MAY 1938 TECHNICAL APPLICATIONS OF SECONDARY EMISSIONS 139

AUDI:rORIUM ACOUSTICS AND INTELLIGIBILITY

secondary emission has heen employed and a slopeof from, 12 to 14 mAjV has been 'reached at ananod~ current of 8 mA, \vith a cathode current ofonly 2.5 mA, of which about 2 mA go to the auxil-iary cathode 5, and 0.5 mA to the screen grid

(fig. 9). In principle is is possible to employ morestages, .when the construction of course becomesmore complicated and the required voltage Va.higher. For these reasop.s only one stage has beenused in this amplifier valve.

. by R. VERMEUL~N.

The phenomena which influence the intelligibility of speech in an auditorium are discussed.Starting from knowledge gnined by experience about intelligibility as a function of theintensity of the signal and of the noise, it is shown that in a large room with much rever-beration the spoken word is only intelligible at certain places. Details are given of thecontribution to intelligibility by the direct and the reflected sound. From the assumptionthat only those waves which reach the listener within 1/15 of a second after the directsound contribute effectively to the intelligibility, are derived the principles for the designof an auditorium with good acoustics. .

An, architect who is designing a theatre audi--torium will not fail to provide an unhindered viewof t\le stage from every seat. There is, ,however,some doubt whether he always makes as good aprovision for the intelligibility of the spoken word,which is certainly equally important in the en-joyment of a play. It is not sufficient simply toprovide for the proper reverberation time in anauditorium, the significanee of which was explainedin a previous article 1), because the intelligibility,especially of the spoken' word, is determined alsoby other factors. Although the reverberation timeis the same at all points in the hall as required bySabine 's law, nevertheless it may be more' dif-ficult to follow speech at one point than at another,more favourable position. It is mainly due to ~hegreat importance of intelligibility of speech for thedevelopment of telephone communication, thatmany experiments have been carried out in thelast few decades on the dependence of intelligibilityon all kinds of disturbing and deforming effects 2).

Factors of in6uence on Intelligihility

It will only be possible here to give severalresults. In the first place intelligibility is to a highdegree dependent on intensity. In order to expressthis fact in figures it must be possible to measureintelligibility. This can be done by having a largenumber of sentences, words or sylables spokenand finding out what' percentage is understood

1) A. Th. van Urk, Auditorium acoustics and' Rever-beration, Philips techno Rev. 3, 65, 1938.

2), Good summaries may be found in the following books:Harvey Fletcher, Speech and Hearing: F. Tren-,delenburg. Klänge und Geräusche.

534.843.5

. correctly.' If syllables with no relation or meaningare used [nonsense syllables), the percentagecorrectly understood, which we shall 'call "intel-ligibility", will of course be much smaller thanwhen a connected sentence is spoken, since in thelatter case the words which are less well heard willbe automatically supplied. The relation betweenthis "comprehensibility" and "intelligibility" isshown in fig. 1.H the intelligibility is determined at different

sound intensities of the speech, without the listenerbeing disturbed by reverberation and foreign sounds,the ratio given in fig. 2 at 0 is 'obtained. 'I'hé other "

%100

t->l--/"

/I

I1/

// - «

71/

80

60

40

20

oo 100%26523

20 40 60 80

Fig. L Abscissa: intelligibility of nonsense syllables, ordinate:percentage of simple questions answered correctly. Thiscurve shows that an intelligibility of 50% is enough to makeit possible to follow an argument.

