(PDF) IRE Recommended Practices on Audio and Electroacoustics: Loudspeaker Measurements, 1961

PROCEEDINGS OF THE IRE

IRE Recommended Practices on Audio and Electro-acoustics: Loudspeaker Measurements, 1961*

61 IRE 30.RP1

COMMITTEE PERSONNELAudio and Electroacoustics Committee-1959-1961

IDEN KERNEY, Chairman, 1959-1961D. S. DEWIRE, Vice Chairman, 1959-1961A. P. EVANS, Vice Chairman, 1959-1961H. C. HARDY, Vice Chairman, 1959-1960

A. B. CohenM. CopelD. L. FavinE. E. GrossR. A. HackleyF. K. HarveyF. S. Hird

J. HirschF. L. HopperW. W. LangA. H. LindL. J. MalmstenA. A. McGeeE. J. McGlinn

H. F. OlsonF. W. RobertsR. H. RoseV. SalmonP. B. WilliamsR. E. Yaeger

J. H. ArmstrongJ. AvinsG. S. AxelbyM. W. Baldwin, Jr.W. R. BennettJ. G. BrainierdA. G. ClavierS. Doba, Jr.R. D. ElbournG. A. EspersenR. J. FarberD. G. FinikG. L. FredendallE. A. GerberA. B. Glenn

Standards Committee-1960-1961

C. H. PAGE, ChairmanJ. G. KREER, JR., Vice ChairmanH. R. MIMNO, Vice ChairmanL. G. CUMMING, Vice Chairman

V. M. GrahamR. A. HackbuschR. T. HavilandA. G. JeiisenR. W. JohnstonI. KerneyE. R. KretzmerW. A/IasoiD. E. MaxwellR. L. McFarlanP. MertzH. I. M\etzE. MittelmannL. H. Montgomery, Jr.S. M\. Morrison

G. A. MortonR. C. MoyerJ. H. Mulligan, Jr.A. A. OlinerM. L. PhillipsR. L. PritchardP. A. RedheadC. M. RyersonG. A. Schupp, Jr.R. SerrellW. A. ShipnaiH. R. TerhunieE. WeberJ. W. WentworthAV. T. Wintringham

Measurements Coordinator

J. G. KREER, JR.

* Approved by the IRE Standards Committee, April 13, 1961. Reprints of IRE Standards 61 IRE 30.RP1 may be purchased whileavailable from the Institute of Radio Engineers, Inc., 1 East 79 Street, New York 21, N. Y. at 0.60 per copy. A 20 per cent discount willbe allowed for 100 or more copies mailed to one address.

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PROCEEDINGS OF THE IRE

1. SCOPEThese Recommenided Practices define terms asso-

ciated with loudspeakers and their testinig, recommendvarious methods of testinig, anid inidicate preferredmethods of presenting information regarding their char-acteristics. Specific informiiation is presen-ted in Sections2-7. Discussions of a nmore qualitative niature aregiven in Sectionis 8 and 9.

In these Practices, the tests recommended involvephysical, steady-state mneasuirements only. Work hasbeeni and is now being done on transient measurementsof loudspeaker performanice, but experience with thesenmethods is still not sufficiently widespread to warranttheir inclusion.

While the physical data which can be obtained asdetailed in Sections 4-9, inclusive, are a helpful guidein designing and in selecting a loudspeaker for a certainpurpose, they are not a complete guarantee that thesubjective performanice will be satisfactory. Wherever itis possible, the quality of reproduction should bechecked by means of listening tests such as those de-scribed in the literature.'

2. INTRODUCTION

2.1 DefinitionA loudspeaker is an electroacoustic transducer in-

tended to radiate acoustic power into the air.

2.2 TypesThere are two general types of loudspeakers, namely,

direct-radiator and horn. The direct-radiator type ofloudspeaker is arranged so that it radiates acousticpower inito the air directly from one or both sides of itsacoustic radiating element, or diaphragmn. One side ofthe diaphragm may be totally isolated by meanis of anairtight eniclosure, or, effectively, by a baffle of ade-quate size. In order to control the response over part ofthe useful frequency range, the energy radiated fromthe back surface of the diaphragm may be acousticallycoupled to the energy radiated from the front.The horn type of loudspeaker radiates the major por-

tion of its acoustic power through a horn. This is a tubewhose shape aind cross section are chosen by designi soas to increase the radiation resistance loading oni thediaphragm of the loudspeaker for a substantial portionof the useful frequenicy range, anid increase its radiatedoutput and electroacoustic coniversion-efficiency overthat obtainable whein it radiates directly.Both direct-radiator anid horni loudspeakers have

been designied for a wide variety of applications whichinfluence their ralnge, directional characteristics, per-imiissible distortion, anid power hanidling capacity. Also,either type may be designed to operate as a single ormultiunit device. One loudspeaker may be used to

