|
|
本帖最后由 Jwang 于 2013-7-24 09:33 编辑
驻波的能量和驻波能够对外传送的能量, 这是两码事.
这里这种讲虽不合理,但至少有进步。我看了下百度的驻波的条文。我可以说这点。它那里说的驻波绝对不是我在谈的驻波,或许是海洋学中的某些现象。
这里在下述的前提下,我可同意你的讲法。
假定说”不能够对外传送的能量”是指不能听见,我不同意你的观点。
假定说”不能够对外传送的能量”但是能听见,我同意你的观点。
但 J版说驻波主要针对 300hz 以下的,不知何解?
这里我抄三段英文,它们是从下面三本书中的。
第一本是:
David M. Howard; James Angus:Acoustics and Psychoacoustics
Because all rooms have modes in their lower frequency ranges
there will always be a frequency below which the modal effects
dominate and the room can no longer be treated as diffuse. Even
anechoic rooms have lower frequency limits to their operation.
One of the effects of room modes is to cause variations in the
frequency response of the room, via its effect on the reverberant
field. The frequency response due to modal behaviour will also
be room position dependent, due to the spatial variation of
standing waves. An important consequence of this is that the
room no longer supports a diffuse field in the modal region and
so the reverberation time concept is invalid in this frequency
region. Instead an approach based on modal decay should be
used. But at what frequency does the transition occur, can it be
even calculated? Consider the typical frequency response of a
room, shown in Figure 6.39. In it, three different frequency
regions can be identified.
The cut-off region: the region below the lowest resonance,
sometimes called the room cut-off region. In this region the
room is smaller than a half wavelength in all dimensions.
This does not mean that the room does not support sound
propagation, in fact it behaves more like the air in a bicycle
pump when the end is blocked. This means that the environment
‘loads’ any sources of sound in the room differently
(such as loudspeakers or musical instruments), and often the
effect of this loading is to reduce the ability of the source to
radiate sound into the room and so result in reduced sound
levels at these frequencies. The low frequency cut-off can be
calculated simply from:
• The modal region: the next region is the modal region in
which the modal behaviour of the room dominates its
acoustic performance. In this region the analysis based on
the assumption of a diffuse field is doomed to fail.
• The diffuse field region: the final region is the region in which
a diffuse field can exist and therefore the concept of reverberation
time is valid. In general this region of the frequency
range is the one that will sound the best, providing the
reverberation characteristics are good, because the effects of
room modes are minimal and so the listener experiences an
even reverberant sound level throughout the room.
The transition boundary between the region of modal behaviour
and the region of diffuse behaviour is known as the critical
frequency. As is usual in these situations, although the critical
frequency is a single frequency it is not a sharp boundary, it
represents some defined point in a transition region between the
two regions.
第二本是:
Floyd Toole: Sound Reproduction, Loudspeakers and Rooms
Our understanding of these perceptual factors is not yet complete, but there
is a lot of information in the accumulated literature of architectural acoustics.
Complicating the situation is the fact that several of these effects can coexist,
interacting with each other, and that the relationships can be different, at least
in some degree, for different kinds of sounds. A lot of the pioneering work was
done using speech at the test signal and, although it is fundamentally important,
it is not the only sound we listen to. Similarly, many experiments examined the
effects of a single refl ection auditioned in an otherwise refl ection-free environment.
It will be found that some conclusions need to be modifi ed for normally
reflective spaces. When looking at the results of data gathered in “scientifi c”
circumstances, it is essential to think carefully before drawing conclusions about
what may or may not be important in real-world situations.
We know that in real rooms there are multiple refl ections. However, to
understand the infl uence of many, it is useful to begin by understanding the
infl uence of a few, or even one. It also makes experiments practical and controllable.
As will be seen, there is a logical progression of effects from a single to
multiple refl ections, giving us, in the end, a better insight into the perceptual
mechanisms at play.
All of the effects being discussed have portions of the frequency range over
which they are most noticeable. Figure 5.2 includes a repetition of Figure 4.12,
which illustrates that, in terms of physical acoustics, the frequency range is
divided into two regions connected by a broad transition zone. Under it is an
attempt to show the frequency ranges over which various audible effects of
refl ections are most likely to be heard. As we will see, these are very approximate
divisions, subject to variations with different program material, reproduced in
different environments, and so on. They will be shown at the beginning of each
relevant chapter and will be discussed at that point.
第三本是:
F.Alton Everest & Ken C Pohlmann: Master Handbook of Acoustics
The audible spectrum is very wide when viewed in terms of wavelength. At 16 Hz,
considered the low-frequency limit of the average human ear, the wavelength is
1,130/16 = 70.6 ft. At the upper limit of hearing, say 20 kHz, the wavelength is only
1,130/20,000 = 0.056 ft or about 0.7 in. The behavior of sound is greatly affected by the
wavelength of the sound in comparison to the size of objects encountered. In a room,
sound of 0.7-in wavelength is scattered (diffused) significantly by a wall irregularity of
a few inches. The effect of the same irregularity on sound of 70-ft wavelength would
be insignificantly small. The heart of the acoustical problem is that no single analytical
approach can cover sound of such a wide range of wavelengths.
In considering the acoustics of small rooms, the audible spectrum can be arbitrarily
divided into four regions: A, B, C, and D, as shown in Fig. 13-6. Room size determines
how the audible spectrum must be divided for acoustical analysis. Very small rooms,
with too few modal resonances spaced too far apart, are characterized by domination of
a great stretch of the audible spectrum by modal resonances.
Region A is the very-low-frequency region below a frequency of 1130/2L or 565/L,
where L is the longest dimension of the room. Below the frequency of this lowest axial
mode, there is no resonant support for sound in the room. This does not mean that such
very-low-frequency sound cannot exist in the room, only that it is not boosted by room
resonances because there are none in that region.
Region B is that region in which the dimensions of the room are comparable to the
wavelength of sound being considered. It is bounded on the low-frequency end by the
lowest axial mode, 565/L. The upper boundary is not definite but an approximation is
given by what has been called the cutoff or crossover frequency given by the equation:
where F2 = cutoff or crossover frequency, Hz
RT60 = reverberation time of the room, sec
V = volume of the room, ft3
Region C is a transition region between region B, in which wave acoustics must be used,
and region D in which ray acoustics are valid. It is bounded on the low-frequency end
approximately by the cutoff frequency F2 and on the high end approximately by F3 = 4F2.
This region is more difficult to analyze, dominated by wavelengths often too long for
ray acoustics and too short for wave acoustics.
Region D describes the spectral area above F3 that covers higher audible frequencies
with short wavelengths; geometric acoustics apply. Specular reflections (angle of incidence
equals angle of reflection) and the sound ray approach to acoustics prevail. In
this region statistical approaches are generally possible.
In summary, as an example, consider a room measuring 23.3 × 16 × 10 ft. Volume is
3,728 ft3, and reverberation time is 0.5 second. Region A is below 565/23.3 = 24.2 Hz.
There is no resonant boost for sound. Region B is between 24.2 and 130 Hz. The wave
acoustical approach of modal resonances is used to predict response. Region C is
between 130 Hz and (4)(130) = 520 Hz. This is a transitional region. Region D is above
520 Hz. The modal density is very high, statistical conditions generally prevail, and
geometrical acoustics can be used.
|
本帖子中包含更多资源
您需要登录才可以下载或查看附件。没有帐号？注册
x
|
发表于 2013-7-23 08:14:33
| 
发表于 2013-7-23 23:11:08
| 
提升卡
置顶卡
沉默卡
喧嚣卡
变色卡