Classroom Audio Systems

Classroom Audio Systems – Intelligibility

When talking about classroom audio systems or any audio system for training environments, the key objective is intelligibility. Intelligibility is just what it sounds like, the quality of being intelligible or understandable.

Audio that is intelligible has the quality of clarity, it has a “crispness” about it. The opposite of intelligible audio is sometimes referred to as the “Peanuts” effect. When Charlie Brown was talking to an adult, all you could hear was “Wha whu waa waa whu wha”. A similar effect can be heard when someone has a TV turned up in the room next to you with the door closed. Speech becomes muffled, not very intelligible. Why, because some of the frequencies that make speech intelligible are being muted while some of the other frequencies are coming through the wall and door more effectively. As we know, bass carries and higher frequencies diminish much more quickly.

Studies have shown that most of speech intelligibility comes from hearing consonants clearly. The sounds “ch” and “sh” and the consonants b, c, f, h, k, p, q, s, t, and x are among the higher frequency sounds that make words intelligible. Whereas vowels tend to carry well and easily at lower frequencies, typically from 100Hz to 280Hz. The fascinating part is that the human ear tends to be most sensitive to frequencies around 3000Hz and this is the range where the above consonants can typically be heard when pronounced clearly and properly. Most cell phones and even land lines have a hard time transmitting frequencies much over 4000hz. You may have noticed that it can be difficult to understand people over the phone, when you have no problem at all hearing them in person. Above 4000hz there is still a lot of audio that promotes speech intelligibility. Really, frequencies up to 8000Hz play a strong roll in enhancing speech intelligibility.

What makes things more challenging is that kids today frequently have slightly impaired hearing for frequencies above 3000Hz. Yes, too much music played too loudly. I guess our parents were right when they said rock n’ roll would make us dumber.

So what works when it come to classroom audio systems and getting the best bang for the buck. You need a system that handles higher frequencies with respect. You don’t want a “muffled” system. When Roemtech designs a classroom audio system, we always put speech intelligibility at the top of the list. You need speakers and amplifiers that accurately recreate the higher frequencies found in the speech range. That’s one reason all of our speakers have dedicated tweeters. For more information, check out our “Speakers” page. You’re going to like what you hear!

8 ohm vs. 70V Classroom Audio Systems

When considering all your options, eventually you will need to decide which type of amplifier is best for your application. If you are considering an amplifier for a classroom audio system, you may be confronted with many options. Some may recommend an eight ohm amplifier and some may recommend a 70V system. They each offer their own advantages for different applications. Here we will review what makes sense and what doesn’t.

Quite simply, eight ohm systems are typically less expensive and work quite well for rooms of 50 people or less when talking about a classroom environment. Obviously most classrooms fall in this category. Eight ohm systems are less expensive because an eight ohm speaker does not have the extra expense of a transformer. In addition, an eight ohm amplifier is less expensive to manufacture. All of this means cost savings.

So when would a 70V amplifier make sense? It can start making sense when you need more than four speakers. Speaker cable can be thinner and wiring becomes easier when more than four speakers are needed. As mentioned in a previous post, when using more than four speakers, typically you will need to be a bit creative when wiring them to an eight ohm amplifier. With a 70V amplifier it is pretty straight forward as speakers can easily be “daisy chained” together until enough speakers have been connected.

70V systems definitely have their place, however most classroom audio systems do not warrant it. Therefore, eight ohm amplifiers typically work best in most classroom applications.

Roemtech has a variety of eight ohm amplifiers that are designed specifically for high quality classroom audio that keep the budget on track.

How Many Speakers can I put on an 8 ohm amp?

The question arises: How many speakers can be put on an 8 ohm amp?

That’s a tricky question. The answer is, as many as you want, theoretically speaking. No, that’s not a joke, the explanation follows.

Just to be clear, when we refer to an 8 ohm amp, we are talking about a typical stereo amplifier with two outputs. The kind that make up 99% of commercial 8 ohm amplifiers.

An 8 ohm amplifier is called such because typically it performs very well with an eight ohm load attached to it. When dealing with most commercial ceiling speakers, you will find that the majority have an 8 ohm rating for audio program material (AC). There are a few out there that are rated at 16 ohms and I know of none that are 4 ohms at the time of this writing.

Let’s say you want eight speakers in a large classroom. You purchase eight of the finest ceiling speakers you can find (otherwise known as the SP-230N). You hire a tech to put them in and taking the easy way out, he wires them in parallel. That means he took the speaker wire pair (+ and -), went into the first speaker and then jumped from that speaker straight to the next speaker, then to the next and so on. This gives you four speakers per channel (rated at 8 ohms each). Since your tech took the easy way out he has just loaded your amp with 2 ohms on each channel. What effect will that have on the amplifier? Most amplifiers will either overheat or shutdown as soon as the volume is turned up. “What? But you said…” I know, at the beginning of the article I said you could hook up as many as you wanted, but it has to be done properly. Almost no amplifier on the market is made to drive much below 4 ohms. Remember, the lower the ohm rating the harder the amplifier has to work to supply the current, this always causes heat and eventually a shutdown or damage.