1 H. F. Olson, "Subjective loudspeaker testing," IRE TRANS. ONAUDIO, vol. AU-I, pp. 7-9; September-October, 1953.

cover the desired frequency range. Alterniatively, twoor imiore loudspeakers, usually interconniiected by imieansof frequency-selective electric networks, mlay be oper-ated together so that each covers a portioni of the de-sired frequency range, while the combination of loud-speakers covers the entire ranige.

3. MEASUREMENT CONDITIONS3.1 Loudspeaker M1ounting

The direct-radiator loudspeaker unit should bemounted in or on the enclosure or baffle called for by themanufacturer, or it should be mounted in or onl a baffleor enclosure which is completely described. The hornloudspeaker unit should be equipped with a horn speci-fied by the manufacturer, or with a horn which is com-pletely described.

3.2 Electrical ConnectionsThe loudspeaker should be connected as specified

by the manufacturer, with the proper networks andcontrols, if any, and these networks and controls shouldbe adjusted as recommended. If not specified, theconnections and adjustments used must be completelydescribed.

If the loudspeaker is of the excited-field type, thefield should be energized with the rated current.

3.3 Acoustic EnvironmentThe acoustic environlment should simulate free-field

conditions to the extent that the inverse-pressure vsdistan-ce law should hold within plus or minus I db atall frequencies at which measurements are made.2Ambienit noise should not affect the measurements toan extent greater than plus or minus 1 db. Departuresfrom these conditions should be measured and de-scribed.

3.4 Test SignalsFixed frequency, swept frequency anid warbled sinu-

soidal signals, as well as banids of nioise, mzay be usedfor loudspeaker testing as dictated by the test and thetype of instrumenitation being utilized. Sweep andwarble rates should be sufficiently slow that response-frequency measurement results do not differ signiif-icantly from the steady-state readings. When banids ofnoise or warble signals are used their characteristicsshould be specified.Each performance parameter of a loudspeaker m11ust

be expressed as a function of frequeincy if it is to be ex-pressed completely. However, for many purposes it isdesirable to condense the iniformationi to a single runm-ber which represents a weighted average over a pre-scribed portionof the spectrum. Such an average imiaybe obtained by means of a signal which sweeps re-

2 H. F. Olson, "Acoustical Engineering," D. Van Nostrand Co.,Inc., New York, N. Y., 3rd ed., ch. 10, pp. 445-450; 1957.

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IRE Recommended Practices on Audio and Electroacoustics: Loudspeaker Measuirements

peatedlyr through the required portion of the spectrumat a rate sufficient to give a steady reading on a suitableindicating device.3The electric power inlput for pressure-frequency re-

sponse, pressure-directional response, and impedancemeasurements, should be kept as low as possible con-sistent with the ambient inoise and the sensitivity of themeasuring system, in order to minimize the effects ofheating and nonlinear distortion in the loudspeaker.However, when it is desired to test a loudspeaker athigh levels, adequate time should be allowed for theattainment of steady-state thermal conditions. Like-wise, when testing excited-field loudspeakers, steady-state thermal conditions should be attained.

3.5 Measuring SystemThe responise of the measuring system including the

microphone should be known. Nonlinear distortionshould be limited to a level consistent with the type ofdata being obtained; however, moderate levels of svs-tem distortion are permissible for tests of maximumpower capacity based upon mechanical or electricalfailure. For nonliinear distortion measurements, thelevel of distortioni products in the measuring systemshould have a negligible effect oIn the readings of dis-tortion products produced by the loudspeaker. TIherefereince-pressure response (fundamental) and hence,the pressure-frequency response (fundamental) by def-inition require measurement of the pressure at thefundamental frequency. A frequency-selective indicat-ing or recording system may be required in order tominimnize errors arising from harmonics in the outputsounid wave.The microphonie used for measuring the sounid pres-

