Low Frequency Limitations Of Loudspeakers

It is obvious that the acoustic energy radiated must be related to the amount of air in contact with the vibrating radiator and to its amplitude of vibration. Now the amplitude of vibration is limited by the mechanical characteristics of the diaphragm suspensions and therefore for some fixed amplitude the size of the diaphragm is the determining factor.

The use of two loudspeakers doubles the area of the radiator; this is equivalent to a diameter increase of 40%. Therefore, less amplitude of vibration would be required.

Often increased low frequency output is sought by increasing the electrical power (by equalizing the amplifier); this can only result in increased output if greater amplitudes of vibration result. When the suspensions reach a certain displacement they cause distortion and may even fail mechanically. The only satisfactory solution is to actually or effectively increase the size of the radiator.

The above discussion applies only to maximum output in the low frequency region. When reproducing at relatively low levels, the low frequency output can be emphasized by equalizing the amplifier and such practice is recommended. The low frequency efficiency can also be increased somewhat by proper cabinet design. This subject will be treated later.


When choosing a loudspeaker one is often limited by the maximum physical dimensions that can be accommodated. If no such restriction exists, then the choice can usually depend on the low frequency performance desired. The following table can be used as a rough guide to the low frequency limit for various loudspeaker sizes:

Size Low Frequency Limit

15" 50

12" 60

10" 70

8" 80

6" 95

5" 115

This table assumes an adequate baffle as discussed later. The high frequency range of direct-radiator loudspeakers depends on the dynamics of the radiating structure. It is possible to obtain useful output to approximately 10,000 cycles for all sizes up to and including the twelve inch size; the 15 and 18-inch sizes are limited to about 8500 cycles. No such limitation exists for multiple channel systems.

The audio frequency power available in most amplifiers is more than adequate for most home applications. In some cases the output power available is quite high and provision must be made to protect the loudspeaker if it does not have a sufficiently high rating.

The power rating of a loudspeaker should be the maximum reading of a VU meter for music signals which can be reproduced without distortion or damage to the loudspeaker after prolonged use. Usually distortion is the controlling factor; a power of twice the rated value can usually be reproduced without damage. However, it must be remembered that the maximum power output of an amplifier may exceed its rating by perhaps a factor of two also.

The power rating of a loudspeaker is a limiting sort of thing and it must not be presumed that such a power must be available to use such a loudspeaker. The power rating is related somewhat to the efficiency, i.e. the higher rated speakers are usually the more efficient. The question often arises - Can a large loudspeaker having desirable characteristics but also high power rating (and usually high efficiency) be connected to a small, low power amplifier? The answer is that the large speaker will give more sound output for the same input power than does the original small speaker because of the added efficiency; therefore the substitution of a larger loudspeaker is always feasible from this viewpoint.

From these considerations it becomes obvious that one can usually improve the average radio receiver to a marked degree by merely changing to a more adequate loudspeaker system. It should also be noted in this connection that for a loudspeaker with higher efficiency less distortion in the amplifier will result for a given acoustic output since less electrical power is required.

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