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Recording location

The sound produced by an appliance can depend greatly on the location in which it is placed. For example below are two sounds of a kettle recorded in two very different rooms. One of them is recorded in a semi-anechoic chamber which has its surfaces covered in absorbent so you here just the sound straight from the kettle. The other is recorded in a normal kitchen. The sound quality is very different between the two sound files, and the kettle in the anechoic chamber sounds a little unnatural.

Three kettle picture

(These kettles do not match the sound files)

If we are going to get people to listen to sounds, it is important that the sounds we used are representative of the product in real use, otherwise the results are likely to not be very useful. We compared peoples' opinions of kettles when recorded in the two different locations, and found the results were quite different.

There are several ways in which the location of the product affects the sound it produces. Consider the cases of washing machines and kettles:

Structure borne noise

If the washing machine is placed on a suspended wooden floor, the washing machine may cause the floor to vibrate and make noise and so create a different sound in comparison to when the washing machine is placed on a concrete floor. In a similar way, kettles can cause worksurfaces to vibrate unless they are very heavy, and this can create a very different sound quality. The next time you are boiling a kettle, try lifting it (carefully!) a little way off the surface, and for many kettles you will hear the sound change.

Immediate enclosure

If the washing machine is built into cupboards then the enclosure can reduce or increase the noise (depending on how it is made). Certainly you would expect fewer high pitched (frequency) sounds, and more bass (low frequency) sounds. If the kettle is placed near walls and cupboards (as is common), then the reflections from these surfaces interfere with the sound direct from the kettle to change the sound quality. Again, you could test this yourself by trying a kettle in different places in a room.

The recording room

Rooms affect sound quality in several ways, but the most obvious is reverberation. This is the echoeing of sound around a room, most obvious in large spaces such as churches and railway stations. Again the reverberation affects the sound quality produced by products.

The problem in deciding exactly what test conditions to use. Certainly, it is best to chose a location which is typical of everyday use; specialist test chambers such as anechoic chambers are not best. But the problem is that there is not a typical installation that everyone will use for a product in many cases. And it isn't possible to try all possible installations, because the test time would take far too long. Consequently, it is necessary to pick a representation case to test, maybe the one most often used or the one which causes the most noise problems. If there is a British or International Standard for measuring the sound power level from the device, it is worth using the same set up for the sound quality tests [e.g. 1].

It is important that each product is put in exactly the same place in the room, and the recording conditions are the same for every product. Otherwise, the differences in sound that you get from having the product in different places might be want people listen to, and not the intrinsic difference in sound from the different products.

In this project we tested three different products, and these are the conditions we would recommend to use for these.

Washing Machine

The washing machine should run in normal room conditions, i.e. in a kitchen or in a room with an acoustic close to that for a normal kitchen. We took a room which was a typical size for a kitchen. The room was very reverberant, so we brought in some absorbing materials to make it sound closer to a real kitchen. This was done using acoustic absorbent (mineral wool similar to that used in loft insulation). The product should be placed along a wall, if possible far from corners. The washing machine should be placed under a worktop and between two side boards to represent the typical set up of a kitchen. In any case, the washing machine must not touch the worktop, the wall and the side boards to prevent structureborne sound. The washing machine must be well balanced on the floor. We placed ours on a solid concrete floor. The background noise level of the room should be low. You should not be recording significant amounts of noise from anything other than the washing machine.

A distance of 1m from the kettle is chosen because this is a typical listener distance and also a standard test distance in acoustics. The height of the microphone is chosen as a typical standing height.

Set up for testing washing machine

(surrounding worksurfaces not shown)

Leaf Blowers

To simulate outdoor conditions, we recorded the leaf blowers in an anechoic chamber as shown in the right picture. An anechoic chamber has absorbent walls, so sound does not reflect off them, so this is like using the leaf blower in a large open space. These facilities are not available to most people, in which case it is easiest to make recordings outdoors. However, it is important that you chose a day that isn't too windy or raining (as these can cause noise), or a place where there is noise from other sources. We found it didn't matter much over what ground we used the leaf blower, the sound levels were similar. Again the microphone was placed about 1m from the device at a normal standing height. The microphone system shown to the right is a dummy head. Hearing defenders should be worn to protect the hearing of the user.

Testing a leaf blower and recording on the summy head

Kettle

The kettle should run in normal room conditions, i.e. in a kitchen or in a room with an acoustics close to the room acoustics of a kitchen. We used a small kitchen within the University. The kettle was placed in a typical position, on a worktop near some cupboards as shown in the right picture. The height of the microphone is chosen as a typical standing height. A distance of 1m from the kettle is chosen because this is a typical listener distance and also a standard test distance in acoustics. The background noise level of the room should be low. You should not be recording significant amounts of noise from anything other than the kettle.

Layout for kettle testing

[1] EN 60704-2-4:2001 "HOUSEHOLD AND SIMILAR ELECTRICAL APPLIANCES – TEST CODE FOR THE DETERMINATION OF AIRBORNE ACOUSTICAL NOISE –

Part 2-4: Particular requirements for washing machines and spin extractors"