sure should preferably conform to the standards givenin American Standard Specificationi for LaboratoryStandard Pressure Microphones, Z24.8-1949, or thelatest revisioni thereof, anid be calibrated by the methoddescribed thereini. Other types of microphoines may beused if calibrated as described in American StandardMethod for the Free-Field Seconidary Calibration ofMicrophones, Z24.11-1954, or the latest revision there-of. If the microphone employs a pressure-gradienit ele-menit, it is niecessary to correct for proximity effect atlow frequenicies.4The measurinig device used for sinigle frequency mleas-

uremetit* should provide readinigs which represent therms values. For complex waves or bands of noise, thereadings should be obtainied by means which indicatepeak and rmns values for the type of complex wave used5or by a means with which these can be accurately cor-

3 H. F. Hopkins and N. R. Stryker, "A proposed loudness-effi-ciency rating for loudspeakers and the determination of systempower requirements for enclosures," PROC. IRE, vol. 36, pp. 315-355; March, 1948.

response GL, in db, may be expressed in the followingforms:

GL = h - H = 20 logio (PLIPO) - 10 logio (PE/PO)= 20 1og1o PL - 20 logio EG + 10 log1o ZR

+ 20 1ogio (1 + ZG/ZR)- 20 logio po + 10 logio Po

and, on substitution of recommended reference quan-tities, the expression

-20 logio po + 10 logio Po = 44 db,

where

h is the axial free-field sound pressure level pro-duced by the loudspeaker at a distance of 1 meter,in db referred to 0.0002 microbar (dyne per cm2).

H is the electric power level delivered to the rating-impedance, in dbm (db referred to 0.001 watt).

PL is the axial free-field rms sound pressure at a dis-tance of one meter, in microbars.

po is the reference sound pressure, 0.0002 microbar.PE is the electric power, in watts, delivered to a re-

sistance equal to the rating-impedance of theloudspeaker, where

EG2ZRPE = -

(ZG + ZR)2

Po is the reference electric power, 0.001 watt.EG is the rms value, in volts, of the open-circuit volt-

age of the source.ZR is the loudspeaker rating-impedance, in ohms.ZG is the loudspeaker measurement source im-

pedance, in ohms.

5.1.2 Method of Measurement. The loudspeakershould be mounted, connected, and tested in a suitableacoustic environment as discussed in Section 3.The microphone should be placed on the principal

axis of the loudspeaker as specified by the manufac-turer, or, if no axis is specified, on the normal to thecenter of the radiating area or horn mouth, and at adistance at least three times the maximum transversedimension of the radiating area or horn mouth in orderto approximate free-field conditions.

Fronm the free-field pressure Pr, measured at a dis-tance D(feet), the pressure PL at one meter can be com-puted by the relation:

PL = Pr(D/C1),

or, in logarithmic form

PL in db = 20 logio Pr + 20 logio D - 20 logio C1,

where

Ci = 3.28 (number of feet in one meter).

5.2 Average Reference Pressure Response

When it is desired to rate a loudspeaker by mneans ofa single value of the reference pressure response, it ispreferable to take an average over a frequency banid3rather than to cite the value obtained for any one fre-quency. The method used should be described.

5.3 Reference Pressure-Frequency Response

5.3.1 Definition. The reference pressure-frequency re-sponse of a loudspeaker is the reference pressure re-sponse presented as a function of frequency.

Note: The relative pressure-frequency response char-acteristic, commonly known as frequency response,is obtained by referring the response values at variousfrequencies to that at some reference frequency.

5.3.2 Presentation of Data. When presenting data onthe reference pressure-frequency response of a loud-speaker, the following quantities should be specified:measured distance between the loudspeaker radiatingarea or horn mouth and the microphone; electric powerinput to the loudspeaker rating-impedance; value of thisloudspeaker-rating impedance; and value of the meas-urement source impedance.

Note: Unless there is a special reason for using othertypes of graphic presentation for pressure-frequencyresponse curves, it is recommended that the fre-quency scale, or abscissa, be logarithmic, and thepressure scale, or ordinate, be linear in db, and thatthe length of a 10 to 1 frequency interval be the lengthof 30 db on the ordinate scale.

