Environmental Effects and Control of Noise Lecture 2

Transcription

1 Noise, Vibration, Harshness Sound Quality Research Group NVH-SQ Group University of Windsor Environmental Effects and Control of Noise Copyright 2015 Colin Novak Copyright 2015 by Colin Novak. All rights reserved no part of this publication may be reproduced or distributed in any form or by any means, or stored in a data base or retrieval, without the prior written permission of the author.

2 OUTLINE Review of last day The sinusoidal wave Sound Pressure vs. Sound Pressure Level (SPL) Conversion of SP to SPL Examples 2

3 WHAT IS SOUND? What is the definition of sound? Sound is the propagation of a disturbance through a medium. For air, sound propagates at the speed of sound or approximately 340 m/s at STP. How would you define noise? Animation courtesy of Dr. Dan Russell, Grad. Prog. Acoustics, Penn State Noise is generally considered to be any unwanted sound. Environmental Noise is generally referred to as unwanted sound produced by human activities which interfere with communication, work, rest, recreation and sleep. 3

4 where c Noise, Vibration, Harshness Sound Quality Research Group SOUND PRESSURE RT M = γ simplifies to: c T λ λ = c f λ Wavelength, λ [m] k 2 k 5 k 10 k Frequency, f [Hz] 4

5 SINUSOIDAL WAVE Amplitude T is the period of the waveform T f (the frequency) = 1/T Peak RMS Average Peak- Time Peak RMS = 1 T T 0 x 2 (t)dt Average = 1 T T 0 x(t)dt T is the averaging time and T is much greater than T 5

6 REAL SOUND WAVE Amplitude Peak- Peak Peak RMS Average Time 6

7 TYPES OF SIGNALS Stationary signals Non-stationary signals Deterministic Random Continuous Transient 7

8 Sound Pressure Level Magnitude of sound pressure affecting the ear varies from 2x10-5 Pa at the threshold to 200 Pa at instantaneous damage. To account for this, we use a log scale to describe sound pressure level (SPL). SPL = 20log(P/ P ref ) Where P ref = 2x10-5 Pa (the threshold of hearing) Units of decibel or db Expands to: Lp = 20log(P) + 94[ db] 8

9 P RMS vs P O For the equation: SPL = 20log(P/ P ref ) P is taken as P RMS P O is absolute pressure. Their relationship is as follows: P RMS PO = [ a ] P P For this course, it will always be assumed that P is P RMS unless given otherwise. O 9

10 PERCEPTION OF dbs Change in Sound Level (db) Change in Perceived Loudness Just perceptible Noticeable difference Twice (or 1/2) as loud Large change Four times (or 1/4) as loud 10

11 Class Examples 1. The fundamental frequency of a note played from an organ was 400 Hz with an absolute sound pressure amplitude of 600 x 10-5 Pa. (a) What is the wavelength of the tone in air at 10 degrees Celsius? (b) What is the effective or rms value? Ans: (a) m, (b) 424 x 10-5 Pa. 2. During a fireworks display, it takes 0.29 seconds between the initial flash of a display and the sound reaching an observer. What is the distance between the observer and the ignited display if the air temperature was 23 C? Ans: 100 m. 3. The sound pressure level measured at 10 m from a transformer was 89 db for the dominant 240-Hz tone. What was the rms sound pressure at the measurement location? Ans: Pa. 11

12 Solutions Q1 Q2 Q3 f = 400 Hz t = 0.29 sec Lp = 89 db Ppeak = 6.00E- 03 Pa T = 23 C d = 10 m T = 10 C f = 240 Hz T = K T = K c = 345 m/s Pref = 2.00E- 05 c = 337 m/s x = c*t 100 m/s λ = m Prms = 5.64E- 01 Pa Prms = 4.24E- 03 Pa 12

13 Adding Noise Sources Another important concept is to have the ability to add the contributions of several sources. i.e. Lp t Let s assume we have two noise producing machines where: and now gives Aside For we can arrange to get: or in general for N sources, 13

14 Adding Noise Sources 1.4 db L + db L 1 L 2 ΔL L + L t = = = = = Example: 55 db 51 db 4 db 1.4 db = 56.4 db L p L 1 p 2 10 L = 10 log( pt ) 0 ΔL 0 4 db db 14

15 Averaging SPL Let s say we want to quantify the noise emissions from a large and complicated piece of machinery. How would we do this? If we were to take several measurements around the source, we can derive at a single number which represents the acoustics of the machine by calculating the Logarithmic Mean Sound Pressure Level (Lpavg). Lp7 Lp8 Lp1 Lp2 Lp3 Lp6 Lp4 Lp5 15