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MPA204

GLOSSARY

SPL

: Sound pressure level is a logarithmic measure of the rms sound pressure of a

sound relative to a reference value. It is measured in decibels (dBSPL).

AES

: Established in 1948, the Audio Engineering Society (AES) draws its membership

from amongst engineers, scientists, manufacturers and other organizations and
individuals with an interest or involvement in the professional audio industry. They are
mainly engineers developing devices or products for audio, and also people working
in audio content production.The Audio Engineering Society is the only professional
society devoted exclusively to audio technology. Its membership of leading engineers,
scientists and other authorities has increased dramatically throughout the world,
greatly boosting the society’s stature and that of its members in a truly symbiotic
relationship. The organisation develops, reviews and publishes engineering standards
for the audio and related media industries, and produces the AES Conventions, which
are held twice a year alternating between Europe and the USA. The AES and individual

national “Sections” also hold AES Conferences on different topics during the year.The

AES publishes a peer-reviewed journal, the Journal of the Audio Engineering Society
(JAES).

RMS

: Root-mean-square (rms) refers to the most common mathematical method of

defining the effective voltage or current of an AC wave. The name comes from the fact

that it is the square root of the mean of the squares of the values.

PARALLEL

: Either when a signal is split and routed down two separate paths before

being re-combined, or when speakers are con to + and - to -. Opposite of
series.

a filtered pink noise signal with a pass band of a decade and with a crest factor (average
value and peak value ratio) of 6 dB. For common applications which require both a
high power and a high reliability level, it is advisable to use power amplifiers with an
output rating equivalent to the AES power rating of the enclosures. This ensures that the
enclosures will operate within their thermal limit specification. For applications where
the user desires to take advantage of the full applicable power of the enclosures, it is
possible to use amplification with output power ratings equal to the program output
rating of the enclosures. In this configuration, the full capacity of the system to handle
transient power is utilised, however it will be necessary to constantly monitor and
regulate the system very carefully in order to avoid overloading the speakers.
It is also necessary to avoid that loudspeakers are subject to signals which could cause
them to exceed their mechanical limits or their applicable frequency limits. In practice, it
is possible to damage a loudspeaker with a power well below its AES applicable power
rating if it is required to reproduce a distorted signal at a low frequency that causes a
cone excursion beyond the physically consented limit.

Brief notes on acoustic.

Diffusing sound into an environment means to distribute sound signals to a given audience

and the results are dependent on several ambient factors (room shape, volume, etc...), the
number of the people present and their precise location, the type of sound source (live or
recorded music or speech), and the level of the background ambient noise.

EFFICIENCY

Sound pressure (

SPL

) of a speaker depends on three factors:

efficiency, dimensions and use in combination with other speakers.
Efficiency, the quantity of energy generated by the amplifier trasformed into sound,
determines the volume that can be obtained by an amplifier of a given power rating. A 50W
amplifier combined with highly efficient speakers may be able to produce a higher volume
than a 100W amplifier combined with less efficient speakers.

IMPEDANCE

One of the electrical features of a speaker is its impedance (resistance to the passage of
alternate current). Both resistance and impedance

varies at different frequencies so different frequencies can be delivered with different soiund

pressure  levels.  In  some  cases,  confusion  may  arise  when  combining  impedence  values  of
power amplifiers with those of loudspeakers.
Adapting the output impedance of the amplifier to the loudspeaker’s input impedance means
that all the energy from the amplifier will be transferred to the speakers with no signal losses,
so that the system works in optimum conditions (and the risk of damage is minimised).
A  louspeaker  having  an  higher  impedance  than  power  amplifier’s  may  be  used,  although
this will result in a power reduction; loudspeakers with an impedance lower than amplifier’s
minimum load, must not be connected.
If more complex systems (e.g. several speakers connected to the sameamplifier) are adopted,
you  must  be  sure  that  the  overall  speaker  impedance  value  corresponds  to  the  amplifier
output impedance.
There are two possible connection systems:

series

parallel

mode.

Connecting two speakers in series means to connect the positive pole of the first speaker to
the negative pole of the second one and
then to connect the two free poles to the amplifier.
In this case the impedance values are summed up: e.g. Two 8 Ohm speakers connected in
parallel give a 16 Ohm load.
To connect two speakers in parallel mode, simply interconnect the two speakers terminals of
the same sign. To obtain the total value, in this case a calculation is required. Indicating R1 and
R2 as the two loudspeaker values, the following formula has to be used: (R1 x R2) / (R1 + R2).
E.g.: with two 8 Ohm speakers, we have that:
(8x8)/(8+8) = 64/16 = 4 Ohm, that is to say that when identical speakers are connected in
parallel, the impedance value is halved.

CHOOSING THE RIGHT AMPLIFIER

According to the

AES

standard, tests run on the components require a two-hour stressing oF

» BRIEF NOTES ON ACOUSTIC