Download Projector Sound Measurement University of Windsor

Projector Sound Measurement
Author: Colin Novak, Ph.D., P.Eng.
Noise Vibration Harshness and Sound Quality (NVH-SQ)
January 2010
University of Windsor
www.nvhwindsor.ca
Projector sound measurement
Whitepaper
Executive Summary
Noise emissions for high powered projectors, and how they are
quantified, are not widely understood. Several measurement
procedures exist, but they do not adequately quantify these
emissions. The fundamental theory and application of the
relevant acoustic metrics are described using case results.
Sound pressure
level (dB)
Typical example
0 dB
Threshold of hearing
35 dB
Bedroom at night
40 dB
Library
Introduction
58 dB
Conversational speech
High power projectors are designed with cooling systems
to neutralize the heat generated by the lamps. One of
the challenges with a projector cooling system is that the
fans are a source of noise no matter how well they are
designed. For the manufacturers of high power projectors,
to reduce the noise emissions adds to performance and
compliance concerns. However, the biggest challenge
is that the methods for measuring and the numbers
used to describe the acoustic emissions are not widely
understood by the industry or its customers.
Through the presentation of the acoustic measurement
results for three similar projector models, the different
types of measurement metrics for quantifying noise and the
relevant standards are presented in this paper. A significant
aspect associated with noise emissions is missed if one only
considers the problem to be a one-dimensional sound level
issue. From a consumer’s perspective, the perceived quality
of the noise emitted takes precedence over what traditional
acoustical analysis techniques of projector fan noise may imply.
Here, the use of psychoacoustic or sound quality metrics
may be a more applicable analysis method as it provides the
quantification of these qualitative human impressions of noise.
66 dB
Business office
80 dB
Factory floor
110 dB
Stage front at rock concert
140 dB
25m from jet takeoff
Quantifying Projector Noise
Sound pressure level: Noise is the result of slight disturbances
in the ambient atmospheric pressure measured in Pascal (Pa).
The perceived magnitude in pressure variation is from 2 x 10 -5
Pa up to 200 Pa. The order of magnitude is 107 and represents
an inconveniently large order of values. The human ear does not
respond to sound pressure amplitude in a linear manner; but it
has been found that by using a logarithmic scale with units of
decibels (dB) it is easier to relate human response to sounds.
Using this scale, 0 dB represents the threshold of human hearing
and 140 dB in the region of irreversible physical damage to the
inner ear. Table 1. illustrates several sound pressure level (SPL)
values along with common examples relating to each level.
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Table 1. - Typical sound pressure levels for common human activities
It is also important to appreciate the perceived change in
loudness of a sound for a given change in sound pressure level.
Table 2. demonstrates that a change in sound level less than
3 dB is not perceptible by those with normal hearing abilities.
Change in sound
level (dB)
1-3
Change in perceived loudness
Barely perceptible
5
A noticeable difference
10
Twice (or half) as loud
15
Very large difference
20
Four times (or one quarter) as loud
Table 2. - Perceived loudness for corresponding change in sound level.
A change of 5 dB is a noticeable difference while a
10 dB increase in sound level is perceived as twice as
loud. From this, it can be determined that it is important
to always put measured sound level data into perspective
when evaluating and comparing noise sources.
Projector sound measurement
Whitepaper
The auditory field in Figure 1. illustrates the threshold limits
of the human auditory system. The solid line denotes, as a
lower limit, the threshold for a pure tone to be just audible.
Figure 1. - Range of the human auditory system illustrating
typical sounds with frequency and the nonlinear nature
of the frequency response in the human ear.
The upper dashed line represents the threshold of pain. If the
limit of damage risk is exceeded for a longer time, permanent
hearing loss may occur. Normal speech and music levels are
in the shaded areas, while higher levels require electronic
amplification. It is important to note that the human hearing
system is extremely nonlinear. This means that for sounds
that are very low or very high in frequency it is much more
difficult for the human ear to detect them. For example, a 20
Hz tone must have an sound pressure level of 51 dB to be
barely perceptible as opposed to a 1000 Hz tone which can just
be heard at 0 dB. A similar characteristic exists for the upper
frequency range of human hearing at approximately 18,000 Hz.
