LOUDNESS COMPENSATION BY MICROPHONE
FREQUENCY RESPONSE AND MICROPHONE PLACEMENT
Bruce Bartlett, AES Member, Crown
International,
AES Working Group SC-04-04
INTRODUCTION
"Specs don't tell how a microphone sounds in the
real world!" This common complaint is being addressed by the AES working group, SC-04-04. The group was formed to
propose new microphone specifications that help users better determine whether
a microphone is suitable for certain applications. With these new
specifications, manufacturers can present information to users
that helps them choose a microphone best suited to their needs.
Abstract. Rock bands produce high SPLs
when heard live. But recordings of these bands are reproduced at much lower SPLs. Consequently, the reproduction may sound weak in the
bass and treble compared to the live band. Some microphones compensate for this
loss: they boost the lows with proximity effect (and nearfield
placement) and boost the highs with a "presence peak." These
microphones are often chosen as being the "best sounding" for
recording loud instruments. The author suggests a new specification that
quantifies this compensation.
Here is a recording situation that suggests the need
for a new specification
A typical rock band produces 110 dB SPL at the listener's ears. But a recording of this band is
typically played back at 85 dB SPL. As shown by
Fletcher & Munson and others, a program heard at a low SPL
sounds weaker in the bass and treble than the same program heard at a high SPL. So the reproduction (at 85 dB SPL)
sounds deficient in lows and highs compared to the original live event (at 110
dB SPL).
Some microphones partly compensate for this effect by
boosting the bass (with proximity effect) and the treble (with a "presence
peak"). These microphones are often chosen as being the "best
sounding" for recording loud instruments.
The author suggests a new specification that
quantifies this compensation. Such a specification may help users determine
whether a microphone will be useful to record loud music and reproduce it with
a realistic tonal
The new specification is called "Loudness
Compensation." It indicates how well a microphone compensates for the
subjective change in tonal balance between loud and quiet music levels. The
author suggests that this new specification be included in the revised
microphone standard IEC 268-4.
The loudness compensation spec is an overlay of two amplitude
/ frequency curves: (1) the microphone frequency response at 2 inches (5 cm)
from a 12-inch (30.5 cm) diameter loudspeaker (including any proximity effect),
and (2) the difference between Robinson-Dadson
equal-loudness contours at 110 phons and 80 phons. This preprint explains the need for such a
specification, and the reasons behind its parameters.
BACKGROUND: EQUAL-LOUDNESS CONTOURS
The proposed specification is based on the
equal-loudness contours, which are explained below.
A simple way to quantify the loudness of a sound is to
compare it with some standard sound-a 1000 Hz tone. The loudness level of a
tone at any other frequency is the SPL of a 1000 Hz
tone that sounds as loud as the other tone. The unit of loudness level is the phon [1 ].
For example, if a 100 Hz tone (at any SPL) sounds as loud as a 1 kHz tone at 90 dB SPL, the 100 Hz tone has a loudness level of 90 phons.
Fletcher and Munson, and others, made measurements to determine the loudness levels of pure tones (or narrow bands of noise) as a function of frequency and sound pressure level. The family of curves from these measurements is called the equal-loudness contours. Fletcher and Munson derived the equal-loudness contours in the following way
[1 ][2]:
A large group of listeners with normal hearing sat one
at a time in an anechoic chamber.
2. Each subject listened to test tones from a
loudspeaker directly in front
at a distance greater than 1 meter.
3. Loudspeaker playback was set to a certain SPL at 1 kHz.
4. Using a volume control, each
subject matched the perceived loudness of the 1 kHz tone with the perceived
loudness of tones of other frequencies. The amplitude compensation in dB for
each frequency was noted.
The sound pressure levels of the tones were measured
with the listener out of the sound field, at a point corresponding to the center of the head.
This procedure was repeated at several phon levels from 0 to 120 phons.
The results for all the listeners were averaged.
