H:\Test-and-Analysis\Acoustic\Acoustic-Primer.doc Page - 3
A Little Acoustic Primer for BioServe
· Sound is a pressure wave with amplitude and frequency. Microphone measures pressure.
· 20mPa is threshold for human hearing (at 1000 Hz). This pressure is used as reference for the dB logarithmic scale to measure sound pressure. Pref = 20mPa.
· Frequency and wavelength: C = l * f. Example, in air, with C = 340 m/s, low frequency (bass) has long wavelength, difficult to ‘trap’ in absorptive material. Higher frequencies easier to trap in open-cell foam due to small wavelength.
· Human hearing is typically from 20 Hz to 20,000 Hz, with speech in the 500 to 1000 Hz range. Most noise measurements are from 63 to 8000 Hz, as the other frequencies do not affect hearing / communication as much.
· Sound pressure is measured as pressure, but referenced to 20 mPa, using a logarithmic scale to the hearing threshold of human hearing (therefore, negative dB is possible). The accepted unit is the SOUND PRESSURE LEVEL, SPL. Therefore, at threshold (20 mPA, we measure 0 dB),
· dB = 20 * log (P/Pref):
20 mPa / dB = 20 * log ( P / 20mPa) / = 0 dB / Threshold of human hearing200 mPa / = 20 dB / Forest (15 dB typ.)
2000 mPa / = 40 dB / Library (35 dB typ.)
20,000 mPa / = 60 dB / Office (65 dB typ.)
200,000 mPa / = 80 dB
2,000,000 mPa / =100 dB
20,000,000 mPa / = 120 dB / Pain (130 dB)
· Typical center frequencies for full (1/1) octave frequency bandwidth: 63 (=44-88), 125 (=88-177), 250 (177-355), 500 (355-710), 1000 (710-1420), 2000 (1420-2840), 4000 (2840-5680), 8000 (5680-11,360) Hz. The upper limit of an octave is twice the frequency than the lower limit.
· Weighting – adjust for human hearing, which is frequency dependent (hear less at high and low frequency).
Center Frequency / DB(A) weighting for 1/1/ (full) octave)32.5 Hz / -39 dB
63 Hz / -26 dB
125 Hz / -16 dB
250 Hz / - 9 dB
500 Hz / -3 dB
1000 Hz / 0
2000 Hz / +1 dB
4000 Hz / +1 dB
8000 Hz / -1 dB
16,000 Hz / -7 dB
dB(A) = dB(-) + weighting factor
dB(-) = dB(A) – weighting factor
· Background noise: measurements are accurate without compensation if background noise is 10dB lower than ‘to-be-measured’ noise. Background noise MUST be 3 dB lower than to be measured noise. Between 3 and 10 dB, the noise readings must be corrected to eliminate contribution of background noise.
· Environmental factors: temperature, pressure, humidity, vibration, wind factor (mass flow) may all affect microphone readings, but good microphones compensate for most but absolute pressure. Biggest impact for Colorado is absolute pressure, which may affect readings by 1-2 dB (lower than at sea level).
Sound Pressure
· Limits imposed by NASA for various carrier locations:
Frequency / MDKNSTS-21000-IDD-MDK 03/07/02 / MDK
SSP 52000-IDD-ERP, Issue C. 8/7/01
Note: extrapolated 2-ft measurement / ISS-EXP / MDK
[Hz] / [1 ft.] / [2 ft.] / [2 ft.] / [3 ft.]
63 Hz / 64 / 60 / 58 / no limit given
125 Hz / 56 / 52 / 50 / no limit given
250 Hz / 55 / 51 / 42 / no limit given
500 Hz / 51 / 47 / 38 / no limit given
1000 Hz / 52 / 48 / 32 / no limit given
2000 Hz / 53 / 48 / 32 / no limit given
4000 Hz / 48 / 49 / 32 / no limit given
8000 Hz / 44 / 40 / 31 / no limit given
Overall Unweighted / 66 / 62 / 58.8 / no limit given
Overall A-weighted / 58 / 54 / 42.0 / no limit given
· Calculating overall (weighted and unweighted) sound pressure from frequency bandwidth data:
Remember: dB = 20 log(P/Pref) and (P/Pref) = 10^(dB/20)
(Ptotal/Pref) = (P1/Pref) + (P2/Pref) + ………. Total pressure is the sum of all pressure contributions.
OA = 10*LOG(10^(dB1/10) + 10^(dB2/10) + 10^(dB3/10) + ...... )
Example: OA = 10*LOG(10^(64/10)+10^(56/10)+10^(55/10)+10^(51/10)+10^(52/10)+10^(53/10)+10^(48/10)+10^(44/10)) = 66 dB
Overall unweighted and Overall A-weighted can be calculated from their corresponding unweighted / A-weighted frequency bands using the same summation of ‘partial’ pressures.
· For NASA sound pressure readings, one measurement must be taken from the loudest area of EACH surface (front, back, left, right, top, bottom). Measurements are taken at 1 ft. (MDK) and 2 ft (ISS). If the 1-ft reading exceeds the limits for middeck, then another reading at 3 ft is taken (see evaluate how fast the sound pressure drops off). Typically (but not as a law, if the emissions at 3 ft are below the limits given for 1 ft, a waiver can be obtained. If the 3-ft readings are still above the 1 ft. limits, that typically means – no joy.
SOUND POWER
· In addition to SOUND PRESSURE (above), nowadays SOUND POWER readings are taken. Sound power readings are a single number, which is independent from, or normalized, to distance. The Sound Power value can be calculated form 10 or 20 microphone measurements at certain locations of an imaginary ‘dome’ surrounding the measurement object. The radius of the dome must be at least 2 times the largest dimension of the object, but can be larger than that (will normalize during calculations). Acoustic chambers will typically measure all 10 microphones simultaneously, but one could measure the 10 locations with one microphone and calculate the final sound power measurement. To further average the readings from the 10 microphones, 2 readings of 10 microphones, with the object rotated 180 degrees around the dome axis, are taken.
· NASA is not yet using Sound Power for payload evaluation, but instead uses sound pressure readings from each surface. Sound power has been used widely since 1992 / 1996 in industry.
Some examples:
Limit / CGBA / PGBASound Power without muffler (distance independent) / n/a / 58.9 dB / 57.0 dB(A)
Sound Power with muffler (distance independent / n/a / 54.3 dB / 53.4 dB(A)
Sound Power reduction by mufflers / target: -5dB / -4.6 dB / -3.6 dB(A)
Sound Pressure Overall A-weighted, 2ft, without muffler / 54.0 dB(A) / 55.6 dB(A) / 52.6 dB(A)
Sound Pressure Overall A-weighted, 2ft, with muffler / 42.0 dB(A) / 44.3 dB(A)
exceed 2.3 dB(A) / 47.9 dB(A)
exceed 5.9 dB(A)
Sound Pressure reduction by mufflers, A-weighted, 2 ft / n/a / -11.3 dB(A) / -4.7 dB(A)
Sound Pressure Overall A-weighted, 1ft, without muffler / 58.0 dB(A) / 59.8 dB(A)
exceed +1.8 dB(A) / 57.7 dB(A)
pass –0.3 dB(A)
Sound Pressure Overall A-weighted, 3ft, without muffler / 58.0 dB(A) / 55.1 dB(A)
pass / 48.9 dB(A)
pass
4/8/02 - 7:52 PM