"

140 PHILIPS TECHNICAL REVIEW Vol. 3, No. 5

> '

curves in this figure refer to cases where the listeneris disturbed by noise of different intensity. Whilethe nature of the noise is not immaterial, the char-acter of the curve remains practically unaltered.Considering that the curves are practically parallel,the conclusion may be drawn that the influence ofnoise corresponds to a decrease in the intensitylevel of the speech, or in the other words, to an

, increase of the threshold of the ear. It may be seen. from the figure that intelligibility at equal inten-sities of noise and speech is about 50 pe~ cent,which may be considered sufficient for following anargument, although an intelligibility of 70 per 'centmust be required for good acoustics (see fig. 1).A second factor which may affect intelligibility,

and which is especially important when amplifierinstallations with microphones and loudspeakers.are to be used for transmission or amplification, isthe change in the distribution of sound energy overthe various frequency ranges. Although the fun-damental tones of the voice lie in the neighbourhoodof 125 cycles/sec (male) to 250 cycle.s/sec (female),the range from 1000 to 2000 cycles/sec is the mostimportant from the standpoint of intelligibility.This follows from' experiments whose results' aregiven infig~ 3. In this figure intelligibility is plottedas a function of the limiting frequency of a filterwhich only passes those tones of the voice which liebelow (L = low), or above (H = high) that limitingfrequency. The inscription under the figure givesfurther details. Itmay be concluded from the figurethat an intelligibility of 60 per cent can be obtainedeven though all the tones below Têûû cycles/secare missing from the speech. That the quality of

%fOO

90

80

./~ ~>~ P;= ~ i-

J J J / J J J /v"1/ 1/ V 1/ V 1/ 1/ 1/ 1/

Jo, /32 /45 /55 /65 /75 J85 /95 :5t IT1/ 1/ 1/ / I I I I I I /0\15V '1125

/ / / / / / I I I I ,I1/ 1/ I1 V 1/ 1/ V I1 I1 1/ I1I I / 'I J J I / / I /

1/ 1/ 1/ V V 1/ 1/ V 1/ 1/ 1/

70

60

50

40

30

20

10

oo 20 40. 60 .80 100 f20 db26524

Fig. 2. The percentage of nonsense 'syllables plotted as afunction of the level of the spoken word above that of thethreshold value and that of the noise. The successive curveswere recorded in each case with an increase in noise such thatthe threshold value increased by 10 decibels. It may be seenthat the curves are practically parallel and that the thresholdvalue lies constantly 25 decibels below the level of the noisewhen the latter is not greater than 40 decibels.

%10

9

0~ V

/"0 "-1\ IJ·0

[\J0 V0 (\0 / \',0 17 1\

IJ \0

I1 -~

0

L/o .JII\..

3

8

4

3

2

4 6 8104Hz26525

Fig. 3. The percentage of nonsense syllables understoodcorrectly by the listener plotted as a function of the limitingfrequency of a filter included in the circuit of the soundproducer. Curve L was determined with a filter which passesthe tones below the limiting frequency so that the influenceof the lack of higher frequencies is shown here. In the absenceof frequencies above 2000 cycles/sec,75% of the syllables werestill correctly understood. Curve H is determined with a'filter passing the high, frequencies: from this curve it may:be deduced that when all tones below 1000 cycles/sec arelacking the intelligibility is still above 80%.

.10 4 6 8 fa 2'2

the spoken word is thereby very much altered, isobvious.

Reverberation as a disturbing factor

Noise which decreases the intelligibility of thespoken word does not consist only of that cau~edby the audience, and such other sounds as the humof fans and street noises, etc. In addition, the soundof previously spoken words which remains in theauditorium as reverberation disturbs the listeneras he tries to understand the new words.

Even if it is possible, by increasing the strengthof the voice', and, to a greater degree, by the useof sound amplifying apparatus, to raise the levelof the spoken word sufficiently above that of theintruding noise, these methods fail when we are con-cerned with the reverberation of that spoken worditself: the intensity of the disturbing reverberationwill increase in proportion to the direct sound. The'result is, that a speaker in a hall with bad rever-beration can only make himself understood overa limited area of surface which cannot be increasedby the use of loudspeakers.

