Direct Acoustics

FREQUENCY RESPONSE

The most important thing about any speaker is its frequency response. It is here that designers spend most of their effort, for the wider the frequency response (the higher and lower the musical tones it can reproduce), the more you hear.

Various frequencies of vibration generate various pitches. For example, middle C on a piano creates a vibration or sound wave at 256 cycles per second.

A good example of high frequency sound is the almost inaudible (for many people) hiss or whistle given off by the flyback transformer of a tube-type television set, which vibrates at 15,750 cycles per second in order to generate the 15,570 lines per second displayed on the TV screen. If you can’t hear this on your own set, try listening in a TV showroom. This is a good test to try out the top end of your hearing, for 15,750 cycles per second is near or past the top limit for most people’s hearing.

WOOFERS AND TWEETERS

The search for speakers with the widest possible frequency response led to the specialization of drivers as woofers and tweeters. Bass or low frequency response depends on a speaker moving large quantities of air, and therefore generally requires a speaker cone of large size and area (and also a long voice coil).

A woofer becomes more efficient as frequency rises. This is because higher frequencies have shorter wave-lengths, closer to the size of the woofer cone itself, resulting in more effective coupling of the woofer cone and the air.

Generating high frequency sounds is a totally different problem, requiring very little mass and size . Usually, the smaller the tweeter, the higher its frequency response. A large tweeter will play more loudly, but with less range and relatively poorer accuracy at the higher frequencies.

VOICE COIL

One of the moving parts and hence moving mass of a dynamic speaker or driver. The design and shape of a voice coil is a critical aspect of speaker design. The coil must not only perform perfectly as an electrical component, accepting and converting current from the amplifier into mechanical energy; but through its length and its mass it also influences the physical behavior of the speaker.

Low frequencies require a relatively heavy long voice coil.

Initial design, winding accuracy, quality of material and workmanship and precise quality control are all factors in voice coil production.

DRIVER

A speaker. Since the term “speaker” refers both to an individual component and to a system of several speakers, the word “driver”, which refers only to a single element, is often used to avoid ambiguity. Examples of drivers include a woofer, a mid-range, a tweeter, a horn, an electrostatic unit.

CROSSOVER

In speaker design, a mechanical or electrical device to split a speaker’s input signal into high-frequency and low-frequency components. In a two-way speaker system, the crossover is essentially two filters: an inductor which allows only low frequencies to reach the woofer, and a capacitor which allows only high frequencies to reach the tweeter.

More complex crossovers have combinations of coils, capacitors, resistors and other components. The use of more crossover components allows the designer either to make the crossover slopes steeper (so that there is less overlapping of the sound from the two drivers) or to shape the frequency response as desired to improve the frequency response of the drivers.

CROSSOVER POINT

In a two-way system, the low end of the tweeter’s frequency response and the high end of the woofer’s frequency response. The tweeter’s low end therefore should be just about the same as the high end of the woofer. One advantage to a speaker company which manufactures its own drivers, rather than buying them ready made, is that the designer can precisely control frequency response through precise variations in such factors as coil mass and cone shape.

EFFICIENCY

Speaker efficiency is the ratio of sound energy to power input. The range of efficiency in speakers runs from about one third of a percent to about ten percent. Very few speakers have better than ten percent efficiency, and most run about one percent.

The loss or wasted energy is mostly heat. In this sense, a speaker is essentially a heater which produces music as a by-product, which is why speakers occasionally burn out or blow up. The trend towards very high-powered amplifiers dramatically increases the possibility of such mishaps.

A general rule of thumb is that more efficiency in a loudspeaker actually means a narrower frequency response. Consider the speaker as analogous to a filter and the reasons become clear. Any filter allows only a certain bandwidth to pass through. A narrow band has higher amplitude. If the filter is designed to allow a wide-range signal to pass, there will necessarily be less amplitude. In this sense a speaker can be perfectly represented by symbols which are the same as electronic symbols. (Also see THIELE.)

DISPERSION

Uniform dispersion of sound exists when you can sit anywhere around a speaker and hear the same sound. Dispersion is an important challenge in speaker design. Ideally a speaker should produce a spherical sound wave at all frequencies. In reality both woofers and tweeters begin to beam sound as frequency gets higher.

The smaller a speaker, the higher the frequency up to which it gives uniform dispersion (spherical waveform). For example, a one-inch tweeter is able to give spherical dispersion up to about 12,000 or 13,000 Hz. A half-inch tweeter is good to nearly 20,000 Hz. A woofer usually begins to beam at about the range of the female voice (800-900·Hz).

Most of the sound that you hear is reflected off the wall. If there’s a frequency range where dispersion is poor, then you don’t hear the reflected sound wave and the overall power response is off in that frequency range.

Because of the difficulty in obtaining drivers that provide ideal dispersion, some designers try to take advantage of the limited dispersion to enhance some other aspect of the performance, such as stereo imaging, or absence of microphone feedback.

