Technical testing of speakers and subjective sound quality evaluation

2. Specific misunderstandings about speakers

1. The statement that large voice coil has long stroke is not accurate enough

I often hear people mention that large voice coils have long strokes. The so-called large voice coil refers to a relatively large voice coil diameter, which is easy to understand. But long stroke is another matter. Because long stroke is only a relative concept. Under the premise of true comparability, the larger the voice coil, the shorter the stroke can only be and it is impossible to extend it at all.

Another factor that determines the travel of the woofer is the support plate attached to the bottom of the speaker cone. Because the speaker cone can only move back and forth reliably when it is fixed at two points, the folding ring and the support plate. The support plate is made of cotton, linen and synthetic materials with glue. The elastic range of the support plate is limited. Therefore, it is not the rubber folding ring that restricts the travel of the woofer, but the support plate. Since the diameter of the support plate cannot be made very large and will never reach or exceed the diameter of the rubber folding ring, the larger the diameter of the voice coil and the limited diameter of the support plate, the smaller the range of movement of the support plate, in other words, the smaller the travel of the speaker. Only when the voice coil is small, the range of movement of the support plate is relatively wide, so the woofer with a large voice coil and a long travel is only a relative statement and is not accurate.

2. A small-diameter bass unit with a long stroke cannot replace a large-diameter bass unit

It is often said that a small-caliber, long-stroke bass unit can produce enough bass as long as the stroke is long enough. This is a wrong view. Theoretically, as long as the same volume of air is driven in the same unit time, the same level of sound pressure can be generated. But specifically for bass units, this is unrealistic. Because too large a stroke and too strong an air compression ratio will cause serious distortion of the reproduced sound. For large-caliber bass units (diameter above 200mm), when sufficient sound pressure is reached, due to the large driving area of ​​the speaker cone and the short stroke, the distortion during playback is small and the tone is better. Even in a relatively ideal state, the current speaker sound distortion can only reach 1%. When a small-caliber, long-stroke bass unit is working, the rapid increase in distortion due to the excessive stroke is a false impression of maintaining quantity but not quality. Therefore, for most small and small-caliber speakers, even if the low-frequency test of the speaker is OK, the gap is still quite large in actual use compared with large and large-caliber speakers, which is an essential difference. So when conditions permit, it makes sense to choose large speakers.

3. Low-sensitivity speakers have different timbres

There was a time when there was a popular saying that low-sensitivity speakers have good sound quality. In fact, this is only a rather one-sided and unscientific statement.

The main factors that determine the quality of a speaker's sound are frequency response, transient characteristics, damping characteristics, and power handling. It has nothing to do with sensitivity. In other words, for speakers with the same power handling, at the same playback volume, the lower the sensitivity, the greater the input power required, the closer it is to overload, and the distortion will increase relatively.

20 years ago, because the technical quality of the speaker unit itself had not yet reached a relatively good level, when making speakers, we could only add a lot of attenuation correction circuits to the crossover. The final result was that the frequency response curve was straightened, but the sensitivity was greatly reduced, only reaching about 82dB. In this case, a high-power amplifier is usually required to drive the low-sensitivity speaker well, but under high-power drive, the low-sensitivity small speaker is prone to overload distortion and even cannot play some large dynamic music works.

Due to the development of audio technology, there are now many models of monitoring-level and HiFi-level speakers with a sensitivity of more than 100dB, and the highest sensitivity is close to 110dB.

4. The sound-absorbing cotton in the speaker does not represent the quality grade

The statement that "speakers without sound-absorbing cotton are low-end speakers" is inaccurate.

It is a reality that there is no sound absorbing material in low-end speakers.

In the packaged sound box and the cheap finished sound box, there is basically no sound-absorbing material. Therefore, some people have come to the conclusion that there is no sound-absorbing cotton in the low-end sound box, and adding filling material to the low-end sound box can improve the replay effect.

In fact, the above conclusions have no causal relationship and no inherent connection.