Without going into too much detail a roughcalculation will demonstràte this fact. When' a,number of sources of sound with a total power Ware set up in a hall whose walls have an absorptionequivalent to an open window of area A, the averageenergy density of the reverberation is:

MAY 1938 AUDITORIUM ACOUSTICS AND INTELLIGIBILITY 141

4W.Em = --,

cA

where c is the speed of propagation of sound(van Drk, loc.cit.). The density of energy. of.the direct sound from the source indicated by thesubscript i, at a distance Ti from the source,

WiEd = . . . . . . .. (2)4 :reTi2 c

From these two formulae it follows that onlyover a limited distance' wili the intensity of thedirect sound be greater than that of the disturbingreverberation as was found necessary for reasonablygood intelligibility' (50 per cent). This maximurndistance is given by

Wi ATi2= W . 1610 . . .. (3)

If to choose the most favourable situation, it, ,is assumed that each source of sound is open tolisteners on all sides, the area where the sound isintelligible is then:

. .2Wi A A(1= :reZTi2 = ui 16 = 16 (4)

Equation: (4) may be used to describe the situationin large rooms with much reverberation where onlya small part of the space is occupied by an audience.It w~uld for instance be possihlë to calculate bymeans of this equation, over how large a part ofan indoor swimming pool the results of a competi-tion could be announced at the same moment.If, however, the' audience occupies a large part

of the area, the intelligibility area can be increasedby making use of the fact that the absorption co-efficient of the audience is nearly 100 per cent. .This obviously completely nullifies the basis ofSabine 's reverberation theory. The sound whichreaches the audience directly will not be reflectedand will not contribute to reverberation. The restof the sound will give rise to reverberation whoseaverage energy density is given by equation (1).If p is the part of the sound which reaches theaudience directly, the total power of the sources ofsound which contribute to reverberation is W (l-p),.and equation (1) mûst now be replaced by

. 4WEm =' - (l-p) (la)

. cA

The area' which can be reached with inteJ~gibilitythereby becomes greater:

A(4a)(1=

16 (l-p)

(1)This result is important chiefly in the case when loud-speakers are used as sources of sound. In order tomake p as great as possible loudspeakers withdir~ctional effect may be used, and they may bedirected toward the audience. In this way, evenunder unfavourable circumstances, an adequateintelligibility can be reached over the whole areaoccupied.

Influence of the growth of sound

. The above' considerations are based on the as-sumption that only the direct sound contributesto intelligibIlity, and that all the reflected soundmust be considered as noise. This assumption will~pptoach the actual situation in very large closedspaces; it does not, however, give a correct pictureof the acoustic relations in a smaller room.The truth of this will be seen when the intelligi-

bility in a room is compared with that which canbe attained out of doors. The direct sound is equallyintense in both cases, but indoors reverberation ispresent as a disturbing factor. One might thereforeexpect that the voice of a' speaker in a hall wouldonly be intelligible over a shorter distance than outof doors.Experience, however, shows that the opposite

is true. Although at the rear of a hall one directsound often supplies only a very small part of thetotal intensity, a speaker can usually be understoodmuch farther away in a closed room than out ofdoors. It is therefore apparent that sound reflectedby the walls and ceiling is, at least partially, in-telligible, and is not experienced as noise. The sizeof this part will depend upon the way in whichthe sound grows at the beginning of each newsound.It will be a surprise to many to learn that when

a constant source of sound is turned on the soundintensity grows in a way analogous to the wayin which it dies out after the source has been inter-rupted. This is surprising, because while the echoingof a voice in a large church is a common experience,one can not 'recall having observed a gradualgrowth. A concise, although not entirely correctrepresentation of this difference in the observationof reverberation and growth, is obtained by as-suming that the sound impression is proportionalto the logarithm of the intensity, whieh comes downto this, that the ear judges equal percentages- ofchange in intensity as equal differences. 'In fig. 4.b the growth 'and decay of sound in a

room is shown diagrammatically, with a linearscale of intensity for the variation of power radiatedaccording to fig. 4a. The intensity Ig during growth

