Besides the purely aesthetic, there are quite big differences in how sound behaves in different cases. Pure plastic cases usually have a richer and deeper sound profile than aluminum cases. Aluminum is a fairly light metal, which can create resonance (reverberation). However, the resonance can be counteracted in several ways: 

- By adding more mass to the case, e.g. a steel or brass weight. The heavier the case is, the lower the case’s natural frequency will be. In other words a deeper sound with less reverberation. 

- Damping. By adding damping materials to the bottom of the case, a tighter sound profile is achieved - with less reverberation (more on this in the damping chapter). 

- Force Break: To minimize vibrations in keyboards that have screwed-together cases, silicone pads are attached on the frame to stop vibration transmission. If your keyboard does not have these pads, you can use small pieces of tape on the frame to create a slight separation between the top and bottom case. 

Airborne and material-borne sound 
When you type, the sound of the keystrokes will spread upwards (into the air) as well as downwards (through the components and towards the chassis). While airborne sound is hard to control, the latter can be greatly influenced. 

Reverberation and hollow sound  
If the bottom inside of the keyboard case is completely even and smooth, sound waves will be reflected directly (specular reflection), resulting in an echoing effect. Keyboards with a large cavity beneath the PCB often have a more hollow sound profile due to the ample space for sound waves to bounce around. This can be mitigated by filling the cavity with sound-absorbing materials, or having acoustics ribs in the bottom case. 

Acoustic ribs  
Acoustic ribs and other uneven surfaces in the bottom of the keyboard case are designed to disperse sound waves in various directions (diffuse reflection). This approach aims to reduce echo and resonance, as well as to distribute sound evenly and prevent amplification of specific frequencies. Acoustic ribs are typically included only in high-end keyboards due to the increased machining costs during production. 

Tape mod: the fastest way to a thocky sound
One common keyboard modification is called "Tape mod" and involves covering the entire underside of the PCB with tape (preferably electrician's tape). The tape will act as a low-pass filter, removing much of the high frequencies but allowing the low frequencies to pass through. Think of the tape as a wall. Through the wall, the bass is heard the most, while the treble from the people talking on the other side of the wall is absorbed. The result is that the sound profile of the keyboard becomes slightly louder and significantly deeper in tone. 

The sound and tactile response of a keyboard are significantly influenced by the presence of flex-cuts in the PCB. These cuts are designed to enhance the flexibility of the board, allowing the keys to flex vertically upon keystrokes, thereby reducing the stiffness of key presses. The extent of this flexibility is determined by the number, location, and size of the flex-cuts. Individuals who prefer a softer, more bouncy feel often favor PCBs with such cuts.  

Conversely, some may argue that this softer sensation diminishes direct feedback, resulting in a somewhat mushy typing experience. 

Regarding acoustics, flex-cuts permit a greater transmission of soundwaves compared to a keyboard without these cuts. Consequently, PCBs with flex-cuts tend to produce a slightly louder sound, typically characterized by increased reverb and bass.

Since the switches are mounted directly to the plate, it has a noticeable effect on both the keyboard's sound and feel. The stiffness (Young’s modulus) and density of the material influence how the plate vibrates when typing. Generally, stiffer and lighter materials produce a brighter, sharper sound, while softer and heavier materials tend to give a deeper, more muted tone. If you're aiming for a “thocky” sound with lower frequencies, choose a plate material with a lower stiffness-to-mass ratio. A bread dough might fit the bill acoustically – but it’s sadly not a viable plate material. 

Plate material properties 
Materials such as polycarbonate, POM, and polypropylene (PP) are all plastics, and will produce a deeper sound than metals like aluminum and brass. Copper is the metal best suited for producing a deeper sound. Carbon fiber is very stiff, and is well-suited if you are looking for a dry and "clacky" sound profile. The all-rounder that sits in the middle is FR4 (fiberglass epoxy). This is a good starting point and a safe choice if you wish to experiment. 

The stiffness of the plate significantly affects the tactile experience when a key is fully pressed. Plates made from materials such as carbon fiber and steel provide a firm and rigid feel, whereas plastic plates offer a softer, less direct response. Many plates are also equipped with flex-cuts, similar to those found in PCBs. To achieve a softer and more buncy typing experience, it is advantageous to use both a PCB and a plate with flex-cuts. 

Switches are undoubtedly the single most influential component on the sound profile of a keyboard. However, it is important to note that the same switch can produce significantly different sounds in two different configurations/keyboards. 

Material properties and sound of switches  
The materials used in the construction of a switch significantly influence its sound profile. The bottom housing of the switch primarily affects the sound produced when the key is pressed, while the top housing predominantly influences the sound during key release. 

The materials most commonly utilized in the top and bottom housing of switches are nylon blends (PA66, PA12, etc.), polycarbonate (PC), and ultra-high-molecular-weight polyethylene (UHMWPE). Nylon has numerous different blends available. Generally, nylon tends to produce a deeper sound, while polycarbonate and polyoxymethylene (POK) are known for producing a sharper, higher-frequency sound. 

The stem of a switch is typically composed of a distinct material from the housing to minimize friction. Commonly utilized materials include POM, POK, and UHMWPE. UHMWPE, in particular, is known for its soft yet exceptionally smooth texture, resulting in a silky soft tactile experience. 

