In the previous article we talked about how sound is composed by a superposition of mechanical waves. These waves have different frequencies and “colour” the sound in different ways. Knowing this, the task of a good sound system is to reproduce these frequencies in a balanced way: playing around with them is the job of musicians and composers, and we don’t want to mess around with their work when we play it for an audience.

The first problem to solve for a sound system maker is that different frequencies require different technical choices. Generally speaking low frequencies are best reproduced by “slowly” moving larger membranes, while high frequencies come out better from smaller and stiffer objects moving “fast”.

There are several ways, mechanical and electronic, to ease this task. In many cases the first approach is to use a transducer for the low and mid/low frequencies, often called woofer, and another one specialized in high and mid/high frequencies. You may hear many different names for this one, corresponding to different technologies, but let’s just call it tweeter for now. When this double approach is used we call it a two way speaker. As of 2020, all Partybag products are two way speakers.

A nice example: Partybag!

In a Partybag 7 for example the rectangular opening on top of the “face” is the tweeter, and reproduces high frequencies, while the large round loudspeaker under it and covered by the grid is the woofer. The electronic board inside, that includes a DSP (Digital Signal Processor), between many other jobs distributes the sound between the two allowing each transducer to work with its favourite frequency range. In this way we make sure that the sum of the two contributions is a clean continuum. In order to get more bass frequencies, that between other things are the ones that put more stress on the battery, the woofer works in synergy with the two circular openings you see next to it. These form a sounding board, kind of like the one you see on a classic guitar, and that configuration is called bass reflex. This is another solution very widely adopted on professional and semi-professional sound systems.

So what about this “Frequency Response”?

In an approximated and very down-to-earth way we can describe the Frequency Response as the measure, often shown in a 2D chart, of how loud a sound system reproduce each frequency or frequency range when given a constant-level input. For example, we can feed a Partybag Mini a sound signal including all the frequencies in a way that no one is louder than the others (something quite horrible to listen at), and then measure the level of the output from our beloved speaker at different frequencies. Ideally we would like for them to be all at the same level, that would mean same level in – same level to your ear for each frequency, in which case the chart would appear flat. But reality is never ideal, and looking at the Frequency Response you can see how much your speaker differs from being acoustically “transparent”. The easiest thing is to just show it, here is our Partybag Mini:

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  Partybag Mini frequency response plot

A common way to summarize charts like this is, as we do as well, to declare “Frequency Response (-3dB) 100-20000Hz”. Meaning? That between 100 and 20000Hz there will be not a point on the chart lower than 3dB from the maximum. Hey, that’s not bad at all! Another common way is to talk about the frequency response “+/-3dB”, more permissive and so more appreciated by marketing departments.

Unfortunately in advertisements you can find the most bizarre things, and it’s not rare to read vague statements such as “Frequency Response 40-20000Hz” without further specifications: could be +/-6dB, +/-10dB, who knows? It depends on how much the producer has been generous with himself. This is just a quick way to showcase the speaker as extra capable, expecially when it comes to low frequencies: since the line unavoidably falls down in the lower part of the spectrum, using a larger tolerance allows a seller to say “we got extra bass!” and play with the unknowing customer. We are more inclined to use the verb “play” in association with “music”, that’s why we are writing this.

What Frequency Response doesn’t say

• It doesn’t say how the situation will change at different volumes. Ideally it remains the same, but in the real world almost nothing does. By the way, some producers use techniques to increase bass frequencies that only work at low volume. This certainly has a nice effect in some situations, but makes the behaviour of the speaker very different at higher levels such as during an event. We at Partybag prefer to avoid this, because for us “normal” means “at high volume”!
• It doesn’t say how the speaker will sound when you are not “on axis”, so not exactly in front of it. To describe this there are directivity measures, but that’s another story and possibly an idea for more articles. In the Partybag products so far we have tried to privilege wide coverage over long throw, because it fits better their most common usage cases. That’s why we don’t provide as accurate directivity data as the ones you can find, for example, on the datasheet of line array systems for large concerts in stadiums.
• It doesn’t say how the speaker will interact with the surrounding environment depending on the positioning, on the reflecting surface, on atmospheric conditions such as temperature and humidity.

For these reasons, but also because we believe in our products, what we suggest is to try them hands-on in our shops or even directly in our headquarters, where we are always ready to meet you, answer to your questions and offer you a good Italian coffee. In any case a look at the technical specifications can’t hurt, and we hope these articles can be helpful in giving non experts a better understanding of them.