142 PHILIPS TECHNICAL REVIEW Vol. 3, No. 5

.t 1111111111111111111111111tf

I

ti-

lo

71-Ig Ivr--- -I.U .L

b

ot/

- -I--I"-I-

0,4 0,8 ~ 1ft 2,0 2,4 2,8 ~ 3,6 4,0 4,4 4,8sec26526

Fig. 4. Diagrammatic representation of the growth and decayof sound when a source of constant strength is switched on att = 0 and stopped at t = tt' a) Radiated power of the sourceas a function of the time. b) Intensity in the room as a functionof time. The growth is the complement of the reverberation(the decay), i.e. Ig + Iv = 10' c) Logarithm oftheintensityas a function of time. The last curve represents approximatelythe subjective impression, and shows that growth seems tobe of shorter duration than reverberation.

.is here described as a function of the time by

Ig = 10 (I-e-at),

while the decay takes place according to the formula

Iv = 10 e-a(t-~)

(see van Urk, loc.cit.). The growth and rever-beration are complementary, i.e., the intensitiesIg and Iv measured at the same time .. afterswitching on and off respectively satisfy the relationIg + Iv = 10, which is understandable when oneconsiders the interruption of the source as the ad-dition of a second equally intense source with anegative sign.If the same Ivariation is now drawn on a loga-

rithmic scale of intensity (fig. 4c), the differencebetween growth and reverberation is clearly seen:growth takes place suddenly and is completed ina fairly short time, reverberation proceeds slowlyand uniformly. Long after the intensity (fig. 4b)could no longer be drawn, the reverberation isstill above the threshold value.

It is not easy to represent in numbers the in-fluence of growth on intelligibiÏity because it isquite different for different sounds. By speakingslowly the vowels can be so much drawn out thatthey always have the opportunity of growing tosufficient intensity. Since during that time the rever-beration of the previous sound has completelydisappeared, the speaker will always be able to

reproduce the. vowels without disturbance andwith sufficient intensity. This is the basis of thepreaching 'tone uS{lalin churches. 'The consonants, and especially b, p, d, t, g, k, etc.

cannot be drawn out by speaking slowly. Theycannot, therefore, on the one hand reach their fulldevelopment, and, on the other hand, they sufferinterference from the previous sounds. For theseletters, therefore, a gradual growth of sound is per-missible only 'to a ,much smaller degree than forthe vowels.In order to find out to what degree reflections

from the walls may produce an increase in intel-ligibüity even for such short sounds, an approx-imation of growth by an exponential curve is notsufficient. Actually the intensity varies by jumpswhich are connected with the arrival of, the wavesreflected by the various walls, and this variationwill differ essentially for example at the backof an auditorium from that in the neighbourhoodof the speaker.In: order to make this clear, in jig. 5 the pro-

pagation of a sound wave is followed step by stepin an imaginary room. The 15 diagrams representsuccessive instantaneous states at intervals of1/150 sec. The waves start from a source of sound A,and it is obvious that the front B of the "direct"wave is the first sound to reach every listener.The intensity of this wave decreases with thesquare of the distance from the source. Actuallythe direct sound will be even more weakened inmany cases, since it passes over an audience whichabsorbs sound strongly. In such a case it may beassumed that the sound pressure, which is thedetermining factor in observation, is given by thecurrent of sound energy which is incident uponone unit of area of the absorbing surface, and notby the intensity of the sound wave which wouldexist at that point in the absence of an absorbingsurface. The sound pressure will therefore usuallybe very small, because the direct sound from aspeaker or ,an orchestra nearly always travels in apractically horizontal direction, and a given areatherefore receives energy from the source onlywithin a very small solid angle. Only with loud-speakers, or by placing the speaker on a high pro-jecting rostrum, is it possible .to direct the soundtoward the audience at a steep angle. Without theseaids very little of the direct sound reaches the backof the hall.r· In fig. 5 it ~ill be seen, however, that it is at theback of the hall, where th~ direct wave is so muchweakened, that a large' number of reflected waveswith slight differences in time reach the audience.