The results are shown in Figure 1, Equal=Loudness Contours (FletcherMunson, 1933) [3].
Figure 2 shows the same data taken by Robinson and Dadson in 1956 [4]. In 1986 these curves became the International Standard ISO 226.
Figure 3
shows the equal-loudness contours calculated
Each phon value describes a
contour, or curve, of equal loudness as a function of frequency. When the SPL at frequencies from 20 Hz to 12.5 kHz is adjusted to
follow a phon contour, the typical listener hears all
frequencies to be equal in loudness to the 1000 Hz reference tone. At the
reference frequency of 1 kHz, the loudness level in phons
is the same as the actual value of SPL.
The equal-loudness contours show the subjective
frequency response of the human ear at various sound pressure levels.
CHANGE IN EQUAL-LOUDNESS CONTOURS AT
DIFFERENT SPLs
(SCALE DISTORTION)
Note that the contours at different phon levels are not parallel. In other words, the ear is
less sensitive to low frequencies and high frequencies (around 4 kHz) at low SPLs than at high SPLs.
This fact has implications for the tonal balance of
live vs. reproduced music. As the listening level is decreased, the listener
hears less bass and treble. Music played at a low SPL
sounds weaker in the bass and treble than the same music played at a high SPL. For example, we hear less bass and treble (around 4
kHz) at home listening levels than at live rock music levels.
M.G. Scroggie proposed a term "scale
distortion" for the change in subjective "frequency response"
(strictly "spectral distribution") that occurs when sound is
reproduced at other than its original level [6]. Scale distortion is another
term for the normalized difference in SPL vs.
frequency between two equal-loudness contours. Subjectively, it is the change
in perceived tonal balance at different average SPLs.
Suppose a rock band produces 110 dB SPL average at the listeners' ears. A recording is made of
this band using microphones with a flat frequency response. If the recording is
played back at 85 dB SPL-atypical home listening
level-the reproduction will lack bass and treble compared to the live band. In
other words, the reproduction will not sound as "punchy" as the live
band, because we hear tonal balance differently at different loudness levels
That is the theory, and it has also been the author's
experience. Rock bands recorded with flat-response microphones sounded dull in
the upper midrange and weak in the bass compared to the live band.
For simplicity, the author suggests that the proposed
specification be based on just two SPLs or phon levels: 110 dB and 80 dB. The 110 dB contour
represents a live rock-band level, and the 80 dB SPL
contour represents a typical home playback level.
How were these figures derived? To determine a typical
live rock-band level, the author researched various sources and made
measurements.
Listed below are some typical SPLs
of loud live music: 'Orchestra playing triple forte at 30 feet: 100 to 130 dB SPL [7]. '4 trumpets playing triple forte at 5 feet: 90 to
118 dB SPL [7]. 'Piano at 4 feet: 85 to 110 dB SPL [7]
'Loud
parts at a rock concert: 110 d8 SPL A-weighted (distance not given) [8].
'Disco with a decent sound system: 110 dB SPL
A-weighted (distance not given) [8].
'Drum set played hard: 108 dB SPL
unweighted at 10 feet.
'Small practice guitar amplifier played at full
volume: 102 dB SPL unweighted
at 10 feet.
The last two values were measured by the author using
a Bruel & Kjaer'/V
free-field microphone and a Crown TEF-20 sound analyzer running RTA software.
Based on all these measurements, the typical SPL of live rock-band instruments appears to be around 110 dB SPL.
A typical home listening level is 85 dB SPL [9]. Because there is no equalloudness
contour for 85 dB SPL, the author suggests that the 80
dB SPL
So far we have determined two useful phon levels for the loudness compensation specification.