6. RATING-EFFICIENCY (')

6.1 Definition

Rating-efficiency is the ratio, usually expressed indecibels, of the output acoustic power to the electricpower delivered to a resistance equal to the loud-speaker rating-impedance, that is,

77 = 10 log,o (PL/PE),

where'1

PLPE

is the loudspeaker rating-efficiency, in db.is the total radiated acoustic power, in watts.is the electric power, in watts, delivered to a re-sistance equal to the rating-impedance of theloudspeaker, where

EG2Z,R

(ZG + ZR)2

6.2 Discussion

The acoustic output of a loudspeaker can be deter-mined under free-field conditions by combining a meas-urement of the pressure on the reference (usally prin-cipal) axis with the directivity index. The formula for

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loudspeaker rating-efficiency , in db, may be expressedin these terms as follows:

= 20 log10 PL - 20 log1o EG + 10 10g1O ZR

+ 20 logio (1 + ZG/ZR) - KL - C

where

KL is the loudspeaker directivity index, in db (seeSection 7.1.3).

C is a constant which relates the pressure, in micro-bars at a distance of one meter to the acousticpower, in watts, transmitted through a sphere of1 meter radius. For air, under standard conditionsof temperature and barometric pressure, thevalue of this constant C is 35.2.

Note: Another m-nethod of determiniing loudspeakerefficienicy which does not require a nmeasuremenit orestinmate of directivity inidex, utilizes a reverberantroom. 7

7. DIRECTIONAL PROPERTIES

In many cases, the directional characteristics of aloudspeaker can be sufficiently described by mleasurinigone or more pressure-frequency responses along axesat chosen angles to the iiormiial axis as specified by themanufacturer. 'rhese will inidicate how well the loud-speaker response characteristics obtained oni the prin-cipal axis are duplicated over anl area in which lis-teners may be located for a given application.

In order to determin-e the loudspeaker rating ef-ficiency when operating uinder free-field coniditionis, it isnecessary to compute the directivity index. This re-quires that pressure-frequency responses be obtained ata large number of angles, or pressure-anigle responses ata large number of frequenicies, to provide adequatedata. For many applicationis it is sufficiently accurateto use computed inistead of mneasured values of thedirectivity index.8 An average value, using a test signalwhich covers a wide baind of frequeiicies, has beenfound useful in public address and sound systemsengineering.'

7.1 Directivity Factor and Directivity Index of a Trans-ducer

7.1.1 Directivity Factor-Definition. The directivityfactor of a transducer used for sound emission is theratio of the intensity of the radiated sound at a remotepoint in a free field on the principal axis, to the averageinitensity of the sound tranismiiitted through a spherepassing through the remote poinlt and concenitric withthe transducer. The frequency must be stated.

7H. C. Hardy, H. H. Hall, and L. G. Ramer, "Direct measure-ment of the efficiency of loudspeakers by use of a reverberationroom." IRE TRANS. ON AUDIO, No. AU-10, pp. 14-24; November-December, 1952.

8 C. T. Molloy, "Calculation of the directivity index for varioustypes of radiators," J. Acoust. Soc. Am., vol. 20, pp. 387-405; July,1948.

Note: The point of observation nmust be sufficientlyremote from the transducer for spherical divergenceto exist. See Fig. 2.

PRINCIPAL AXIS'

Fig. 2.

7.1.2 Directivity Index Definition. Ihe directivitvindex of a tranisducer is anl expression of the directivityfactor, in decibels, viz., ten times the logarithm to thebase ten of the directivity factor.

7.1.3 Directivity Index-Alathematical Expression.The directivity inidex miiay be expressed inl the followingforml:

KL = - lo1log ffJ J (plpr) sin Odod,',

where

KL is the loudspeaker directivity index, in db.Pr is the axial free-field sound pressure, in micro-

bars, at distance r.p is the free-field sound pressure, in microbars at

the poinit (r, 0, 41).6 and 41 are the anigular polar coordinates of the

system, and the principal loudspeaker axis is at0 -O.

7.1.4 Directivity Index-Method of Measurement. Theloudspeaker should be mounted, coinniected, and testedin a suitable acoustic environment as discussed ini Sec-tion 3.The reference axis for the positioii of the miiicrophonie

should be the principal axis of the loudspeaker as speci-fied by the manufacturer, or, if nio priilcipal axis isspecified, the normal from the ceniter of tile radiatingarea or horn mouth. The microphonie should be placedat a distance at least three times the maximumn trains-verse dimension of the radiating area or horni 11)o0ith.The anigular steps should be as smlall as nieeded todelineate accurately the directional pattern in theplane chosen. If the loudspeaker has no axis of syvmi-miietry, the measuremiients should be repeated inl otherplanes until the field is mapped.The directivity index mlay theni be calculated by

niumerical iintegration of the expressioni given under thedefinition above.9

9"Calibration of Electroacoustic Tranisducers," ASA Z24.24;1957. Contains information helpful in the calculation of directivityindex.

