Loudspeaker Enclosures
- nontasb
- 13 Φεβ
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Intro
The enclosure or cabinet of the loudspeaker is the wooden construction that the actual loudspeaker drivers & crossover circuit need to be mounted on. If the loudspeaker is active it also houses the power amp module/circuit in some section of the cabinet (usually at the back side).
The main role of the enclosure is to contain the sound waves of the back side of the loudspeaker drivers (preventing them from interfering with the front side), minimizing the creation of comb filters/cancellations, which would have ended up (if the enclosure didn't exist) to the distortion of the original audio signal that is intended to be heard from the listener.
Comb filtering: Is the result of two (or more) copies of sound or identical audio signals, when combined with a slight delay between them. This delay is introducing a difference in phase, so we say that they are "out of phase" or that they "cancel each other out". When this occurs, very steep notches are created in the frequency response and the sound is starting to have a hollow or a "flangy" effect. The actual "comb filter sound" differs, depending on which frequency it occurs.
Another basic purpose of the enclosure is to manage vibrations created by the drivers. That's the main reason the enclosure needs to be thick & braced (19mm MDF is sufficient for sides-top-bottom, double thickness 38mm is optimal for the baffle/front side that the drivers are mounted on).
Braces: The internal support structure of the cabinet which can be made from MDF or plywood and binds together internally, all of the external walls (sides/up-down/front-back) of the speaker. Example here.
Decoupling the cabinet externally from the floor or desk surface also helps eliminate the vibrations and contributes to the clarity & openness of the sound of the speaker.
IsoAcoustics iso-puck & iso-stands are a very good option for that.
Internal Cabinet Treatment
The internal volume/walls of the enclosure can and should be covered with 2 different layers of insulation/absorption material (polyester & felt textile are a good combo). Absorption helps to tame the resonances inside the cabinet, making the driver act like it was in a bigger enclosure and also prevents high frequency resonances to radiate from the port/bass reflex.
All the internal walls can be covered with damping material except braces & the back side of the front baffle. Placing a double layer of polyester on the back wall of the woofer and on the upper and down walls of the enclosure as well, is a good rule of thumb.
Another enhancement which increases the rigidity and mass of the walls, is to place bituminous felt sheets before adding the insulation materials, making it even more difficult for the walls to vibrate.
Types of enclosures
Closed/Sealed Box
Description
This type of enclosure is simply an air tight box so when the driver cone moves, it only alters the pressure inside the box. It's the easiest enclosure to build because the only thing that you need is to calculate the volume of the cabinet (Liters) for the individual bass/sub driver. Filling the internal volume of the box with absorptive material helps tame the stationary waves produced in the box from the back of the speaker.
Enclosure example
Frequency Response example
Driver Dayton 10" (model: RSS265HF-8) measured with DATS
-3db @ 42Hz / -6db @ 29Hz
Pros
Easy to design/build
Small group delay
Greater transient response (than bass reflex enclosures)
Smooth roll-off (12 dΒ/octave)
Transient: Is a high amplitude short duration sound at the beginning of a waveform. Some examples are the initial peak of a waveform of a kick or a snare drum.
Roll-off: refers to the steepness of the low end attenuation of the driver (the section below 100Hz & down to 10-20Hz region in our examples).
dB/octave: expresses & determines how aggressively this attenuation reaches beyond the "cutoff" frequency (below the -12db point in our diagram).
Cons
Low efficiency
Frequency response in very low frequencies not as full & close to the reference axis (0 db), as in bass reflex enclosures
More about sealed design ➡ here
Bass Reflex / Ported / Vented
Description
This type of enclosure uses a port or vent cut into the woofer/sub cabinet and a section of PVC tubing or pipe, stuck & sealed to the baffle port/hole. This port is tuned (by port length) specifically for each driver-cabinet volume pair. It enhances the reproduction of the very low frequencies (sub-bass range 20-60Hz) making good use of the energy/waves from the back side of the woofer driver. This way the woofer combined with the port (radiating in the specific tuned frequency) are providing a very full/fat & flat frequency response down to the low end of the speaker.