Using the equal-loudness contours, the author calculated the dB difference vs. frequency between the 110 phon contour and the 80 phon contour. This difference curve was normalized at 1 kHz. Results are shown based on the Fletcher-Munson contours (Fig. 4),
The general trend is a low-frequency rise below 100 Hz
(+7.5 dB at 30 Hz) and a 4 dB rise centered at 3 to 4
kHz. In theory, that is the equalization required to make a program reproduced
at 80 dB SPL match the tonal balance of the same
program heard at 110 dB SPL. Note that the difference
vs. frequency for the
COMPENSATING
FOR TONAL DIFFERENCES AT DIFFERENT SPLs
To compensate for the subjective loss in lows and
upper-mids at low listening levels, recording
engineers may use any or all of the following methods:
'Use equalization to boost low and high frequencies.
'Place each microphone near the sound source so as to
pick up stronger lows or highs.
'Choose microphones with boosted lows and highs in
their frequency
response.
In the last method, the boosted lows come from
proximity effect, and the boosted highs come from a "presence peak"
in the microphone's frequency response-typically a rise of several dB around 4
to 6 kHz.
Proximity effect occurs in pressure-gradient
(directional) microphones with a "single-D" design. In this design,
all the rear sound entries are the same distance from the microphone diaphragm.
Proximity effect is the progressive rise in
low-frequency response as the microphone is placed closer to the sound source.
For example, most cardioid microphones sound more bassy close to the mouth than
far from the mouth.
As the microphone is placed closer to a point source,
the wavefront changes gradually from plane to
spherical.
Proximity effect increases as the wavefront becomes
more spherical.
The bass and treble boost of a microphone with
proximity effect and a presence peak compensates somewhat for the subjective
change in the ears' frequency response when the playback level is less than the
original live level. Hence, microphones with proximity effect and a presence
peak are very popular on loud instruments such as drums and guitar amplifiers.
The microphones have a contoured frequency response that makes the reproduced
instruments sound more realistic or natural than would microphones with a flat
frequency response.
Let us state this another
way. In loud rock music, a typical sound pressure level at the listener is 110
dB SPL. But recordings of this music are typically
played back at 85 dB SPL. The difference in
equal-loudness contours between 85 and 110 dB SPL is
a "smile-shaped" curve: rising low frequencies and rising high
frequencies (around 4 kHz). Recording engineers get this response curve when
they close-mike with a pressure gradient microphone having a presence peak.
These microphones with "smile-shaped" frequency-response curves are
often picked as the best sounding [10].
To summarize, the effect of scale distortion is a
perceived loss in bass and treble frequencies when playback level is less than
the original live level. Compensation for scale distortion is a complementary
bass and treble boost. In a microphone, this boost comes from proximity effect
and a presence peak.
For several years, many consumer preamplifiers
included loudness controls which boosted the bass (and sometimes the treble)
for low-level listening. A typical boost is 20 dB at 100 Hz relative to 1 kHz
[2]. Fig. 7 shows the frequency response of the IRC loudness control Model LC-1
[11 ].
A SUGGGESTED
NEW SPECIFICATION
The author suggests that the microphone industry
create a new microphone specification-loudness compensation-that indicates how
well a microphone compensates for the subjective change in tonal balance
between loud and quiet music levels.
Such a specification may suggest how well a microphone
can record loud music and reproduce it with a realistic tonal balance at
less-than-live SPLs The new specification may answer
such questions as, "Will this microphone sound good on loud guitar amps
and drums?" "Will the tonal balance conveyed by this microphone at
normal listening levels sound like the tonal balance heard at a live
concert?"
The current specification for microphone frequency
response is not useful for that purpose. Typically, microphone frequency
response is measured in (approximately) a plane wave at 2 feet or 1 meter from
the sound source. But in real-world studios, the miking
distance is usually just a few inches or centimeters,
so proximity effect is usually part of the microphone's "sound."
Also, the frequency-response graph in current data
sheets does not show the frequency compensation needed to give loud sounds a
natural tonal balance at a typical home listening level.
The proposed specification includes three elements
The usual dB vs. frequency grid with a logarithmic
frequency scale.
