Leq SEL

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Leq, SEL WHAT? WHY? WHEN? by Pierre Bernard, Bruel&Kjaer

What are Leq and SEL? L eq an d SEL ar e en er gy p ar a mwhich has the same amount of enerThus, SEL SEL is similar to Leq in that ete rs wh ic h pr ov id e a ve ry use ful gy in in one one second as as the the original the total sound energy is integrated me ans of de sc ri bi ng fl uc tu at in g noise event. over the measurement period, but sou nds . In this ap pl ic at io n note instead instead of then averaging averaging it over over the we will look at what L eq a n d S E L e n t i r e me as u reme nt p e r i o d , a r e fe r h e n e n c e d u r a t i o n of 1 s. is used. Sound are, wh y we use them and w we use t h e m . E x p o s u r e Level m ea s ur em en ts us ing A-weighted sound pressure levels L eq ( e q u i v a l e n t c o n t i n u o u s s o u n d a r e d e n o t e d by t h e s y m b o l LAE . level) is defined as the steady sound p r e s s u r e l ev el w h i c h , ove r a g i v e n T h e d i f f e r e n c e s b e t we e n t he f l u e perio d of time, has the same total tuatin g sound level, the Leq and SEL ene rgy as the act ual fluc fl uc tuat tu at ing in g Fig.2 ig.2.. The defini definitio tion n of SEL can be seen in Fig. 3. noise. Thus, the L eq is in fact the RMS sound level with the measure ment duration used as the averaging time.

Fig.t. The definition of L eq





Why use Leq and SEL ? L eq an d SEL ar e us ed be ca us e in man y me as ur em en t si tu at io ns the wi de ly fl uc tu at in g di sp la y of a tr ad it io na l so un d level met er ma ke s it e xt re me ly di ff ic ul t to de te rm in e the co rr ec t so un d level. Traditi onal sound level meters are designed and built accor ding to intern atio nal (or National ) Stan dard s. These Standards define the time weig htin g of the sound level meter that is the speed with which the detector and indicati ng meter or digital display respond to sound. Most sound level level measureme nts are RMS values, because of the di rect relation ship between the RMS value and energy. Thus, a tradi tiona l sound level meter displays a time varyi ng RMS value using either the "F " or "S" time weig hting .

The response known as "F" (fast) gives a steady steady reading when the measured sound is steady, if the sound level level fluctuates, the meter meter tries to follow the fluctua tions. The "F" response is used, for example, when finding the maximum soun d level produced by a motor vehicle in a drive -by test. The "S" (slow) response attempt s to steady the readings when the noise is of a fluc tua ting nature. Regulations governi ng sound level measurements may stipulate the use of either "F" or "S" time weighti ngs. Often national legislation and standards are based on "A-we igh ted " sound energy measure ments, for both industrial and environ mental situatio ns. This is normally expressed in terms of an A-wei ghte d Leq. For example , in many countri es the maximu m allowable noise exp osure for worke rs is defined as an

LAeq of either 85dB(A) or 90dB(A) over an eight hour working day. SimMarl Marly, y, environ mental noise — for example, the noise at a construction site or noise due to traffic flow — is also frequently expressed in terms of either an A-wei ghted Leq or SEL.

The tradi tiona l sound level meter can only be used to obtain an accurate Leq if sound levels are steady, or are steady for relatively long period s of time , so that levels and their dur ations can be noted and recorde d, and an Leq calculated. However, if the sound levels fluetuate too much to allow a meaningful reading Integrati ng Sound Level Meters should be used. These instr uments summate noise energy on a relatively long- term basis and divide the value obtained by the elapsed time, thus, provid ing a direct Leq readin g.

When and how do you use L eq and SEL ? Let us look at some typi cal cases which illustrate when a clearer an swer can be easily obta ine d by using an Integr atin g Sou nd Level Me ter which provid es data directly in the form of either an Leq or an SEL. 1.

