When choosing whether to install a non-acoustically transparent screen (non-AT) or an acoustically transparent (AT) screen, the choice isn’t an easy one. I personally struggled with this same decision for years. This is why I’ve decided to write about it so you can hopefully make an informed decision.
Types of Projector Screens
The varying types and kinds of projector screens will be covered more extensively in a separate article. However, for our purposes here, projector screens can be split into non-acoustically transparent and acoustically transparent screens.
Non-acoustically transparent projector screens don’t allow audio to pass through them efficiently and they degrade audio performance greatly if speakers are placed behind them.
Acoustically transparent screens are designed to have speakers placed behind them. While there is still some audio degradation, it is minimised.
Within acoustically transparent screens, there are two major types:
Perforated screens are made from a solid material such as PVC – although they can be made of ANY solid material. During manufacturing, lots of little pin-prick holes are punched into the material to allow sound to pass through the screen.
Perforated screens can be mini-, standard- and micro-perforated. Mini and standard perforated screens have around 1.7% of the surface area as perforations, while micro-perforated screens can have as high as 5%. The main issue with micro-perforated screens is possible dust build-up clogging the perforations so they need to be installed in a dust-free environment. However, all perforated screens should be dusted regularly so acoustic performance is not affected.
The main benefit of perforated screens is their retention of brightness and contrast. Generally, micro-perforated screens can retain 1.0 gain or even higher, but that’s not always the case. They start from around 0.8 gain screen and can go unto around 1.1 to 1.3 gain (actually measured versus manufacturer claimed).
However, speakers need to be placed further back from a perforated screen to negate the audio degradation caused by AT screens called comb filtering – more on comb filtering a bit later. For now, keep in mind that ideally, a minimum of 30cms (12 inches) should be between the speakers and the screen. However, 50cms (20 inches) is likely more ideal for excellent audio performance. This allows the effect of comb filtering to be reduced and audio performance to increase.
Woven screens are made from a woven material that can be natural or synthetic, and can be coated with a reflective coating to aid in reflecting light back to the viewer. However, due to how woven materials scatter light more than non-woven materials, the reflectance value of woven screens are lower than perforated screens. They generally start at around 0.8 and go DOWN from there. Manufacturer quoted reflectance values higher than this are generally fiction and bear no resemblance to the actual measured value with properly calibrated equipment.
The upside of woven materials is that they cause less acoustical problems than perforated screens. Firstly, the attenuation of high frequencies is less, and secondly, comb filtering is also reduced. This allows speakers to be placed closer to the back of the screen, so woven screens are the choice if there is not much room behind the screen to place speakers at a distance. While placing speakers further back will aid in reducing comb filtering even further with woven materials as well, it isn’t as critical to do with a woven screen as it is with perforated screens.
Another advantage of woven screens can be the lack of visual artefacts, but it is highly dependent on the type of weave used and the reflective coating applied, if any. Tighter, smaller weaves generally tend to be less visible at closer distances. But ultimately, the viewing distance will determine how visible the weave is. The very best screens don’t produce any sort of artefacts from 2m / 6 feet away, while other screens produce artefacts up to 5m / 16 feet away. So it is critical that you try and get a sample of the material before buying.
Pros and Cons of Acoustically Transparent Screens
Let’s review the pros and cons of acoustically transparent screens and whether it is the right choice for your application.
1. Placing speakers at ear height
Being able to place front speakers behind the screen at ear height is a massive plus. The front soundstage feels more seamless and actors’ voices are actually coming out of the screen as opposed to over or under it. The centre channel especially benefits.
If you have multiple rows of seating, the only other alternative would be to place the centre speaker above the screen or have one both above and under. Unfortunately, have more than one centre speaker will also introduce comb filtering issues, so there is little benefit acoustically.
2. Ability to lower the projection screen
By not having the centre speaker below the screen, the screen can come down further to viewer eye level. This can massively aid in immersion so should not be forgotten as a plus. If you are installing an AT screen, plan to lower it to achieve ideal viewing height for all rows in your home theatre.
3. Ability to have a larger projection screen
As long as your projector is able to handle it, you can go bigger with an AT screen as you don’t need to worry about having left and right speakers fitting to the side of it. By placing them behind the screen, you free up room on your front wall and can go as big as your wall – and projector – allows.
Now that we’ve covered the pros, let’s turn our focus to the downsides of having an acoustically transparent screen.
1. Reduction in Brightness
This is less of an issue with perforated screens, but woven screens reflect around 20 – 40% less light back than a unity-gain white screen. 20% is not that noticeable, but 40% is. So you either have to plan for this with your projector’s output or you should choose a screen that has a higher reflectance value of between 0.8 (for woven) to 1.0 (for perforated) while also taking into account and balancing other drawbacks we are about to cover.
2. Reduction in Contrast
There is a slight reduction of contrast with AT screens – around a maximum of 10% with the black backing installed. The reason there is a black backing for AT screens is partly to reduce the chance of “backlighting”, which happens when light penetrates the screen, bounces around and then shines back through the perforations or weave. This can massively reduce contrast.
