Acronym Definition
PNV Park 'n View (trucker's term for park in view at truckstops)
PNV Partido Nacionalista Vasco (Basque Political Party)
PNV Potential Network Value
PNV Prenatal Vitamins
Psychoacoustic Noise, Vibration, and Harshness
Acoustical measurements and instrumentation
Acoustics
Digital signal processing
Resonance
Sound pressure level
Vibration
Vibration isolation
Psychoacoustic Noise, Vibration, and Harshness, also known as Noise and Vibration,
abbreviated to NVH and N&V respectively, is the name given to the field of
measuring, and modifying, the noise and vibration characteristics of vehicles,
particularly cars and trucks. Harshness is a bit of a historical misnomer.
Noise, and vibration, can be measured, but harshness is more of a subjective
assessment. There is a psychoacoustic measurement called harshness but it does
not correlate very well with many harshness issues.
Interior NVH is the noises and vibration experienced by the occupants of the
cabin, while exterior NVH is largely concerned with the noise radiated by the
vehicle, and includes drive-by noise testing.
NVH is mostly engineering, but often objective measurements fail to predict or
correlate well with the subjective impression on human observers. This is partly
because the human body has its own frequency response, eg the ear's response is
approximated by A weighting, but this does not mean that two noises with the
same A-weighted level are equally disturbing. The field of psychoacoustics is
partly concerned with this correlation.
In some cases the NVH engineer is asked to change the sound quality, ie adding
or subtracting particular harmonics, rather than making the car quieter.
Sources of NVH
The sources of noise in a vehicle are many, including the engine, driveline,
tire contact patch and road surface, brakes, and wind. Noise from cooling fans,
or the HVAC, and alternators, is also fairly common. Many problems are generated
as vibrations, transmitted via a variety of paths, and then radiated
acoustically into the cabin. Others are generated acoustically, and this is then
attenuated via various barriers until it excites the cabin. Vibrations are
sensed at the steering wheel, the seat or the floor and pedals. Some problems
are sensed visually - such as the vibration of the header rail or rear view
mirror on open topped cars.
Tonal vs Broadband
NVH can be tonal, such as engine noise, or broadband, such as road noise or wind
noise, normally. Some resonant systems respond at characteristic frequencies,
but in response to random excitation. Therefore, although they look like tonal
problems on any one spectrum, their amplitude varies considerably. Other
problems are self resonant, such as whistles from antennas.
Tonal noises often have harmonics. Here is the noise spectrum of Michael
Schumacher's Ferrari at 16680 rpm, showing the various harmonics. The x axis is
given in terms of multiples of engine speed. The y axis is logarithmic, and
uncalibrated.
more check out Instrumentation
Psychoacoustics is the study of subjective human perception of sounds.
Alternatively it can be described as the study of the psychological correlates
of the physical parameters of acoustics.
Background
Hearing is not a purely mechanical phenomenon of wave propagation, but is also a
sensory and perceptual event. When a person hears something, that something
arrives at the ear as a mechanical sound wave traveling through the air, but
within the ear it is transformed into neural action potentials. These nerve
pulses then travel to the brain where they are perceived. Hence, in many
problems in acoustics, such as for audio processing, it is advantageous to take
into account not just the mechanics of the environment, but also the fact that
both the ear and the brain are involved in a person’s listening experience.
The ear for example, takes a spectral decomposition of sound as part of the
process of turning sound into neural stimulus, so certain time domain effects
are inaudible. MP3 compression makes use of this fact. In addition the ear has a
logarithmic dynamic response. Telephone networks make use of this fact by
logarithmically compressing data samples before transmission, and then
exponentially expanding them for playback. Another side effect of the ear’s non
linear logarithmic response is that sounds which appear on the ear drum in close
spectral proximity produce phantom beat notes. This is the same principle that
is used for down conversion of carrier frequencies in radio front ends by a
non-linear amplifier. Such physiological effects due to the ear’s anatomy are
properly called physiology-acoustic effects, though people commonly lump them in
with psycho-acoustic effects.