Sound-absorbing materials only play two roles in speakers: one is to eliminate certain resonances and coloration of the speaker body; the other is to appropriately reduce the size of the speaker. It has nothing to do with the grade of the speaker.

Some people think that adding padding to a speaker is a panacea, but they are totally wrong.

First, as long as the speaker cabinet is reasonably designed, has no obvious resonance, and is large enough, high-quality speakers can be made without adding filler materials. There are many successful examples of this in the field of speaker manufacturing around the world. Not adding filler materials to the speaker cabinet is good for the transient characteristics of the speaker.

A carefully designed and manufactured speaker has basically reached an ideal state when it leaves the factory. In this case, arbitrarily changing the presence or amount of filling materials in the speaker will have many effects on the speaker's playback, and most of these effects are negative.

Too much filler will make the sound fleshy, with poor transient characteristics and weak performance during playback. Although the curve will be improved during testing, the sound performance will be deteriorated during subjective listening. One thing must be made clear, that is, speakers are for listening, not for seeing.

5. About the speaker crossover

In the speaker crossover, there are only three main types of components: inductors, capacitors and resistors.

The resistor acts as an attenuator to balance the sound ratio of each frequency band. When selecting, just use a resistor with enough power.

As for resistors, metal film resistors will have better results.

The function of the inductor is to filter out high tones and select low tones. In recent years, many types of inductor coils made of special-shaped enameled wires have become popular. Among them are multi-strand enameled wires, hexagonal enameled wires, and strip enameled wires. Each type of inductor coil made of special-shaped wires is said to have some kind of magical power. But is this really the case? Actually, it is not.

There are only three requirements for inductor coils used in speakers: one is that the inductance value is accurate, one is that the DC resistance is low, and one is that it is not easy to produce saturation distortion.

As for the accuracy of the values, it is sufficient to use high-precision instruments to measure and calibrate each one during the production process.

In order to reduce the resistance of the inductor coil, the conductivity of the enameled wire must be improved. The conductivity of the enameled wire is directly and closely related to its cross-sectional area and cross-sectional shape. The efficiency is highest when the cross-section of the wire is circular, square, or hexagonal.

Specifically for winding inductor coils, enameled wire with a hexagonal cross section can effectively reduce the gap between turns and improve the efficiency of the inductor. Especially for multi-layer coils with a large number of turns, the improvement effect will be very obvious. However, when using hexagonal enameled wire, the production cost will also increase significantly. Therefore, if it is not used in very high-end occasions, the effect is already quite good if round oxygen-free copper enameled wire with a purity of more than 4N is selected. When choosing the wire diameter of the enameled wire, there is no saying that the thicker the wire diameter, the better. As long as the inductance meets the requirements, its DC resistance is about one-tenth of the DC resistance of the bass speaker voice coil. If the DC resistance of the inductor is too large, it will directly affect the low-frequency damping characteristics of the speaker; if the DC resistance of the inductor is too small, it will increase the production cost unnecessarily.

As for the selection of crossover capacitors, it is impossible for a single capacitor to fundamentally change the playback of a speaker. For those who want to make their own speakers or improve finished speakers, they must first have a clear purpose. When making speakers, the grade of crossover capacitors should be selected according to the investment in the speaker unit.

For example, if you buy a tweeter that costs tens of yuan, it is not worthwhile to spend 20 yuan to select capacitors for it. You might as well buy a tweeter that costs hundreds of yuan and choose a crossover capacitor that costs a few yuan. If you have already bought a tweeter that costs 300 yuan, spending tens of yuan on a crossover capacitor is a good match.

For audio capacitors, different brands and materials have different effects on playback. However, these intrinsic and subtle differences can only be better reflected in tweeters above the mid-range.

For low-frequency crossover capacitors, the main requirements are accurate capacitance, voltage resistance, and reliability. Compared with high-frequency crossover capacitors, the requirements can be relatively lower, because high-end capacitors with large capacitance are too expensive. They are often in a state where the gains do not outweigh the losses.