Basic information customer and some concepts
Reflexions on the powers and the acoustic pressure levels.
Since I put the ear in the world of the hifi, I have to reexamine, with the listening of the specialists, all the basic concepts which I had acquired at the school of electronics. I wanted, just to clean me the neurons, to revise all these basic concepts a little more closely and to make you share my rediscoveries.
In the books of popularization, in the shops, the unhappy consumer is often face to face with quantities of contradictory data concerning the POWER of the systems. Let us take again the usual terms used in this field and give again a their right place and especially a right reality. In spite of the fact that the human feelings tend to be controls by exponential laws, for reasons of facilitates they are translated scientifically by logarithmic laws (for the revisions of these cruel terms, it will be necessary for you to seek in your attic your old books of maths)
Synopsis of the technical terms
:
Sound pressure Level ( SPL )
Mesuring unit of sound pressure
Comparative table of the sound pressure level in dB(A)
Mesuring unit of frequency
Mesuring unit of electric power
About the powers
Power felt by the listener and history of efficiency
Conclusion
Sound Pressure Level (SPL)
corresponds to the sound power of listening, that is to say the pressure of the air on the tympanums.
The unit of mesurment of the sound pressure is the dB or decibel, it is a unit with logarithmic progression curve. The 0 dB corresponds scientifically to a pressure of 2.105 Pascals, which correspond to the threshold of hearing of the human ear, and 1 dB is the finest variation that our ear (or our brain) can perceive. Each increase of 3 dB corresponds to a doubling of the power of emission. The threshold of painful hearing is towards 120 dB. For information in sonorisation the 0 dB at exit of mixing table corresponds to 2 Vcc or 775 mV effective. The dB(A) sound pressure level balanced A, is used in acoustics industrial and environmental because it takes into account the organic risk of the ear and is a function of the physiological curve of hearing. The decibel per Watt or dB/W is representative of the sensitivity of a loudspeaker or of an enclosure, it is the relationship between the electric power in entry and the sound pressure level obtained. The standard is to measure level SPL for 1 Watt with 1 meter. This efficiency is very weak, less than 1% for HP industrialists and up to 5% for the HP high efficiency, but after all those of a thermal engine or incandescent lamps are not better.
Comparative table of the noise levels in dB(A)
ACOUSTIC
PRESSURE Pascal (Pa) 
SOUND LEVEL PRESSURE dB (A) 
SOUND FEELING SUBJECTIVE IMPRESSION 
NATURE
OF THE NOISE 
< 2 .105 
< 0 
Unperceivable