So you have two possibilities to fix this:
- Black out the enclave behind the screen with triple velvet material
- Put on the optional black backing
While the black backing can introduce a bit more high-frequency attenuation, which we will cover in a bit, it more than makes up for it in visual performance. Even though my speaker enclave is completely black, light still bounced around and reduced contrast (and sharpness) so I had to put the black backing on. I wasn’t willing to take the hit to picture quality.
3. Possible Visual Artefacts
There are a few visual artefacts that can happen with some acoustically transparent screens so let’s summarise them here.
- Reduction in sharpness: you can get a reduction in sharpness either because of backlighting or because of the weave or perforations. Some screens show this less than others. The material I am using doesn’t have this issue at all, as long as the black backing is on. With the black backing off, there is a noticeable drop in resolution and a softening of the image.
- Moire Effect: this can happen if the weave or perforation pattern matches that of the pixel grid of your projector. The higher-resolution the projector, the less likely this is to happen. The finer the weave, the less likely this is to happen as well. Also, how the weave is oriented makes a difference for some screens. This is not at all an issue with my JVC NZ8 8K projector as there is virtually no pixel structure to speak of. But this is something to worry about with LCD projectors especially due to the higher inter-pixel gap.
- Visible weave or perforations: these can show up especially on brighter scenes or on solid colours. Again, some screens do better with this than others. It is important to contact the manufacturer for a sample or read reviews on the screen / screen material you are planning to install (see at the end of the article for more on this.) Again, zero issue with the screen material I have: no visible weave beyond 2m, and I am highly sensitive to this.
4. Acoustical Issues – Attenuation
AT screens introduce some attenuation of the mid-range but especially the high frequencies – with perforated screens doing worse than woven screens. Perforated screens can reduce high frequencies up to around 10dB in some instances, while woven screens only around 3 – 6dB.
However, this is generally not an issue, as equalisation can and will correct for this. Ultimately, this is something that all commercial theatres need to deal with and use equalisation to bring the levels back up.
The only major issue to consider here is to over-size your front speakers. My THX Select 2 speakers are rated correctly for my room size, but once I switched to an AT screen, and applied equalisation, even with a woven screen, I was worried about blowing a tweeter when listening to music using aggressive equal-loudness curves. The speakers simply don’t have that extra headroom to do so without possible damage – at least not at or near reference level. So ultimately, I need to upgrade to either an Ultra 2 set for behind the screen – or equivalent, so they can take the abuse.
5. Acoustical Issues – Comb Filtering
Comb filtering is the result of two identical sound waves interacting and causing peaks and troughs in the frequency response. Unfortunately, the small holes on an acoustically transparent screen behave like distinct sound sources – as if you had lots of tiny speakers – and cause comb filtering. It is more pronounced on perforated screens than woven screens – but happens on both.
The closer the speaker is to the screen, the more this issue is amplified. Hence we said earlier, you need to aim for putting around 30 – 50cm distance between the screen and the speakers. If that is not possible, angling the speakers by toe-in and angle-up / angle-down (in the case of the centre speaker) can help!
This is what I had to do with my current speakers as there’s very little space behind the screen. In fact the speakers we are building for the enclave will have 2cm distance between the screen and the speakers. Hence, I had to get a woven screen and the speakers will be ever slightly toed in – as much as those 2cms will allow. A perforated screen would introduce nasty comb filtering at this distance, however, so that was out of the question.
Conclusion / Is an AT Screen Right for You?
Let me get this out of the way: I do not regret going to an acoustically transparent screen. To me, the pros greatly outweigh the cons. The immersion you get by putting the front speakers behind the screen is immense. What’s more, the screen can be at the right height, can be somewhat bigger and the speakers can be as large as the space behind the screen allows.
However, in my case the projector is bright enough to stand the 20% brightness loss, which is not large. This is about what a lamp can lose in its first 500hrs. I simply opened up the manual iris by a couple of clicks, but even without that, it was fine. In addition, the screen I selected has no visible structure or artefacts from our 3m viewing distance, which was the most critical consideration for me.
However, you have to weigh up the pros and cons for your setup to make that decision.
To help you with that decision, there is a very comprehensive review of screen materials on AVSForum here. It is recommended to read it before choosing a screen material. I personally selected a weave that was identical to the XYScreens Soundmax 4K due to this review. I am more than pleased with the results in terms of its sharpness and acoustical performance. However, your use-case might be different so make sure to read this article carefully again as well as the above excellent resource, before making a choice.
Lastly, feel free to ask questions in the comments section below. We’re pretty prompt with replies.
Recalibrating the Projector
It is recommended to recalibrate your projector – or touch up your existing calibration when installing a new screen. See the Simple Home Cinema Display Calibration Guide for more on how you can accomplish this relatively cost-effectively yourself. It is a skill every home cinema enthusiast should learn – at least on a basic level.