DISTORTION

When light goes through a lens, a certain amount of fuzziness is always introduced to the original image. When an electrical sound-wave enters a loudspeaker, a certain amount of fuzziness or distortion is always introduced while converting the electrical energy into sound energy.

This distortion comes about because the motion of the speaker is never exactly the same as the input. When the voice coil moves, the force behind the motion may not be uniform because the coil moves in relation to the stationary magnetic field and because opposition to the coil motion is not uniform because of the varying resistance and stiffness of the suspension

Ways of decreasing distortion are:

• Longer voice coils
• Uniform suspension elements which allow a lot of play (excursion).
• Oversized elements. (Large drivers, long voice coils, large suspension elements.)

Distortion will occur: if the force from the motor driving the cone is non-linear, if parts of a cone move independently, if air is escaping in and out of small vents in the enclosure, if there is ringing from parts which should not vibrate, or if components emit their own resonant frequencies when excited by other frequencies.

Distortion also occurs when dispersion of the sound wave is uneven, or if the phase response is not uniform as a function of frequency.

If the magnetic field of the magnet is unequal on two sides, this will also cause distortion.

Since the room in which the speaker functions can also introduce distortion into the speaker’s output, placement of speakers in the room, particularly bass reflex designs, can be crucial in improving sound.

Forms of distortion:

• HARMONIC: easiest to measure
• LIMITED BANDWIDTH: most important
• INTERMODULATION OR IM: most unpleasant
• TRANSIENT INTERMODULATION or TIM
• UNEVEN DISPERSION: difficult to hear
• UNEVEN PHASE RESPONSE: difficult to hear
• DOPPLER: difficult to measure
• SPURIOUS BUZZES, RESONANCES OR VIBRATIONS: from poor construction
• ENVIRONMENTAL EFFECTS: room acoustics.

Q

Q is the formula for what happens electrically and physically at the resonant frequency. Q is related to how slowly a natural vibration dies down. Q is a neat term that combines many elements of speaker design, so it’s an important term to speaker designers.

Q is important to the woofer because in a speaker system where the Q is too high, you’ll get a whomp in the bass. If a speaker system’s Q is too low, then the bass output is relatively weak, because frequency response is highly damped in that region.

The best Q for a speaker to have is one that gives flat response down to the resonant frequency.

If you do nothing to a speaker but make the cabinet smaller, you increase stiffness and therefore Q goes up. But Q is not a word for stiffness alone. It’s a ration involving mass, stiffness, and resistance (or damping). Designers often juggle these three factors. Good bass is not enough. Good bass with uniform frequency response and spherical wave-form is the goal. Obtaining that goal without too much sacrifice in efficiency is the challenge.

EFFICIENCY AND MAGNETIC FORCE

In a dynamic speaker, the voice coil and the magnet structure interact to force the speaker to move. Send an electric current through a coil in or near a magnet and a force is produced. The force results in an alternating movement or motion which is converted to variations in air pressure by a cone. The pressure is radiated as a sound wave.

The stronger the magnetic field, the higher the force, the higher the efficiency. Bigger magnets often lead to stronger magnetic fields, but not always, because the strength is related, not only to magnet size, but also to the size and shape of the gap that the voice coil moves in.To get a more efficient speaker of the same size, you can use a stronger magnetic field. However, this gives you lower Q. You’ll get more output at the middle frequencies, but less at the high and low ends. This can give relatively less bass even though you now have higher efficiency. Therefore, if you increase magnetic field, you have to go back and redesign the tuned port of the cabinet to get back to correct response.

CABINET DESIGN

Speaker cabinets have two jobs: they serve as a baffle for mounting of drivers, and serve to modify, re-phase, improve, radiate, suppress, or redirect the back wave from the woofer and keep it separate from the front wave.

As a baffle, the speaker cabinet is usually less than ideal. Since the cabinet sticks out from the wall, and has sharp edges and corners, acoustic problems develop as sound waves pass over the edges of the cabinet. One relatively effective response to this problem is a cabinet whose shape flows smoothly “into the wall” because of beveled or curved panels.

Cabinets should ideally be designed to avoid:

1. Motion of the walls of the cabinet.

2. Transmission of sound through the cabinet walls.

Therefore, the ideal cabinet is heavy and rigid. These considerations are particularly important in acoustic suspension designs, where the back wave of the woofer must be completely “contained”.

Today, almost all speaker cabinets are made from particle board. This material is particularly good because it has no spaces into which air can flow (unlike plywood), which often does). However, particle board is relative flexible, therefore failing to avoid condition 1 above, and often requires astute bracing and very solid construction techniques (screws, glue, reinforcement, etc.) in order to approach required rigidity.

There are many types of speaker designs where the cabinet design is integral to the speaker design.

Some that come to mind are:

• Transmission line
• Isobaric
• Omni-directional (bi-polar)
• Band-pass
• Horns
• Limited dispersion