Long pole vs classic stem
The switch stem is typically manufactured in two designs: classic and long-pole. The long-pole design features an extended pole, allowing only the pole to bottom out on the floor of the bottom housing. In contrast, the classic switch stems have side walls that contact the floor of the bottom housing. Practically, switches with a long-pole design exhibit a more firm tactile feel and produce a crisper sound compared to classic switches, due to having a single point of contact rather than dual points of contact. 

Classic switches provide a softer touch and a rounder sound. On the other hand, long-pole switches, when combined with tight tolerances, produce a crisper and firmer sound profile. This can be compared to hitting a drum with two sticks simultaneously versus using only one stick. 

Silent switches  
Switches with integrated damping mechanisms are available in the market. Various manufacturers offer different solutions, typically involving the attachment of small silicone pads either to the switch stem or the impact area of the housing. While these switches are not entirely silent, they produce significantly lower noise levels compared to standard switches without damping features. When paired with a keyboard that has been optimized for a quiet sound profile, it is possible to achieve a very quiet typing experience. 

Springs, resistance, and force curves  
The spring within the switch determines the amount of resistance required to press a key. Resistance is measured in gF (gram force) or cN (centinewton).* A medium resistance is generally considered to be between 45 and 60 gF. High resistance ranges from 60-80 gF, while low resistance ranges from 30-45 gF. When manufacturers specify the resistance of a switch, they provide two values: the actuation force, which is the force necessary to actuate the switch, and the bottom-out force, which is the force required to fully depress the switch. In terms of acoustics, high-resistance springs tend to produce a deeper sound profile compared to low-resistance springs, simply because they have more mass. 

Springs can exhibit varying force curves, which indicate changes in resistance levels during compression. The most prevalent force curves are categorized as linear, progressive, or slow. 

Linear springs are considered standard and deliver a consistent increase in resistance throughout the keystroke. Progressive springs demonstrate a more rapid increase in resistance as the spring compresses further. Conversely, slow springs also provide a gradual increase in resistance similar to linear springs; however, they necessitate a higher initial force, resulting in a less steep power curve compared to linear springs. 

Lubrication of Switches 
Unlubricated switches may exhibit a dry or scratchy feeling due to increased friction. Additionally, unlubricated switches can present with more rattling noises compared to their lubricated counterparts, as the grease serves as a silencer. Non-lubricated springs often produce a metallic pinging sound upon key release and spring decompression. 

Lubricated switches offer a smoother operation by reducing friction and produce a deeper, less rattling sound. For those seeking a deeper acoustic profile and a smoother tactile experience from their switches, it is advisable to apply lubrication. Pre-lubricated switches are also available for purchase, which generally provide an enhanced feel and sound quality. However, as mechanical application processes can vary, there may be inconsistencies in lubrication quality within the same batch or package. 

*1 gF = 0.98 cN* 

The design, including the profile of the keycaps and the material they are made of, significantly affects the sound profile of the keyboard. Generally, heavier (higher mass) keycaps produce a deeper sound. Thus, keycaps made of metal will produce a deeper sound compared to ceramic keycaps, which in turn produce a deeper sound than plastic keycaps. 

ABS vs PBT
When comparing different plastic materials, PBT is known to be harder than ABS. As a result, PBT tends to produce a sharper, more crisp sound, while ABS usually yields a softer, rounder sound profile. This difference can also be observed in the tactile experience to some extent. 

Influcene of keycap thickness
When comparing two keycaps made from the same material, the thickness determines the depth of the sound profile, with thicker keycaps producing a deeper sound due to their greater mass. Regarding keycap profiles, low-profile keycaps tend to produce a brighter sound compared to high-profile keycaps, such as SA or MDA, which emit a deeper sound because of their larger size and increased mass. 

To dampen sound and reverberation, the keyboard can be stuffed with damping materials. The most common variants are as follows: 

Casefoam: inserts that are placed in the bottom of the case, below the PCB. Dampens vibrations in the case itself, and/or fills cavities under the PCB that contribute to reduced echo/reverberation and a less hollow sound profile. 

PCB/switch foam: Placed between the PCB and the switches. These help to reduce the transmission of vibrations and sound from the switches to the PCB during keystrokes. 

Platefoam: Are positioned between the PCB and the mounting plate. With MX design switches, platefoam typically has a thickness of 3.5mm, making it the most significant damping component in most keyboards. The 3.5mm platefoam effectively fills the space and reduces sound dispersion between the plate and the PCB. 

There are several different dampening materials that can be used, where each material has its unique properties. What you want to achieve is crucial for the choice of materials. As a case foam, Poron will be an excellent choice, while IXPE will be excellent as a material in PCB/Switch foam to attenuate high frequencies. 

Less is more
Keep in mind that while adding foams to your keyboard can enhance its sound profile, overdoing it may mask the keyboard's true sound signature. As a general guideline, the more foam you pack into your keyboard, the more it will resemble the sound of all other foam-packed keyboards. Approach this process thoughtfully—use damping materials in moderation and try out various configurations to discover the sound that best complements your specific build.

Perhaps not surprising, but whether the keyboard has feet or not, and the size of them, has an impact on how the sound spreads further down into your desk.

Of course, how the sound propagates from the case to the desk also depends on whether there is direct contact between these two, or whether there is a desk mat or two in between them. The material the mat is made of, as well as how thick it is, will also be important.

Try typing with the keyboard on your lap. Does it produce a different sound compred to when it's sitting on your desk? If so, the variation arises from how your desk vibrates and amplifies the sound of your keyboard.