In order to do this properly so that your classroom audio system sounds great and you are not overworking your amplifier, a series-parallel configuration must be used. For an illustration of this, please refer to our “Tech Tips” section of our website, click on the “Multiple Speaker Diagram”. Basically you take two speakers wired in series to make a 16 ohm pair, then wire those two pairs in parallel, this brings the load back down to eight ohms. So now you have the ideal load of 8 ohms, but you have a total of eight speakers in your classroom. This has many benefits. One big one is that your classroom audio system will actually be louder for the same power. More speakers makes your system more efficient. Also, audio dispersion is a bit better.

This parallel-series configuration can be used to add more and more speakers as long as the load is balanced back to 8 ohms. However, if you need more than eight speakers, it is probably time to consider a 70V system. The practical drawbacks start to become a big factor over eight speakers.

So really, the answer is eight – practically speaking.

Classroom Audio Systems – Mono or Stereo?

When considering classroom audio systems you may be pondering the above question. What are the benefits of having one over the other?

Typically classroom audio systems needs to do two things well.
First, they need to amplify a voice that is instructing the students. This voice may be live or prerecorded. Either way it needs to be heard clearly. This is known as speech intelligibility and it is not affected much by the difference of a mono or stereo setup. For instance, if you have two speakers in your classroom and they are wired in a stereo configuration while amplifying the voice of a single instructor, you would be hard pressed to tell the difference between a stereo configuration versus a mono one. So for strictly voice-lift applications, it makes almost no difference. This brings us to the next scenario.

Second, classroom audio systems need to amplify what we are going to refer to here as non-vocal program material. Let’s say we have an amplifier in a classroom that’s being used to instruct racing car drivers. While showing the program material you want the classroom to hear the engine noises and squealing tires come in from the left and fade out to the right as the car travels across the projection screen showing the corresponding images. This is where stereo shines. Is it an integral part to educating students? No, but it’s cool, and never underestimate the impact of cool. Part of educational effectiveness is making a lasting impression on students. Having an impressive sound system certainly doesn’t hurt.

All of this being considered, if it is voice-lift only, then mono makes sense. If you show the occasional program material, stereo can add to the realism and the price typically isn’t much different.

Our PMA-245, PMA-245H and PMA-350H are all capable of both mono and stereo. For more information, please refer to our amplifiers page.

Ground Loop Humming in Classroom Audio Systems

If you have ever listened to an amplified sound system you have probably heard the dreaded hum. It’s common. From presidential speeches to movie theaters to concerts to classroom audio systems the hum can be heard. What do you do? There are so many factors and so many things that can cause that horrible humming.

The typical culprit are mishandled high voltage grounds. When the facility has several electrical circuits (and they all do), some of the circuits have different grounds with slightly different voltages or ohm values. Many people say that this can cause DC to creep into the electrical system and cause humming issues. While this is typically correct, frequently that DC component has some AC characteristics, but that’s another discussion.

Once the DC creeps in, it can travel a long distance from one component to another. Let’s say that you have a classroom with a laptop that is plugged into your plenum rated amplifier in the ceiling via a 3.5mm audio cable. The devices now have a shared ground over the shield of the audio cable. If the laptop’s ground has a small bit of voltage on it, it will try to lift the ground level of the amplifier and will probably create a humming issue. At this point you have several options.

1. Disconnect the power supply from the laptop. (Not a good choice, since your battery will eventually run out).
2. Get a ground pin eliminator for the power cord. (Not a good choice since this is dangerous).

3. Cut the shield to the audio cable. (Not a good choice as this could cause other audio issues).
4. Get a ground-loop isolation transformer that maintains the signal integrity but, through magnetic decoupling does not allow ground differentials to transfer from one device to the other. (Bingo!)

Yes, number 4 is the correct answer. Roemtech’s HummBuster technology does exactly that. We put several ground-loop isolation transformers in each amplifier that has the HummBuster feature. So if the user ever hears a hum in the speakers, he/she can simply flick the switches until the problem goes away.

The PMA-350H takes this technology to the next level. The amplifier can actually search all of the inputs and deduct which one is causing the problem, once it is identified it engages the ground-loop filters until the problem is gone. This happens with the simple push of a button. Only Roemtech has this technology and it is so unique, we filed a patent on it.

To see how this feature works, refer to our “Videos” page for a demonstration.