MAY 1938 AUDITORIUM ACOUSTICS AND INTELLIGIBILITY 143

The question, now is: .to what degree can these Useful and detrimental soundwaves help- to increase the intelligibility? Since the ear, like the eye, possesses the charac-

o

G

c• H

4'

A•

7

A•

10

A,.13 '

2

5

3

Ij

A•

8

A•

9

A•

11

A•

12

A• A•

Fig. 5: Let the above figure'represent the cross section of an auditorium. A source of soundis placed at A, and.it begins to radiate at the moment t = O.In the successive pictures thewave front is drawn at intervals of 1/160 sec. The walls are considered to be completely.reflecting, while the strength of the wave is represented approximately by the thicknessof the lines. The waves are indicated by the same letters as the walls by which they arereflected. The most important of these are:

B: the "direct" wave, i.e. the wave which has not bcen reflected by anywall;

C: the reflection by the floor;D: the reflection by the wall behind the source;E: the reflection by the sloping roof above the source;F: the reflection by the ceiling;G: the reflection by the curved rear walls;H: the reflection by the vertical part of the rear wall.

Because of the complexity of the combinations of repeated reflections, it is impossibleto continue this method of indication consistently.

144 PHILIPS TECHNICAL REVIEW Vol. 3, No. 5

teristic of blending' stimuli received during a certainperiod of time to a single impression, the waves, which are incident one after an~ther can reinforce .each other, if their succession is rapid enough. Thetime within which the ear is unable to perceivesuccessive reflections as separate from each otheris 'of course not sharply limited, but it is of theorder of I/IS sec. The waves in fig., 5, which reachthe listener within 1/15 sec after the first sound im-pression, therefore act together to increase theintelligibility: they are "useful" sound. That whichcomes later may be "detrimental" because of thefact that it is observed as a new impression and,as such, may be disturbing. It is now possibleto 'find out in a simple way where "detrimental"sound first occurs in fig. 5. In the first 10 statesonly useful sound can reach the listener, since tbese10 states occur within 1/15 -sec, In the next few •states also the audience receives no "detrimental"sound, because at those places. reached byreflected sound the direct wave. B is incidentonly several hundredths of a second after the sourcehas begun to work. From a comparison of the 4thwith the 14th state and of the 5th with the 15th,it will be seen that at the points of the arrows

, Pand Q respectively, the reflection of the rear.wall is incident just 1/15 sec after the direct sound.The wave H, which up to this moment as usefulsound could make a contribution, to the intel-ligibility, now begins to be detrimental. To an evengreater degree is this the case with the wave Greflected by the curved rear wall and concentratedin a focus. This gives an excessively reinforcedsound wave which arrives 0.08 sec later than thedirect wave. It is distinguished from the otherreflections which make up the reverberation byits greater intensity, and is therefore observedseparately as an echo.The analysis as carried out in fig. 5 is too time-

.26527

Fig. 6. If it is necessary that the ceilingof an auditorium shouldbe higher at the rear it can to advantage be constructed in theform of steps. . "

consuming for practical application and, more9ver,it extends only over 1/10 of a second. It-must usuallysuffice to find out how much energy the differentparts of the audience receive from the direct beamand from the principal reflected waves which differin length of path covered by not more than 20'metres (difference in time about 1/15 sec). This can bedone quite simply by drawing a beam of sound raysstarting from a speaker at various suitable places,and following these rays through their first andsometimes second reflection, until they reach thelisteners. It must hereby be kept in mind that theaim is to throw' the sound as much as possible on ,the audience and, in addition, to distribute theavailable energy as uniformly as possible to preventone listener from obtaining too much at the expenseof another.In this investigation those rays of sound are also

di~covered which differ in length of path by morethan 20' metres from the direct sound, and whichtherefore, if they are sufficiently intense, can giverise to an echo.