Soundproof room

2
.105 
0

Threshold of audibility 
Audiometry Test 
6,3
.105 
10

Silencer

Recording Studio

2
.104 
20

Very calms 
Sailing ship, garden, cave 
6,3
.104 
30

Calm 
Bedroom , low voice 
2
. 103 
40

Calm 
Office or apartment Calms normal voice 
6,3
. 103 
50

Moderated 
Office or apartment Calms normal voice 
2
. 102 
60

Bearable 
Apartment + television, strong voice 
6,3
. 102 
70

Noisy 
Momentary street 
2
. 101 
80

painful 
Park, workshops 
6,3
. 101 
90

Lesions
if 8H/day 
Mechanics, weaving 
2,00

100

Very intense 
Press, motor bikes, undercutting 
6,30

110

Unbearable

Riveter, hammer rammer 
20

120

Deafening 
No conversation, discotheque, concert 
63

130

Threshold of pain, disturb, deafness 
Reactor

The Mesurment unit of frequency is the hertz (Hz). The theoretical human audiofrequency range (with new ears all and under good conditions of listening) is between 15 Hz and 20000Hz but more usually this range is between 20 and 16000 Hz. This frequency band is divided into frequency bands:
Infra
bass : 15 à 80 Hz, bass : 80 to 200Hz, low medium : 200 to 1600Hz, high medium : 1600 to 7000 Hz and high : 7000 to 20000 Hz.
The higher frequencies are described as ultrasound and are unperceivable by the ear. The infrasounds as the ultrasounds increase the comfort of listening but these frequencies are not perceived any more by the ears but directly by the body.
Warning : the ultrasounds, after 20000 Hz can,si their intensity exceeds 140 dB, to cause physical disorders, just like infrasons (lower than 20 Hz) towards 100 dB are very traumatisants and can cause unexplained tiredness, feelings of oppression, nauseas etc.
The Mesurment unit of electric power is Watt (W) this unit is with linear progression. 1 Watt gives place to an energy production equal to a joule by second.
About the powers :
history of Watt and the origin of its drift.
First of all it is advisable to remember that the power provided by an amplifier depends on the load which is applied to him.
P = U.I = R.I²
(with
U the tension in volts, I the intensity in Amps and
R the resistor of load in Ohms).
For example if the amplifier delivers a tension of 10 Volts, one a:
With a loading charge of 8 Ohms , P = 10² / 8 = 12.5 W
With a loading charge 4 Ohms , P = 10² / 4 = 25 W
History to want some always more, the power of the amplifiers and the loudspeakers was seen allotted many parallel units : Watts peak; musical Watts, Watts PMPO, Watts DIN . Iy is easy to lose himself when one seeks has to answer to a question as simple as "which power must I provide to my loudspeaker, of which I know the efficiency, to obtain a sound pressure level chosen and reciprocally. Here thus a recall of these units.
Start with the real Watts, the RMS power ( root mean square
) or
effective power, in Watts RMS or effective Watts
Let us see the others
:
The peak power ,
peak watt
It is obtained by multiplying the effective power by root of 2.
The peak to peak power : peak to peak watt
It is obtained by multiplying the effective power by 2 root of 2.
The musical power ,musical watts
It is obtained while multiplying by two the effective power.
The DIN power ,
Watt DIN
The DIN power is est
a German standard where 100 W DIN corresponds to 60 W RMS
.
The impulse power
Very little used it can be 2 to 100 times higher to corresponding RMS power.
The average power
Also very little used it is obtained by multiplying the effective power by 0.9.
La
PMPO power
, PMPO Watt ( peak music power output )
PMPO Watts are used for the amplified loudspeakers of multimedia type (PC), it is necessary to divide this unit by 30 to have an idea of the effective power .
Reminds us that it is always possible to exceed the power indicated by the manufacturer of the amplifier because this one is given for a precise rate of distortion harmonic and as low as possible. Iincrease the power increases the distortion as well as the risk to damage the loudspeakers.
Power felt by the listener and history of efficiency :
The ear is stimulated by a pressure. This acoustic pressure for a given amplifier depends directly on the efficiency of the loudspeakers. This efficiency expressed in dB/Watts/1 m, is the number of decibels produced by the HP for one Watt effective applied to its terminals and measurement is taken by a microphone placed at one meter of the HP.
Exemple :
Let us take 2 loudspeakers having respectively efficiency of 93 and 102 dB/W that is to say with a difference of efficiency of 9 dB. Knowing that the decibel is the logarithm of a ratio of pressures
G dB = 10 x log (P2 / P1) with P2 the mesured pressure and
P1 the reference pressure that is to say 2 .105 Pa.
When we double the power we obtain a profit of 3 dB.
.
What is to say that if we feed the 2e loudspeakers with 10 W, it will be necessary to feed the first with 10 X 2 X 2 X 2 = 80 W to obtain the same acoustic pressure, the same impression of power to hearing. This shows at which point it is important to have loudspeakers having the most possible efficiency in order to be able to use an amplification of low power. Indeed the harmonic distortion is directly related to the power of amplification. Between 2 HP which produces the same level of acoustic pressure but one with 10 W and the other with 80 W, the first will suffer from a harmonic distortion less good.
Moreover less one HP is sensitive more it packs the dynamics.
Conclusion :
Of this article it is necessary for you to retain three paramount concepts with the choice of its loudspeakers:
 Only effective Watts or RMS are significant

It is always necessary to pay attention to the efficiency of its HP to deduce its qualities from operation and to envisage from them what it is able to produce like acoustic pressure.
 The power of a loudspeaker defines only its acceptable maximum capacity .
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