Site surveys are a necessary part of all construction trades.

The ground must be firm enough to support the building. The water must drain properly.

There must be adequate ingress and egress. The list goes on.

Sound system designers should also perform a site survey of the auditorium to determine the conditions in which the new sound system must operate.

There are a multitude of parameters that can be measured in a building, but it won’t be possible, practical or even necessary to measure the majority of them.

As the list of essential measurements is trimmed, there remain three items that should always be addressed when sizing up a room for a sound system. Two of them require acoustical measurements and one requires an electromagnetic measurement.

The Acoustical Environment
There are several acoustical parameters that must be known about the room. These can be gathered with two measurements. An impulse response can answer the following questions:

• Is the room reverberant?
• Will low frequency modes be a problem?

The impulse response is usually plotted on a positive decibel vertical scale (log-squared response) or displayed as an energy-time curve (ETC).

This makes it easier to interpret relevant to the human listener.

Large-Room Characteristics
The time span of room decay gathered during the impulse response measurement should be longer than the time required for the stimulus to fully decay into the room’s noise floor (Figure 1).

This can range from less than one second to several seconds. At least one second is recommended as a minimum, as this will allow a frequency resolution of about 1 Hz to be observed in the frequency domain.

It’s always better to gather too long a time record than too short a time record.

Figures 1 & 2. Click to enlarge.

A room can be “live” but not be reverberant (Figure 2). In reverberant spaces, the time response will have a dense “tail” that decays over time (Figure 3).

The reverberant tail will be similar from point-to-point in the space (it’s a statistical room parameter), so a single impulse response measurement can characterize the entire space to the accuracy needed at the design stage. I like to pick a spot away from room boundaries but not exactly in the center of the room.

The purpose of this initial impulse response is to evaluate the late decay characteristics of the space. There’s probably no point in trying to analyze specific reflections at this time.

Figures 3 & 4. Click to enlarge.

These will vary dramatically with loudspeaker placement and aiming, as well as seating position and can be minimized by good design practice.

It may be necessary to evaluate them after the loudspeakers have been selected and installed to determine if and where room treatment is justified.

An acoustically large space is one in which the wavelengths of the sound waves are small relative to the spacing between surfaces. This is true of most rooms at high frequencies.

As frequency decreases, a different type of acoustical behavior occurs. The region of transition between large and small room behavior (critical frequency) can be revealed by the impulse response.

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