An Inexpensive Type of Sound-Proof Room Suitable for Zoological Research

Modern developments in electronic technique have now rendered possible and attractive the electrophysiological study of auditory organs and the critical analysis of the sounds produced by animals. But in order to undertake such experimental work the provision of some form of sound screening or insulation is often required. The standard type of sound-proof room giving virtually full insulation at all audiofrequencies is of course ideal and for some types of work essential. Such rooms are, however, very expensive to construct, since, in order to be impervious to the lower frequencies in the auditory spectrum, there must be at least two constructional shells sufficiently massive to be themselves non-resonant and to support between them the considerable weight of sand which is necessary for insulation. Moreover, while a completely an-echoic room will probably be an unnecessary or unattainable refinement, most types of experiment will necessitate that the internal walls of the room have a high coefficient of sound absorption (i.e. be more or less non-reflective) over a wide range of audio-frequencies. This will require, in addition, a lining of glue-foam, ‘fibre-glass’ board or other similar material. Specifications for the construction of sound-proof rooms of the standard type will be found in the book by Constable & Constable (1949) and in the publications of the Acoustical Materials Association.

Fortunately, complete sound-proofing over the full range is not necessary for many types of zoological work. It may be sufficient to eliminate the higher frequencies only, provided the others are reduced by a known amount. This is particularly true of work with insects, and with the songs and call notes of the passerine birds, and with the sounds produced by the smaller mammals. The equipment here described was developed at the Madingley Ornithological Field Station of the Cambridge University Department of Zoology for the investigation and analysis of the innate and learned components of the sounds produced by passerine birds (Thorpe, 1954, 1955 and 1956). For such experiments it was necessary to be able to isolate birds, from the earliest nestling stages onwards, from the songs and call notes of the same and related species in sound-proof rooms with a low degree of echo. For this purpose sounds below 2 kc./sec. could be disregarded, and it was possible to adopt a mode of construction very much cheaper than the standard type. The details are published now in the hope that they may be of interest to biologists working in a number of different fields.

The essence of the constructional plan consists in the use of Thermacoust (woodwool 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 soundproof 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 :

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.

There are two shells, each consisting of 2 in. Thermacoust supported on a 2 × 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.

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. 2A).

Since the Thermacoust sheets were too narrow for the outer door to be covered by one sheet, special precautions to prevent the plaster cracking over the joint were necessary. The door was therefore covered with a thin metal sheet mounted over fibre glass; this gives increased rigidity without a great increase in weight or clumsiness.

Each shell has a double window—one sheet of Perspex and one oi 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 Thermacoust in the inner shell, and outside in the outer).

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; 10, hardboard sheet; it, 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, Thermacoust; 19, rubber facing to door frame.

Window: 20, Perspex; 21, glass; 22, rubber seating; 23, wood block; 24, Thermacoust; 25, plaster skin.

Ventilators. The room is ventilated by two Thermacoust ducts, the space insidebeing 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 ventilator 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 constructed of two layers of Thermacoust mounted on either side of a common wooden frame, and in having the floor similar to the inner shell.

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 110 db.

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 db. (i.e. approximately by a factor of over 3500) was felt to be eminently satisfactory. Thus at an average of 6000 cycles (6 kc.) sound outside the room at 110 db. was reduced inside the room to an intensity less than that produced by the respiration, blood circulation and heart-beats of the observer.

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 construction. 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.