Speakers: Instrumental to Listening

Speakers are instrumental to our music listening experience. Since the early 1920s, speakers have been the default method of reproducing audio signals. With the advent of wireless connectivity, digital signal processing, multi-room capabilities, smart speakers, and the emergence active speakers, our listening experience is transformed into an immersive experience. In this blog, we will examine some of the key aspects of next-generation speaker design.

Many modern loudspeaker designs are active units. In an active speaker design, the audio amplification, signal processing, and filtering are all conducted within the speaker cabinet itself. In a two-way system, there are two audio amplifiers: One for the mid-/low-frequency drive unit (woofer), and one for the high-frequency drive unit (tweeter). The input signal is split into two distinct frequency bands by electronic filters (crossovers), the outputs of which supply the two amplifiers. This system yields many advantages over the traditional passive speaker. First, the crossover components themselves are smaller, cheaper, and more accurate than their passive counterparts. The amplifiers function over a more limited frequency spectrum, reducing intermodulation distortion. The use of digital signal processing (DSP) and wireless connectivity is widespread in such designs. The units can also be joined together for multi-room listening. Their output frequency range, extended and controlled, can provide a hearty bass response without damaging the drive unit. Finally, many can stream music directly from the raft of streaming services available today, and feature a host of digital and analog inputs.

Class D offers highly efficient audio amplification along with low power consumption and small size. Traditionally this technology, used in cars and portable devices, was considered inferior in terms of audio quality to its linear counterparts (class A, class AB). However, advances in DSP and increased transistor speeds have made this type of amplifier a more attractive proposition, and it is now common within modern active designs. 

In an untreated listening room, sound waves—depending on their frequency—bounce off walls or are absorbed by soft furnishings, while reflected waves can collide with each other. As a result, some frequencies can appear boosted while others attenuated, and the listener can hear a very colored rendition of the audio. The positioning of the speakers within the room, particularly with regard to room boundaries, can have a significant effect on this. One technique to overcome this issue is to use the DSP in the design to compensate for the room’s interaction with the audio, known as digital room compensation (DRC). The process involves an analysis of the room’s sonic characteristics and design, and implementation of suitable filters to mitigate these effects. Many active units can function in this way, and there are several add-on DRC units available for those that don’t. The combination of DRC, intelligent speaker placement, and some basic sound treatment can make a huge difference.

The main purpose of the enclosure (cabinet) is to provide rigid support for the drive units at all frequencies (Figure 1). The enclosure must avoid responding to the vibrations of the drive unit; in effect, it must avoid being a drive unit itself. There are challenges in terms of the materials used, the weight, and the cost of the final design. Braces and chambers are often used to lessen the likelihood of sympathetic vibrations.

Figure 1: Cabinet design is important for rigid support to prevent vibrations. (Image Source: Nerthuz - Shutterstock.com)

Additionally, the sound produced by the drive unit as the cone moves forward is out of phase between the front of the cone and the back. Therefore, it is important that the listener does not hear this rear signal unless chambers reverse its phase within the speaker enclosure before it is projected forward. Some designs feature reversed conical shapes behind the drive units to attenuate this unwanted sound for the listener. The use of bass extension tubes, or tubes mounted on the front of the enclosure (baffle), are also common. These produce resonances at lower frequencies to extend the range of the cabinet beyond that of its natural size.

The dynamic loudspeaker is the most common drive unit encountered as a transducer for mid/low frequencies. A permanent magnet (often neodymium), a voice coil, a frame or basket, and a speaker cone are included in such a drive unit. The cone needs to be very light so as not to present inertia (resistance to movement); also, it needs to be rigid; any distortion across the surface of the cone could color the audio produced. Historically, there has been a variety of cone materials, from treated paper (very light), to aluminum (rigid) and ceramic materials. More recently, manufacturers have employed aerospace materials, often with a golf-ball texture to allow for a smooth passage of air.

High-frequency transducers (tweeters) can fall into two main types. The dynamic variety is constructed like its low/mid counterpart, but using different materials optimized for high-frequency response in its construction. Manufacturers can sometimes use synthetic diamonds for their dome material. This material is extremely rigid and also light, so it does not warp and can push air around efficiently. However, the synthetic diamonds are grown in 1,500°C ovens and are expensive to produce. Other tweeters include ribbon types. These feature an ultra-thin metal diaphragm (the ribbon) suspended in an electromagnetic field. Because the weight of this type of diaphragm is negligible, it can respond to even the tiniest subtlety and thus yields a very detailed picture of the incoming high-frequency audio. The disadvantage of this type of tweeter is its very low impedance, requiring a transformer to match it to the amplifier. They are expensive and also delicate.

Speakers remain at the heart of our music listening pleasure, and active designs, with inbuilt streaming and multi-room capabilities, are becoming commonplace within the domestic audio sphere. Digital room compensation and innovative driver control can help a small speaker achieve a really big sound. The active architecture gives the designer control over important parameters such as drive unit choice, protection, and the placement of features within the digital/analog domain, allowing for circuit optimization in these areas, with the added aesthetic benefit of reduced cabling.