A-weighting correction: To account for the nonlinearity
of human hearing, a correction factor called the
A-weighting scale is often applied to sound pressure
level measurements. If used, the units of the sound
level are instead given as dBA. A-weighted sound level
measurements are preferred over linear sound pressure level
measurements for projector noise, as it better represents
the perceived sound level that is detected by a person.
When conducting projector noise measurements, the sound
field in which the projector is located must be given careful
consideration as the measured sound pressure level (dB) or the
sound level (dBA) will be influenced by the environment in which
the source is located. This means that the level measured in an
environment free of reflecting surfaces will be very different
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then the level measured in a reflective room. For example,
the sound pressure level of a source located in the corner of
a room can be as much as 9 dB greater than the same source
located in an open area. Because of this, sound measurements
should always be conducted within a controlled environment.
The most common of these are anechoic (no reflecting surface)
or semi-anechoic (one reflecting surface) rooms. These rooms
have wedged surfaces designed to absorb acoustic waves such
that only the radiating acoustic sound waves of the source are
measured. In the case of a semi-anechoic room the reflected
energy from the one reflecting surface is easily determined.
Sound power level: One of the inherent disadvantages
of quantifying the acoustic emissions of projector noise
using sound pressure level is that the levels can vary with
changes in distance between the source and the point of
measurement. The preferred approach is to measure and
report the sound power level. The sound power represents
the total acoustic power generated by a source and has the
advantage that it is an intrinsic property of the noise source
alone and is not influenced by the environment in which the
source is located or the space in which it is operated.
The sound power level is measured either by using a sound
intensity analyser or by measuring the sound pressure levels
all around the source in an anechoic room and calculating the
sound power level. The advantage of using a sound intensity
probe that complies with the measurement standard ISO 9614
is that measurements can be conducted in most sound fields
which removes the need for requiring a special room. The
disadvantage is that the equipment can be very expensive
and the measurements may take several hours to perform.
The alternative is to use the standard microphone method as
specified by ISO 3745 wherein the required tests are performed
in a semi-anechoic room where the floor is reflective and the
walls and ceiling absorb all radiating acoustic sound waves.
Projector sound measurement
Whitepaper
In this standard, the microphones are usually located on an
imaginary hemi-spherical surface at a distance of at least 1
meter from the noise source as illustrated in Figure 2.
at four locations – the front, rear and both sides – in both idle
and operating modes of the projector in a room simulating
semi-anechoic conditions. The procedure also details the
height and location for each of the microphones. The caveat
of this simple procedure is that the results do not fully reveal
all the acoustic characteristics of a projector including tonality
and frequency spectra of the noise. Unfortunately, the intent
of the measurement procedure can also be circumvented to
show satisfactory results for a poor performing product if the
projector design is such that the cooling fan intake and exhaust
ports are located away from the measurement locations as
sounds from such sources can be highly directional in nature.
Sound Quality
Figure 2. - Hemi-spherical microphone placement for measuring
and calculating projector sound power in a semi-anechoic room
Once the sound pressure levels are measured and
averaged together, the sound power level of the
projector can be calculated using Equation 1.
Equation 1: LW(Power)=LP(SPL)+20log(r)+11±correction_factors
This approach is very fast and very repeatable with little
chance for error. As such, it is the preferred method for
quantifying the physical noise emissions of projector
products. Irrespective of the measurement method
used, sound power ratings are typically approximately
11 dB greater than sound pressure levels. This is due to
a constant value within the relationship expression which
relates the sound pressure level to sound power level.
Japan Business Machine Makers Association: Another
procedure often followed for qualifying the noise emissions of
digital projectors is the “Guidelines for LCD Projector Measuring
Procedure and Measuring Conditions” established by the Japan
Business Machine Makers Association (JBMMA) - Data Projector
Committee . The general procedure here is to measure and
logarithmically average the A-weighted sound pressure level
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In the evaluation of the acoustic comfort of a sound,
fundamental quantities such as the acoustic sound pressure
and sound power level do not adequately represent the actual
hearing sensations perceived by the human cognitive system.
The science of psychoacoustics involves the quantitative
evaluation of these subjective sensations using sound quality
metrics. The application of sound quality metrics provides
a means to visualize the complicated relationship that exists
between the physical and perceptual acoustic values.
