Sound pressure in notification — selection of acoustic systems.
Determining the required power and sound pressure level of acoustic devices in notification systems has always been a significant problem for designers.
Some manufacturers of notification systems, trying to make their work easier, provide all sorts of graphs, tables or programs for calculating these parameters.
Most often, an attempt to practically apply such recommendations or programs generates more questions than answers, or baffles the absurdity of the solutions obtained.
Most designers simply do not have the time to independently study acoustics problems, so in this article it makes sense to outline the basic principles of acoustic calculations and the selection of sound-reproducing devices.
The main difficulty in designing warning systems is the correct selection of the number, switching power and optimal placement of alarms in the premises.
The installation locations of alarms should be selected not based on ease of installation or design considerations, but based on achieving maximum audibility and intelligibility of the information transmitted.
We will not go into the theory of sound propagation and the structure of the human ear. Let us just say that the frequency range of speech most perceptible to the human ear is between 400 Hz and 4 kHz. Any expansion of this range, especially in the low frequency range, really worsens the intelligibility of the transmitted information.
The choice of the number and power of the alarms in a particular room directly depends on such basic parameters as: the noise level in the room, the size of the room and the sound pressure of the installed alarms.
Very often, the volume level of the sound emitted by the alarm is associated with the electrical power of its inclusion in the transmission line — this is not at all true.
The volume of sound depends on the sound pressure level that the alarm can provide (the designation SPL is often used — an abbreviation for the English «sound pressure level»).
The unit of measurement for this parameter is the decibel (dB). The characteristic of each alarm is the sound pressure level measured at a distance of 1 m along the radiation axis.
The energy characteristic of the alarm is the power it consumes from the transmission line (switch-on power).
This is what is measured in Watts (W).
This parameter is used, first of all, to calculate the required amplifier power.
There is an indirect relationship between these values, since the volume of the sound is determined by the sound pressure, and the power ensures the operation of the loudspeaker. Of the supplied power, only a part is converted into sound and the value of this part depends on the efficiency of a particular loudspeaker.
Most manufacturers of acoustic systems indicate in the technical documentation the sound pressure in Pascals or the sound pressure level in decibels at a distance of 1 m from the emitter.
If the sound pressure is indicated in Pascals, while it is necessary to obtain the sound pressure level in decibels, the conversion of one value to another is carried out according to the following formula:
For a typical omnidirectional loudspeaker, we can assume that 1 watt of electrical power corresponds to a sound pressure level of approximately 95 dB. Each doubling (decrease) in power results in an increase (decrease) in sound pressure level by 3 dB. That is, 2 watts — 98 dB, 4 watts — 101 dB, 0.5 watts — 92 dB, 0.25 watts — 89 dB, etc.
There are loudspeakers with sound pressure per 1 W of power less than 95 dB, and loudspeakers providing 97 and even 100 dB per 1 W, and a loudspeaker with a power of 1 W with a sound pressure level of 100 dB replaces a loudspeaker with a power of 4 W with a level of 95 dB/W (95 dB — 1 W, 98 dB — 2 W, 101 dB — 4 W), it is obvious that the use of such a loudspeaker is more economical. It can be added that with the same electrical power, the sound pressure level of ceiling loudspeakers is 2?3 dB lower than that of wall loudspeakers.
This is because the wall speaker is either in a separate enclosure or at a highly reflective rear surface, so the sound emitted backwards is almost entirely reflected forwards.
Ceiling speakers are usually mounted on false ceilings or suspensions, so the sound emitted backwards is not reflected and does not affect the increase in frontal sound pressure.
Horn speakers with a power of 10?30 W provide a sound pressure of 12?16 Pa (115?118 dB) or more, thus having the highest decibel-to-watt ratio.
Today, there is a wide range of alarms on the market, and they all have different characteristics that are unique to them.
As a rule, the manufacturer specifies these characteristics. Sometimes manufacturers do not provide this data or do not provide it in full. It remains to hope that at least what they provide is true.
So, there are directional and non-directional loudspeakers.
Non-directional loudspeakers include speakers, ceiling speakers, and all kinds of sound columns (although it should be noted that speakers occupy an intermediate position between directional and non-directional systems).
The sound distribution area of non-directional loudspeakers (directional pattern) is quite wide (about 60°), and the sound pressure level is relatively low.
Directional loudspeakers primarily include horn emitters, the so-called «bells».
Horn loudspeakers concentrate acoustic energy due to the design features of the horn itself; they have a narrow directivity pattern (about 30°) and a high sound pressure level.
Horn loudspeakers operate in a narrow frequency band and are therefore poorly suited for high-quality reproduction of music programs, although due to the high sound pressure level they are well suited for sounding large areas, including open spaces.
The choice of loudspeakers by frequency range depends on the purpose of the system.
It should be noted that the sound level of the signal for normal operation of the warning system should be loud enough to be immediately heard and identified, but should not be too loud, because this can cause a negative impact on both the health and psyche of people.
According to the Technical Regulations, the sound level at any point in the protected premises should not exceed 120 dB. In order to ensure clear audibility of sound signals in accordance with SP 6.13130.2009 «Fire protection systems.
Electrical equipment.
Fire safety requirements» the warning system must ensure that the sound level of the signal exceeds the constant noise level in the premises by 15 dB.
Measurements of the permissible sound level of constant noise in the protected premises must be carried out at a level of 1.5 m from the floor level.
If people are in the protected room and are wearing noise protection gear, and if the sound level is more than 95 dB, in order to avoid exceeding the sound norm (120 dB), it is necessary to use light alarms in addition to sound alarms, and the use of flashing light alarms is also permissible.
(Note 3 to paragraph 6 of SP 3.13130.2009: «In buildings with permanent presence of people with hearing and vision impairments, flashing light alarms or specialized alarms must be used»).
The product range of Arsenal Bezopasnosti also includes an option for such a case: the combined internal alarm device “Grom-12-KPS IP55”, which in its technical characteristics is a complete analogue of the combined alarm device Grom-12KP IP55, plus it is additionally equipped with a strobe flash.
For sleeping areas, the sound level limit is set at 70 dB (it should also exceed the constant noise by 15 dB), and measurements should be taken at the head level of a person sleeping in this room.
The types, power and location of detectors must be selected in such a way as to ensure a sufficient sound level in all places where people are or may be temporarily located.
The warning system includes alarms (placed in a certain way throughout the premises), communication lines that perform power supply functions, and devices that monitor operability in automatic mode.
The required levels of warning signals must be provided by the system constantly — not only during the absence of an emergency, but also during fires, that is, it is necessary to take into account the impact of extreme conditions when selecting equipment.
Such conditions can be overheating of conductors on the communication line, rupture and short circuit, which can lead to the impossibility of monitoring its operability and failure of the warning system.
The equipment of the Arsenal Bezopasnosti Group of Companies is designed in such a way as to meet the requirements of the Technical Regulations as much as possible.
In particular, the Sonata voice warning system, in addition to all other capabilities, has in its functionality the ability to monitor the line for rupture and short circuit.
Thus, in an emergency, Sonata is guaranteed to notify about a malfunction on lines.
