ECE 465 – ACOUSTICSLECTURE ON LOUDSPEAKERS

Definition:LOUDSPEAKER is a device that converts electrical AC audio current to an

acoustical sound wave.

Types:1) according to Transduction:

a) Electromagnetic principle – also known as “dynamic” type, employs a coil of magnetic wires wounded on a bobbin (this is the voice coil) in between a strong magnetic field of North and South field poles of a circular permanent magnet. The bobbin moves the speaker’s diaphragm, thus sound is produced in relation with the electrical audio current.

b) Piezoelectric principle – a speaker type employing a bi-morph element as its transducing element, the electrical audio current will flow thru’ the element, and once this happen, the element will vibrate mechanically moving forward and backward, and since this element is mechanically connected to speaker’s diaphragm, the diaphragm will move air molecules in front of it, thus sound is produced in relation to electrical audio current and mechanical vibration of the bi-morph element.

2) according to Frequency Response:a) High frequency (HF) or tweeters – ranging from 4 kHz to 20 kHz

(sometimes up to 30 kHz)b) Mid-frequency (MF) or squakers – ranging from 250 Hz to around 8 kHz.c) Low frequency (LF) or woofers – ranging from 20 Hz to 500 Hz.d) Full range speakers – speakers with frequency response ranging from 20 Hz

– 20 kHz.

Specifications to consider in choosing loudspeakers:1) Power Capacity – refers to the power that can be handled by the speaker from

normal to maximum power under a specified condition.Types:

a) Continuous power – the power that can be handled by the loudspeaker continuously (for 24 hours) without damaging its voice coil; usually a pink noise is used as a signal while this is measured.

b) Program power – the power that can be handled for a definite period of time (typical length of a live program) and usually larger than continuous power in magnitude as well as band-limited using C-weighting filter.

c) Peak power – the power based on transients or surge currents, usually 4 times as high as continuous power, but can be handled for the typical duration of a transient signal (less than a second). This power when applied to speaker continuously may burn and destroy the voice coil of the speaker.

Peak Music Power Output (PMPO) – a FALSE power commonly used to specify the peak sound power produced by a loudspeaker that usually misleads most of speaker buyers at electronics or music store. This power has no basis whether a particular speaker can deliver the said sound power without damaging or destroying its voice coil. Most consumer type speakers used in home entertainment systems such as CD players are specified in terms of this power which is NOT TRUE.

2) Frequency Response – as mentioned above. The frequency sound a particular speaker can reproduce depending on the frequency spectra of the audio current.

3) Impedance – in AC, we defined this as the TOTAL opposition to the flow of current considering the effects of reactive components produced by inductors and capacitors (this forms the loudspeaker equivalent circuits). This parameter is frequency dependent, usually measured at 400 Hz tone. Typical value is 8 ohms.

4) Sensitivity – the ability of the loudspeaker to produce a loud sound (measured in dB-SPL) at a distance of 1 meter in its front using a pink noise signal feed at an electrical power of 1 Watt.

5) Directivity factor (Q) or Directivity Index (DI) – this tells the directional loudness of a loudspeaker in a particular direction. This parameter tells us the location of the loudspeaker; usually more SPLs can be heard on its front direction, minimal on back and sides.

DI (dB) = 10 log Q

Speakers utilizing Horns got its Q on horizontal (H) and vertical (V) coverage angles, all measured in degrees:

Q = _________180___________ sin-1[ sin(H/2) sin (V/2) ]

Note: The higher the Q value, the smaller or the narrower the speaker’s coverage area.

More presentation of Q value:a. If Q = 1, sound transmission is isotropic, equal loudness level will be

distributed in all direction. This may be noticed on sound outdoors such as flying aviation vehicles, considering the sound of its engines.

b. If Q = 2, sound transmission is hemispherical in shape but doubles its sound intensity level where sound is directed. This may be noticed on fast moving vehicles considering the ground as the dividing plane, sound transmitted under the vehicle will be reflected by the ground in opposite direction depending on the angle of incidence, and thus no transmission will take place under the ground. The coverage area is reduced to 50% of the target area.

c. If Q = 4, sound transmission will be more directive, this is equivalent to a source located at a corner (two planes intersecting) equivalent to a speaker

located at two intersecting walls, and the horizontal coverage will be limited to 90º coverage angle thus the area of coverage is reduced to almost 25%.

d. If Q = 8, this is equivalent to the octet of a sphere, equivalent also to three (3) intersecting planes (similar to speaker located at a corner of a room, 2 walls and the room ceiling). This reduced the coverage area to 12.5%.

Assignment: Preparation for Exercise No. 3.Look for professional speaker showing the following specifications discussed above. Study its specifications. Make a discussion of its specification discussed above.

Prepared by:

Engr. Efren T. PinedaInstructor