2. The frequency response of the microphone measured
at 2 inches [5 cm] from a 12-inch (30.5 cm) diameter loudspeaker in order to
include proximity effect.
3. An overlay of the difference between two Robinson-Dadson equalloudness contours:
110 phons and 80 phons.
Figure 8 is an example of such a specification.
4 THE REASONS FOR EACH SPECIFICATION
PARAMETER
The suggested specification includes parameters for
the mic-to-source distance, sound source, and
difference between two equal-loudness contours. This section explains why each
parameter was chosen.
4.1 Mic-to-source distance
In a typical popular-music recording, the recording
engineer attempts to reduce pickup of room acoustics and leakage. To do this,
microphones are placed a few inches or centimeters
from their sound sources. Also, the engineer often uses cardioid
microphones, most of which have proximity effect (up-close bass boost).
To measure a microphone for this specification, I
suggest that the measurement be done with the microphone grille 2 inches (5 cm)
from the sound source. 2 inches (5 cm) is a typical mic-to-source
distance for recording guitar amplifiers and drums. For example, a microphone
is typically placed 2 inches (5 cm) from a guitar-amp loudspeaker (nearly
touching the grille cloth), and 2 inches (5 cm) from a drum head.
The proposed specification dictates a standard test
distance of 2 inches (5 cm). But the manufacturer also can mention test results
for other distances that are closer to the product's design center
distance. For example, if a microphone is likely to be used'/, inch (0.6 cm)
from the mouth, the manufacturer may want to show the response curve at that
distance as well as 2 inches (5 cm).
4.2 Sound Source
An artificial mouth is one simulation of a real-world
sound source. Some standard artificial mouths are IEEE 269, CCITT
51.1, and ITU-T. The latte is a 3-inch (7.6 cm)
diameter cone speaker in a small sealed housing having a defined geometry and
reference mic position. It makes nearly a point
source below a few hundred Hz [12]. The IEC P.51
standard specifies an artificial mouth.
In the real world, loud instruments are seldom point
sources.
Guitar -amplifier loudspeakers, cymbals, and drum heads are
approximately
Although it is not a point source, a loudspeaker does
produce some proximity effect .To illustrate, Figs. 9 and 10 show the proximity effect
of a microphone near a point source and near a loudspeaker,
respectively.
Fig.9 shows the frequency
response of a cardioid microphone 1.5 inches [~4 cm]
from an artificial mouth, referenced to a flat-reponse
pressure microphone. Fig. 10 is the same, but 1.5 inches from a 12-inch (30.5 c
diameter guitar loudspeaker. Results: The piston-like source produces some
proximity effect, but less than the point-like source.
Figs. 9 and 10 mentioned above are for a cardioid dynamic microphone with a bass rolloff
in its free-field frequency response. Figs. 11 and 12 the
same for a different cardioid dynamic microphone.
Figs. 13 and 14 the same for a cardioid condenser
microphone with a flat low-frequency
As will be shown later, the proximity effect produced
by this loudspeake correlates more closely with scale
distortion than the proximity effect produced by the artificial mouth. The
artificial mouth produces much m proximity effect than real-world sound
sources, such as loudspeakers a drum heads. For this reason, the author
suggests that the specification include the
amplitude/frequency response of the microphone 2 inches cm) from a 12-inch
(30.5 cm) diameter loudspeaker, referenced to the amplitude/frequency response
of a flat-response pressure microphone the same position.
This sound source is similar to a typical
guitar-amplifier loudspeaker or drum head. It is a repeatable sound source that
generates a partly spherical wavefront, which in turn
may generate proximity effect in the microphone under test.
Because a 30 cm diameter
loudspeaker does not reproduce high
Above 1000
Hz, the measurement can be made using a wide-range
loudspeaker at 2 feet (61 cm) to 1 meter, and the two curves can
be spliced together.