.Influence of the shape of the hallIn connection with the foregoing we shall discuss

several points about the influence exerted by theshape of the hall and its walls on intelligibility.This discussion cannot be a complete one, but onlyan explanation of the ways in which the usefulsound can be increased and the detrimental sounddecreased.

Useful soundAs we have already shown, except with a raised

source of sound (a loudspeaker for example), the'direct sound radiation will only be of importancefor the first rows of seats. The rest of the audienceWill have to' rely upon sound reflected by the walls.The ceiling, considered as one of the walls, willfulfil the most important function in this reflection,and a plane surface will in ordinary cases be foundfavourable. The front part of the ceiling, how~ver,throws the sound into the front of the hall whereit is not needed, and it is therefore an advantage tomake this part of the ceiling sloping as shown infig. 5 at E. The rear wall also, especially under thebalcony; becomes effective if it leans forward at asuitably-chosen angle. In some cases there will bea conflict between the position of a wall desirablefor other reasons and that necessary for the correctreflection of sound. This is true for instance whenthe ceiling slopes upward toward the rear, wherebytoo much sound is .reflected to the extreme rear. of the hall. A step-shaped design, as 'indicated infig. 6 may then often solve the difficulty. Care must.

MAY 1938 AUDITORIUM ACOUSTICS AND INTELLIGIBILITY

be taken not to exaggerate by carryingthe principle of the reflecting actionof the walls too far. It will in generalbe satisfactory if the audience receivesreflected sounds from several direc-tions. This can, for example, be achievedby making the reflection somewhatdiffuse in the transverse cross section.This is not only desirable for themore uniform distribution of the soundand for the greater independenee ofthis distribution on the position ofthe source, but also in order to avoidthe speaker being disturbed by noisefrom the public by a reversal of theray diagram.As a check on our calculations in

the case of complicated shapes of halls,optical models can be made of the dia-grams constructed. In these models,

a

b

145

Fig. 7. Optical model of the large assembly hall in the Leagueof Nations Palace in Geneva. The place of the speakers isrepresented by an electric lamp. At the intended positionsof the audience, plates of frosted glass are introduced. Theintensity of illumination at these places is a measure of thesound intensity available for the audience.

D

c c

H

6

F

26550

Fig. 9. Cross section of the large assembly hall in the Leagueof Nations Palace. The speakers rostrum is at A and is sur-rounded by a marble sound reflector B. The ceiling is partiallystrongly absorbent (C), and partially occupied (B) by a glasslighting element. The audience is situated in the centreportion of the hall E and in the various balconies F, G and H.

D

B

Fig. 8a). Distribution of light on the floor and rows of seatsaround the floor.b). Distribution of light when the quasi-parabolic reflectorbehind the speaker rostrum is inactive. Not only at the rearbut also along the sides of the hall is the illumination andtherefore the sound intensity, less.

2464S

Fig. ID. Course of the sound rays in the large assembly hallmade visible in the smoke-filled optical model. This figuremakes particularly clear the importance of the ceiling andof the reflector for the distribution of sound in the directionof the horizontal axis of the room. The letters have the samesignificanee as in fig. 9.

146 PHILIPS TECHNICAL REVIEW Vol. 3, No. 5

\I

an example of which was described previouslyin this periodical ê) the source of sound is replacedby a small lamp and the coefficient of optical re-flection of the walls of the model are made to cor-respond somewhat to' the acoustic coefficient ofreflection in the actual structure. The intensityof illumination of every surface ~s then a measureof the sound intensity at that place ..In the attempt to detect the cause of undesired

deviation the rays may be made visible by fillingthe model with a mist, Fig. 7 shows such a modelof the large assembly room of the League of NationsPalace in 'Geneva, the cross section of which isgiven in fig. 9. Figures Ba, band 10 îllustrateseveral investigations carried' out on this model.The inscriptions under the figures furnish an ex-planation. ,

Detrimental sound

Echo occurs especially in the case of concavecurved, surfaces, an example of which was given infig. ~ at G. Such surfaces must therefore be avoidedas much as possible, or, when' they cannot beavoided experiments must be carried out to findout whether that shape is really permissible.Such a form is permissible when the radius ofcurvature is>sufficiently great: it may be assumèdas a rule in the transverse cross section that aradius equal to twice the. ,he,i~ht is desirable. Inthat mise the wave is projected on the audience asa fairly ,parall~l beam. .

