How does speaker design affect audible sound?

Question

Speaker design for guitar/bass cabinets seems to have been the same for the past 50 years. A traditional guitar/bass cabinet is usually made a wood, with 8-12 inches speakers mounted to the frame, etc.

PA speaker (and some bass cabinets) have adopted a "big" and "little" design

• large bass speaker to cover bass frequencies
• small tweeter to cover the rest of the EQ spectrum
• sometimes has a cross-over to handle EQ variances between the two speakers

Some of the latest mixing speakers have what are called coaxial speakers. These involve a bass speaker with a tweeter mounted directly on top. All of the sound comes from one source, rather than two.

Obviously these designs are not limited to the paradigms mentioned.

I am looking for a credible source to explain how these designs differ from each other in terms of how we perceive sound.

For example, most guitar player's a familiar with the fact that their guitar amp sounds different depending on where they stand in the room.

Apparently, some modelers use powered coaxial monitors to alleviate this issue. Why is this so? What about the design makes this possible?

- What designs are best (acoustically speaking, no matter what) at delivering the full spectrum of EQ?
- Is it impossible for sound to leave two speakers at the same time and reach your ears?
- Are all of these differences negligible to an acoustic engineer and the design doesn't matter?

Answer 1

If we first simplify things to consider that the speakers are in an infinite space with no walls, floors or reflective surfaces, many of the things you mention have to do with, as you say, the amount of time the sound takes to get to the ear from each driver.

Let's also pretend for a moment that people have just one ear!

If you were to place two speakers in space, there will be a plane of points halfway between them where you could stand and the sound would arrive simultaneously. This is a 'good thing' as you don't then get the effects of out-of-phase waveforms reinforcing and cancelling each other, and you aren't smearing the frequency spectrum in time.

Observers not on this plane are going to receive the sound from each speaker at different times. If, however, you could put the two drivers in exactly the same place, then observers everywhere in space will hear synchronized waveforms from both drivers.

In reality, even a single person is aurally 'in two places at once', as we have two ears - so the coaxial design can be seen as advantageous for stereo imaging.

Of course in practice drivers are not point sources, and if one is mounted in front of the other then there are still time-alignment issues that need to be dealt with. In some cases the more important advantage of the coaxial design is being able to put a 2-way system in a smaller space.

Moving on to guitar / bass speakers - guitar speakers have an advantage that they are typically not required to reproduce the full frequency range. Also, if multiple speakers are not acting as a 'point source', this is seen as just part of 'the sound of the cabinet', rather than being a problem with fidelity.

Bass speakers have the advantage that the phase cancellation /reinforcement effect due to slightly differently-positioned drivers will be less marked at lower frequencies, because as the wavelengths are longer the same time difference is a smaller phase difference.

In reality, speakers exist in acoustic environments where reflections cause sound reinforcement and cancellation just as multiple drivers would - so the acoustic environment is often as important as (and should be matched with) the monitoring system.

If we finally consider that it's easier to drive a bigger speaker to produce low frequencies, and a smaller one to produce high frequencies, that crossovers aren't perfect, and that it's easier to manufacture two separate drivers than one coaxial one - you can see that there is really no one best design. It's a question of how 'hi-fi' you need to be, what your acoustic environment is, where your audience is in relation to the speakers, cost considerations, and so on.

Answer 2

Not quite sure exactly what you are asking (recording, live sound?) but as a guitar player who also is responsible for the PA for my band, and as a songwriter with an in- home recording studio - I have studied on this and experimented over the years.

Here are some thoughts to consider. Different speakers are designed differently to do different things. A guitar cabinet or bass cabinet has a speaker designed to reproduce the sounds of the instrument (within a specific sonic frequency) in a manner that the guitar player (or bass player) can hear. In many live applications the guitar cab is mic'ed and the sound is fed to the main and monitor speakers through one or more mixers - so it is sent to the audience through the main PA speakers and the musicians on stage through floor or flying monitor speakers.

Guitar cabinet speakers are not designed to direct sound to a large audience and tend to be very directional. So they would sound different depending on the angle of the speaker to the listeners ears, and of course whether the cab was facing or behind the listener. That's why in live settings with higher volumes for larger audience, the musicians will usually be listening to their instruments through the monitors, not the amps (even though they may also hear some of what is coming out of the amp).

