In acoustics, the omnidirectional noise source is used in many fields.
To know the sound insulation of a wall or reverberation time In a room, a certain sound level must be produced. For acoustic isolation, a sound field is created in one room, and the sound level is measured in the other room. For reverberation time, the decrease in sound level is measured after the noise source is stopped.
This is not the only possible source of noise; there are also directional speakers (like the ones in our living room) or you can also excite rooms with sound pulses such as balloons.
But why then use a sound source that sends sound in all directions?
An omnidirectional sound source is composed of several speakers (often twelve, hence the name dodecahedron).
The speakers are distributed to ensure good homogeneity of sound directivity around the sound source, and thus avoid generating sound in a given direction.
Like any speaker, this one needs to be powered and must be connected to an amplifier which will also be used as an interface to generate specific noises (either directly in the amp or externally via a jack). The most common noises are the pink noise, white noise or specific signals such as MLS (Maximum Length Signal) or sine sweep (Sweep Signal).
This is one of the main characteristics, indeed THE characteristic, to check before buying an omnidirectional source. In fact, standards require that maximum deviations in directivity around the sound source be respected.
The table below shows, for example, the maximum directivity deviation in ISO 3382-1.
Frequencies (Hz)
125 Hz
250 Hz
500 Hz
1000 Hz
2000 Hz
4000 Hz
Maximum difference (dB)
+ / - 1
+ / - 1
+ / - 1
+ / - 3
+ / - 5
+ / - 6
An example of directivity is shown below.
La sound power The sound source is also a key point, both from the point of view of sound level and its spectral response.
Omnidirectional sources can emit up to 120dB of power.acoustic.
But why such power?
In the case of sound insulation, this allows for the measurement of very high-performance walls and, for reverberation time, ensures a high signal-to-noise ratio.
Let's take the example of a wall with 55dB insulation at 1000Hz and a background noise of 40dB; this means that more of 100dB sound level in the emitting room.
And don't forget that this power must be generated across the entire useful frequency range, which can range from 50Hz to 5000Hz.
And it is also important to ensure that the spectrum is more or less flat at least 100Hz to 5000Hz.
There are also other factors to consider
Le weight of the sound source, especially for measurements in buildings (and when there are several floors to climb). The clutter This could be a criterion if one is travelling by train, for example.
La remote controlThis is an essential component for remotely switching the source on or off. It's also important to ensure the signal can penetrate thick concrete slabs.
Avoid saturation, Because of the sound levels generated, it's important to have a clean signal and also to avoid damage. Safety features may be present on amplifiers supplied with the source.
There are 3 parts to the ISO 3382 standard. The first two deal in particular with the measurement of reverberation time (but not only that).
ISO 3382-1 and 3382-2 specify that " the source should be as omnidirectional as possible"See table above. The values are obtained by moving averages every 30°.
Note that an impulse source such as a balloon does not meet this criterion even if it is an easy source to use during testing.
She must ensure a sound level allowing for temporal decays with a dynamic (signal-to-noise ratio) of 45dB for the calculation of T30 (extrapolated reverberation time over the decay at 30dB) and 35dB For the calculation of T20 (extrapolated on the decay at 20dB). Only a powerful sound source allows these dynamics to be achieved in noisy environments and in medium-sized rooms.
ISO 16283-1 describes the measurement methods in situ acoustic insulation of buildingsISO 140 is applicable for laboratory sound insulation measurements.
The standard also defines directiveness requirements: » For one-third octave bands, the loudspeaker(s) can be considered to have uniform omnidirectional radiation if the values of DI are within the limits of ± 2 dB in the frequency range between 100 Hz and 630 Hz, ± 5 dB at 800 Hz and ± 8 dB in the frequency range between 1,000 Hz and 5,000 Hz. «
The sound produced in the broadcast room must be stable and have a continuous spectrum in the measured frequency range.
Other standards, particularly for measuring degrowth in the offices advocate an omnidirectional source. For example in ISO 22955 for the quality of open spacesIt is specified that the source must be omnidirectional for the measurement of speech-related attenuation (also for the'sound level reduction by doubling the distance covered by ISO 3382-3).
An omnidirectional sound source ensures that you have a homogeneous acoustic field in all directions.
In-situ and laboratory measurement standards require this omnidirectionality, which can only be achieved with this type of source. The sound source must also be verified to meet these requirements.
La acoustic power of the sourceThis is also a key point in order to obtain sound levels that will stand out from background noise.
Finally, when choosing a source, pay attention to the weight, the amplifier, and the battery.
