The answers to the most commonly asked questions on loudspeakers.
dB Decibel - The units of how loud something is
W - Watts A measurement of power – same as for cookers, light
bulbs, heaters etc
Hz - Hertz One Hz means backwards and forwards once in one
second. Thus, a loudspeaker which is radiating 50Hz
is moving in and out 50 times a second
SPL - Sound Pressure Level Technical term for “how loud!” (Measured in dB)
Decibels (dB): General Information
The dB scale is used when referring to how loud something is. 0dB is regarded as the threshold of
hearing and 110dB is when it starts to hurt!
Double the power = 3dB increase in SPL
Power (in Watts) is related to dB in the following way: For every 3dB increase in SPL you will need to
double how much power is sent to your loudspeaker.
At low power levels, for example 5 Watts to 10 Watts, this is not a problem; however, to get the same
increase at high levels may require you to go from 1000W to 2000W for the listener to be aware of the
same increase in sound level.
To the average ear: 3dB is just a noticeable increase
10dB is twice as loud
140dB 30m (at the side of the runway) from a military aircraft at take off!
130dB Pneumatic drill at the operator’s position
120dB Ships engine room at full speed
Peak level in bunker at PLASA was 126dB
110dB Automatic Punch Press at the operator’s position
Average Nightclub level playing mainstream dance music
100dB Platform of Underground station at rush hour with a train
90dB Heavy lorries at 6m distance
Health and Safety upper limit for noise in an average working day
80dB Kerbside of busy street
70dB Loud radio in average domestic room
60dB Restaurant/Department Store
50dB Conversational speech at 1m. General Office noise
40dB Residential area at night. Whispered conversation at 1m
<30dB Background in TV and recording studios
Power Handling This is probably the most commonly asked and quoted specification.
How many Watts a speaker uses is often regarded as the best way of
assessing how loud it will be. However, the keyword here is uses. A
car engine can use a lot of petrol (Watts) without moving the car very
quickly. In the same way, a speaker can use a lot of Watts without
being very loud!
Sensitivity This parameter tells how much sound you will get per Watt. To
continue the above analogy this is the “mpg” rating of the
loudspeakers. The higher, the better.
Speakers are rated in terms of how loud they will be when 1 Watt is fed
to them. This loudness is measured at a distance of 1 metre from the
Thus, if a speaker is rated as 99dB 1W/1m then you know that if you
put 1 Watt in, at 1 metre distance you will hear 99dB.
Note: This is significantly more than a speaker rated at 96dB 1W/1m,
because you will then need twice as much power to generate the
same sound level. (See Decibels: General Information)
Power Compression The Watts that are not turned into sound get transformed into heat.
This heat increases the electrical resistance of the speaker, making it
harder for the amplifier to drive. The harder you drive the speaker, the
more heat is generated.
To return to the motoring analogy: This is the “aerodynamics of the
speaker”: The faster you drive, the more wind resistance there is.
Thus, even more power is required to make up for this. The lower the
power compression figure is, the better.
Dispersion Horn loaded loudspeakers do not radiate sound in all directions. They
radiate a much greater proportion of this sound within a “dispersion
angle”. This angle is affected by the design of the horn and can vary
quite dramatically from one model to another.
They are quoted in the following manner; 90o x 50o (HxV) or
something similar. This refers to the angles, horizontal and vertical
from which you can move away from “straight out” and still hear the
sound properly. 90o is a right angle, thus the quoted speaker will cover
all areas when mounted in a corner within a 50o vertical (up & down)
Frequency Response The frequencies covered by the loudspeaker. They are measured in
Hz. The human ear can hear from 20Hz to 20000Hz to varying
degrees of sensitivity. The average “shot to pieces” DJ/Rock ‘n’ Roller
ear, probably only manages up to 15000Hz.
Watts – dB increase: How many dB increase for full power input relative to 1 Watt
Watts 1 50 75 100 150 200 300 350 600 800 1200W
dB 0 +17 +19 +20 +22 +23 +25 +25.5 +28 +29 +31dB
Maximum Sound Output To calculate the maximum SPL available from a loudspeaker
Max SPL = SENSITIVITY + dB at Full Power – Power Compression at Full Power
Sensitivity and power compression are found from the specification sheet. dB at full power can be
found from the table above. If you cannot find the power compression figure then quote “excluding
power compression” and omit it from the calculation.
Example: Max SPL at 1m = 99 +25.5 = 124.5dB (excluding power compression)
Effect of Distance: To calculate the level at a distance from the speaker relative to 1m:
Distance 1 3 5 7 10 15 20 30 50m
dB 0 -9.5 -14 -17 -20 -23.5 -26 -29.5 -34dB
This can be taken off the figure at 1m to give the level at any of the above distances.
Example: Max SPL at 15m: 124.5 – 23.5 = 101dB (excluding power compression)
NOTE: This does not account for any meteorological changes such as temperature, humidity
or wind direction. However, it provides a good rule of thumb for indoor work.
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