Morset Sound Development Sound System Calibration Example - Performing Monitor Calibration in...

Morset Sound Development

Sound System Calibration

Example - Performing

Monitor Calibration

in Studios


IntroductionIntroduction

• Setting up the measuring equipment• Ambient sound levels• The effect of reflections• Impulse and Frequency Responses• Acoustical problems in rooms• Reflection diagnosis & causes of reflections• Tone controls on different monitors• Calibration standard• Practical calibration of monitors• Subjective evaluation


Connecting the Measuring SystemConnecting the Measuring System

Laptop Computer

XLR LeftLine Output

Digigram VXpocketPCMCIA Sound Card

XLR RightOptional loop-back(for synchronisation)

XLR LeftMic. Input

AKG PhantomPower Supply

GENELECR

Neutrik 3382Microphone

Active Monitor

- example using VXpocket sound card and Neutrik mic.


Positioning the MicrophonePositioning the Microphone

• At the listening position

• At average ear height (1.4m)

• Vertically NOT horizontally

• On a good microphone stand


Why is the microphone angled at 90Why is the microphone angled at 90ºº


Ambient Sound LevelAmbient Sound Level(Signal-to-Noise Ratio)(Signal-to-Noise Ratio)

Amb ient Sound in Listening R oomTest Level vs Ambie nt Level

-4 0

-3 0

-2 0

-1 0

0

1 0

2 0

3 0

4 0

1 0 10 0 1, 00 0 1 0 ,00 0 1 0 0,0 0 0Fre qu e n cy, H z

Le vel, dB

E xa m p le S pe a ker Te st L e ve l

Am bie n t L eve l S a m pl e


The Effects of ReflectionsThe Effects of Reflections

0Time

0Freq

0Time

0Freq

td 1/td

Time domain Frequency domain


Subjective Perception of ReflectionsSubjective Perception of Reflections

Time, ms

Level, dB

0

-10

-20

-300 20 40 60 80

IMAGE SHIFT

ECHOS

USEFUL EARLY ENERGY

RE

VE

RB

ER

AT

ION

TO

NE

CO

LOU

RA

TIO

N

NO REFLECTIONS HEARD

0 6.8 13.6 20.4 27.2 Distance, m


Room Impulse ResponseRoom Impulse Response

t = 0Time

Input DirectSound

1st Reflection

Discrete Early Reflections

Noise Floor

Reverberant(Diffuse) Field

Time of Flight Initial Time

Gap, td

50-80 ms80+ ms


Typical Studio Impulse ResponseTypical Studio Impulse Response

Time Data - 1039AL reflections example

Name: ...\Measurements\1039AL reflections example.wmb Comment: Measured - 17:23:58, 13Aug2001 Plotted - 17:54:14, 25Sep 2001

Distance [meter]12011511010510095908580757065605550454035302520151050

Pres

. [Pa

] / [v

olts

], Li

near

0.055

0.05

0.045

0.04

0.035

0.03

0.025

0.02

0.015

0.01

0.005

0

-0.005

-0.01

-0.015

-0.02


First 15First 15mm (44ms) (44ms) of of the the Impulse ResponseImpulse Response

Time Data - 1039AL reflections example

Name: ...\Measurements\1039AL reflections example.wmb Comment: Measured - 17:23:58, 13Aug2001 Plotted - 17:54:14, 25Sep 2001

Distance [meter]1514131211109876543210

Pres

. [Pa

] / [v

olts

], Li

near

0.055

0.05

0.045

0.04

0.035

0.03

0.025

0.02

0.015

0.01

0.005

0

-0.005

-0.01

-0.015

-0.02

Time of Flight

Impulse from Monitor (direct sound)

