Test and Measurement 1617among contractors and consultants, who must often
perform sound system calibrations in the real world of air
conditioners, vacuum cleaners, and building occupants.
While other measurement methods exist, the ones
outlined above make up the majority of methods used
for field and lab testing of loudspeakers and rooms. Used
properly, any of the methods can provide accurate and
repeatable measured data. Many audio professionals
have several measurement platforms and exploit the
strong points of each when measuring a sound system.
46.3.5 Preparation
There are many measurements that can be performed on
a sound system. A prerequisite to any measurement is to
answer the following questions:
- What am I trying to measure?
- Why am I trying to measure it?
- Is it audible?
- Is it relevant?
Failure to consider these questions can lead to hours
of wasted time and a hard drive full of meaningless
data. Even with the incredible technologies that we have
available to us, the first part of any measurement
session is to listen. It can take many hours to determine
what needs to be measured to solve a sound system
problem, yet the actual measurement itself can often be
completed in seconds. Using an analogy from the
medical field, the physician must query the patient at
length to narrow down the ailment. The more that is
known about the ailment, the more specific and relevant
the tests that can be run for diagnosis. There is no need
to test for tonsillitis if the problem is a sore back!
- What am I measuring? A fundamental decision that
precedes a meaningful measurement is how much
of the room’s response to include in the measured
data. Modern measurement systems have the ability
to perform semianechoic measurements, and the
measurer must decide if the loudspeaker, the room,
or the combination needs to be measured. If one is
diagnosing loudspeaker ailments, there is little
reason to select a time window long enough to
include the effects of late reflections and reverbera-
tion. A properly selected time window can isolate
the direct field of the loudspeaker and allow its
response to be evaluated independently of the room.
If one is trying to measure the total decay time of
the room, the direct sound field becomes less
important, and a microphone placement and time
window are selected to capture the entire energy
decay. Most modern measurement systems acquire
the complete impulse response, including the room
decay, so the choice of the time window size can be
made after the fact during post processing.- Why am I measuring? There are several reasons for
performing acoustic measurements in a space. An
important reason for the system designer is to char-
acterize the listening environment. Is it dead? Is it
live? Is it reverberant? These questions must be
considered prior to the design of a sound system
for the space. While the human hearing system can
provide the answers to these questions, it cannot
document them and it is easily deceived. Measure-
ments might also be performed to document the
performance of an existing system prior to
performing changes or adding room treatment.
Customers sometimes forget how bad it once
sounded after a new or upgraded system is in place
for a few weeks.
The most common reason for performing
measurements on a system is for calibration
purposes. This can include equalization, signal
alignment, crossover selection, and a multiplicity
of other reasons. Since loudspeakers interact in a
complex way with their environment, the final
phase of any system installation is to verify system
performance by measurement. - Is it audible? Can I hear what I am trying to
measure? If one cannot hear an anomaly, there is
little reason to attempt to measure it. The human
hearing system is perhaps the best tool available for
determining what should be measured about a
sound system. The human hearing system can tell
us that something doesn’t sound right, but the
cause of the problem can be revealed by measure-
ment. Anything you can hear can be measured, and
once it is measured it can be quantified and
manipulated. - Is it relevant? Am I measuring something that is
worth measuring? If one is working for a client,
time is money. Measurements must be prioritized
to focus on audible problems. Endless hours can be
spent “chasing rabbits” by measuring details that
are of no importance to the client. This is not neces-
sarily a fruitless process, but it is one that should be
done on your own time. I have on several occasions
spent time measuring and documenting anomalies
that had nothing to do with the customer’s reason
for calling me. All venues have problems that the
owner is unaware of. Communication with the
client is the best way to avoid this pitfall.