There are true psycho-acoustic effects introduced by the brain. For example,
when a person listens to crackly and needle-on-vinyl hiss-filled records, he or
she soon stops noticing the background noise, and enjoys the music. A person who
does this habitually appears to forget about the noise altogether, and may not
be able to tell you after listening if there was noise present. This effect is
called psycho-acoustical masking. The brain’s ability to perform such masking
has been important for the adoption of a number of technologies; though in this
age of digital signaling and high fidelity playback the effect is typically used
to hide losses in compression rather than to cover up analog white noise. As
another example of a psycho-acoustic effect, the brain appears to use a
correlative process for pattern recognition; much like is done in electronic
circuits that look for signal patterns. When the threshold for acceptance of a
correlative match is very low a person may perceive hearing a sought after
pattern in pure noise or among sounds that are somewhat indicative, as the brain
fills in the rest of the pattern. This is a psycho-acoustic phantom effect. For
example when a radio operator is straining to hear a weak Morse Code signal in a
noisy background, he or she often perceives hearing the pitch of tiny dots and
dashes even when they are not present. In general psycho-acoustic phantom
effects play an important role in any environment where people have heightened
perceptions, such as when danger may be perceived to be near. (There is an
analogous visual effect experienced by people standing watch in very dark
places.) The psycho-acoustic phantom effect is conceptually distinct from
hallucination, where the brain auto generates perceptions. Also, the
psycho-acoustic phantom effect is distinct from the physiology-acoustic phantom
effect. It is the estimation of masking threshold level.
Limits of perception
The human ear can nominally hear sounds in the range 20 Hz to 20,000 Hz (20
kHz). This upper limit tends to decrease with age, most adults being unable to
hear above 16 kHz. The ear itself does not respond to frequencies below 20 Hz,
but these can be perceived via the body's sense of touch. (Some recent research
has demonstrated a hypersonic effect which is that although sounds above 20 kHz
cannot consciously be heard, they can have an effect on the listener.)
Frequency resolution of the ear is, in the middle range, about 2 Hz. That is,
changes in pitch larger than 2 Hz can be perceived. However, even smaller pitch
differences can be perceived through other means. For example, the interference
of two pitches can often be heard as a (low-)frequency difference pitch. This
effect of phase variance upon the resultant sound is known as 'beating'.
However, the effect of frequency on the human ear has a logarithmic basis. In
other words, the perceived pitch of a sound is related to the frequency as an
exponential function. The 12-tone musical scale is an example of this; it
evolved due to the way tones are perceived. When the fundamental frequency of a
note or tone is multiplied by approximately (this factor is true in the average,
but varies slightly depending on the tuning), the result is the frequency of the
next higher semitone. Going 12 notes higher — an octave — is the same as
multiplying the frequency by , which is the same as doubling the frequency.
The impact of this is that the semitone scale used in Western musical notation
is not a linear frequency scale but logarithmic. Other scales have been derived
directly from experiments on human hearing perception, such as the Mel scale and
Bark scale (these are used in studying perception, but not usually in musical
composition), and these are approximately logarithmic in frequency as well.
The "intensity" range of audible sounds is enormous. Our ear drums are sensitive
only to the sound pressure variation. The lower limit of audibility is defined
to 0 dB, but the upper limit is not as clearly defined. The upper limit is more
a question of the limit where the ear will be physically harmed or with the
potential to cause a hearing disability. This limit depends also on the time
exposed to the sound. The ear can be exposed to short periods in excess of 120
dB without permanent harm — albeit with discomfort and possibly pain; but long
term exposure to sound levels over 80 dB can cause permanent hearing loss.
A more rigorous exploration of the lower limits of audibility determines that
the minimum threshold at which a sound can be heard is frequency dependent. By
measuring this minimum intensity for testing tones of various frequencies, a
frequency dependent Absolute Threshold of Hearing (ATH) curve may be derived.
Typically, the ear shows a peak of sensitivity (i.e., its lowest ATH) between 1
kHz and 5 kHz, though the threshold changes with age, with older ears showing
decreased sensitivity above 2 kHz.
The ATH is the lowest of the equal-loudness contours. Equal-loudness contours
indicate the sound pressure level (dB), over the range of audible frequencies,
which are perceived as being of equal loudness. Equal-loudness contours were
first measured by Fletcher and Munson at Bell Labs in 1933 using pure tones
reproduced via headphones, and the data they collected are called
Fletcher-Munson curves. Because subjective loudness was difficult to measure,
the Fletcher-Munson curves were averaged over many subjects.