Musical instruments are not recommended as sound
sources for this specification. The radiation characteristics of the source
affect the response curve because the amount of proximity effect depends on how
curved the wavefronts are. For many musical
instruments, wavefront curvature varies substantially
with frequency [13J. Because musicalinstrument sound
sources are not repeatable, they are not recommended.
4.3 Overlay of Scale Distortion
So far, this specification includes a grid and a
frequency response curve taken at 2 inches (5 cm) from a 12-inch (30.5 cm)
diameter loudspeaker. On this grid, overlay the difference in equal-loudness
contours (scale distortion) between 110 dB SPL and 80
dB SPL. As stated earlier, 110 dB SPL
is a typical level of live rock music at a typical listener's position. 80 dB SPL is a typical home listening level for elements within
an 85 dB SPL mix.
The Robinson-Dadson contours
should be used because they are the current standard. The differences between
the contours derived by
If the frequency response of the microphone approximates the difference in equal-loudness contours between 110 phons and 80 phons, then the microphone has some loudness compensation. In theory, the microphone would be a good choice for recording guitar amps, drums, or other loud sources.
The suggested name of the specification is
"Loudness Compensation (110 dB SPU80
dB SPL)." The manufacturer has the option of
also presenting the loudness compensation for other SPLs
that may be encountered in the intended application.
LOUDNESS COMPENSATION OF TYPICAL
MICROPHONES
Figs. 15, 16, and 17 show the loudness compensation of
three commercially available microphones. Each figure is an overlay of the
loudness compensation curve (110 dB/80 dB, Robinson-Dadson)
and the amplitude/frequency response of the microphone. These are the
microphones tested:
Fig. 15. A cardioid
condenser microphone with an essentially flat free-field response at 2 feet (61
cm).
Fig. 16. A cardioid dynamic (moving
coil) microphone whose frequency response at 2 feet (61 cm) shows a low-end rolloff and a presence peak
Fig. 17. Another cardioid
dynamic microphone with a. fairly similar frequency response as used in Fig.
16.
Below 1000 Hz, the amplitude/frequency response of the
three microphones was measured 1.5 inches (4
cm) from the center of a 12-inch (30.5 cm)
diameter loudspeaker. For high-frequency accuracy above 1000 Hz, the response
was measured 2 feet (61 cm) from a coaxial loudspeaker, approximately in the
free field.
Figs. 18, 19, and 20 correspond to 15, 16, and 17, but using the FletcherMunson equal-loudness contours.
Results: The cardioid
microphones with a presence peak show a bass and treble rise in their frequency
response, which is the same trend as the scale distortion. However, the match
between the two curves is not exact. The response of the flat-response
condenser microphone matches the scale distortion fairly well at low
frequencies, but not in the upper midrange. The cardioid
dynamic microphones exhibit a bass and treble rise, but the bass rise and
treble rise occur at higher frequencies than the scale-distortion rises.
From these results, the user can get an idea of how
well each microphone would reproduce loud instruments at a normal listening
level. The more closely matched the curves are, the more tonally accurate the
reproduced instrument should sound.
Another way of showing this data is to difference the microphone frequency response from the scale distortion. If the microphone has perfect loudness compensation, the difference curve would be flat. Such a curve could be called "Subjective frequency response for loud sounds," or "Deviation from loudness compensation (110 dB/80 dB SPL)." If the curve is flat, the subjective tonal balance is natural, i.e., the same as the tonal balance heard at the original SPL.
Figs. 21, 22 and 23 show the difference in dB vs.
frequency between the microphone frequency response and scale distortion, for
each microphone, normalized at 1 kHz. In other words, the figures show the
subjective frequency response for loud sounds (three different microphones).
None of these curves is flat, but the deviation from flat is not extreme.
LOUDNESS COMPENSATION BY MICROPHONE
PLACEMENT
Proximity effect is not the only thing that causes
bass boost in a microphone signal. Certain microphone placements can boost the
bass as well.
