An Inexpensive Type of Sound Proof Room Suitable for Zoological Research

AN INEXPENSIVE TYPE OF SOUND-PROOF ROOM

SUITABLE FOR ZOOLOGICAL RESEARCH

BY W. H. THORPE AND R. A. HINDE

Madingley Ornithological Field Station, Department of Zoology, University of Cambridge

{Received 22 June 1956)

INTRODUCTION

CONSTRUCTION

The essence of the constructional plan consists in the use of Thermacoust (wood-wool slabs) as the main building material. This has the great advantage that it is itself sufficiently strong and rigid to constitute the actual framework of building and yet, when smoothly plastered on the outer surface, it confers a high degree of sound insulation (from the outside), while the unplastered inner surface is reasonably absorptive of sound. It is thus possible to avoid the expense of building the two massive and independent double frameworks necessary to contain the two separate layers of insulating material (sand, broken cork, dried eel-grass, etc.) required for the standard type of sound-proof room. All that is required is to construct two independent shells of Thermacoust, plastered on the outsides, each supported by a separate light wooden framework.

The present paper is based on the experience gained in building two sound-proof rooms. The description given below refers primarily to the second (room A) which incorporates lessons learned in the building of the earlier one (room B). Although, as will be seen from the tables, room B actually produces a slightly better sound attenuation than does room A, this is because room B was at first unsatisfactory in certain respects and was subsequently modified at considerable extra cost. In view of this elaboration it must be regarded as the less efficient of the two. The design of room A, if properly carried out, is both efficient and fully adequate for its purpose. The construction of room A is illustrated in Figs. 1—3. The principal features are as follows:

Floor. The room is built on the wooden floor of a pre-existing hut, the foundations

of which consist of four dwarf brick walls on concrete footings, running lengthwise of the hut. The floor of the room consists of a bitumen-bonded fibre-glass mat lying on fibre board which in its turn lies on the floor of the hut. The area of the bitumen-bonded fibre-glass mat is slightly greater than the plan area of the outer shell, so that it projects slightly round the edges.

Walls and roof. There are two shells, each consisting of 2 in. Thermacoust

supported on a 2 x 1 in. wooden framework. This framework lies in the 3 in. space between the two Thermacoust shells—so that there is a 1 in. space between the battens on the inside of the outer shell and those on the outside of the inner shell. The nails holding the Thermacoust to the wood penetrate only a short distance into the wood. The outside of each shell is covered in a fairly thick layer of plaster.

Doors. The doors open outwards in order to give the maximum of useful space

inside; the outer door is therefore somewhat larger than the inner. All edges of the doors are bevelled and shut against sheet sponge rubber so that there is a tight seal all round (Fig. 2 A).

Windows. Each shell has a double window—one sheet of Perspex and one 01

glass (Fig. 2B). Each sheet is supported in sponge rubber held in a wooden frame, the latter being attached to the side of the Thermacoust (inside the Therma-coust in the inner shell, and outside in the outer).

12

-13 , _ / _

Inchei

Fig. i. Front elevation with front of outer shell removed.

Explanation of Figs. 1-3

1, plaster skin of outer shell; 2, Thermacoust of outer shell; 3, wood frame of outer shell; 4, space between inner and outer shells; 5, wood frame of inner shell; 6, plaster face of inner shell; 7, face of wooden frame of inner shell; 8, door; 9, bitumen-bonded mat; io, hardboard sheet; 11, ventilator shaft; 12, packing to ventilator shaft; 13, open end of ventilator shaft.

Door: 14, wooden beading frame; 15, galvanized iron sheet; 16, fibre glass; 17, plaster; 18, Therma-coust; 19, rubber facing to door frame.

753

Inches

15 16 17 18

ESThermacoustraWoodEB R u b b e r , , Plaster mFlbre-glass ^«Sheet metal

Inches

Fig. 2. Construction of A door and B window.

Inches

Ventilators. The room is ventilated by two Thermacoust ducts, the space

inside-being 3 in. square. Each duct has two right-angle bends, one in the plane of the roof, the other at right angles to this, leading down into the interior. These ven-tilator shafts thus form the only points of contact between inner and outer shells above floor level; where the shafts penetrate the shells they are insulated from them by a packing of sponge rubber. The outer surface is plastered (Fig. 3).

Room B. This differed from room A primarily in that the inner shell was

con-structed of two layers of Thermacoust mounted on either side of a common wooden frame, and in having the floor similar to the inner shell.

EFFICIENCY

The performance of the rooms was tested by generating tones of known frequency and amplitude immediately outside and testing their intensity inside by means of a Ribbon Microphone and a Dawe Sound Level Meter (type No. 1400 c). In order to overcome the position errors produced by standing waves, the sound was produced by means of a Warble Tone Generator at n o db.

Table 1. Showing efficiency of sound-proof rooms A and B expressed as attenuation

in decibels of a sound of n o db. produced by a Warble Tone Generator at different average frequencies {expressed in cycles per second)

The internal ambient noise level due to the presence of the observer, etc., was 38 ± 1 db. This was increased to 54 ± 1 db. by fluorescent lighting. Figures for sound level are taken relative to the human auditory threshold reckoned as coooz dynes/cm.1 _{at 1 kc.}

Frequency (c./sec.) 1000 2000 4000 6000 8000 IOOOO IOOO 2000 4000 Sound level

outside (db.) Sound levelinside (db.)

Room A no no no no no no 55 43±2 42±2 4O±3 Room B no no no 53±i 4 5 ± i 43 ±2 Attenuation (db.) 55 67 68 70 >7o >7o 57 67

The results, shown in Table 1, are self-explanatory. Since the sounds produced by the main species being investigated contain practically no audio-frequencies below 2000 c./sec. (2 kc), a screening which attenuates this frequency and above by

not less than 67 db. (i.e. approximately by a factor of over 3500) was felt to be

SUMMARY

The design of an inexpensive sound-proof room, virtually sound-proof to audio-frequencies of 2 kc. and above, is described. Since for the purposes of many zoological experiments low frequencies can safely be disregarded, it is felt that on account of cheapness and relative ease of construction, this type of sound-proof room has much to recommend it.

We are greatly indebted to Mr H. R. Humphreys, of the Engineering Division of the British Broadcasting Corporation, and to Mr N. Fleming, of the National Physical Laboratory, for most valuable technical advice on the details of construc-tion. We are also very grateful to the British Broadcasting Corporation for the loan of a Warble Tone Generator and a Dawe Sound Level Meter. Mr J. A. Popple gave indispensable advice and help in measuring the efficiency of the rooms.

REFERENCES

CONSTABLE, J. E. R. & CONSTABLE, K. M. (1949). The Principles and Practice of Sound Insulation. Pitman: London.

THORPE, W. H. (1954). The process of song-learning in the chaffinch as measured by means of the sound spectrograph. Nature, Land., 173, 465—9.

THORPE, W. H. (1955). Comments on The Bird Fancyers Delight; together with notes on imitation in the subsong of the Chaffinch. Ibis, 97, 247-51 _•

THORPE, W. H. (1956). Learning and Instinct in Animals. London: Methuen.