The frequency response of a PA speaker

Strictly speaking, there are several 'frequency responses'. However, the frequency response of a speaker usually refers to the frequency response of the sound pressure. For practical applications, this is an important detail for the installer and user.

An audio measurement system generates test signals containing frequencies in adjustable ranges. This frequency range is often the range 20-20,000 Hz because it corresponds to the range of human hearing. The test signals are fed to the speaker to be measured via an amplifier. The reproduction of the frequencies by the speaker is then measured with a high-quality sensing microphone.

A good speaker should reproduce all frequencies with the same sound pressure level across the entire frequency range to be measured. You will see that the sound pressure level drops in the lower frequency range (bass) and in the upper frequency range (treble). The radiation range of the speaker is stated within defined limits in which the SPL has fallen by a certain max. value in dB. Depending on the manufacturer, these limits are specified at -3 dB, -6 dB and often also at -10 dB.

We are displaying the frequency response with a graph in a coordinate system. This coordinate system shows how sound pressure and frequency of the reproduced sound are related to each other in a speaker. Hence, if we want to evaluate a speaker on the basis of the frequency response, we have to consider two quantities:

1. The frequencies which the speaker reproduces, i.e. pitches in Hertz. This is shown on the X-axis in the coordinate system.
2. The sound pressure (sound level), i.e. in simple terms the volume level in decibels. This is shown on the Y-axis in the coordinate system.

Be careful when reading the coordinate system: the Y-axis does not start at 0 dB. It starts at the volume level which the speaker features at 20 Hz. This, in turn, is the starting point of the X-axis.

Please pay particular attention to upward deflections in the frequency response. Small fluctuations of 1 to 3 dB are perfectly normal in 100 V technology. From 4 dB upwards, however, we perceive these fluctuations as increasingly irritating, especially at higher frequencies. You have to distinguish here: in case of an equal fluctuation, we rather notice an upward deviation than a downward deviation. Small dips in the frequency response are therefore not as significant as peaks. Depending on the frequency, we may also have a different perception of the irritation.

However, it also depends on the level of deviation. If the speaker increases the level by 5 dB at 1,600 Hz, this also becomes unpleasant, depending on the application.

The bass range in the frequency response (up to 150 Hz)

Please have a look at the left side of the frequency response first. The low bass range is 20-40 Hz. Especially in the PA range, there are hardly any speakers which reach their average volume level at 40 Hz. Even at 40-100 Hz, many PA speakers and 100 V speakers have a problem. This is acceptable and by no means surprising: especially small speakers which are often used in the 100 V range, have a problem in this bass range due to their size. This is often insignificant because PA speakers often provide background sound and frequencies below 100 Hz are not as important. As a guidance: the lowest fundamental note of a normally tuned guitar is at 82 Hz.

Orientation for the bass range:

• Too much emphasis in the bass range will result in a dull and somewhat muffled sound.
• Most male voice have a fundamental note of 100-150 Hz and a frequency range of 80-12,000 Hz.
• A valley of 100-150 Hz can take the warmth from a music reproduction. However, this is often uncritical with background music. For primary PA applications, a speaker should be selected which already reaches a certain volume level from 100 Hz on. This could, for example, be the PA speaker PAB-308/WS.

The midrange in the frequency response (150-2,000 Hz)

The range between 200 and 400 Hz is particularly important for a high-quality music reproduction. It is responsible for the rich, wide and pleasant sound which complements the sound pattern. Most acoustic instruments lie between 200 and 1,000 Hz.

Orientation for the midrange:

Significant increases just below 400 Hz make the sound a bit softer but also make the sound less dynamic.

Boosts above 1,000 Hz tend to provide more pressure, more punch and a rather obtrusive, bright sound.

Female voices often have a fundamental note of just above 200 Hz. The vocal range with treble covers the range of about 150-13,000 Hz.

With valleys of 100-1,000 Hz, you can expect a speaker which emphasises on high frequencies. If this range on average significantly decreases, the speaker does not provide a full range sound. The speaker may not reproduce such a wide and warm sound as a viola or grand piano will do at a live performance.

The treble range in the frequency response (2,000-20,000 Hz)

When the high frequencies are high, the speaker emphasises instruments such as acoustic guitars, violins or parts of high-pitch voices. Strong peaks are particularly irritating between 2,000 Hz and 6,000 Hz which lets the speaker screech to a certain extent. The human ear is particularly sensitive to this range.

Orientation for the high range:

• If a speaker particularly emphasises on the high frequencies, it may provide a more metal sound.
• Too much relative power in the range around 3,000 Hz can result in a highly dynamically and somewhat agressive sound.
• With strong valleys in this range, the sound becomes muffled and undynamic or in the worst case, the speaker 'mumbles'.
• The range from 14,000 Hz on, however, can be neglected for backbround music applications.

The graphs for the frequency response do not comply with a standard. Hence, it is possible to change the visual appearance of the frequency response by adjusting the coordinate system. If the graph is created with a larger spread, it looks straighter and thus, 4-5 dB or a large deviation do no longer have such an impact. So, do look carefully at the axes:

1. How big are the steps in decibels on the X-axis? Is it necessary to stretch the length of the axis like this?
2. The same applies to the Y-axis: where does it begin and where does it end? Does it start at 50 Hz instead of 20 Hz and thus, simply leaving out an audible frequency range?

We hope that this article has given you a little insight. You can also browse through our magazine on additional topics such as 100 V technology, planning PA systems or voice alarm systems according to EN 54.

  _{^{Image source header grafic: snowing12 – stock.adobe.com}}