Consider an electric-guitar loudspeaker with an open-back cabinet. At a distance from the loudspeaker, the rear wave from the speaker combines in opposite polarity with the front wave from the speaker, canceling low frequencies. Close to the speaker, the front wave is emphasized at the microphone, so that low frequencies are not canceled. Thus there is an apparent bass boost compared to the spectrum recorded at a distance.
To illustrate, Figure 24 shows the spectrum of a
guitar amplifier loudspeaker picked up by a pressure (omnidirectional)
microphone at 1 meter. It was fed pink noise and equalized flat. Figure 25
shows the spectrum with the microphone placed 2.54 cm (1 inch) from the grille clc at the center of the
loudspeaker cone. There is a sizable increase in bass
over that picked up at 1 m. This bass boost is not due to microphone proximity
effect since an omnidirectional microphone was used
to recon the spectra.
Note that most of the bass boost is below 100 Hz, like
the 110 dB/80 dB loudness compensation curve. So the author suggests that this
tonal effect, due to microphone placement near an open-back loudspeaker,
contributes more to loudness compensation than proximity effect. The necessary
bass boost is provided by placement in the speaker's nearfield
more so than by proximity effect.
Drums produce different spectral balances at different
playing levels. T is a physical effect, not a psychoacoustic one. As a tom-tom
is struck progressively harder, its spectrum becomes weaker in the lows
relative 1 the highs (Fig. 26). So a lot of bass boost is needed in the
microphone signal to make a loudly played tom-tom sound "full."
Close-miking a tom- tom
creates such a bass boost because the microphone is in the bassy
nearfield of the drum head, and also because its partially spherical
Compared to cymbals heard in front, cymbals miked overhead produce spectrum that is weaker in the highs
relative to the lows (Fig. 27). So overhead cymbal
microphones probably need some high-frequency boo: to sound as naturally bright
as cymbals heard out front. This boost can I in
the microphone frequency response or in the channel equalization.
As shown, microphone placement has a strong effect on
the recorded spectral balance, apart from proximity effect. Instrument
radiation patterns and the closeness of the mic to
parts of the instrument, create a different tone
quality in each microphone position (14). Since this effect varies highly with
each musical instrument, it cannot be included in the proposed new microphone
specification. But it is important to be aware of the effect.
7 CONCLUSION
Equal-loudness contours were derived by Fletcher &
Munson, Robinson & Dadson, and Moore et al. As these
contours show, we hear less bass and treble in a program reproduced at typical
listening levels (say, 85 dB SPL) than in the same
program heard live at, say, 110 dB SPL. Microphones
with proximity effect and a presence peak compensate approximately for these
bass and treble losses. That's one reason why
these microphones are popular for recording loud instruments such as guitar
amps and drums
Some key findings are listed below:
*The equal-loudness contours calculated by Moore et al
do not agree with experience. Although they predict that we hear bass more
weakly at low SPLs, they also predict that we hear
treble stronger at low SPLs-the opposite of
experience.
*A suggested typical SPL for
loud rock instruments is 110 dB SPL at the listener.
A suggested typical SPL for reproduced rock
instruments is 80 d8 SPL at the listener. Reasons are
given in this paper.
*The difference in the Robinson-Dadson
equal-loudness contours (scale distortion) at 110 phons
and 80 phons is a broad rise (up to 4 dB) centered at 3 kHz, plus a low-frequency rise below 100 Hz
(+7.5 dB at 30 Hz). In theory, that is the equalization (or the microphone
frequency response) needed to make a program reproduced at 80 dB SPL have the same balance as the same program heard live at
110 dB SPL.
*A cardioid microphone
measured near an artificial voice shows much more proximity effect than the
same microphone measured the same distance from a 12" (30.5 cm)
loudspeaker. Because loud instruments (guitar amps, drum heads) are shaped more
like loudspeakers than like point sources, a loudspeaker is a more realistic
sound source for testing loudness compensation for these loud sources.