Enclosure example
Frequency Response example
Driver Dayton 10" (model: RSS265HF-8) measured with DATS
-3db @ 19Hz / -6db @ 17Hz
Pros
Higher efficiency than sealed enclosures
Drivers can reach lower frequencies below their Fs point in frequency response
Resonant Frequency (Fs): is the point at which the weight of the speaker's moving parts are compensated by the force of the speaker suspension as it moves, so the cone remains perfectly balanced and doesn't move at all. Around that point & below, the frequency response of the driver starts to roll off towards the low end.
Reduced distortion
Relatively higher power handling capability
Cons
Not as good transient response as sealed enclosures
Larger enclosure than sealed
Steep roll-off (24 dΒ/octave)
Port noise only if smaller diameter size than required or a non-flared tube is used (vs flared)
More about bass reflex design ➡ here
Sealed vs Ported
Same driver in same cabinet Liters (125L)
Passive Radiator
Description
Works in a similar way to a bass reflex enclosure replacing the bass reflex port/tube with a passive driver which is tuned with weights/mass adjustments (instead of changing/"tuning" the length of the port) that are mounted directly at the
passive driver cone.
Enclosure example
Sealed vs Ported vs Passive Radiator (Frequency Response)
Pros
Can be used in smaller or very small cabinets with low tuned designs where port is impossible to fit
resulting in more compact enclosure designs
Improved low end frequency extension over sealed enclosures
Cons
Steeper low end roll off than bass reflex 29dΒ/octave
Starts to roll off at higher frequencies than bass reflex
The passive driver component is significantly more expensive than a port tube
More about passive radiator design ➡ here
Transmission Line
Description
In this type of enclosure, unlike bass reflex, a line or labyrinth is created on the back side of the driver, through which the waves travel.
Labyrinth is not the only line shape here, although it' s the most popular because it saves room and the enclosure won't end up gigantic.
The length of this labyrinth is correlated to the free air resonant frequency (Fs) of the driver. In practice it's tuned to the 1/4 wavelength frequency of the driver (see example below). In this way we invert by 90° the phase of the back waves of the speaker and the back waves are combined with the front waves on the front (& out) side of the speaker baffle.
The tricky part is to fill the labyrinth with various thicknesses & densities of damping material, in random patterns, until all the upper frequencies are absorbed, leaving the low end only (in a similar way as in the bass reflex enclosure).
This is the main reason eventually that there isn't a reliable way to predict/emulate the output of the labyrinth "tunnel", which compensates for the lack of energy of the driver at it's resonant frequency (Fs) and makes this enclosure type, less popular than the bass reflex type.
Frequency Response Notes:
similar to bass reflex enclosure
low end starts to roll-off a bit higher in frequency than bass reflex
with smoother roll off, similar to sealed box
cut off is a bit lower than bass reflex into the subsonic region (below 20Hz)
In our example Fs=27,83Hz
Wavelength= 343(speed of sound)/27,83=12,32m
1/4 Wavelength= 12,32/4=3m long line/labyrinth
Enclosure example
Ported vs Transmission Line
(Frequency Response)
More about transmission line design ➡ here
Notes
Infinite baffle or Free-air
Finite or Open baffle
Bandpass
Folded Horn
Outro
Some final thoughts and rules of thumb about enclosures, drivers & ports
The bigger the driver diameter the larger the enclosure will be and this results to a speaker with full-blown, fat and flat low end. This affects more than you can imagine the sound stage and stereo image of the speaker & results to an enhanced listening experience.
Smaller enclosures mean smaller baffle sizes and that means more diffraction issues that should be compensated for, in the crossover circuit down the road.
Thin, not dense enough enclosure materials or not braced internal designs and non decoupled setups, results in more vibrations reaching the drivers which smears the stereo image, clarity & openness of the loudspeaker.
When choosing a ported enclosure, pick the biggest port diameter possible & choose a flared design at least at the baffle side (if not both ends) to minimize port turbulence & noise.
Round ports have the best linearity overall, try to avoid 90° elbow tubes if possible or choose a custom 45° pieces to minimize port turbulence.
Choose the driver with the greater excursion (Xmax), the "flattest" frequency response & keep in mind that the materials of the drivers (woofer/mid/tweeter) play a major part (if not the greatest) in the tonality, resonances and the overall character of the sound of the speaker.
Some material examples are paper, silk, aluminum, carbon fiber & diamond to name a few...
Go big or go blown...🔥