Most of the time, guitar cabinets designed strictly for guitar (or bass), have one or more speakers (usually identical), that are optimized to produce the frequencies for the intended instrument - whether bass guitar or standard guitar. Some acoustic amps with microphone inputs may also have two different sized speakers in order to cover the more varied range of vocals in addition to guitar.

Coaxial speakers were originally designed primarily to save space. By positioning a higher frequency driver inside the cone of a lower frequency driver, two drivers can occupy the same space as one. A true coaxial speaker is actually two separate speakers (drivers) mounted concentrically, each with their own terminals and usually with a built in crossover.

Recent developments in the design of high fidelity coaxial speakers have been geared more towards taming inconsistencies in the off axis sound reflections that are characteristic of a vertically stacked driver orientation. By putting both drivers in concentric relation to one another, the sound radiates from the same point in space and will reflect off of horizontal planes (walls) AND vertical planes (such as a ceiling) at the same angle.

PA (Public Address) speakers are designed to deliver sound to a large audience over a wide area. The cabinets and speakers ideally should be designed for as wide a dispersion pattern as possible in order to broadcast clear sound to a wider area. Typically at least two PA speakers will be used to broadcast sound to a wider area. In larger venues many more may be used. The placement and orientation of these PA speakers will depend on the acoustic properties of the venue (if indoors) and where the audience is in relation to the speakers.

A typical PA speaker for most popular music will have two speakers (known as drivers) in the enclosure - a low frequency driver and a high frequency driver. You can find them with a low, mid and high frequency driver known as a 3 way speaker.

The PA speaker must cover the entire frequency spectrum as they will be tasked to deliver not only the sound of the instruments, but the vocals as well, to the audience. For bass heavy music you will often have one or more sub-woofers incorporated in the the PA speaker array to handle the lowest frequencies (bass guitar and kick drum).

The low frequencies will be directed to the sub woofers through a crossover either in the speakers themselves or in the mixer, depending on the equipment used. The cabinet of a sub-woofer (as well as many bass guitar cabinets) are designed to optimize the low frequencies. The cabinets are engineered to reflect and radiate the sound waves coming from a large speaker cone moving over a larger area at a slower rate. So the cabinet design itself will have a significant impact on the way these low frequency speakers sound.

PA speakers are constructed to withstand higher wattage amplifiers required to render distortion free sound to a larger audience. Lower wattage amplification may not drive them sufficiently to sound good so the same type speaker would sound bad in a 100 watt guitar amp - but the speaker cone in the guitar amp would be reduced to shreds as soon as a 800 watt PA amp sent an electrical signal to the voice coil.

Some PA speaker systems feature an articulated array of speakers angled to point in different directions to widen the dispersion pattern. In addition to the articulated speakers, the Bose L-1 and L-1 compact speakers for small to medium venues, also feature a line array system that uses the physics of the sonic wave form to allow the sound waves from multiple speakers to push each other along, thereby reaching a greater distance with lower volume. In large venues, this line array phenomena is used by arranging multiple PA speakers in a vertical array - either stacked on the floor or stage or flying overhead from trusses.

Because of advancement in amplification technology allowing for high powered amplifiers that are extremely light weight, there is a trend in PA speaker design towards what are known as powered PA speakers. These speaker cabinets have a built in amplifier just for the drivers inside that one cabinet - and tend to offer more headroom and higher volume before distortion than many of the passive speaker systems offer.

In live settings there is one other type speaker that you will see. The stage monitors are designed to allow the musicians to hear what they are playing. They are very similar in design to PA speakers in that they must deliver the entire frequency range, but are usually smaller and often wedge shaped so they will angle up towards the musician's ears when sitting on stage.

For studio use, the speakers are designed for close proximity monitoring (listening) in a very quiet room with controlled sonic properties. They don't require as much amplification as a PA speaker and the design can focus more on optimal sound quality than ability to withstand high wattage amplification. Studio monitors will generally be two or three way with possibly a low and mid and high frequency driver to cover then entire sonic frequency range.

But while studio monitors sound great in that very controlled listening environment, they would not be effective at all for live sound reinforcement.

It would be great if we could get the same high fidelity sound that studio monitors are capable of producing in a live music sound reinforcement application. But even if we could, the varying demands (indoors, outdoors, acoustic properties of the room, ambient noise) of the various settings where live music is played, would have as much of an impact on what the listener perceived than the speaker design itself.

But I have been really impressed with the sound of some of the newer powered PA speakers.

Hope this will help shed some light on speaker design properties for different applications.