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IRE Recommended Practices on Audio and Electroacoustics: Loudspeaker AMeasurements

8. NONLINEAR DISTORTION

8.1 Nonlinear Distortion in a Loudspeaker8.1.1 Definition. Noinliniear distortion in a loud-

speaker is an undesired change in waveform due todeviation from a linear relationship between acousticoutput and electric input.

8.1.2 Discussion. The effect of this distortion onquality constitutes one of the factors which limit theuseful magnitude of the output of a loudspeaker. Someof the more important causes of this type of distortionare: (1) nonlinear force-displacement relationships inthe mechanical vibrating system of the loudspeaker,(2) nonuniform flux linkage throughout the displace-ment range in moving-coil loudspeakers, and (3) non-linearity in the air itself, especially in horn loud-speakers. The first two causes produce significant dis-tortion at low frequencies for which the displacementis greatest; the third cause is significant at high fre-quencies.

In consequence of nonlinearity, components havingfrequencies other than those appearing in the electricinput signal will be found in the acoustic output. For asingle frequency input, these nonlinear distortion prod-ucts will be harmonics and subharmonics of the originalinput. In the case of a complex wave input there willbe, in addition, intermodulation products, i.e., sum anddifference frequencies of the original components andof various multiples of these components.

Nonlinear distortion is commonly determined by ameasuremient of the distortion products, using a micro-phone to convert the acoustic output of the loudspeakerto a corresponding electric signal and appropriate net-works to identify and measure its various components.Distortion products fall in various parts of the spec-trum, having frequencies either above or below the fun-damental signal or signals by a small or large frequencyinterval, and may be harmonically or nonharmonicallyrelated to the original signals that produce them. Eachof these products may have a different subjective im-portance. Furthermore, loudspeaker distortion will varymarkedly with the signal frequency or frequencies evenwith constant input power. For these reasons, the sub-jective evaluation of distortion through listening testsmay be more useful than measured data; and for thesame reasons, no single method of measurement of dis-tortion is standardized herein.

8.1.3 Methods of Measurement. The loudspeakershould be mounted, connected and tested in a suitableacoustic environment as discussed in Section 3.The microphone should be placed as specified in Sec-

tion 5.1.2.Each component of the output signal may be meas-

ured individually by means of a wave analyzer whichprovides a narrow pass band movable over the spec-trum. The pressure amplitude of each component, ex-pressed in per cent of the amplitude of the fundamental,may be plotted vertically above a frequency scale.

Harmonics of a single fundamental may be measuredby the use of a high-pass filter which effectively sup-presses the fundamental and passes the harmoniics with-out attenuation. The result, as indicated by a square-law meter (see Section 3.5), will be the root-sum-squareof all harmonics present. Subharmonics will not be in-cluded.

Subharmonics may be measured with the use of alow-pass filter which suppresses the fundamental andtransmits all lower frequencies without attenuation.When measured with a square-law meter the result isthe rms value of all subharmonics.Two input signals of equal amplitude are recom-

mended for measurement of intermodulation distortion.A filter may be considered to "suppress effectivelv" a

fundamental if it attenuates the fundamental to an am-plitude at least 10 db below that of the distortion prod-uct closest to it in frequency.

In selecting fundamental frequencies for a measure-ment, it should be recognized that (a) the greatest meas-ured distortion occurs most often at the lowest fre-quencies; (b) distortion due to nonlinearity of the airoccurs to a significant degree only in loudspeakerscoupled to horns and is greatest at the higher frequen-cies; (c) harmonics and sum-products of plural test fre-quencies may not be radiated if the fundamental fre-quency or frequencies approach the upper limit of theeffective radiating frequency range of the system, andnonlinearity may therefore be important principally inthe production of difference-products; (d) difference-products may fall below the cutoff of a loudspeaker orhorn and thus escape measurement; (e) harmonics andsum-and-difference-products falling in the middle por-tion of the frequency range of the loudspeaker, or of theear, may be much more important subjectively thanthose at the extremities of the range; (f) subharmonicsmay be more important subjectively than harmonics;(g) nonharmonically related distortion products may bemore important subjectively than harmonically relateddistortion products.

