Back-to-Front Cancelation

While the front surface of the cone of a loudspeaker is pushing forward to create a sound wave by increasing air pressure, the back surface of the cone is lowering the air pressure. Since the wavelengths of low frequency sound are large compared to the size of the speaker, and since those low frequencies readily diffract around the speaker cone, the sound wave from the back of the cone will tend to cancel that from the front of the cone. For most bass frequencies, the wavelength is so much longer than the speaker diameter that the phase difference approaches 180°, so there is severe loss of bass from this back-to-front cancelation.

This is one of the reasons why even the best cone-type loudspeaker must have an enclosure to produce good sound.

Loudspeaker Resonance

Direct-radiating cone-type loudspeakers must be mounted so that they are free to vibrate. This mounting is elastic, so there is an inherent resonant frequency of the speaker cone assembly -- like a mass on a spring. This free cone resonant frequency distorts the sound by responding more strongly to signals near its natural vibration frequency. This non-uniform response changes the frequency content in terms of the relative intensities of the harmonics and thus changes the timbre of the sound. Since the cone is undamped, it tends to produce "ringing" or "hangover" with frequencies near resonance. If the resonance is in the bass range, the bass will be "boomy".

Because of this inherent non-uniformity in frequency response, it is common practice to use two or three loudspeakers connected together by a crossover network to achieve a more-nearly uniform frequency response.

Coupling Loudspeaker to Air

How hard can you punch a handkerchief? Not very hard, because it offers so little resistance. A loudspeaker has a similar problem when it tries to punch sound energy into the air. The usual language is that the speaker has a poor "impedance match" to the air.

A loudspeaker without an enclosure does a very poor job of producing sounds whose wavelengths are longer than the diameter of the loudspeaker. For an 8-inch speaker, diameter of speaker equals wavelength at about 1700 Hz. Even for a 16-inch speaker, the diameter equals the wavelength at 850 Hz. Besides the severe bass loss, the overall efficiency of such loudspeakers is low, about 3-5% compared to 25-50% for well designed horn type loudspeakers.

Impedance Match to Air

One of the reasons for the low efficiency of direct-radiating cone-type loudspeakers is the poor impedance match to the air that they are driving. The impedance which the air offers to the motion of a speaker cone can be mechanically modeled. The specific acoustic impedance of free air is approximately 42 ohms per square cm. For optimum efficiency the radiation resistance of the speaker cone should also be 42 ohms/cm² , but for sound wavelengths longer than the diameter of the speaker, this impedance drops rapidly . The smaller the speaker, the poorer its low frequency production.

A loudspeaker without an enclosure does a very poor job of producing sounds whose wavelengths are longer than the diameter of the loudspeaker. For an 8-inch speaker, diameter of speaker equals wavelength at about 1700 Hz. Even for a 16-inch speaker, the diameter equals the wavelength at 850 Hz.

This is one of the reasons why even the best cone-type loudspeaker must have an enclosure to produce good sound. The enclosure increases the effective size of the loudspeaker.