Leq m e a s u r e m e n t s

Fluctuating machinery noise noise Fig.4 shows a rec ord ing of the noise produce d by a typewr iter. Even Even with the "S " time -wei ghti ng, the fluctua tions make it difficul t to put a value on the noise level. It can

be even more diff icul t if the machi ne has a comple x work ing cycle (Fig.5). (Fig.5). The reason for the measu reme nt uncert ain ty is that the tra dit ion al sound level level meter does not provide time cons tan ts which are of suffi cien t du ration. However, this can be overcome by measuring the overall energy of the noise. When measu ring Leq the choi ce of measurement durat ion depends on the type of noise. For For relatively steady noises such as ma chinery noise the read ing can be take n when

the displ ay of the instr umen t has stabilized. With cyclical noise, the measureme nt pe rio d should inclu de an integer number of cycles so that an Leq measurement effectively represents the true aver age SPL, havin g the same energy as the fluctuating sound. Long-term noise exposure It is often necess ary to rate noise on a long-t erm basis. In industry, work ers' exposu re to noise during work is assessed to establish it's accep tab ili ty. In town s and dwel ling s,





environmental noise produced by traffic, industry, etc. is measured to estimate annoyance caused by noise and thereby the degree of communi ty reaction to be expected. In both cases, noise may vary considerably over the exposure period. Record ings of a time-varying RMS (as shown in Figs.6 and 7) do not allow either direct compr aris on or simple

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Fig.6. Typica l reco rdin g of noise in an office

rating of noise exposure. In such cases, the data analysis provided by traditional sound level meters is not sufficient. Leq measurements for long-term exposure rating, require the selec tion of representative measurement periods. In the case of occupational noise exposure, this may be a whole working day, or alternatively parts of the day in which representative mea surements of the entire working day can be made. Similarly, community noise expo sure can be assessed by measure ment of Leq over a whole day, or by taking "period" Leq measurements of representative intervals within typi cal periods (e.g. quiet periods during daytime, rush hours, evening, night).

2. SEL measurements

Fig.7. Typical recording of traffic noise a) in a busy street and b) close to a motorway

Transient noise As seen in the above examples, Leq measurements give us very use ful results in many situations. How ever, an Leq measurement does not remove ambiguity in the case of transient noise, for example from an aircraft fly-over or a vehicle drive-by (Figs.8 & 9). These situ atio ns often result in wide variations from back ground noise to maximum level and if only the maximum level is report ed, information on the duration of the noise (an important feature for rating annoyance) is lost. This also makes it difficult to compare be tween rapid and slow events, and to combine

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9- - Typical recording of aircraft fly-over noise

prediction purposes. Since L

is an energ y aver age,

In such a case, use of an Sound

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same time scale. Although the differ ence between the maximum levels is approximately 16dB, the difference between the SEL values is only 12dB, indicating that the low level event lasts longer. This example shows that measurement of only the maximum noise level is an insuffi cient description of a transient noise. The second application of SEL is to calculate the corresponding Leq for a given period using the individ ual SEL values for different events occurring within that period. For just one event occuring during the time interval T, the relationship between SEL and L eq over this time interval T is: Leq = SEL - 10 log — wit h T0 = To

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If there are several events with dif ferent individual SEL values, the ex posure levels can be added on an energy basis as shown below: n

SELj

j

Leq = 10 log log ^T 10 ~^ -1 0l og j = 1

T0

Fig.9. Comparison of transient noises

The SEL values may for example be those from different types of ve hicles. They may also be fro m diff er ent types of machines performing cyclical operations provided that the number of cycles is known for the exposure period. For such cyclical noises, measurements of SEL should start and end in the quieter part of the cycle ensuring that the noise en ergy in a whole cycle is measured. Fig.10. Example of L eq derivation from SEL values

The Integrating Sound Level Meter The int egr atin g sound level meter consists basically of a micro phon e with it's pream plifie r, an A-we ighti ng filter and and an integra tor which summates the noise energ y over the measurement perio d. The measured result may be e xpr esse d in two

in our homes, whic h is is in fact an integra ting wattme ter. It does not provi de a record of our our in stanta neous cons umpt ion; we just read it fro m time to time to know how much energy we have consum ed since the last ins pec tio n. The bill does not de





