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Glossary of Acoustic Terms

Here are explanations of some acoustic and electro-acoustic terms or concepts that you may come across, the list is by no means exhaustive but these are some of the more common terms.

Background noise

This is noise break-in either internal e.g. heating, air-conditioning, lighting etc. or external e.g. traffic noise.

Calibration and commissioning

To optimise the dynamic range of every component in the range, the sound engineer has to set the gain structure of the equipment chain before it is used. Other engineering jobs like equalisation also have to take place and this is all done at the critical commissioning stage. All good design work is wasted if the system is not commissioned effectively.

Comb filtering

If the listener is not equidistant from two loudspeakers, the two sounds arrive at different times. Some frequencies will cancel (because the two signals are out of phase) and others frequencies combine (in-phase) to give a comb-shaped frequency response.

The same effect occurs when the direct sound from a loudspeaker meets the same sound reflected from a wall, both having travelled different distances and so arriving at different times.

Direct to reverberant energy

The direct sound from a loudspeaker gives good speech intelligibility but as you move further away you hear more and more reflected (reverberant) energy. In effect the reflected energy becomes little more than background noise - a real nuisance.

Dynamic range

When we want to put emotion into our voice we use the contrast of softly spoken words with shouted commands. Music can also use this contrast to great effect. A sound system should be capable of relaying all that contrast. To do that it has to cope with low level sound (without excessive background noise) and very high level sound (without distortion). This operating range is called the dynamic range.

Flutter echoes

These are produced by sound bouncing between parallel surfaces, with unpleasant audible results. A flutter echo is usually initiated by a percussive sound.


This specifies the “loudness” of the system before it starts to squeal and go into feedback. It is controlled by the distances from talker to mic, mic to loudspeaker, loudspeaker to listener, and listener to talker. Background noise is also relevant.

Feedback is avoidable. Let us calculate the potential and advise you how to avoid it.

Hum & buzz

Hum is usually due to poor wiring practice. If a circuit is constructed which connects from one earth point to another earth point of ‘different potential’, a large current flows between them. The effect is a low frequency hum through the sound system. A hum from a sound system is something that we are all familiar with. Most buzzes are also due to poor wiring technique and are usually caused by external interference imposing itself onto the audio signal.

Loudspeaker choice and locations

There are many options in loudspeaker type and their dispersion characteristics. The choice is critically linked to the results of acoustic measurements and design criteria.

Loudspeaker time alignment

Electricity travels approximately a million times faster than sound in air. The result can cause problems when you hear the loudspeaker before you hear the talker or other loudspeakers.


Reflections are sound waves that are bounced off reflective surfaces and when mixed with the direct sound can cause deterioration in the intelligibility of the sound. By plotting the time arrival of reflections we can calculate where they are coming from.

Reverberation time

The reverberation time of the room is critical to speech intelligibility and musical clarity. Reverberation time (RT60) is the time, measured in seconds, for the average sound in a room to decrease by 60 decibels after the sound source stops generating sound.

Room modes

See Standing Waves

Speech intelligibility

This is the big test. It measures how successful the overall system design has been in the given acoustic environment. The index applicable to sound systems is the STI-PA measurement which stands for Speech Transmission Index for Public Address systems and the test can be performed using suitable test equipment with results within about 15 to 20 seconds per room position. The index ranges from 0 to 1with 1 being perfect and an acceptable figure being generally accepted as >0.5.

Standing waves

Also called room modes, these are frequencies at which the room is naturally resonant. Room modes are typically set up at the lower frequencies and are exacerbated by parallel surfaces. Room modes will make the bass response seem muddy and a listener would hear uneven base response as the long wavelength sound waves add to or cancel each other out. Room modes can be broken up by suitably placed absorptive materials.

Training and documentation.

A good sound system can be nullified by poor operation so training is an essential ingredient. It is also important to any future understanding of the system or system maintenance that correct “as built” documentation is provided with the equipment or at the completion of the installation.