8.1.4 Presentation of Data. Loudspeaker distortionmust be expressed as a function of frequency if it is to beexpressed completely. The electric input connections tothe loudspeaker and the type of equipment used formeasurement must be specified.

9. RATED POWER (HANDLING) CAPACITY

9.1 Power Capacity Rating of a LoudspeakerBecause of the many factors which influence the

power capacity rating of a loudspeaker, no generalagreement has been reached on what physical measure-ments should be made nor on a method of weighting thenumerical value so obtaiiied to arrive at a single ratedpower capacity. There is, however, general agreement oInthe object of such a rating.The objective is to obtain a power capacity rating

which indicates to the user that the loudspeaker will

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operate satisfactorily with an amplifier having the samepower rating. Some of the more importanit factors toconsider in the rating and in deciding whether operationis "satisfactory" are:

1) operating life,2) freedom from spurious noises (buzzes, rattles,

etc.),3) acceptable nonlinear distortion for the intended

application.

The operatin g life may be limited by electrical failureor mechanical failure arising from mechanical or ther-mal causes. The acceptable noise and nonlinear distor-tion depend on the application. For example, morenoise anid distortion might be tolerated in high-level,narrow-frequency-band, public-address speech systemsthan in broad-band, music systems. The standardiza-tion of power capacity ratings is further complicated bythe fact that the important rating factors vary greatlywith frequency and the frequency dependence differs invarious designs intenided for the same application. Atpresent, subjective tests are an essential part of loud-speaker rating procedures. In making physical tests forrating procedures, the following mininium requirementsshould be considered.

9.1.1 Testing Arrangements. The loudspeaker shouldbe mounted anid connected to the test equipmenit as dis-cussed in Sections 2.1 anid 3.2. The acoustic eniviroIl-ment should simiiulate approximately that in which theloudspeaker will be used. The test enclosure should be ofsufficient size and should emlploy sufficient damlpingthat nio abnormiial resonaiices occur which mllight lead toabniormal mechaniical fatigue in the loudspeaker.

9.1.2 7ype of Test Signal. The test signial should similu-late the energy distributioni, both in frequency anid time,of the intenided signal source mlaterial. This is importantin order to excite all modes of vibration that may giverise to physical fatigue or nioise anid to insure niormal andnot excessive heating. This is particularly inmportant in

moving-coil loudspeakers where the ratinig may be lim-ited by the temperature of the voice coil.The rated power capacity of the loudspeaker is nor-

mally based on the type of signal obtainied from a liniearsystem. Short-time-peak to long-time-average powerratios may be in the order of 20 db in linear speech andmusic systems. If the system designer initends to repro-duce a modified signal, for example, a speech signal al-tered by substantial compression and limitinig or clip-pinig in which both the frequency and tinme distributionof the eniergy are modified, a special rating will, in genier-al, be required.

Because of the difficulty of producing a satisfactoryartificial test signal, speech and music are frequenitlyused. This is preferably selected recorded material.Other test signals, such as pulsed sine wave, band of

nioise, warbled or swept frequency, have been used. Theswept frequency signal is often one in which the fre-quency is varied liniearly with time over a specifiedranige.

Note: For accelerated life tests the signial is fre-quenitly supplied to the loudspeaker through a limiiiterso operated as to give approximately a 1/8 secondl p)eakto 15 second average power ratio of 6 db. TIhis altersthe eniergy distribution and substantially increasesthe heating effects. The rated loudspeaker capacity issaid to be equal to the rated amnplifier output powerif nlo failure or signiificant perform-anice degradationoccturs in 100 hours. WVhat is obtainied relates to a sys-teimi ratinig sinice the overload characteristic of theamiiplifier is also involved.

9.1.3 Duration of Test. Tlhe test should be conitiniuedfor a sufficient lenigth of time to permit (a) miiaximlunmsteady-state temiiperatures to be developed at all poinitsin the loudspeaker structure under the specified condi-tions of amlbienit air temperature anid venltilation, anid(b) fatigue failures of materials to develop (in order of107 stress alterniations).

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