*Typical cardioid dynamic mics with proximity effect and a presence peak show the
same general response trend as scale distortion. However, in the microphone
response, proximity effect near a loudspeaker tends to peak at 120 - 150 Hz,
not at 30 Hz as in scale distortion. Also, the presence peak tends to be at 5
kHz, not at 3 kHz as in scale distortion.
*Additional loudness compensation (bass boost) is due
to microphone placement in the nearfield of the
loudspeaker in an open-back guitar-amp cabinet, and in the nearfield
of a drum head. In fact, this microphone placement appears to be the main
factor in loudness compensation at low frequencies, more so than proximity
effect.
Whether or not a new specification comes out of this
research, it is helpful to know exactly what is happening to the tonal balance
at different SPLs, and how microphones (or mic placement) may compensate for this effect.
SUGGESTIONS FOR FURTHER RESEARCH
1. The equal-loudness contours were derived from pure
tones, not music.
The difference in tonal balance between loud and soft
music may not the same as the difference in tonal balance between loud and soft
tones. Experiments could be done in which listeners hear a piece of music
alternately at 110 dB SPL and 85 dB SPL, and equalize the 85 dB program to sound like the 110
dB program. That equalization would be the correct loudness compensation.
2. If a spec results from this research, we need to
find the best way to present the spec to the user. Some possibilities are:
"An overlay of two curves (mic
response and scale distortion)
'A single curve showing their
difference (a flat curve indicates correct compensation).
"A number
that indicates the correctness of the compensation (For example, Loudness
compensation: 8 on a scale of 10.)
In any case, we need to ask whether
a typical user can understand and use such a specification.
3. For an open-back guitar amplifier and a drum head,
loudness compensation at low frequencies appears to be mostly due to mic placement in the nearfield of
the sound source. Perhaps a more realistic
sound source for the proposed spec would be a loudspeaker
in an openback cabinet of specified dimensions.
19
9 ACKNOWLEDGEMENTS
The author thanks the members of the AES working group SC-04-04 wt contributed many helpful
ideas to this paper.
10 REFERENCES
[1] L. Beranek, Acoustics,
[2] J. Eargle, Sound Recording, New York: Van Nostrand Reinhold Company, pp. 34-35 (1976).
{3) H. Fletcher
and W. A. Munson, "Loudness, Its Definition, Measure and
Calculation," J. Acoust. Soc. Am., vol. 5, pp. 82-108 (1933).
[4] D. W. Robinson and R. S. Dadson,
"A Re-Determination of the Eq Loudness Relations
for Pure Tones," Brit. J. Appl. Phys., vol. 7, pp. 1 181 (1956).
[5] B. J. Moore, B. R. Glasberg,
and T. Baer, "A Model for the Predictic
Thresholds, Loudness, and Partial Loudness," J. Audio Eng. Soc., vc
no. 4, 1997 April.
[6] M.G. Scroggie,
"Scale Distortion," Wireless
World, Sept. 24, 1937 "Scale Distortion-Again," Wireless World,
Vol. LIV, No. 11, p. 392, 1948. Thanks to John Woodgate for supplying this reference.
[7] D. Rosmini, Teac Multitrack
Primer, TEAC Corporation, p. 11 (1971
[8] Live Sound! International magazine, July/August 1996
[9] D. Huber and R. Runstein,
Modern Recording Techniques, Fourth
20
[10] Email from David Josephson
to working group SC-04-04,
[11 ] H. Tremaine, Audio Cyclopedia,
[12] Email from David Josephson
to working group SC-04-04,
[13] Email from John Woodgate
to working group SC-04-04, May 1997. [14] B. Bartlett, "Tonal Effects of
Close Microphone Placement," Jour. Aud. Eng. Soc. Vol. 29 no. 10 (October 1981), pp.
726-738.