## 1 Introduction  
Impulse response is the temporal evolution of the sound pressure observed at a point in a room as a result of the emission of a Dirac impulse at another point in the room [1]. Usually the measurement of an impulse response utilizes a dodecahedron speaker following the sound source requirements according to ISO 3382-1:2009 [2]. For a more complete description of the acoustical conditions in a space, acoustic parameters such as Reverberation Time (RT), Early Decay Time (EDT), Clarity (C\({}_{80}\)), Definition (D\({}_{50}\)), Centre Time (T\({}_{s}\)) and Source Strength (G) [2] can be obtained directly from the impulse response. These parameters can be used to assess the acoustic quality of a space and provide guidance for possible improvements. Appropriate values of the acoustic parameters are required in spaces according to their usage (classrooms, theatres, music halls, etc.). As ISO 354:2003 [1] states, it is impossible in practice to create and radiate a true Dirac impulse. However, alternative measurements techniques exist which utilize a dodecahedron speaker. Maximum-Length Sequence (MLS) [3] and Exponential Sine Sweep (ESS) [4] type signals are commonly used, which transform the measured response back to an impulse response. The appropriate choice of excitation signal can be dependent on the background noise [5]. Also, white or pink noise can be used as excitation signals. Dodecahedron speakers are the most widely used sources for room acoustics measurements, as they produce a good approximation of omnidirectional radiation. Other polyhedron loudspeakers can also be used, which in some cases can be viewed as equally omnidirectional [6]. The directivity of a dodecahedron speaker can be considered uniform in the low-frequency range (namely below 1 kHz), while at higher frequencies sound radiation shows greater deviation [7]. Stepwise rotation of dodecahedron speaker can be employed to improve the accuracy of room acoustic measurements [8]. Constructive interference of the pressure field across the spherical baffle surface and not individual loudspeaker pistonic radiation characteristics is the most significant factor with respect to deviations from omnidirectional radiation [9]. However, there are certain drawbacks associated with the use of dodecahedron speakers for acoustic measurements. The main disadvantage is their high cost which limits their widespread use. Also, their heavy weight combined with large volume makes transportation difficult e.g., transfers to airports. In addition, there are cases where their usage is required in places without electric supply. An external generator or an appropriate dodecahedron speaker with an internal generator can be used, which further increases the cost. Although the dodecahedron speaker is quite expensive, it is possible to use a 3-D printer for its manufacture [10]. Nonetheless, there are alternative low-cost sound sources. Numerous examples can be found in the literature where alternative sound sources were used instead of a dodecahedron speaker e.g., measuring impulse responses in churches [11], measuring the acoustics of catacombs [12], measurements in Stonehedge [13], in Notre-Dame cathedral [14], Hagia Sofia [15], measurements in urban environments [16] or for the acoustic of caves [17]. The most common low-cost sources are balloons, pistol shots and firecrackers. Other sound sources that have been reviewed in the literature are handclaps, wooden clappers, shotsell primers and inverse horn designs. Important questions concerning their performance are their directionality as a source and the repeatability of measurements. Source omnidirectionality ensures uniform space excitation necessary for correct impulse response measurement. Source repeatability ensures that a similar impulse response is measured each time. The balloon as a sound source is a common, affordable solution for acoustic measurements. Considering the directionality, there seems to be some evidence to indicate that it cannot be considered as a broadband omnidirectional source.