ASHRAE Journal - July 2013 - 17

Figure 1: NC curves (1997 ASHRAE Handbook—Fundamentals).

Figure 2: RC curves (1997 ASHRAE Handbook—Fundamentals).

each time until it reaches 8000 Hz in the eighth band. Although
manufacturers typically take readings across the full spectrum,
by industry convention only bands 2 – 7 are reported.
Certain octave bands are critical to the performance of certain products. For instance, fan-powered products produce
their highest sound levels in band 2 (125 Hz), although they
may shift into band 3 (250 Hz) at higher speeds. This low frequency noise can be difﬁcult to attenuate because the wavelengths are longer. Air outlets produce their highest sound
levels in bands 4 – 6 (500 – 2000 Hz). These bands are referred
to as the speech interference bands because these are the same
frequencies used for verbal communication. Although these
frequencies are more easily attenuated, quiet selections are
necessary to provide good speaking environments.

to collect test data at sound levels in the test chamber that are
at least 10 dB higher than the background level in each octave
band to prevent background interference. This condition is easy
to achieve with fan-powered products, but often requires test
runs at elevated airﬂows for quieter products such as air outlets.
Reverberant chambers are qualiﬁed and all sound measurements made in accordance with AHRI Standard 220-2012,
Reverberation Room Qualiﬁcation and Testing Procedures for
Determining Sound Power of HVAC Equipment. This standard
is referenced in both ASHRAE Standard 70-2006, Method
of Testing the Performance of Air Outlets and Air Inlets, and
ASHRAE Standard 130-2008, Methods of Testing Air Terminal Units. After terminal units have been tested, their performance is rated in accordance with AHRI Standard 880-2011,
Performance Rating of Air Terminals.
To determine the sound power performance of a product,
tests must be conducted using a device known as a reference
sound source (RSS). This device is certiﬁed each year by its
manufacturer in accordance with AHRI Standard 250-2008,
Performance and Calibration of Reference Sound Sources. It
creates a relatively ﬂat sound spectrum and is used to determine the room attenuation in each octave band. This is done
by measuring the sound pressure in each octave band and subtracting it from the certiﬁed sound power. This octave band
attenuation will remain constant regardless of the magnitude
of the sound levels produced in the test chamber. Because all
sound power determinations are made in comparison to the
RSS, this is known as the substitution method of testing.

Sound Power Determination
To determine acoustical performance, manufacturers test
their products in the controlled environment of a test chamber. Two types of rooms are used for measuring equipment
sound levels: anechoic and reverberant chambers. Anechoic
chambers are used for testing loud products while reverberant chambers are used for testing relatively quiet products like
diffusers, terminal units and room fan coils. The reverberant
chamber provides an ideal acoustical test environment of high
isolation and low absorption.
It is important to conduct tests in a chamber that provides
high isolation from background noise to eliminate complications due to background interference. The typical procedure is
July 2013

ASHRAE Journal

ASHRAE Journal - July 2013

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