ROOM ACOUSTIC

MEASUREMENTS

ACOUSTICS OF CONCERT HALLS AND ROOMS

Handbook of Acoustics, Chapter 9Schroeder (1965)

IMPULSE RESPONSE

IMPULSE RESPONSE IS THE BASIC SOURCE OF INFORMATION ABOUT THE AUDIBLE PROPERTIES OF THE SOUND FIELD IN A ROOM.

WHEN RECORDED WITH A MULTICHANNEL SYSTEM THAT PRESERVES INFORMATION ABOUT DIRECTION OF INCIDENCE IT CONTAINS ALL RELEVANT INFORMATION ABOUT THE ACOUSTICS OF A ROOM WITH A GIVEN SOURCE AND RECEIVER POSITION

SCHROEDER’S INTEGRATED-IMPULSE METHOD FOR RECORDER SOUND DECAY

TRADITIONALLY REVERBERATION TIMES WERE MEASURED BY RECORDING THE RESPONSE TO AN IMPULSIVE SOUND (PISTOL SHOT OR LOUD PERCUSSIVE SOUND). THE ACCURACY WITH WHICH REVERBERATION TIMES CAN BE BE DETERMINED FROM DECAY CURVES IS LIMITED BY RANDOM FLUCTUATIONS IN THE DECAY CURVES

THESE RANDOM FLUCTUATIONS RESULT FROM THE MUTUAL BEATING OF NORMAL MODES OF DIFFERENT FREQUENCIES AT THE MOMENT THE EXCITATION SIGNAL CEASES

NOISE-DECAY CURVES IN PHILHARMONIC HALL (NY)

1/3-OCTAVE NOISE BAND (167 Hz); NOTE LARGE DIFFERENCE IN DECAY RATES IN 4 SAMPLES

BACKWARD INTEGRATION

SCHROEDER HAS SHOWN THAT THE REVERBERATION CURVE CAN BE MEASURED WITH INCREASED PRECISION BY BACKWARD INTEGRATION OF THE IMPULSE RESPONSE h(T):

WHERE R(t) IS THE EQUIVALENT OF THE SQUARED PRESSURE DECAY. THE METHOD IS CALLED BACKWARD INTEGRATION BECAUSE THE FIXED UPPER INTEGRATION LIMIT (INFINITE TIME) IS NOT KNOWN WHEN THE RECORDING IS STARTED.

EDT (EARLY DECAY TIME) IS EVEN MORE SENSITIVE TO RANDOM DECAY FLUCTUATIONS

FREQUENCY RANGE OF MEASUREMENTS

RT IS DETERMINED FROM R(t) DATA BY LINEAR REGRESSION.

RT CAN BE MEASURED IN 1/3 OCTAVES BUT FOR MOST OTHER OBJECTIVE PARAMETERS IT MAKES LITTLE SENSE TO USE FREQUENCY RESOLUTION SMALLER THAN OCTAVES.

THE FREQUENCY RANGE FOR ROOM MEASUREMENTS IS NORMALLY THE SIX OCTAVES FROM 125 TO 4000 Hz, BUT IT MAY BE EXTENDED TO INCLUDE THE OCTAVE BANDS CENTERING ON 67 Hz and 8000 HZ

SOUND SOURCES

ROOM MEASUREMENTS REQUIRE AN OMNIDIRECTIONAL SOUND SOURCE EMITTING SHORT SOUND PULSES (OR SIGNALS THAT CAN BE PROCESSED TO BECOME IMPULSES) COVERING FREQUENCY RANGE OF INTEREST

LOUDSPEAKERS OFTEN CANNOT EMIT SUFFICIENT PULSE POWER SO SPECIAL SIGNALS OF LONGER DURATION, SUCH AS MAXIMUM-LENGTH SEQUENCES OR TONE SWEEPS HAVE BEEN DEVELOPED.

THEY HAVE THE INTERESTING PROPERTY THAT THEIR AUTOCORRELATION FUNCTIONS ARE BAND-LIMITED DELTA PULSES, WHICH MEANS A LOUDSPEAKER CAN EMIT A LARGE AMOUNT OF ENERGY WITHOUT CHALLENGING ITS POWER CAPABILITY (GADE. 2007)