26528

'Fig. 11'.By means of a ceiling with a radius of curvature oftwice its height above the source of sound, the sound is reflectedupon the audience in the form of a nearly parallel beam.

Cupolas must always be regarded with suspicion,as well as. elliptical forms and the like. Roundedangles with a sufficiently small angle of curvatureare often succesfully used. In such casès the focusmust be far above the audience which serves to

3) R. Vermeulen and J. de Boer, Optical model ex-periments for studying the acoustics of theatres, Philipstechno Rev. 1, 46, 1936.

give a strong spreading of the sound. This methodcan be used to advantage in the angle formed bythe ceiling and the rear wall which has a strongtendency to throw the sound back to the speakeras a disturbing echo.If the reverberation is too strong and absorbing

materials must be applied, the. best places are theside, walls which usually do not play an importantpart in the distribution of the useful sound.

sec2,5

2,0

b

V .a

./:---.

t5

1.0

0.5

oo 20000 30000 40000 5()000 m3

2662S10000

Fig. 12. Reverberation time for the spoken word as a functionof the volume of the room, according to Knudsen. Uppercurve: recommended values for ordinary theatre auditoria,lower curve: reverberation' times at which intelligibility is ata maximum.

In' general it is not wise to reduce the rever-'beration more than necessary; in the first place

. because the spoken word becomes dull and Iess.pleasing, and in the sècond place ..because there is nosharp difference between walls which refle?t us'eful,and those which reflect detrimental sound, ~o that·'a decrease in reverberation is in fact always accom-panied by a' decrease' in the intelligible sound. BY1means of measurements on a large number of halls.with unusually good acoustics and on theoretical'considerations, various investigators have found'optimum values of the reverberation 'time. Thesevalues increase with the volume of the room (seefig. 12), which means simply that in large hallsa longer reverberation must be allowed in oldernot to suppress the desired reflections (from theceiling for example).In general it may be said that the acoustics of

a hall become pleasanter when the wall surfaces,which do not contribute to the useful reflection' areso constructed that they scatter the sound whichis not absorbed. For this purpose a' coarse roughsurface is necessary since unequalities of the sur-face which are small with respect to the wave lengthof sound are ineffective.

In many cases it may be doubted whether modernarchitecture, with its preference for simple lines andsmooth surfaces, is always as efficient from thestandpoint of acoustics as it pretends to be.

PUMPING PLANT FOR RADIO VALVES

MAY 1938

The illustration shows the method employed forevacuating radio valves. Unlike the evacu~tionof incandescent lamps great care must be takenhere to out gas the parts mounted in the valvesuch as cathode, molybdenum grids and nickelanode. The rotating pumping plant has a numberof points of connection which move through aseries of positions on the circumference of acircle. Each connection point has its own pumpwhich accompanies the valve from one positionto the next. The small mercury vapour pumpshave a common backing pump which is dis-connected during the passage from one positionto the other. The connection between the valve

147

and the pump is formed by a thin glass tube.The inner parts are outgassed by means of highfrequency fields induced by currents throughspiral tubes lying around the valves at certainpositions. These spiral tubes are connected inseries and are supplied with current from aninstallation not visible in the drawing. At otherpositions the outgassing is continued by electronbombardment if necessary. The gas is liberatedfrom the cathodes by electrical heating. In thephotograph the electrical contacts for connectinggrids, plate and cathode can be seen. The blackcoating on part of the bulb is for the purposeof eliminating secondary emission.