Since there are so many different speaker designs out there, the answer to that question could never be an absolute yes or no, at least without knowing the characteristics of that particular speaker. However, in our experience, grilles usually do not improve loudspeaker performance. In this article, we will explain how and why grilles tend to have an adverse effect on the speaker's performance, and we will also talk about how much they can actually spoil the sound quality.
In order to talk about how a grille can affect the performance, we have to talk about diffraction. Generally speaking, diffraction is how a wave is modified when it is deflected by some obstacle. In loudspeaker engineering, diffraction occurs when the sound waves reflect off the speaker enclosure. Very simply put, when the sound pressure wave is produced by the driver, it will disperse at a wide angle, unless the wavelength itself is not as large as the surface area of the diaphragm acting on the air, such as the woofer cone or tweeter dome. A good speaker is designed so that the diameter of the driver's diaphragm is smaller than the wavelength being reproduced. This ensures wide-angle dispersion, instead of beaming like a flashlight. The sound waves can emanate at such a wide angle that it reflects off of the front baffle of the cabinet. The reflected sound waves from the front baffle of the speaker can reach the listener (or microphone) at a very slight time delay with respect to the sound coming directly from the driver diaphragm and cause comb filtering: cancellation and summation effects with the original sound.
To restate loudspeaker diffraction more simply, it is when sound produced by the woofer or tweeter bounces off the front of the speaker and interferes with the sound coming directly from the woofer or tweeter. One important point to make here is that the wavelength of the sound frequency has to be shorter than the speaker's baffle surface in order to be reflected, so bass frequencies that have long wavelengths don't experience diffraction- they just bend around smaller obstacles and surfaces as though they weren't even there. It is this reason why subwoofer grilles will not affect subwoofer sound at all; the wavelengths of those low frequencies are much larger than the speaker enclosure itself, let alone any grille frame. But sound waves of shorter wavelengths will be affected by the size of the front baffle- and everything on it, including the grille. The grille is just more "stuff", acoustic obstacles, for higher frequency sound waves to diffract off of and cause interference patterns.
Table 1 shows some examples of sound frequency wavelengths. We can see that the wavelengths at 1,000 Hz and below are too large to reflect off of a typical home audio speaker baffle, but 2,500 Hz and higher frequencies have wavelengths that are much more common sizes for speaker cabinet widths, and so are more likely to be subject to diffraction effects.
We are not talking about the fabric that is often stretched over the frame of the grille; we are talking about the frame of the grille itself. The fabric does very little to interfere with the sound, assuming the fabric is tautly stretched over the frame. Most grille fabrics are acoustically transparent and generally do not affect the sound in an audible manner or even in a scarcely measurable manner. The fabrics might attenuate the sound slightly in very high frequencies, but the effects would be minor compared to the diffraction caused by the grille frame. The grille frame is a more substantial obstacle. Of course, not all grille frames are the same, and some are more obtrusive than others. A grille with a thick, protruding frame is going to cause a lot more diffraction than a grille with a slight, minimal frame. But the frame of the grille is frequently just a diffracting object which can cause the speaker to have a rougher frequency response than otherwise.
Pictured below are two grille frames: one that incurs a lot of diffraction (Fig. 1) and one that does not (Fig. 2). It can be seen that the thick grille in Figure 1 uses two layers of wood that add up to a half inch thick frame. The top layer of the frame is the more constricting of the two and its proximity to the drivers will increase the severity of diffraction. The frame of the grille in Figure 2 is very thin and keeps its distance away from the drivers. Furthermore, there is also some perforation in the grille frame that may help a bit as well. This grille barely affects the speaker's response at all.
To see how much grilles can affect speakers' responses here is are a few examples of speakers from past reviews, picturing both the grilles and their response difference:
Now let's talk about the impact on the sound from the diffraction effects of grilles. One complication is that the angle and distance from the speaker will somewhat change the frequencies where these diffraction effects occur at. The sum total output of the speaker per frequency is not changed, but the output per frequency does become modified depending on distance and angle. Since the overall acoustic energy generated by the speaker per frequency remains the same, and also since much of what we normally hear from the loudspeaker is not just direct sound from the speaker itself but also the many acoustic reflections from the various surfaces of typical rooms, baffle diffractions effects may not be significantly audible in ordinary listening situations unless it is severe. Whatever the audibility if diffraction effects, in practice they are going to be more significant in the near-field than in the more reverberant far-field, since grille/baffle diffraction does not affect the speaker's total acoustic power output. The audibility of diffraction is even more questionable in modern speakers that have relatively narrow cabinets. The point here is that while we can easily see the effects of the extra diffraction from grilles in these graphs that were measured in quasi-anechoic conditions, the effects would likely not be easily heard amidst in the acoustic conditions of ordinary in-room listening situations. What you hear from a sound system in a domestic environment is far more complex than what a microphone picks up in a reflection-free environment, and the diffraction effects of grilles would be more subtle.
