Engineer Noise and Vibration flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Sound pressure level formula

Lp = 20 log10(p/p0), where p0 = 20 µPa. A pressure of 0.2 Pa corresponds to 80 dB and 2 Pa to 100 dB.

Decibel addition of two sources

For incoherent sources, L_total = 10 log10(10^(L1/10) + 10^(L2/10)). Two equal 70 dB sources combine to 73 dB, an increase of 3 dB.

A-weighting purpose

A-weighting approximates the ear's sensitivity at moderate levels, based on the 40-phon equal-loudness contour. It is the standard weighting for occupational and environmental noise assessment.

Sabine reverberation formula

RT60 = 0.161 V/A in metric units, where V is room volume in m³ and A is total absorption in m² sabins. Doubling absorption halves reverberation time.

Simple harmonic acceleration

For displacement amplitude X and angular frequency ω, acceleration amplitude is ω²X. Acceleration rises 12 dB per doubling of frequency for constant displacement.

Octave band center frequencies

Standard octave bands run 31.5, 63, 125, 250, 500, 1000, 2000, 4000, 8000, and 16000 Hz. Each band's upper edge is twice its lower edge.

Class 1 versus Class 2 meters

IEC 61672 defines Class 1 and Class 2 sound level meters. Class 1 has tighter tolerances, about ±0.7 dB at reference conditions, versus ±1.0 dB for Class 2.

Time weightings Fast Slow Impulse

Fast is 125 ms, Slow is 1000 ms, and Impulse has a 35 ms rise time with slower decay. LAeq is an energy average and does not depend on time weighting.

Sound exposure level LAE

LAE = LAeq + 10 log10(T/T0) with T0 = 1 s. It condenses a single event's energy into a one-second equivalent level for comparison.

FFT frequency resolution

Frequency resolution equals sampling frequency divided by number of samples, and also equals the reciprocal of record length. A 1 s record gives 1 Hz resolution.

Calibration drift acceptance

A post-measurement calibration drift greater than 0.5 dB generally invalidates the measurement set. Record pre- and post-calibration values with the instrument serial number.

OSHA permissible exposure limit

OSHA sets a PEL of 90 dBA as an 8-hour time-weighted average with a 5 dB exchange rate. The action level is 85 dBA TWA.

NIOSH recommended exposure limit

NIOSH recommends 85 dBA for 8 hours with a 3 dB exchange rate. Permissible time is T = 8/2^((L−85)/3) hours.

Noise dose calculation

Dose D = 100·(C1/T1 + C2/T2 + …), where C is actual exposure time and T is allowable time for that level. A dose of 100% equals the criterion level.

Standard threshold shift definition

OSHA defines an STS as an average change of 10 dB or more at 2000, 3000, and 4000 Hz in either ear, relative to the baseline audiogram.

NRR derating method

Estimated exposure = dBA − (NRR − 7). Derating factors of 25% (OSHA), 50% (NIOSH), or 75% may be applied to reflect real-world fit.

Barrier insertion loss limit

Practical barrier insertion loss is limited to about 20 dB. The barrier must break line of sight; small gaps and flanking paths reduce performance sharply.

Lden weighting periods

Lden uses day 07:00–19:00, evening 19:00–23:00 with a +5 dB penalty, and night 23:00–07:00 with a +10 dB penalty, averaged over 24 hours.

Atmospheric absorption with frequency

Atmospheric absorption rises with frequency: under 1 dB/km at 125 Hz but roughly 10–30 dB/km at 4 kHz, depending on temperature and humidity.

Mass law transmission loss

For a single-leaf partition, TL ≈ 20 log10(m·f) − 47 dB with m in kg/m² and f in Hz. Doubling mass or frequency adds about 6 dB.

Vibration isolation transmissibility

Transmissibility T = 1/|1 − (f/fn)²| for an undamped system. Isolation begins above f/fn = √2 and improves about 12 dB per octave beyond it.

Static deflection and natural frequency

Natural frequency fn ≈ 15.76/√δ Hz, where δ is static deflection in mm. A 25 mm deflection gives about 3.2 Hz.

Coincidence dip in panels

The coincidence dip lowers transmission loss when the trace wavelength in the panel matches the airborne wavelength. It occurs near the critical frequency of the panel.

Noise-induced hearing loss notch

Noise-induced hearing loss typically shows a notch at 3000–6000 Hz, most often at 4000 Hz, with partial recovery at 8000 Hz.

Hand-arm vibration A(8)

A(8) = a_hv·√(T/T0) with T0 = 8 h. The EU action value is 2.5 m/s² A(8) and the exposure limit value is 5 m/s² A(8).

Whole-body vibration action values

The EU action value for whole-body vibration is 0.5 m/s² A(8) and the exposure limit value is 1.15 m/s² A(8), or a VDV of 21 m/s^1.75.

Vibration dose value formula

VDV = [∫a_w⁴(t)dt]^(1/4), using frequency-weighted acceleration. The estimated eVDV equals 1.4·a_w·T^(1/4) for constant exposure.

Report measurement uncertainty

Environmental LAeq measurements typically carry an expanded uncertainty of about 2–3 dB. Compliance decisions should account for uncertainty rather than relying on a single point value.

Report instrument and settings

A defensible report states instrument class, calibration records, time and frequency weightings, measurement duration, positions, and meteorological conditions, so results can be reproduced independently.

Modeled versus measured data

Modeled results must be labeled as predictions, with model name, version, inputs, and any validation against measurements. Presenting predictions as measurements is misleading.

Speed of sound in air

Speed of sound in air is approximately 331.3 + 0.606·T m/s with T in °C, giving about 343 m/s at 20 °C. Wavelength equals c/f.

Background noise correction

If background is 10 dB below the total, subtract 0.4 dB; at 3 dB difference the correction is about 3 dB but uncertainty is large. Below 3 dB, results are unreliable.