Robinson and Dadson refined the process in 1956 to obtain a new set of
equal-loudness curves for a frontal sound source measured in an anechoic
chamber. The Robinson-Dadson curves were standardized as ISO 226 in 1986. In
2003, ISO 226 was revised as equal-loudness contour using data collected from 12
international studies.
Interpretation of sound
Human hearing is basically like a spectrum analyzer, that is, the ear resolves
the spectral content of the pressure wave without respect to the phase of the
signal. In practice, though, some phase information can be perceived.
Inter-aural phase difference, that is the difference in sound between the ears,
is a notable exception by providing a significant part of the directional
sensation of sound. The filtering effects of head-related transfer functions
provide another important directional cue.
Masking effects
Main article: Auditory masking
In some situations an otherwise clearly audible sound can be masked by another
sound. For example, conversation at a bus stop can be completely impossible if a
loud bus is driving past. This phenomenon is called masking. A weaker sound is
masked if it is made inaudible in the presence of a louder sound. The masking
phenomenon occurs because any loud sound will distort the Absolute Threshold of
Hearing, making quieter, otherwise perceptible sounds inaudible.
If two sounds occur simultaneously and one is masked by the other, this is
referred to as simultaneous masking. Simultaneous masking is also sometimes
called frequency masking. The tonality of a sound partially determines its
ability to mask other sounds. A sinusoidal masker, for example, requires a
higher intensity to mask a noise-like maskee than a loud noise-like masker does
to mask a sinusoid. Computer models which calculate the masking caused by sounds
must therefore classify their individual spectral peaks according to their
tonality.
Similarly, a weak sound emitted soon after the end of a louder sound is masked
by the louder sound. Even a weak sound just before a louder sound can be masked
by the louder sound. These two effects are called forward and backward temporal
masking, respectively.
'Phantom' fundamentals
At the lower end of the ears' response, low notes can sometimes be heard when
there is no sound at that frequency. This is due to the brain synthesising the
low frequency sound from the differences of audible harmonics that are present.
This effect is used in some commercial sound systems to give the effect of
extended low frequency response when the system itself cannot reproduce that
frequency adequately. See missing fundamental.
Psychoacoustics in software
The psychoacoustic model provides for high quality lossy signal compression by
describing which parts of a given digital audio signal can be removed (or
aggressively compressed) safely - that is, without significant losses in the
(consciously) perceived quality of the sound.
It can explain how a sharp clap of the hands might seem painfully loud in a
quiet library, but is hardly noticeable after a car backfires on a busy, urban
street. This provides great benefit to the overall compression ratio, and
psychoacoustic analysis routinely leads to compressed music files that are 1/10
to 1/12 the size of high quality original masters with very little discernible
loss in quality. Such compression is a feature of nearly all modern audio
compression formats. Some of these formats include MP3, Ogg Vorbis, WMA, Musicam
(used for digital audio broadcasting in several countries) and ATRAC, the
compression used in MiniDisc and walkman.
Psychoacoustics is based heavily on human anatomy, especially the ear's
limitations in perceiving sound as outlined previously. To summarize, these
limitations are:
* High frequency limit
* Absolute threshold of hearing
* Temporal masking
* Simultaneous masking
Given that the ear will not be at peak perceptive capacity when dealing with
these limitations, a compression algorithm can assign a lower priority to sounds
outside the range of human hearing. By carefully shifting bits away from the
unimportant components and toward the important ones, the algorithm ensures that
the sounds a listener can hear most clearly are of the highest quality.
Psychoacoustics and music
Psychoacoustics include topics and studies which are relevant to music
psychology. Theorists such as Benjamin Boretz consider some of the results of
psychoacoustics to be meaningful only in a musical context.
Applied psychoacoustics
Psychoacoustics is presently applied within many fields from software
development where developers map proven and experimental mathematical patterns;
in the design of (high end) audio systems for accurate reproduction of music in
theatres and homes, as well as defense systems where scientists have the
capability to create new acoustic weapons {some of which emit frequencies that
may impair, harm, or kill [with very limited success . It is also applied today
within music, where musicians and artists continue to create new sonic
sensory-breaking perceptions of sonic reality by masking unwanted frequencies of
instrument while others are enhanced by the absence of the masked ones. Yet
another application is to provide listeners of small loudspeakers the impression
that they hear low notes by masking what is subsonic and enhancing what is
perceived as low the frequencies.
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