate/apacoust