The most fundamental of the sound quality metrics is loudness
which is an ISO standardized metric that describes the human
perception of how loud a source is perceived as opposed
to a simply reported sound pressure level. The human
body, head and outer ear act as spatial and spectral filters
on an acoustic signal. The inner ear also imparts nonlinear
characteristics on a signal that are not reflected by a simple
sound pressure level measurement. The application of the
loudness metric in a measurement includes the effect of
temporal processing and audiological masking effects of
sounds across the frequency range. As a result, a reported
loudness value (having units of sones) for projector noise
emissions is a much better indication of how an end user will
perceive the acoustic desirability or harshness of the product.
There are other sound quality indicators that exist which
are good metrics for the subjective evaluation of projector
noise. Acoustic sharpness is an indicator of the annoyance
of the high frequency component of noise which can be a
significant characteristic of fan generated noise. Modulation
metrics, such as fluctuation strength and roughness, are
commonly used for applications when several cooling fans
interact with each other and produce pulsating or beating
sounds. Such sounds are very annoying but this acoustic
unpleasantness is not adequately represented by simple
sound pressure or sound power level measurements.
Projector sound measurement
Whitepaper
Case Study
In order to illustrate the measurement approaches described
above, a study of the acoustics of three projectors was performed
by the Noise Vibration Harshness and Sound Quality (NVH-SQ)
research group located at the University of Windsor.
Figure 3. - Christie HD10K-M (left) and Panasonic PT-DW1000U
(right) in the semi-anechoic room during testing.
For this investigation, acoustical measurements were conducted
on the Christie HD6K-M, Christie HD10K-M and Panasonic
PT-DW10000U projectors (Figure 3.). The A-weighted
sound power level for each projector was measured under
different power operating conditions. Sound pressure
level measurements were conducted in a semi-anechoic
environment for each of the four sides of the projectors
to determine the averaged A-weighted sound pressure
levels as per the JBMIA guideline. At the same time, the
sound quality metric of loudness was also measured.
The acoustical results of the various tests are presented in
Table 3. The Panasonic projector had a significant 10 dB
higher A-weighted sound power level compared to the
Christie HD6K-M projector and a 5 dB higher sound power
level compared to the Christie HD10K-M projector.
Projector
Average
sound
level
(dBA)
Maximum
loudness
(sones)
Sound
power
level
(dBA)
Christie HD6K-M
(full power,
dual lamp operation)
46.3
5.1
55.6
Christie HD10K-M
(full power,
dual lamp operation)
51.1
6.8
60.6
Panasonic PT-DW10000U
(full power)
50.9
8.8
65.5
Table 3. - Noise measurement results for projector models tested.
It was also found that the measured maximum loudness for
the Panasonic PT-DW10000U projector was appreciably
higher than the Christie HD6K-M projector, as well as the
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more powerful Christie HD10K-M for the operating conditions
evaluated. It should be noted that the Christie HD10K-M and
Panasonic PT-DW10000U are the most comparable models
as they both claim similar performance specifications in
term of light output. However, these two projectors showed
little difference in measured sound pressure levels but the
loudness and the sound power of the Christie projectors
was appreciably less than the Panasonic model.
The other important observation is that the average
sound levels of the Christie HD10K-M and Panasonic
machines were virtually identical (a 0.2 dB difference is
imperceptible to even the most well trained human ear as
shown in Table 2.) yet the maximum loudness and sound
power level of the Christie HD10K-M was lower.
In summary, it was found that while few differences were found
with the measured sound pressure levels of the tested projectors,
appreciable differences were realized between the measured
sound power and loudness levels with the Christie models
performing better than the Panasonic projector. This is significant
as it demonstrates that the reporting of sound levels may have
little significance in the overall quantification of projector noise
and that either the sound power or loudness metrics provide
more effective characterisitics of the overall sound emissions.
Conclusion
Noise emissions for high powered projectors and how it is
quantified is not widely understood by both the industry and
customers alike. An attempt is given above to demystify the
fundamental concepts of noise and measurement procedures.
Sound power level and sound quality metrics should be
paramount for the quantification of projector noise as they
provide the less ambiguous results and better correlation with
the perceptual qualities for an end user. This was demonstrated
by the measurement results for the three projectors for which
the measured average sound level for the Panasonic model
demonstrated adequate results but was shown not to be the best
performer from a sound quality or sound power perspective.