The B&K Family of Integrating Sound Level Meters I nt e g ra t i ng S ound Leve l M e t e r s A micr opro cesso r ensures the calT y p e s 2225 a nd 2 2 2 6 c u l a t i o n of t h e t r u e Leq or SEL values Th e se p o c k e t - s i z e s o u n d l e v e l m e a n d it is possible to switch between ters (Fig.11) conform to IEC 6 5 1 t h e tw o c a l c u l a t i o n s d u r i n g th e m e a Type 2 (except for "Max. Hold" f o r s u r e m e n t . Th e r e s u l t s a r e p r e s e n t e d 2225) a n d AN S I S. 1 .4 - 1 98 3 . B o t h m e o n a l arg e d ig i ta l di spla y w i th a r e so t e r s d i s p l a y th e A - w e i g h t e d s o u n d l u t i o n of 0,1 d B . T h e d i s p l a y a l s o ha s p r e s s u r e l e ve l on a 4 0 d B c o l u m n d i s s y m b o l s fo r i n d i ca t i n g o v e r l o a d , u n play and have a total measurement der range, battery state, time e xrange from 25 to 140dB(A). As well ceeded (Leq measurement) and A- or Fjg 12 E xamp le of 60s Leq recording over a as the tra dit ion al "F" and "S " time 2 hour peri period od Linear wei ght in g. Alth oug h the ma xi constants, the th e 2225 provides a mum measurement period for Leq is "Peak Hold" and the 2226 an " I " 2,77 2,77 hours, the equivalent level for mode (a hold function is available on of the event and the final 60s Leq longer periods can be derived from all modes on the 2226). Both meter s value can be con ver ted to the co rr ethe SEL value . have an integrating mode with a sponding SEL by addition of 10 log fixed measurement period of 1 min60 (appr oximate ly 18dB). The instruments are automa tically reset when changing the measur eute, providing an A-wei ghte d 60 s. Uq (LAeq )- During the integration proPrecision Integrating Sound Level ment range or the function selector cess, the display keeps flashing until Meters Type 2221 and 2222 (except between Leq and SEL). A the averaging period is over. The reAs light and slim as Types 2225 2225 pause functi on allows the exclusion suiting 60s Leq value is then held unand 2226, the Preci sion In tegr atin g of unwanted events in Leq and SEL til the instrument is reset. If the DC Sound Level Meter s Types 2221 and meas urem ents and makes it possioutput of the sound level meter is 2222 (Fig.13) conform to IEC 651 , ble to use the inst rume nts for spat ial connected to a level reco rder such Type 1, and to the prop osed IEC averagin g in sound power measur eas Type 2317, the sound level meter stan dar d for integ rati ng sound level ments. The signa l avai labl e at the AC is automatically reset at the end of meters Type 1P. output is either A or linea r-wei ghted , each integra tion peri od, enabling the depend ing on the functi on selecte d. recording of a series of consecut ive They have four basic measur e60s Leq values (Fig.12). ment mode s: an A-weighted Leq Precision Integrating Sound Level (L Aeq ), A-weighted SEL (L AE ), Max. Meter Type 2230 Type 2225 or 2226 may also be Hold "F" (2221) or "S " (2222), (2222), and Type 2230 (Fig.14) is a Type 1 pr eused to calculate the SEL of events Max. Hold "Peak". The three first cision instrumen t. Its compr ehens ive which last for less than 1 minute. The functio ns are A-we igh ted, whilst the const ructi on and versati lity make it instrument is reset at the beginni ng "Peak" value is measure d with a flat ideal for all kinds of sound level frequenc y response . Four over lapmeasurem ents. This includes octave ping sensitivity ranges provide a and 1 /3 octave frequency analysis measurement span from 25dB to when used with a snap-on filter set. 145dB for Leq measurements. The Type 2230 has a measurement range from 24 to 130 dB (30 to 150 dB with supplied attenuator) and carries out five measurements in parallel: SPL, Max., Min., L eq and SEL, all of which can be obtained for the same signal. A choice between 2 detector modes (RMS an d Peak), 3 time weightings ("S", "F", "I"), 4 frequen cy weightings (A, C, Lin and All pass) is available for the measurements. A partial (Max./Min.) reset, a total reset and a pause function increase the capabilities of the instrument.







tions on the front panel of the 2231 are defined by the software package. This flexibility, together with provi sions for connecting Fitter Sets and hard-copy recorders, enable the 2231 to be used for a wide variety of noise measurements. The 2231 fulfils IEC and ANSI Type 1 SLM requirements and also the proposed integrating SLM require ments. A unique parallel-detector al lows display of both RMS and Peak values of the same signal. "A", "C", "Lin." (10Hz to 20kHz) and "All Pass" (2 Hz t o 70 kHz) fre que ncy weightings are available. The DC output allows recording of "moduledefined" parameters. Five of the apFig. Fig.14 14.. The Precisio Precision n Integrating Integrating Sound Level Level Meter Type 2230 2230

p li ca ti o n b e l o w*

digit liquid crystal display. The SPL is continuou sly monit ored on a quasi-analo gue 60dB scale. Indication of overl oad, depleted batteries and illegal setting is also displa yed.

Integrating SLM Module BZ7100: Standard module delivered with the 2231. "F" , "S" and " I " time respon ses and 4 frequency weigh tings. Measurement Parameters: Max. peak hold; Max. peak in 1 s peri od; Sampled RMS RMS in 1 s; Max. RMS in 1 s; Max. SPL SPL hold; Min. SPL hold; Leq (Llm with " I " respon se); SEL (IEL with " I " response). Special Functions: setting quasi- analogu e scale to di splay peak levels; DC out put giving Leq vs. time histo gram; data inhibit using pause pushkey; digita l reado ut faci lity .