Reflection off Mixing Desk

Reflection off CeilingReflection off Rear Wall

Reflection off Side Wall


Reflections in the Frequency DomainReflections in the Frequency Domain

Magnitude Frequency Response - 1039AL reflections example

Frequency [Hz]10 100 1,000 10,000

Pres

. [Pa

] / [v

olts

], dB

5

0

-5

-10

-15

-20

-25

-30 Reflection off Rear Wall

Reflection off Side WallMonitor angled off axis to listening position

Reflection off Ceiling Reflection off Mixing Desk


Typical Acoustical ProblemsTypical Acoustical Problems

Front Wall and Floor• Front wall should have no discontinuities (the front glass

window, doors, etc. should all be flush)• When less than 1.2m from the floor, floors reflections can

dominate• Bass trapping can be implimented in the front wall but

beware of the ¼ wavelength cancellation with the hard wall behind the monitors

• Main monitors are usually mounted with a vertical angle of about 15º - 20º



Console, Equipment and Furniture• First order reflection off the mixing console surface creates

a dip around 1 – 3kHz. Vertical positioning of 2-way is necessary in all cases

• The ¼ wavelength cancellation with the hard wall behind the monitors is often a problem with near-field monitors

• Meter bridge, large tables, computer monitors and racks all create ripples and comb filtering in the mid band

• Computer monitors should be tilted and mounted into the furniture. Flat screens are acoustically better

• Open furniture is acoustically more transparent



The Ceiling and the Side Walls

• Hard reflective ceilings close to the monitors create strong reflections. Bumps and dips can be seen around 3 – 5kHz

• Ceiling construction close to the monitors should not induce first order reflections

• Various design philosophies exist for side wall construction but symmetrical treatment is essential



Control Room Layout Recommendation

• No control room design should have parallel walls• No control room should be installed in a ’shoe box’ type

rectangular room. Standing waves will be hard to control• All equipment in the room should be placed in a symmetrical

layout arrangement• Enough (thick) damping material should be built in the

trapping to handle very low frequency absorption


Typical Causes of Cancellations in StudiosTypical Causes of Cancellations in Studios

• 3kHz – 5kHz Compression ceiling• 1kHz – 3kHz Mixing console• 250 Hz – 1kHz Furniture, etc• 70 Hz – 250Hz Floor• Under 200Hz Rear wall of studio?

Side walls of studio?

Wall behind monitor?


Diagnosis of the Causes of ReflectionsDiagnosis of the Causes of Reflections

• Move the microphone about 30-40 cm in one direction at a time– Left / Right … side wall reflections– Up / Down … floor / ceiling reflections– Forwards / Backwards … rear wall reflection

• No change indicates monitor rear wall reflection• Dip moves down indicates longer path length• Dip moves up indicates shorter path length


Calibration StandardCalibration Standard

• High quality monitor design tolerance is +/-2.5dB over the whole pass band when measured in anechoic conditions

• This can also be achieved in a good studio with well controlled acoustics

• The calibration target is to fit the whole bandwidth of the frequency response within a 5dB window

• WinMLS lets you overlay a 5dB guideline


Calibration StandardCalibration Standard

• WinMLS system gives you about 80% of the information about the calibration

• The listening process (about 20%) lets you check for colouration, harshness, unfocused imaging, tonal changes etc

• The monitor balance has to be right at the mix position where the engineer does most of his/her work


Broken TweetersBroken Tweeters

Effects of Broken Treble Drivers

60

65

70

75

80

85

90

95

100

105

10 100 1,000 10,000 100,000

Gain, dB1033A

1035B


Subjective Evaluation of CalibrationSubjective Evaluation of Calibration

• Use a large variety of source materials:– Orchestral, Big Band, Female vocal, rock, etc.

• Listen for the following:– Smooth bass with no obvious bumps or holes – Bass content should not mask vocals & guitars– No upper midrange hardness at high SPL– Midrange & HF in balance


Calibrations Session SummaryCalibrations Session Summary

• Remove the obvious causes of reflections to get a better response with less ripple– Move the monitors around in the room– Move equipment around in the room– Recommend appropriate acoustic treatment (maybe?)

• Move the microphone to find the source of reflections• Tone controls are for acoustical loading correction and do

not solve the problems of poor acoustical design• Reflections CANNOT be equalised out using tone controls• The calibration standard is +/- 2.5dB


ConclusionsConclusions

• Setting up the measuring equipment• Ambient sound levels• The effect of reflections• Impulse and Frequency Responses• Acoustical problems in rooms• Reflection diagnosis & causes of reflections• Tone controls on different monitors• Calibration standard• Practical calibration of monitors• Subjective evaluation

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Morset Sound Development Sound System Calibration Example - Performing Monitor Calibration in...

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