AC and DC outp uts allow tape or level rec ord ing s to be made and audio mon ito rin g of the sound level with the selected freque ncy weighti ng. Despite its Type 1 perfo rman ce and the numerous functio ns inclu ded, the Type 2230 is only 25 cm long and weighs less than 1 kg. It can easily be held in one hand, even when the Filter Set is connected. Types 2233 2233 and 2234 are spec ial vers ion s of the 2230. Type 2233 has the fun ctio (exce Min.

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Statistical Analysis Module BZ7101: "F" and "S" time responses and 4 frequency weightings. Measureme nt Para mete rs: as for BZ7100 plus L(99.0), L(99.0), L(90.0), L(50.0), L(10.0) L(10.0) and L(1.0) Also calcu late L Cumu

Fig. Fig.15. 15. The Modular Modular Precisio Precision n Sound Level Level Meter Type 2231 and one of its appl ica tion modules

vice such as a data-l ogger , mic roprocessor or printer. Time and frequency weighti ngs as for BZ 710 0. Simultaneous measurement at one second intervals of max. SPL, insta nta neo us SPL and Peak, and at user selectable intervals Leq, SEL, min. SPL, max. SPL and max. Peak. Event Rec ording Module BZ 7107: Enables the 2231 to measur e and record the princip le parame ters con nected with a noise event. An event is taken to exist when the the am bi ent noise level exceeds a predefi ned threshold level for more than a predefined time. The princip le parameters, i.e. i.e. Leq, SEL, max. and min. SPL, dur ati on and thre shold level to gether with other data may be tra nsferred digitally data-l





Noise Level Analyzer Type 4427 The Noise Level Analyzer Type 4427 (Fig.16) offers a wide range of fea tures for accurate on-site analysis of community, airport and traffic noise or any other acoustical event requir ing accurate measurements and ex tensive statistical analysis of collect ed data. The Type 4427 Noise Level Analyz er represents an innovative design concept, complying with the relevant sections of IEC 651 and ANSI S 1,4 1,4 (1983) Soun d Level Mete r Specification Type 0. It permits fast, u se r -fr ie n d ly d ia l o gu e se le ct io n o i nstru me nt se tt in g s and p r o d a ta c o l l e c t i o n , s t o r a g e , level a n s i s a n d p r i n t - o u t in o n e c o m u ni t. T i m e - s a v i n g m e n u - d r i v e n p r c e d u r e s a ll ow easy i n t e r a c t i v e i s t r u m e n t s e t - u p , r e d u c i n g the n e f o r i n s t r u c t i o n m a n u a l s. S o p h ed d a t a p r o c e s s i n g f a c i l i t i e s i n c r a t e d in t h e 44 27 a l l o w c o m p s i v e f r o n t - e n d p r o c e s s i n g of s i

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data. Th e d e t e c t o r c i r c u i t p r o v i d e s F, S I a n d Pe a k p l u s 3 s a n d 5 s T Maximalpe gel responses in parallel with True Linear 1 s L eq re sp on se s. A built-in IEC/IEEE or optio nal RS232 232 C comm unic atio n interfa ce port provides for remote set-up and con trol with the same ease as oper ating the frontpa nel keypad . The 110 dB dynamic range of the detecto r ensures that no infor matio n from the input signal is lost, and a wide range of levels can be measured with extreme accuracy . A built-in graphic printer /plotter allows

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Fig.17 g.17.. The The Noi Noise Dose Mete Meterr Type Type 4428 sound level meters which have been especially designed to monitor the noise exposure of worke rs durin g a work ing day. The digita l display indicates the Noise Dose, which gives the noise exposur e as a percentag e of the maximu m allowable exposu re (which, in most countr ies, cor respon ds to an L Aeq of either 85 or 90dB(A) for an 8 hour work ing day). Conversion tables enable derivation

Summary i n g s o u n d level m e t e r s ca n u s e d t o s o l v e a n u m b e r of n o i s e measuremen t problem s which would be ex tr em el y di ff ic ul t to han dle dl e wi th trad ition al sound level meters. Much noise legi slation is based on Leq measurem ents, both for industria l noise exposure in the work pla ce and noise annoyance in communities. Several intern ational standard s also recomme nd Leq and SEL as basic noise desc ripti ons, for exampl e: ISO ISO 1996: "Asse ssme nt of noise with . respect to commu nity response"

Leq SEL

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